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Obesity. Common and increasing problem in the Western world. Usually defined according to body mass index (see Table 12; p 79). Approximately 20% of UK adults are obese by this definition and this number has trebled in the last 20 years. Morbid obesity is defined as twice ideal body weight and affects 1% of the population; general mortality in this group is twice that of normal. Distribution of fat is thought to be more important than weight per se, with abdominal deposition particularly detrimental. Thus for a BMI ≥ 25 kg/m², a waist (just above the navel) circumference of ≥ 94 cm indicates increased risk and ≥ 102 cm substantially increased risk for men; corresponding values for women are 80 cm and 88 cm respectively.

• Effects:

ent RS:

– increased body O₂ demand and CO₂ production, because of increased tissue mass. Minute ventilation required to maintain normocapnia is thus increased, which further increases O₂ demand.

– reduced FRC because of the weight of the chest wall. FRC is especially reduced in the supine position, due to the weight of the abdominal wall and contents. Thoracic compliance is thus reduced, increasing work of breathing and O₂ demand. mismatch results in hypoxaemia.

– hypoxic pulmonary vasoconstriction increases work of the right ventricle and may lead to pulmonary hypertension and right-sided cardiac failure.

– obstructive sleep apnoea and alveolar hypoventilation syndrome may occur.

ent CVS:

– cardiac output and blood volume increase, to increase O₂ flux.

– hypertension occurs in 60%; thus left ventricular work is increased. Left ventricular hypertrophy and ischaemia may occur, with resultant left-sided cardiac failure. Arrhythmias are common.

– ischaemic heart disease is common due to hypercholesterolaemia, hypertension, diabetes mellitus and physical inactivity.

ent other diseases are more likely, e.g. non-insulin-dependent diabetes mellitus (caused by insulin resistance and inadequate insulin production, the latter worsening with age), hypercholesterolaemia, gout and arthritis, gallbladder disease, hepatic impairment due to fatty liver and cirrhosis, CVA, breast and endometrial malignancies.

Patients may present for bariatric surgery or other procedures. The former is usually done laparascopically and includes gastric banding, partial gastrectomy and gastric bypass, as single procedures or in combination.

• Anaesthetic considerations:

ent preoperatively:

– preoperative assessment for the above complications and appropriate management. Patients may be taking amfetamines or other drugs for weight loss.

– low-mw heparin prophylaxis is routine, because patients are less mobile and risk of DVT is increased. The ideal prophylactic dose is not certain in morbidly obese patients, but suggested regimens include enoxaparin 40 mg sc bd or 0.5 mg/kg od. Intermittent pneumatic calf compression should be used if possible.

– im injection may be difficult because of subcutaneous fat, while anti-DVT stockings may not fit.

ent perioperatively:

– veins may be difficult to find and cannulate.

– hiatus hernia is common, with risk of aspiration of gastric contents. Volume and acidity of gastric contents may be increased. In addition, tracheal intubation may be difficult: insertion of the laryngoscope blade into the mouth may be hindered, the neck may be short and movement reduced.

– hypoxaemia may occur rapidly during apnoea, since FRC (hence O₂ reserve) is reduced, and O₂ consumption increased. FRC is increased if the patient is positioned head-up before induction of anaesthesia.

– airway maintenance is often difficult, because of increased soft tissue mass in the upper airway. Spontaneous ventilation is often inadequate because of respiratory impairment, which worsens in the supine position (especially in the head-down position or with legs in the lithotomy position). Thus IPPV is usually employed; high inflation pressures may be required.

– transferring and positioning the patient may be difficult. The typical maximum weight limit for manual operating tables is 135 kg, and for electrical operating tables is 250–300 kg. Two operating tables placed side by side may be necessary if the patient is too wide for a single table.

– monitoring may be difficult, e.g. BP cuff too small, small ECG complexes.

– surgery is more likely to be difficult and prolonged, with increased blood loss.

– drug use:

– appropriate dosage may be difficult; e.g. neuromuscular blocking drugs are given according to lean body weight. Factors affecting drug pharmacokinetics include: changes in the volume of distribution due to decreased fraction of total body water, increased fatty tissue and increased lean body mass; increased drug clearance due to increased renal blood flow, increased GFR and tubular secretion; changes in plasma protein drug binding.

– increased metabolism of inhalational anaesthetic agents is thought to occur, e.g. increased fluoride ion concentrations after prolonged use of enflurane.

– although regional techniques may have potential advantages over general anaesthesia, they are often technically difficult.

ent postoperatively:

– atelectasis and hypoventilation are common, with increased risk of infection, hypoxaemia and respiratory failure. Patients are often best nursed sitting. Elective non-invasive positive pressure ventilation may be beneficial. Difficulty mobilising may be a problem.

– postoperative analgesia, O₂ therapy and physiotherapy are especially important. CPAP therapy for obstructive sleep apnoea should be restarted in the immediate postoperative period. HDU or ICU admission is often required.

Cheah MH, Kam PCA (2005). Anaesthesia; 60: 1009–21

Obesity hypoventilation syndrome (Pickwickian syndrome, after a character from Dickens’ Pickwick Papers). Obesity, daytime hypersomnolence, hypoxaemia and hypercapnia, often in the presence of right ventricular failure. Causes are multifactorial:

ent most patients have restrictive lung disease, resulting in poor compliance, increased work of breathing, alveolar hypoventilation and increased CO₂ production. Pulmonary hypertension is present in 60% of patients.

ent patients often have coexisting mismatch. Cyanosis and plethora are common, due to polycythaemia secondary to hypoxia.

ent severe obstructive sleep apnoea is almost invariable. In addition there is a disordered central control of breathing, possibly due to leptin deficiency or resistance. Control is especially poor during sleep and sudden nocturnal death is common.

General and anaesthetic management is as for obesity and cor pulmonale. The F_IO₂ should be increased cautiously to avoid depression of the hypoxic ventilatory drive. CPAP may be useful to reduce hypercapnia and normalise O₂ saturations. Respiratory depressant drugs should also be used cautiously; postoperative respiratory failure may occur.

[Charles Dickens (1812–1870), English author]

Piper AJ, Grunstein RR (2011). Am J Respir Crit Care Med; 183: 282–8

Obstetric analgesia and anaesthesia. Strictly, ‘analgesia’ refers to removal of pain during labour and ‘anaesthesia’ is provided for operative delivery and other procedures. Pain during the first stage of labour is thought to be caused by cervical dilatation, and is usually felt in the T11–L1 dermatomes. Back and rectal pain may also occur. Pain often worsens at the end of the first stage. Pain during the second stage is caused by stretching of the birth canal and perineum.

Early attempts at pain relief included the use of abdominal pressure, opium and alcohol. Simpson administered the first obstetric anaesthetic in 1847, using diethyl ether. He used chloroform later that year, subsequently preferring it to ether. Moral and religious objections to anaesthesia in childbirth declined after Snow’s administration of chloroform to Queen Victoria in 1853. Regional techniques were introduced from the early 1900s, and have become increasingly popular since the 1960s.

Choice of technique is related to the physiological effects of pregnancy (especially risk of aortocaval compression and aspiration pneumonitis), and effects of drugs and complications on the fetus, neonate and course of labour. Anaesthesia has until the last 20–30 years been a major cause of maternal death, as revealed in the Reports on Confidential Enquiries into Maternal Deaths.

• Methods used:

ent non-drug methods, e.g. TENS, acupuncture, hypnosis, psychoprophylaxis, audioanaesthesia (‘white noise’; high-frequency sound played through headphones), abdominal decompression (application of negative pressure to the abdomen): generally safe for mother and fetus, but of variable efficacy and thus rarely used, except for TENS and psychoprophylaxis.

ent systemic opioid analgesic drugs:

– morphine was used with hyoscine to provide twilight sleep in the early 1900s. However, it readily crosses the placenta to cause neonatal respiratory depression. pethidine was first used in 1940 and approved for use by UK midwives in 1950; it is the most commonly used opioid (e.g. 50–150 mg im up to two doses), but some units prefer diamorphine. 30–75% of women gain no benefit from pethidine and there is little evidence that opioids actually reduce pain scores. Nausea, vomiting, delayed gastric emptying and sedation may occur, with neonatal respiratory depression especially likely 2–4 h after im injection. Neonatal respiratory depression is marked after iv injection. Subtle changes may be detected on neurobehavioural testing of the neonate.

– other opioids have been used with similar effects. A lower incidence of neonatal depression has been claimed for partial agonists and agonist/antagonists (e.g. nalbuphine, pentazocine, meptazinol), but they are not commonly used.

– patient-controlled analgesia has been used, e.g. pethidine 10–20 mg iv or nalbuphine 2–3 mg iv (10 min lockout), or fentanyl 10–25 µg following 25–75 µg loading dose (3–5 min lockout). More recently, remifentanil has been used (e.g. 30–40 µg bolus, 2–3 min lockout), though severe respiratory depression has been reported.

– opioid receptor antagonists, e.g. naloxone, may be required if neonatal respiratory depression is marked.

ent sedative drugs: rarely used nowadays; promazine, promethazine, benzodiazepines, chloral hydrate, clomethiazole and chlordiazepoxide have been used. All may cause neonatal depression.

ent inhalational anaesthetic agents:

– ether and chloroform were first used in 1847. Trichloroethylene was used in the 1940s, and methoxyflurane in 1970; formerly approved for midwives’ use with draw-over techniques, their use in the UK ceased in 1984.

– N₂O was first used in 1880. Intermittent-flow anaesthetic machines were developed from the 1930s, using N₂O with air or O₂. Entonox was used in 1962 by Tunstall, and approved for use by midwives in 1965. It is usually self-administered using a face-piece or mouthpiece and demand valve. Slow deep inhalation should start just before a contraction begins, in order to achieve adequate blood levels at peak pain. May cause nausea and dizziness; it is otherwise relatively safe with minimal side effects, although maternal arterial desaturation has been reported, especially in combination with pethidine. Useful in 50% of women but of no help in 30% and, like opioids, there is little evidence that it reduces pain scores. Isoflurane has been added with good effect (Isoxane).

– enflurane, isoflurane, desflurane and, more recently, sevoflurane have been used with draw-over inhalation.

ent general anaesthesia: no longer used for normal vaginal delivery. Problems are as for caesarean section.

ent regional techniques: involve blockade of the nerve supply of:

– uterus:

– via sympathetic pathways in paracervical tissues and broad ligament to the spinal cord at T11–12, sometimes T10 and L1 also.

– the cervix is possibly innervated via separate S2–4 pathways in addition.

– birth canal and perineum: via pudendal nerves (S2–4), genitofemoral and ilioinguinal nerves and sacral nerves.

• Regional techniques used:

ent epidural anaesthesia/analgesia:

– caudal analgesia was first used in obstetrics in 1909 by Stoeckel; a continuous technique was introduced in the USA in 1942.

– continuous lumbar techniques were first used in 1946; they have become popular in the UK since the 1960s, with most units now providing a 24-h service. Uptake varies widely, with up to 70–80% for primiparae in some centres. The overall epidural rate in the UK is around 20–30%.

– advantages:

– reduces maternal exhaustion, hyperventilation, ketosis and plasma catecholamine levels.

– avoids adverse effects of parenteral opioids.

– reduces fetal acidosis and maintains or increases uteroplacental blood flow if hypotension is avoided.

– may improve contractions in incoordinate uterine activity.

– thought to reduce morbidity and mortality in breech delivery, multiple delivery, premature labour, pre-eclampsia, maternal cardiovascular or respiratory disease, diabetes mellitus, forceps delivery and caesarean section.

– disadvantages:

– risk of hypotension, extensive blockade, iv injection and other complications. Post-dural puncture headache is more common than in non-pregnant subjects following accidental dural tap (the maximum acceptable incidence of the latter has been set at about 1% in the UK). Shivering and urinary retention may occur.

– motor block may be distressing, and if extensive may be associated with delayed descent of the fetal head.

– requires iv cannulation (although the need for routine administration of iv fluids has been questioned if low-dose techniques are used.

– requires 24-h dedicated anaesthetic cover.

– increased incidence of backache has been reported, but this has been shown to reflect selection of patients prone to backache (e.g. complicated labour, lower pain threshold), plus the tendency of patients to link back pain with any procedure performed on the back, rather than a result of regional analgesia or anaesthesia itself.

– effect on labour:

– temporary reduction in uterine activity has been reported following injection of solution, though this may be caused by the bolus of crystalloid traditionally given concurrently.

– incoordinate uterine activity may improve.

– ventouse/forceps rate is increased; thought to occur because:

– patients likely to require forceps delivery are more likely to receive epidural analgesia.

– muscle tone is reduced, as above.

The relative importance of these two factors is hotly disputed, with non-anaesthetists claiming that epidurals cause an increase in instrumental delivery rates whilst anaesthetists claim that epidural analgesia is merely a marker of abnormal and/or high-risk labours. Randomised clinical trials are few and suffer from practical problems such as lack of obstetric blinding and non-compliance with the allocated treatment. The argument is therefore likely to continue, although studies suggest that low-dose techniques are more likely to result in spontaneous delivery than the older, higher-dose methods (though even with the latter, normal vaginal delivery rates are thought to occur if adequate time is allowed for the second stage). Perineal tears may occur if the second stage is very prolonged.

– technique:

– standard techniques are used, but low doses of local anaesthetic agent are used to minimise motor block and risk of adverse effects. If higher doses are used, smaller volumes are required because venous engorgement reduces the volume of the epidural space. Hypotension is common with higher doses of local anaesthetic, especially in the presence of hypovolaemia; it is reduced by preloading with iv fluid, usually crystalloid (e.g. 0.9% saline/Hartmann’s solution, 500 ml). L2–3 or L3–4 interspaces are usually chosen, although, because identification of the lumber interspaces by palpation is not reliable (especially in pregnancy when the pelvis tilts), anaesthetists often place the catheter at a higher interspace than that intended.

– bupivacaine is traditionally preferred, since fetal transfer is least. Others have been used, e.g. lidocaine, chloroprocaine. prilocaine is rarely used because of the risk of methaemoglobinaemia. Ropivacaine is claimed to cause less motor block than bupivacaine when higher concentrations are used. Levobupivacaine has a better safety profile, but with low-dose regimens this difference becomes less relevant.

– use of a test dose is controversial. With low-dose regimens, the first dose is also the test dose.

– suitable dose regimens:

– bupivacaine 0.1% 10–15 ml with fentanyl 1–2 µg/ml as boluses. More concentrated solutions provide analgesia lasting slightly longer, but with more motor blockade. 0.75% solution is contraindicated in obstetrics. Manual top-up injections are usually given by midwives. Aspiration through the catheter should precede top-ups, which should be given in divided doses except for low-dose solutions. A maximum of 25 mg bupivacaine has been suggested for any single injection. Ropivacaine 0.2% is an alternative.

– infusions: provide more consistent analgesia, with less motor block and hypotension than high-dose top-ups, and reduce the risk from accidental iv or subarachnoid injection. Bupivacaine 10–20 mg/h is usually employed, usually as a 0.1–0.2% solution, and often combined with fentanyl 1–2 µg/ml. Large volumes of more dilute solutions have been used, supporting the concept of an ‘extended sleeve’ of anaesthetic solution over the appropriate segments. The height of the block must be regularly assessed, and the infusion adjusted accordingly.

– patient-controlled epidural analgesia is also used, e.g. with 0.1–0.125% bupivacaine with fentanyl 1–3 µg/ml, and boluses of 8–12 ml without a background infusion, or of 3–6 ml with a background infusion of 3–6 ml/min, and a lockout time of 10–20 min.

– epidural opioids have been used alone, but rarely in the UK (see Spinal opioids). Fentanyl is usually added to weak solutions of bupivacaine as above. In the USA, sufentanil is often used. Epidural pethidine has also been used.

– inadequate blockade includes: ‘missed segment’ (commonly in one groin, the cause is unclear); backache (especially with occipitoposterior presentation); rectal or perineal pain; and unilateral blocks. Remedial measures include further injection of solution, with the unblocked part dependent. Use of a stronger solution, a different local anaesthetic, or fentanyl 50–75 µg, may be helpful. The catheter should be withdrawn 1–2 cm if unilateral block occurs. Resiting of the catheter may be required. Suprapubic pain may result from a full bladder, and may be relieved by urinary catheterisation. Breakthrough pain in the presence of a uterine scar may indicate uterine rupture. Overall about 10% of epidurals require adjustment or extra doses.

– contraindications, complications and management are as for epidural analgesia/anaesthesia. Care should be taken in antepartum haemorrhage (see below). Extensive blockade and accidental iv injection of local anaesthetic are possible following catheter migration. All blocks should be regularly assessed and an anaesthetist should be readily available, with resuscitative drugs and equipment. Maximal doses of local anaesthetic agents should not be exceeded in a 4-h period.
Backache and neurological damage may be caused by labour itself, although epidural analgesia is often blamed by the patient and non-anaesthetic staff.

ent spinal anaesthesia was first used in 1900. Popular in the USA in the 1920s, it only increased in popularity in the UK towards the end of the 1900s. Technique and management are as standard, but with more rapid onset of hypotension and greater incidence of post-dural puncture headache and variable blocks (especially using plain bupivacaine) than in non-pregnant subjects. Dose requirements are reduced, possibly due to altered CSF dynamics, although changes in CSF pH, proteins and volume have been suggested. Effects are as for epidural anaesthesia. Mostly used for caesarean section, forceps and ventouse delivery and removal of retained placenta. Doses for vaginal procedures: 1.0–1.6 ml heavy bupivacaine 0.5%; lower doses with opioids have also been used.

ent CSE has been advocated because of its rapid onset and intense quality of analgesia (from the spinal component), with subsequent management as for epidural analgesia. Its routine place in labour is controversial because of its increased cost, the increased risk of post-dural puncture headache, damage to the conus medullaris and (theoretical) concerns over increased risk of infection. In addition, the unreliability of identifying the lumbar interspaces by palpation may result in insertion of the needle at a higher vertebral level than intended. The lowest easily palpable interspace should therefore be chosen, and an epidural-only technique used above L3–4.

A widely used starting intrathecal dose is 1 ml plain bupivacaine 0.25% mixed with fentanyl 25 µg, made up to 2 ml with saline. 3–5 ml boluses of the low-dose epidural mixture above are also used. Continuous spinal analgesia has been described, using the same low-dose solution.

ent paravertebral block: bilateral blocks are required at either L2 (for sympathetic block) or T11–12 (somatic block).

ent paracervical block: rarely performed because of fetal arrhythmias.

ent pudendal nerve block and perineal infiltration/spraying with local anaesthetic: only of use for the second stage. Pudendal block is used for forceps and ventouse delivery.

ent local infiltration of the abdomen for caesarean section.

• Particular problems in obstetric anaesthetic practice:

ent obstetric conditions, e.g. pre-eclampsia, placenta praevia, placental abruption, postpartum haemorrhage. Haemorrhage may follow any delivery, and facilities for urgent transfusion should be available, including a cut-down set and O-negative uncross-matched blood. DIC may also occur in septic abortion, intrauterine death, hydatidiform mole and severe shock.

ent maternal disease, e.g. cardiovascular, respiratory, diabetes. Epidural blockade is usually preferred.

ent fluid overload associated with oxytocin administration; pulmonary oedema associated with tocolytic drugs.

ent specific procedures/presentations:

– premature labour: spinal/epidural analgesia/anaesthesia is usually preferred, since it allows smooth controlled delivery with or without forceps. The immature fetus may be especially susceptible to drug-induced depression. Tocolytic drugs may have been used.

– twin delivery: epidural analgesia is usually employed. Caesarean section is required for delivery of the second twin in up to 10% of cases. Blood loss at delivery is greater than with a single fetus. The enlarged uterus is more likely to cause aortocaval compression.

– breech presentation: most deliver by caesarean section nowadays because of evidence that neonatal outcome is better.

– manual removal of placenta: spinal/epidural anaesthesia is usually considered preferable to general anaesthesia, since the latter risks aspiration of gastric contents, and inhalational agents cause uterine relaxation.

ent collapse on labour ward:

– causes include: shock associated with abruption and DIC, postpartum haemorrhage, total spinal blockade, overdosage or iv injection of local anaesthetic, amniotic fluid embolism, PE, eclampsia, inversion of the uterus and pre-existing disease.

– CPR is hindered by aortocaval compression, relieved by tilting the patient to one side or manually displacing the uterus laterally. Caesarean section should be undertaken within 5 min if there is no improvement in the mother’s condition.

[Walter Stoeckel (1871–1961), German obstetrician; Michael E Tunstall (1928–2011), Aberdeen anaesthetist]

Obstetric intensive care. Required in 0.2–9 cases per 1000 deliveries, depending on the population served and the ICU admission criteria used. Most common reasons for admission are haemorrhage, pre-eclampsia and HELLP syndrome; a mortality of 3–4% is reported in UK series but up to 20% has been reported elsewhere. Main problems are related to the risks to the fetus and the physiological changes of pregnancy: obstetric patients have increased oxygen demands and reduced respiratory reserves, and are more susceptible to aspiration of gastric contents, aortocaval compression, acute lung injury, DVT and DIC.

General management is along standard lines, with attention to the above complications. Excessive fluid administration should be avoided, since ARDS is a common feature of obstetric critical illness. Fetal monitoring should be ensured if antepartum, although the needs of the mother outweigh those of the fetus. Uteroplacental blood flow may be impaired by vasopressors and the mother may be too sick to receive tocolytic drugs should premature labour occur. Caesarean section may be required to improve the mother’s condition. Breast milk may be unsuitable for use because of maternal drugs; if required, lactation can be suppressed with bromocriptine (although hypertension, CVA and MI have followed its use, hence it should be avoided in hypertensive disorders).

Price LC, Slack A, Nelson-Piercy C (2008). Best Pract Res Clin Obstet Gynaecol; 5: 775–99

Obstructive sleep apnoea, see Sleep apnoea/hypopnoea

Obturator nerve block. Performed to accompany sciatic nerve block or femoral nerve block, or in the diagnosis and treatment of hip pain. The obturator nerve (L2–4), a branch of the lumbar plexus, passes down within the pelvis and through the obturator canal into the thigh, to supply the hip joint, anterior adductor muscles and skin of medial lower thigh/knee.

With the patient supine and the leg slightly abducted, an 8 cm needle is inserted 1–2 cm caudal and lateral to the pubic tubercle, and directed slightly medially to encounter the pubic ramus. It is then withdrawn and redirected laterally to enter the obturator canal, and advanced 2–3 cm. If a nerve stimulator is used, twitches in the adductor muscles are sought. After careful aspiration to exclude intravascular placement, 10–15 ml local anaesthetic agent is injected.

In an alternative approach, the leg is externally rotated and abducted and an 8–10 cm needle inserted behind the adductor longus tendon near its pubic insertion, and directed posteriorly and slightly cranially and laterally. 5–10 ml solution is injected at a depth of 2–3 cm.

Occipital nerve blocks, see Scalp, nerve blocks

Octreotide. Long-acting somatostatin analogue, used in carcinoid syndrome and related GIT tumours and acromegaly. Also licensed for use in treating complications of pancreatic surgery. Has also been used in bleeding oesophageal varices, and to reduce vomiting in palliative care. Plasma levels peak within an hour of sc administration, and within a few minutes of iv injection. Half-life is 1–2 h. Lanreotide is a similar agent.

• Dosage:

ent 50 µg sc od/bd, increased to 200 µg tds if required (rarely up to 500 µg tds in carcinoid syndrome).

ent 50–100 µg iv in carcinoid crisis, diluted to 10–50% in 0.9% saline.

ent 50 µg iv followed by 50 µg/h in bleeding varices.

• Side effects: GIT upset, glucose intolerance, hepatic impairment.

Oculocardiac reflex. Bradycardia following traction on the extraocular muscles, especially medial rectus. Afferent pathways are via the occipital branch of the trigeminal nerve; efferents are via the vagus. The reflex is particularly active in children. Bradycardia may be severe, and may lead to asystole. Other arrhythmias may occur, e.g. ventricular ectopics or junctional rhythm. Bradycardia may also follow pressure on or around the eye, fixation of facial fractures, etc. The reflex has been used to stop SVT with eyeball massage. Reduced by anticholinergic drugs administered as premedication or on induction of anaesthesia. If it occurs, surgery should stop, and atropine or glycopyrronium should be administered. Retrobulbar block does not reliably prevent it; local infiltration of the muscles has been used instead.

See also, Ophthalmic surgery

Oculogyric crises, see Dystonic reactions

Oculorespiratory reflex. Hypoventilation following traction on the external ocular muscles. Reduced respiratory rate, reduced tidal volume or irregular ventilation may occur. Thought to involve the same afferent pathways as the oculocardiac reflex, but with efferents via the respiratory centres. Heart rate may be unchanged, and the reflex is unaffected by atropine.

ODAs/ODPs, see Operating department assistants/practitioners

Odds ratio. Ratio of the odds of an event’s occurrence in one group to its odds in another, used as an indicator of treatment effect in clinical trials. For example, if a disease is suspected to be caused by exposure to a certain factor, a 2 × 2 table may be drawn for proportions of patients in the following groups:

	With disease	Without disease
Exposed	a	c
Not exposed	b	d

Odds ratio	= the ratio of a/b to c/d
	= ad/bc.

Harder to understand (but more useful mathematically) than other indices of risk commonly used.

ODIN. Organ dysfunction and/or infection, see Logistic organ dysfunction system

O’Dwyer, Joseph (1841–1898). US physician; regarded as the introducer of the first practical intubation tube in 1885, although the technique had been described previously by others, e.g. Kite. His short metal tube, used as an alternative to tracheostomy in diphtheria, was inserted blindly into the larynx on an introducer; the flanged upper end rested on the vocal cords. He mounted his tube on a handle for use with Fell’s resuscitation bellows in 1888; the Fell–O’Dwyer apparatus could be used for CPR or anaesthesia. Later modifications included addition of a cuff.

[George Fell (1850–1918), US ENT surgeon]

Baskett TF (2007). Resuscitation; 74: 211–14.

Oedema. Generalised or local excess ECF. Caused by:

ent hypoproteinaemia and decreased plasma oncotic pressure.

ent increased hydrostatic pressure, e.g. cardiac failure, venous or lymphatic obstruction; salt and water retention (e.g. renal impairment, drugs, e.g. NSAIDs, oestrogens, corticosteroids).

ent leaky capillary endothelium, e.g. inflammation, allergic reactions, toxins.

ent direct instillation, e.g. extravasated iv fluids, infiltration. Several causes often coexist, e.g. hypoproteinaemia, portal hypertension and fluid retention in hepatic failure. Characterised by pitting when prolonged digital pressure is applied, although fibrosis reduces this in chronic oedema. Generalised oedema occurs in dependent parts of the body, e.g. ankles if ambulant, sacrum if bed-bound. Treatment is directed at the cause. If localised, the affected part is raised above the heart.

Oesophageal contractility. Used as an indicator of anaesthetic depth and brainstem integrity. Skeletal muscle is present in the upper third of the oesophagus, smooth muscle in the lower third, and both types in the middle third. Afferent and efferent nerve supply is mainly vagal via oesophageal plexuses, but also via sympathetic nerves.

• Normal pattern of contractions:

ent primary: continuation of the swallowing process; propels the food bolus down the oesophagus.

ent secondary (provoked): caused by presence of food, etc., within the oesophageal lumen. Unrelated to swallowing.

ent tertiary (spontaneous): non-peristaltic; function is uncertain.

Measured by passing a double-ballooned probe into the lower oesophagus. The distal balloon is filled with water and connected to a pressure transducer; the other balloon (just proximal) may be inflated intermittently to study provoked contractions.

• Altered by:

ent anaesthesia: provoked contractions diminish in amplitude as depth increases, and spontaneous contractions become less frequent. Oesophageal contractility index ([70 × spontaneous rate] + provoked amplitude) is used as an overall measure of activity. Thought to be analogous to BP, heart rate, lacrimation and sweating during anaesthesia; i.e. suggestive of anaesthetic depth, but not reliable. Activity may be decreased by atropine and smooth muscle relaxants (e.g. sodium nitroprusside) and increased by neostigmine.

ent brainstem death: spontaneous contractions disappear, and provoked contractions show a low amplitude pattern. Has been used to indicate the presence or absence of brainstem activity in ICU, but its role is controversial. Presently not included in UK brainstem death criteria.

See also, Anaesthesia, depth of

Oesophageal obturators and airways. Devices inserted blindly into the oesophagus of unconscious patients to secure the airway and allow IPPV when tracheal intubation is not possible, e.g. by untrained personnel. They have been used in failed intubation. Consist of a cuffed oesophageal tube, often attached to a facemask for sealing the mouth and nose and preventing air leaks. The cuff reduces gastric insufflation and regurgitation but may not prevent it.

The epiglottis is pushed anteriorly, creating an air passage for ventilation. An ordinary tracheal tube may be used to isolate the stomach and improve the airway in a similar way.

• Two main types are described:

ent blind-ended cuffed tube, perforated level with the hypopharynx for passage of air. Inflation is through the tube and via the perforations to the lungs.

ent open-ended tube, to allow gastric aspiration. Inflation is through a separate port of the facemask. If accidental tracheal placement occurs, IPPV may be performed through the tube.

The above features have been combined in a double-lumen device (Combitube), which may be placed in either the oesophagus or trachea (Fig. 121). A distal cuff (15 ml) seals the oesophagus or trachea, whilst a proximal balloon (100 ml) seals the oral and nasal airways. IPPV may be performed through either tube depending on the device’s position; it enters the oesophagus in over 95% of cases initially and ventilation via the longer proximal tube (A) will result in pulmonary ventilation via the proximal openings (C). The shorter distal tube (B) may then be used for gastric suction via the distal opening (D). If the device is tracheal, IPPV may be achieved via tube B and opening D. Has been suggested as a suitable device for non-medical personnel (e.g. for CPR), although trauma is more common than with alternative devices, such as the LMA.

Fig. 121 The Combitube (see text)

Oesophageal sphincter, see Lower oesophageal sphincter

Oesophageal stethoscope, see Stethoscope

Oesophageal varices. Dilated oesophagogastric veins occurring in portal hypertension, e.g. in hepatic cirrhosis; the veins represent one of the connections between the systemic and portal circulations. Account for up to a third of cases of massive upper GIT haemorrhage. Mortality is up to 30% if bleeding occurs, partly related to the underlying severity of liver disease.

• Management:

ent prevention of haemorrhage: β-adrenergic receptor antagonists, e.g. propranolol, have been used to reduce portal BP if hepatic function is not too impaired. Endoscopic sclerotherapy (e.g. with ethanolamine oleate or sodium tetradecyl sulphate; causes variceal thrombosis and fibrosis) and ligation (e.g. with rubber bands) are also used. Portocaval shunt procedures, e.g. distal splenorenal shunts (requiring surgery) or transjugular intrahepatic portosystemic shunts (TIPS; performed under radiological control) decompress the portal circulation but at the expense of hepatic encephalopathy (possibly less common after TIPS). The effect of all these procedures on survival is disputed.

ent if bleeding occurs:

– resuscitation as for acute hypovolaemia. Airway management is complicated by haematemesis and steps to avoid aspiration of blood and gastric contents must be taken.

– pharmacological reduction of portal venous pressure:

– vasopressin 20 U over 15 min iv or its analogue terlipressin 2 mg iv followed by 1–2 mg 4–6-hourly up to 72 h. Controls bleeding in 60–70% of cases.

– somatostatin 250 µg followed by 250 µg/h or its analogue octreotide 50 µg followed by 50 µg/h.

– endoscopic sclerotherapy or ligation may be performed acutely.

– radiological procedures include embolisation or TIPS.

– acute surgical shunt procedures.

– balloon tamponade using a Sengstaken–Blakemore tube.

Garcia-Tsao G, Bosch J (2010). N Engl J Med; 362: 823–32

‘Off-pump’ coronary artery bypass graft, see Coronary artery bypass graft

Ofloxacin. Antibacterial drug, one of the 4-quinolones related to ciprofloxacin. Used for respiratory and genitourinary tract infections.

• Dosage: 200–400 mg orally or iv od/bd.

• Side effects: as for ciprofloxacin. Hypotension and thrombophlebitis may occur on iv administration.

Ohm’s law. Current passing through a conductor is proportional to the potential difference across it, at constant temperature. Thus: voltage = current × resistance. (i.e. V = IR). An analogous form exists for flow of a fluid: pressure = flow × resistance.

[Georg S Ohm (1787–1854), German physicist]

Old age, see Elderly, anaesthesia for

Oliguria. Reduced urine output; definition is controversial but usually described as under 0.5 ml/kg/h. Common after major surgery or in ICU.

• Caused by:

ent urinary retention, blocked catheter, etc.

ent poor renal perfusion, e.g. hypotension, hypovolaemia, low cardiac output. Urine formation usually requires MAP of 60–70 mmHg in normotensive subjects.

ent effect of drugs, e.g. morphine causes vasopressin secretion.

ent increased intra-abdominal pressure (e.g. abdominal compartment syndrome): the mechanism is unknown but ureteric stents do not prevent it, suggesting mechanisms other than ureteric compression.

ent renal failure.

• Management:

ent exclusion of retention or blocked catheter.

ent urinary and plasma chemical analysis (e.g. sodium, osmolality) is useful in distinguishing renal from prerenal causes (see Renal failure). Management is according to the underlying cause.

Omeprazole. Proton pump inhibitor used to reduce gastric acidity. A prodrug converted to its active form by the acidic conditions of gastric parietal cell canaliculi. Effects last for up to 24 h after single dosage.

• Dosage: 10–40 mg orally od. For reduction of risk from aspiration of gastric contents, 40 mg orally the night before, and 40 mg on the morning of surgery. May also be given iv: 40–80 mg.

• Side effects: uncommon and usually mild: diarrhoea, rash, headache, rarely dizziness, hepatic enzyme and haematological changes.

Omphalocele, see Gastroschisis and exomphalos

Oncotic pressure (Colloid osmotic pressure). Osmotic pressure exerted by plasma proteins, usually about 3.3 kPa (25 mmHg). Important in the balance of Starling forces, and movement of water across capillary walls, e.g. in oedema. Although related to plasma protein concentration, the relationship is thought to be non-linear because of molecular interactions and effects of charge.

See also, Intravenous fluids

Ondansetron hydrochloride. 5-HT₃ receptor antagonist, introduced in 1990 as an antiemetic drug following anaesthesia and chemotherapy. Although claimed to be superior to alternative antiemetics for PONV, convincing evidence for this is lacking. However, it does not affect dopamine receptors and unwanted central effects are rare. Evidence suggests greater efficacy for treatment of PONV, than for its prophylaxis. Has also been used to treat intractable pruritus following spinal opioids, although evidence for its effectiveness is weak. Decreases the incidence of postoperative shivering. Only 70–75% protein-bound. Undergoes hepatic metabolism and renal excretion. Half-life is 3 h.

• Dosage:

ent PONV:

– prophylaxis: 4 mg slowly iv/im on induction, or 16 mg orally 1 h preoperatively, or 8 mg orally preoperatively repeated twice 8-hourly postoperatively. In children > 1 month, 0.1 mg/kg slowly iv up to 4 mg.

– treatment: 1–4 mg slowly iv/im.

ent nausea following radiotherapy or chemotherapy: 8 mg orally/iv/im or 16 mg pr, followed by further doses up to 16 mg/day for up to 5 days. In severe cases, a single loading dose of 16 mg pr or iv over 15 min may be given before treatment. In children, 5 mg/m² iv before treatment followed by 2–4 mg orally 12-hourly.

• Side effects: headache, constipation, flushing sensation, hiccups, occasionally hepatic impairment, visual disturbances, rarely convulsions. Prolongation of ECG intervals, including heart block, has been reported. May reduce the analgesic efficacy of tramadol hydrochloride.

Ondine’s curse. Hypoventilation caused by reduced ventilatory drive, originally described following CNS surgery (classically to medulla/high cervical spine). Despite being awake, victims may breathe only on command, with apnoea when asleep. The term has also been applied to a congenital form of hypoventilation and to respiratory depression caused by opioid analgesic drugs.

[Ondine, German mythological sea nymph; the curse of having to remember when to breathe, and thus being unable to sleep for fear of dying, was inflicted on her unfaithful husband by her father, King of the Sea]

Nannapaneni R, Behari S, Todd NV, Mendelow AD (2006). Neurosurg; 57: 354–63

One-lung anaesthesia. Deliberate perioperative collapse of one lung to allow or facilitate thoracic surgery, whilst maintaining ventilation and gas exchange on the other side. Requires the use of endobronchial tubes or blockers. Commonly performed for surgery to the lungs, oesophagus, aorta and mediastinum, but most operations are possible without it (sleeve resection of the bronchus being a notable exception). Its main problem is related to hypoxaemia caused by the mismatch produced, exacerbated by the lateral position used for most thoracic surgery. Perioperative hypoxaemia increases postoperative risk of cognitive dysfunction, atrial fibrillation, renal failure and pulmonary hypertension.

• Effects of lateral positioning on gas exchange:

ent awake:

– ventilation: FRC of the upper lung exceeds that of the lower lung, because of mediastinal movement to the dependent side, and pushing up of the lower hemidiaphragm by abdominal viscera. Thus the upper lung lies on a flatter part of the compliance curve (i.e. is less compliant) whilst the lower lung lies on the steep part of curve, i.e. is more compliant (Fig. 122a). In addition, the raised hemidiaphragm on the dependent side contracts more effectively. Thus most ventilation is of the lower lung.

Fig. 122 Compliance curve for upper and lower lungs in the lateral position: (a) awake; (b) anaesthetised

– perfusion: mainly of the lower lung because of gravity; i.e. is matched with ventilation.

ent anaesthetised:

– FRC of both lungs is reduced; the upper lung now lies on the steep part of the curve and the lower lung on the flatter part (Fig. 122b). Thus the upper (more compliant) lung is ventilated in preference to the lower (less compliant) lung.

– perfusion is still mainly of the lower lung, i.e. mismatch occurs (usually of minor importance in normal patients, since both blood flow and ventilation usually differ by up to 10% between the two sides).

ent one-lung anaesthesia: all ventilation is of the lower lung, whereas considerable perfusion is still of the upper lung. Thus significant shunt occurs in the upper lung, with mismatch usual in the lower lung.

– CO₂ exchange increases via the lower lung; thus CO₂ elimination is thought to be maintained if minute ventilation is unchanged.

– degree of hypoxaemia is affected by:

– side of operation: as the right lung is larger than the left, oxygenation is often better during left thoracotomy.

– pre-existing state of the lungs: decrease in oxygenation is greatest in normal lungs, e.g. during non-pulmonary surgery. Conversely, contribution to oxygenation by the diseased, operative lung is usually reduced; thus the drop in arterial PO₂ is smaller when it is collapsed.

– F_IO₂: increases above 0.5 may not improve oxygenation, since pure shunt is not corrected by raising F_IO₂.

– cardiac output: hypoxaemia worsens if cardiac output falls because of a decrease in the PO₂ of mixed venous blood passing through the shunt. The situation is complicated by altered distribution of pulmonary blood flow caused by changes in cardiac output.

– hypoxic pulmonary vasoconstriction: whether it is attenuated by use of anaesthetic agents, or whether it contributes any protection against shunt, is unclear.

– ventilation strategy: the optimum tidal volume and level of PEEP are controversial. In order to prevent atelectasis in the dependent lung, some advocate using large tidal volumes (e.g. 12 ml/kg) without PEEP (as PEEP may reduce cardiac output or increase shunt through the uppermost lung, exacerbating hypoxaemia). An increasingly common strategy is to use low tidal volumes (e.g. 6–7 ml/kg) with moderate PEEP to prevent atelectasis while reducing the risk of causing volutrauma and acute lung injury.

– content of the collapsed lung: hypoxaemia worsens after about 10 min as contained O₂ is absorbed. Arterial PO₂ is increased by application of 5–7 cmH₂O CPAP using O₂, or by intermittent inflation, e.g. every 10–15 min.

– surgery: e.g. leaning on mediastinum, reduction of venous return. Tying the uppermost pulmonary artery stops shunt to the uppermost lung.

• Practical management:

ent preoperative assessment as for thoracic surgery; patients particularly at risk during one-lung anaesthesia may be identified.

ent close monitoring using oximetry and/or arterial blood gas interpretation.

ent F_IO₂ is usually set to 0.4–0.5.

ent surgical ligation of the pulmonary artery is performed early.

ent management of acute desaturation includes:

– increasing F_IO₂ to 1.0.

– use of fibreoptic bronchoscopy to check tube position and clear secretions.

– administration of O₂ to the uppermost lung, e.g. with CPAP or intermittent inflation.

– performing an alveolar recruitment manoeuvre on the dependent lung.

– altering the ventilation strategy (i.e. changing tidal volume and/or PEEP).

ent suction is applied to the collapsed lung before reinflation, to remove accumulated secretions.

ent slow manual inflation is performed at the end of the procedure, to encourage expansion. The surgeon may request sustained pressures (e.g. 30–40 cmH₂O) to test the integrity of the bronchial suturing.

Karzai W, Schwarzkopf K (2009). Anesthesiology; 110: 1402–11

Open-drop techniques. Common and convenient techniques for administering inhalational anaesthetic agents in the 1800s/early 1900s. The volatile anaesthetic agent (e.g. chloroform, diethyl ether, ethyl chloride) was dripped on to a cloth (originally a folded handkerchief) on the patient’s face from a dropper bottle. Concentration of agent depended on the rate of drop administration. Specially designed bottles and masks were later developed; the best-known mask is that of Schimmelbusch, although this was adapted from Skinner’s earlier model. Some incorporated channels for O₂ insufflation, or gutters around the edge to catch liquid anaesthetic.

[Curt Schimmelbusch (1860–1895), German surgeon; Thomas Skinner (1825–1906), Liverpool obstetrician]

Operant conditioning. Type of learning in which voluntary behaviour is strengthened or weakened by rewards or punishments respectively. May be involved in the development of certain behavioural aspects of pain syndromes. Has been used in chronic pain management, with several weeks’ admission to hospital, involving reduction in drug therapy and encouragement of activity and independence. Thus concentrates on behaviour secondary to pain instead of pain itself.

Operating department assistants/practitioners (ODAs/ODPs). Non-medical anaesthetic support staff; the role arose from the requirements of military surgeons and anaesthetists for specialist non-nursing assistance during World War II, although ‘box carriers’ (so called because they carried the surgeon’s instruments in a box) were in use in the UK in the early 1800s. City & Guilds of London Institute training for ODAs was introduced in 1976, offering specific training in the areas of anaesthesia and surgery without passage through the nursing training system, whilst the term ODP was introduced in 1989 to further the concept that adequately trained staff could equally come from nursing or traditional ODA backgrounds. A National Vocational Qualification (NVQ) in Operating Department Practice was introduced in 1991. Repeated attempts to bring the two ODP career structures (i.e. ODA and non-ODA) closer together was hampered for many years by (1) different pay scales and (2) lack of central registration of ODAs compared with nurse ODPs’ statutory requirement to be registered with the nursing authorities. A voluntary register of ODAs was established in the late 1980s/early 1990s and compulsory registration of all ODPs was established in 2004. The professional body for ODPs is the College (formerly Association) of ODPs, which has almost 5000 members and publishes the bi-monthly Technic: The Journal of Operating Department Practice.

ODPs have an invaluable role in supporting most anaesthetic activity, e.g. preparing and ordering drugs and equipment, setting up the operating theatre for cases, helping to organise operating lists. They may also assist the surgical staff (including ‘scrubbing’) and the concept of ‘multi-skilling’ supports their activity in various roles within the operating theatre suite and beyond, e.g. ICU, trauma teams. More extended practical roles are supported in some units (e.g. assisting at cardiac arrests, placing iv cannulae), although this is controversial.

Ophthalmic nerve blocks. Performed for procedures around the eye, nose and forehead, and certain intraoral procedures.

• Anatomy (see Fig. 76; Gasserian ganglion block):

ent ophthalmic division of the trigeminal nerve (V₁) is entirely sensory and passes from the Gasserian ganglion, where it divides into branches which pass through the superior orbital fissure:

– lacrimal nerve: supplies the lateral upper eyelid and conjunctiva, lacrimal gland and skin of the lateral angle of the mouth.

– frontal nerve: supplies the upper eyelid, frontal sinuses and anterior scalp via the supraorbital branch; upper eyelid and medial forehead via the supratrochlear branch.

– nasociliary nerve: supplies the anterior dura, anterior ethmoidal air cells, upper anterior nasal cavity and skin of the external nose via the anterior ethmoidal branch; posterior ethmoidal and sphenoid sinuses via the posterior ethmoidal branch; medial upper eyelid, conjunctiva and adjacent nose via the infratrochlear branch; cornea, iris, ciliary body and dilator/sphincter pupillae via the long and short ciliary branches. Sympathetic fibres carried in short ciliary branches synapse in the ciliary ganglion

ent supraorbital foramen, pupil, infraorbital notch, infraorbital foramen, buccal surface of the second premolar and mental foramen all lie along a straight line.

• Blocks:

ent supraorbital nerve: 1–3 ml local anaesthetic agent is injected at the supraorbital notch.

ent supratrochlear nerve: 1–3 ml is injected at the superomedial part of the opening of the orbit.

ent both the above nerves may be blocked by subcutaneous infiltration above the eyebrow.

ent frontal nerve: 1 ml is injected at the central part of the roof of the orbit.

ent anterior ethmoidal nerve: 2 ml is injected at the superomedial side of the orbit, at a depth of 3–4 cm.

Ophthalmic surgery. Historically, first performed without anaesthesia and then under topical anaesthesia (e.g. by Koller), because of the eye’s accessibility and the disastrous effects of coughing during general anaesthesia. Subsequently, increasingly performed under general anaesthesia because of patients’ expectations and the ability to control intraocular pressure (IOP). More recently local anaesthesia has been favoured again, especially in the elderly. Children (for strabismus repair) and the elderly (for cataract extraction) form the largest groups of patients.

• Local anaesthesia:

ent cornea and conjunctiva: 4% lidocaine (with or without adrenaline) or 2–4% cocaine is instilled into the conjunctival sac. Cocaine is not used in glaucoma, as it dilates the pupil.

ent retrobulbar block, peribulbar block or sub-Tenon’s block: retrobulbar block is less commonly performed now because of associated complications.

ent prevention of blepharospasm: infiltration between the muscles and bone parallel to the lower and lateral orbital margins from a point 1 cm behind the orbit’s lower lateral corner; alternatively, local anaesthetic may be injected above the condyloid process of the mandible. These injections are rarely required with large-volume modern regional techniques.

ent sedation may be used. Close monitoring is required as the patient’s head is covered by drapes. Supplementary O₂ should be delivered.

• General anaesthesia:

ent preoperatively:

– preoperative assessment of children with strabismus for muscle disorders and MH susceptibility. Cataracts may occur in dystrophia myotonica, inborn errors of metabolism, chromosomal abnormalities, diabetes mellitus, corticosteroids therapy or following trauma. Lens subluxation may occur in Marfan’s syndrome and inborn errors, e.g. homocystinuria. The elderly should be assessed for other diseases, e.g. diabetes, hypertension (see Elderly, anaesthesia for).

– drugs used in eye drops may be absorbed and active systemically, e.g. ecothiopate, timolol.

– opioid premedication is usually avoided because of its emetic properties. Benzodiazepines are popular.

ent perioperatively:

– procedures include the above operations, repair of retinal detachment, vitrectomy, repair of eye injuries (see Eye, penetrating injury) and operations on the lacrimal system.

– the airway is usually not easily accessible to the anaesthetist.

– for children, considerations include those for paediatric anaesthesia, the very active oculocardiac and oculorespiratory reflexes, and the increased incidence of PONV after strabismus repair (thought also to be associated with traction on extraocular muscles). Atropine or glycopyrronium should be available; some advocate routine administration to all patients preoperatively or on induction of anaesthesia. Standard techniques are employed, with tracheal intubation or LMA and spontaneous or controlled ventilation.

– for adults, standard agents and techniques are used. Control of IOP is usually achieved by iv induction, IPPV and hyperventilation, and use of a volatile inhalational anaesthetic agent (for effects of specific drugs, use of sulphahexafluoride, etc., see Intraocular pressure). Administration of iv acetazolamide may be required. Spontaneous ventilation may be suitable for extraocular procedures. The LMA is often used, since coughing and straining are less pronounced than with tracheal intubation. The oculocardiac reflex may still occur in adults.

– systemic absorption of topical solutions, e.g. adrenaline, cocaine, may occur.

– coughing, straining and vomiting may increase IOP, especially undesirable if the globe is open.

ent postoperatively: avoidance of straining and vomiting is desirable. Postoperative pain tends to be mild.

Opiates. Strictly, substances derived from opium. Formerly used to describe agonist drugs at opioid receptors; the terms opioids and opioid analgesic drugs are now preferred.

Opioid analgesic drugs. Opium and morphine have been used for thousands of years; morphine was isolated in 1803 and codeine in 1832. Diamorphine was introduced in 1898, papaveretum in 1909. Other commonly used drugs include: pethidine (1939); methadone (1947); phenoperidine (1957); fentanyl (1960); alfentanil (1976); tramadol (1977); sufentanil (1984) and remifentanil (1997). Drugs with opioid receptor antagonist properties include pentazocine (1962), nalbuphine (1968), meptazinol (1971) and buprenorphine (1968).

May be divided into naturally occurring alkaloids (e.g. morphine, codeine), semisynthetic drugs (slightly modified natural molecules, e.g. diamorphine, dihydrocodeine) and synthetic opioids (e.g. pethidine, fentanyl, alfentanil, remifentanil). May also be classified according to their opioid receptor specificity and actions, or according to their onset and duration of action.

Each drug has slightly different effects on the body’s systems, but their general effects are those of morphine. The ‘purer’ drugs, e.g. fentanyl, alfentanil, sufentanil, do not cause histamine release, and may be used in very high doses with relative cardiostability, e.g. for cardiac surgery. In lower doses, they are used to provide intra- and postoperative analgesia, and to prevent the haemodynamic consequences of tracheal intubation and surgical stimulation. Also used as general analgesic drugs and for premedication, anxiolysis, cough suppression and treatment of chronic diarrhoea.

Opioid detoxification, see Rapid opioid detoxification

Opioid poisoning. Presents with nausea and vomiting, respiratory depression, hypotension, pinpoint pupils and coma. Depressant effects are exacerbated by alcohol ingestion. Hypothermia, hypoglycaemia and, rarely, pulmonary oedema and rhabdomyolysis may occur. Convulsions may occur with pethidine, codeine and dextropropoxyphene. Drug combinations containing opioids include atropine–diphenoxylate for diarrhoea and paracetamol–dextropropoxyphene/codeine/dihydrocodeine for pain. The former combination may cause convulsions, tachycardia and restlessness (hence it has been withdrawn from US and UK markets); the latter may cause delayed hepatic failure.

• Management:

ent supportive: includes gastric lavage, iv fluids, O₂ therapy and IPPV. Activated charcoal may be helpful if oral opioids have been recently ingested.

ent naloxone 0.4–2.0 mg iv repeated after 2–3 min as required to a total of 10 mg; infusion may be necessary as its duration of action is short. Respiratory depression due to buprenorphine may not be responsive.

Opioid receptor antagonists. Different types:

ent pure antagonists, e.g. naloxone, naltrexone: antagonists at all opioid receptor subtypes. Methylnaltrexone is a peripherally acting mu antagonist, currently under investigation as a treatment for postoperative ileus.

ent agonist–antagonists: agonists at some receptors but antagonists at others, e.g.:

– pentazocine: agonist at kappa and sigma, antagonist at mu receptors.

– nalorphine: partial agonist at kappa and sigma, antagonist at mu receptors.

– nalbuphine: as for nalorphine, but a less potent sigma agonist.

ent partial agonists, e.g. buprenorphine, meptazinol (mu receptors); may antagonise mu effects of other opioids (e.g. morphine).

Their main clinical use is to reverse effects of opioid analgesic drugs, e.g. in opioid poisoning. Those with agonist properties are also used as analgesic drugs; some have been used to reverse unwanted effects of other opioids (e.g. respiratory depression) whilst still maintaining analgesia. In practice, this is very difficult to achieve. Also used in diagnosis and treatment of opioid addiction. Receptor-specific compounds have been developed for research and identification of receptor subtypes.

Many result from modification or substitution of the side chain on the nitrogen atom of parent analgesic drugs, e.g. N-allyl group substitution for the N-methyl group (hence the name, nal …).

Opioid receptors. Naturally occurring receptors to morphine and related drugs, isolated in the 1970s. All are G protein-coupled receptors and activation results in opening of potassium channels and closure of voltage-gated calcium channels; this leads to membrane hyperpolarisation, reduced neuronal excitability and thus reduced nociceptive transmission. This effect is enhanced by reduction of cAMP by inhibition of adenylate cyclase. Found mainly in the CNS but also GIT; thought to be involved in central mechanisms involving pain and emotion. Three primary subgroups are now recognised (each subdivided into two or more putative subtypes), although others have been suggested in the past. More recently, data from transgenic mice lacking a single opioid receptor (e.g. MOP) have called into question the validity of further subtype classification.

• Subgroups:

ent mu (MOP):

– activation causes analgesia, respiratory depression, euphoria, hypothermia, pruritus, reduced GIT motility, miosis, bradycardia, physical dependence; i.e. the classic effects of morphine.

– responsible for ‘supraspinal analgesia’; i.e. drugs act at brain level.

– the putative mu₁ receptor subtype is thought to be responsible for supraspinal analgesia; the mu₂ for most of the other effects; and mu₃ receptors have been identified on immune cells.

– agonists: all opioid analgesic drugs.

– partial agonists: buprenorphine, meptazinol (thought to be specific at mu₁ receptors).

– antagonists: nalorphine, nalbuphine, pentazocine.

ent delta (DOP):

– distributed throughout the CNS. Located presynaptically, they inhibit the release of neurotransmitters.

– activation has been experimentally shown to produce analgesia and cardioprotection.

– agonists: enkephalins.

ent kappa (KOP):

– activation causes analgesia, miosis, sedation, different sort of dependence.

– responsible for ‘spinal analgesia’; i.e. drugs thought to act at spinal level.

– agonists: experimental agents spiradoline and enadoline cause analgesia but adverse effects, including diuresis, sedation and dysphoria, preclude clinical use.

Sigma receptors, previously considered opioid receptors, are not considered so now because the effects of their stimulation are not reversed by naloxone. They bind to phencyclidine and its derivatives, e.g. ketamine. All subtypes are antagonised by naloxone and naltrexone (mu and kappa more than delta).

The nociceptin/orphanin FQ peptide (NOP) receptor (previously termed the ‘orphan’ receptor) is related to the above receptors but its lack of sensitivity to naloxone makes it difficult to classify in the original opioid taxonomy. It is found throughout the brain and spinal cord, binds endogenous orphanin FQ, and produces antanalgesia supraspinally and analgesia at spinal level.

Dietis N, Rowbotham DJ, Lambert DG (2011). Br J Anaesth; 107: 8–18

Opioids. Substances which bind to opioid receptors; include naturally occurring and synthetic drugs, and endogenous compounds.

Opium. Dried juice from the unripe seed capsules of the opium poppy Papaver somniferum. Contains many different alkaloids, including morphine (9–20%), codeine (up to 4%) and papaverine. Used for thousands of years as a recreational drug and for analgesia, especially in the Far East. Use as a therapeutic drug is rare now, purer drugs and extracts being preferred.

Oral rehydration therapy. Method of treating dehydration when mild or where facilities for iv fluid administration are lacking, e.g. in the community or developing countries. Particularly useful in gastroenteritis and in children; it has also been used in less serious burns. Various commercial mixtures exist; all contain glucose, the presence of which in the intestinal lumen facilitates the reabsorption of sodium ions and thus water. A simple version can be made by adding 20 g glucose (or 40 g sucrose since only half becomes available as glucose after ingestion), 3.5 g sodium chloride, 2.5 g sodium bicarbonate and 1.5 g potassium chloride per litre of water. Suitable solutions have been made by taking three 300 ml soft drink bottles of water and adding a level bottle capful of salt and eight capfuls of sugar.

Orbeli effect. Increase in strength of contraction of fatigued muscle following sympathetic nerve stimulation.

[Leon A Orbeli (1882–1958), Russian physiologist]

Orbital cavity. Cavity containing the eye and extraorbital structures. Roughly pyramidal with the apex posteriorly, its roof is formed by the orbital plate of the frontal bone (and lesser wing of the sphenoid posteriorly); its floor by the maxilla and zygoma; its medial wall by the frontal process of the maxilla and lacrimal bone anteriorly and orbital plate of the ethmoid and body of the sphenoid posteriorly; and its lateral wall by the zygoma and greater wing of the sphenoid (Fig. 123). Has three openings posteriorly:

ent superior orbital fissure: transmits the third, fourth and fifth (the three branches of the ophthalmic division) cranial nerves. Also transmits branches of the middle meningeal and lacrimal arteries, ophthalmic veins and sympathetic fibres.

ent inferior orbital fissure: transmits the maxillary nerve.

ent optic canal: transmits the optic nerve and ophthalmic artery. The extraocular muscles are supplied by the third, fourth and sixth cranial nerves and have the following actions on the pupil:

– superior rectus: elevates.

– inferior rectus: lowers.

– medial and lateral rectus: moves medially and laterally respectively.

– superior oblique: moves downwards and laterally.

– inferior oblique: moves upwards and laterally.

The rectus muscles attach posteriorly to a common tendinous ring surrounding the optic canal and part of the superior orbital fissure; they attach anteriorly to the sclera of the eyeball in front of the equator. The superior oblique muscle attaches posteriorly above the tendinous ring, hooking round the pulley-like trochlea before attaching posterolaterally to the eyeball, behind the equator. The inferior oblique attaches posteriorly to the floor of the orbit and attaches to the posterolateral surface of the eyeball, behind the equator.

Fig. 123 Frontal view of right orbital cavity

Oré, Pierre-Cyprien (1828–1889). French physician; Professor of Physiology at Bordeaux. Investigated blood transfusion and the effects of iv injection of drugs. Produced general anaesthesia with iv chloral hydrate in 1872, thus becoming the first to employ TIVA. Also treated tetanus with the drug.

Orexins (Hypocretins). Excitatory neuropeptides derived from an amino acid precursor, prepro-orexin. Postulated as playing an important role in arousal, maintenance of the waking state, neural control of food intake and neuroendocrine function, including energy metabolism and reproduction. May be involved in the pathophysiology of neurodegeneration and head injury.

Organ donation. Organs for transplantation may be donated by living subjects, e.g. kidney and bone marrow. The main issue concerns the undertaking of anaesthesia and surgery (with their attendant risks) by a healthy patient for altruistic reasons.

Many organs may be obtained from patients following diagnosis of brainstem death. They include kidney, heart, lung, liver, small bowel, pancreas, skin and cornea. Demand for organs outstrips supply. At present in the UK, members of the public identify themselves as potential donors by joining a national registry (an ‘opt-in’ system); in some countries (e.g. Spain, Austria, Belgium), an ‘opt-out’ system operates in which permission is assumed unless specified otherwise, and this has been proposed in the UK amongst considerable controversy.

Even more controversial is donation after circulatory death (DCD), e.g. patients who arrive in hospital dead, patients in whom CPR is unsuccessful, those in whom cardiac arrest occurs after brainstem death and those in whom treatment has been withdrawn but in whom formal brainstem death testing cannot be done because of sedation, metabolic abnormalities, etc. In the latter group, treatment may be withdrawn once the surgical team is ready; death is pronounced 2 min after asystole and surgery to retrieve organs begins 5–10 min after asystole.

Non-living patients have traditionally been considered unsuitable for donation for a number of relative reasons (e.g. systemic infection, age); with the relative shortage of organs, only HIV infection or Creutzfeldt–Jakob disease is considered an absolute contraindication and DCD now represents 35% of all donors in the UK.

Maintenance of tissue oxygenation, organ function and metabolic and cardiovascular stability should be pursued as for critically ill patients, until and during organ removal. Endocrine therapy with methylprednisolone, triiodothyronine and control of blood sugar with insulin may improve donor organ function.

Thompson JP, Murphy PG, Bodenham AR (2012). Br J Anaesth; 108 (suppl 1): i1–i119

Organe, Geoffrey Stephen William (1908–1989). English anaesthetist, born in India. A major influence on the development of anaesthesia in the UK and abroad, and involved in much research, particularly into the newly introduced neuromuscular blocking drugs. Professor of Anaesthesia at Westminster Hospital, London, and knighted in 1968.

Organophosphorus poisoning. An important worldwide cause of death due to acute poisoning, organophosphorus compounds are acetylcholinesterase inhibitors, commonly used as insecticides but also manufactured as chemical weapons. One, ecothiopate, is used in glaucoma. Those used in insecticides are usually ester, amide or thiol derivatives of phosphoric or phosphonic acids, or their mixtures. They may be absorbed via the GIT, lungs or skin, and are rapidly distributed to all tissues, especially liver and kidney. Half-lives vary from minutes to hours, with metabolism by oxidation, ester hydrolysis and combination with glutathione, and excretion in faeces or urine.

• Toxic effects:

ent peripheral enzyme inhibition:

– phosphorylation of acetylcholinesterase: may be irreversible depending on the compound involved. Features are those of cholinergic crisis and include muscarinic effects (bronchospasm, sweating, increased secretions, abdominal cramps, bradycardia, miosis) and nicotinic effects (muscle twitching, weakness, hypertension and tachycardia). Enzyme ‘reactivation’ may be induced by pralidoxime if administered within 24–36 h.

– phosphorylation of other enzymes, e.g. lipases, GIT enzymes.

ent myopathic effects: weakness may occur within 24 h of poisoning, with recovery taking up to 3 weeks. Muscle paralysis in humans may occur after recovery from the initial cholinergic crisis, 24–96 h after poisoning. Mainly affecting proximal muscles, it is thought to involve postsynaptic dysfunction at the neuromuscular junction.

ent delayed polyneuropathy: usually follows poisoning with non-insecticide compounds. Develops 2–4 weeks after the cholinergic crisis, with weakness and paraesthesiae. Pyramidal signs may be present. Recovery is variable.

ent CNS effects: anxiety, tremor, confusion, coma and convulsions may occur, with EEG abnormalities.
Respiratory failure may result from peripheral weakness, central depression and increased tracheobronchial secretions.

Diagnosis is based on history, tolerance to atropine therapy, acetylcholine assay and measurement of blood and urine organophosphorus and metabolite levels.

• Treatment:

ent supportive measures as for poisoning and overdoses in general. Care should be taken to avoid self-contamination.

ent drug therapy:

– atropine 2 mg (20 µg/ml in children) iv each 5–10 min until dry flushed skin, dilated pupils and tachycardia.

– pralidoxime 30 mg/kg diluted in 10–15 ml water, iv over 5–10 min. May be repeated up to twice if no improvement is seen within 30 min, up to a usual maximum of 12 g/24 h. Rarely, iv infusion of up to 500 mg/h may be required. For children, a bolus dose of 20–60 mg/kg.

Roberts DM, Aaron CK (2007). Br Med J; 334: 629–34

Orphanin FQ (OFQ; nociceptin), see Opioid receptors

Orthopaedic surgery. Anaesthetic considerations may be related to:

ent indication for surgery:

– trauma: presence of other injuries, risks of emergency surgery (e.g. aspiration of gastric contents). Adequate resuscitation is important preoperatively, especially in the elderly, e.g. following fractured neck of femur (NOF). Cases with risk of infection, ischaemia or nerve damage are particularly urgent.

– musculoskeletal disease, e.g. rheumatoid arthritis, connective tissue diseases, muscular abnormalities. There is a higher than normal incidence of MH susceptibility in young patients with musculoskeletal abnormalities.

– congenital malformations: may be accompanied by other system involvement, e.g. cardiac lesions.

– risk of massive hyperkalaemia following suxamethonium if neurological or muscle lesions are present.

ent surgical procedure:

– may involve repeated anaesthesia.

– use of tourniquets.

– use of methylmethacrylate cement.

– problems of specific procedures, e.g. kyphoscoliosis.

– regional techniques are particularly useful, e.g. hip surgery (arthroplasty, fractured NOF). Epidural and spinal anaesthesia are associated with fewer postoperative complications (including DVT) than after general anaesthesia, although mortality after fractured NOF is the same at 1 year.

– DVT and PE are common, especially after hip surgery; prophylactic measures should be taken. Fat embolism may occur after long bone fractures.

Oscilloscope. Device for displaying recorded signals, particularly those of high frequency and when analysis of their shape is required, e.g. ECG or arterial waveform. May also be used without the time-base to plot two signals with respect to each other, e.g. flow–volume loops.

The earliest oscilloscopes utilised a cathode ray tube to generate, accelerate and focus an electron beam on to a fluorescent screen, the beam visible as a bright dot. The signal potential was applied vertically across the beam, causing vertical deflection; a spatial reconstruction of the signal against time was then seen on the screen. The pattern could be made to persist by altering the characteristics of the fluorescent material, or by using a second cathode system. These are now termed analogue oscilloscopes, to distinguish them from the modern digital devices that have replaced them in medical practice. These utilise an analogue-to-digital converter to translate measured voltages (sampled at close, regular time intervals) into digital information that is then displayed on liquid crystal or light-emitting diode panels. Digital devices benefit from greater portability and the option to apply processing algorithms to recorded signals (e.g. for S–T segment analysis and detection of arrhythmias).

Oscillotonometer. Obsolete device for indirect arterial BP measurement, using one (upper) cuff for occluding the brachial artery and a second (lower) cuff for detecting pulsations, often incorporated into a double cuff. Both cuffs are inflated by hand to above systolic BP and then allowed to deflate slowly, using a lever to switch the dial to a sensitive ‘indicator’ mode by which increased and then decreased oscillations of the dial needle indicate systolic and then diastolic pressures respectively. At each of these points, the lever is used to return the indicator dial to a ‘recording’ mode to allow actual cuff pressure to be displayed. Has been replaced by automated devices, which employ similar principles but are more accurate, more reliable and easier to use.

Osmolality and osmolarity. Expressions of concentration of osmotically active particles in solution:

ent osmolality = the number of osmoles per kilogram solvent.

ent osmolarity = the number of osmoles per litre solution.

ent osmoles = the mw of a substance divided by the number of freely moving particles liberated in solution.

Thus 1 mmol of a salt that dissociates completely into two ions provides 2 mosmol. In the body, the solvent is water, with density 1 kg/l; thus osmolality and osmolarity are often used interchangeably, although proteins and fats in plasma give rise to a small difference.

Osmolality of plasma is maintained at 280–305 mosmol/kg. Regulatory mechanisms include stimulation of thirst by osmoreceptors, baroreceptors and the renin/angiotensin system. Osmoreceptors also stimulate vasopressin release. Most contribution to plasma osmolality arises from sodium and its anions, glucose and urea; thus plasma osmolality may be estimated thus:

Alcohols, proteins, triglycerides and mannitol are not accounted for. Proteins usually contribute little since, despite their high concentration, few particles are liberated in solution because of their high mw.

Osmolality/osmolarity is determined by measuring ionic concentration with a flame photometer, measuring osmotic pressure or by employing the colligative properties of solutions (e.g. depression of freezing point, lowering of vapour pressure).

Urinary and plasma osmolality measurement is useful in investigating oliguria and renal failure.

Osmolar gap (Osmolality gap). Difference between calculated and measured plasma osmolality. Normally 10–15 mosmol/kg; increased in the presence of low-molecular-weight substances not included in the formula for calculating plasma osmolality, e.g. alcohols, mannitol, glycine (in the TURP syndrome). May also be applied to urine osmolality, e.g. to indicate the presence of osmotically active substances such as ammonium ions.

Osmoreceptors. Cells in the anterior hypothalamus, outside the blood–brain barrier; respond to changes in plasma osmolality. Control thirst and secretion of vasopressin, possibly via separate groups of osmoreceptors.

Osmosis. Movement of solvent molecules across a semipermeable membrane from a dilute solution to a concentrated one, tending to equalise the concentrations on both sides. Thus water moves across cell membranes from the ECF following dextrose infusion, once the dextrose has been metabolised. Similarly, water in very hypotonic iv fluids may move into red blood cells after infusion, causing haemolysis.

Osmotic clearance, see Clearance, osmotic

Osmotic diuretics, see Diuretics

Osmotic pressure. Pressure required to prevent movement of solvent molecules by osmosis across a semipermeable membrane.

where n = number of particles,

R = universal gas constant,

T = absolute temperature,

V = volume.

Thus proportional to the number of osmotically active particles per unit volume, not mw. Ideal ionic solutions dissociate completely in solution, whereas in the body incomplete dissociation and interactions between ions result in lower osmotic pressure than predicted. Plasma osmotic pressure is approximately 7.3 atmospheres.

Ouabain. Cardiac glycoside, poorly absorbed from the GIT and administered iv. Faster acting than digoxin; thus used when rapid action is required. 5% protein-bound, with half-life about 24 h, and excreted via kidneys and liver.

• Dosage: 100–250 µg by iv infusion.

Not commercially available in the UK.

Outreach team. Concept similar to the medical emergency team, for improving identification and care of acutely ill patients throughout hospitals but especially on general wards. Usually nurse-led, but may also contain experienced medical and physiotherapy staff, often from ICUs, who may provide the following services: rapid response to acutely ill patients in general ward areas, critical care education for ward clinicians, facilitation of early admission to ICU/HDU, early recognition of patients for whom CPR and/or ICU admission is inappropriate and early post-ICU follow-up. Referral criteria include specific clinical scenarios or the results of early warning scores.

Goldhill DR (2005). Br J Anaesth; 95: 88–94

Ovarian hyperstimulation syndrome. Condition caused by pharmacological stimulation of the ovaries in assisted conception programmes. Characterised by ovarian enlargement, pleural effusion and ascites; the latter in particular may be massive and unrelenting. Clinical features range from abdominal discomfort and swelling to hypovolaemic shock, hepatic impairment, acute kidney injury and acute lung injury. DVT may also occur. Mild symptoms occur in up to a quarter of cases of induced ovulation, whilst the severe form occurs in 1–2%.

Treatment is mainly supportive, with correction of hypovolaemia, careful attention to fluid balance and correction of metabolic disturbances. DVT prophylaxis is recommended. Abdominal paracentesis and pleural drainage are usually performed in severe cases; ultrafiltration and re-infusion of the ascitic fluid iv have been used to replace the protein-rich fluid otherwise lost.

Sansone P, Aurilio C, Pace MC, et al (2011). Ann N Y Acad Sci; 1221: 109–18

Overdoses, see Poisoning and overdoses

Oximetry. Determination of arterial O₂ saturation of haemoglobin (S_aO₂) by measuring absorbance of light by blood. Described in 1934 using open blood vessels, and in 1940 using ear/hand probes, but the technique was cumbersome and difficult to perform. Modern pulse oximeters became widespread from the 1980s following advances in microchip technology, allowing manipulation of the recorded signal. Included in the UK/Ireland minimal monitoring standard for anaesthesia since 1987.

Relies on the principle that absorbance of light energy by haemoglobin varies with its level of oxygenation. Oxygenated and deoxygenated haemoglobin (HbO and Hb respectively) have different absorbance spectra (Fig. 124). Isobestic points occur where the lines cross. Thus comparison of absorbance at different wavelengths allows estimation of the relative concentrations of HbO and Hb (i.e. S_aO₂). Earlier machines used two wavelengths, including one isobestic point as a reference; modern pulse oximeters may use two or more wavelengths, not necessarily including an isobestic point.

Blood gas machines estimate S_aO₂ from arterial samples, whereas pulse oximeters read from ear or finger probes measuring light passing through tissue. Analysis of reflected light has also been used to determine S_aO₂; surface probes have been developed which may be stuck on to skin at any site, e.g. the head of a fetus. All oximeters are calibrated using data measured from human volunteers; saturations < 80% are therefore estimated from extrapolated data, and may be inaccurate.

Mannheimer PD (2007). Anesth Analg; 105: S10–17

See also, Beer–Lambert law

Fig. 124 Absorbance of light by oxygenated (HbO) and deoxygenated (Hb) haemoglobin: A and B are isobestic points. The vertical dashed lines indicate the wavelengths (660 nm and 940 nm) most often used by modern pulse oximeters

Oxpentifylline, see Pentoxifylline

Oxycodone hydrochloride. Opioid analgesic drug, described in 1916. Oral preparations are twice as potent as oral morphine due to higher oral bioavailability. Intravenous oxycodone is roughly equivalent to iv morphine.

• Dosage: initially 5 mg 4–6-hourly, increased to a maximum of 400 mg/day. Slow-release oral preparations are available, allowing bd dosing. May also be given sc/slowly iv: 1–10 mg 4-hourly.

Also available as suppositories (as the pectinate) by special order.

Oxygen. Non-metallic element existing as a colourless odourless diatomic gas (O₂) in the lower atmosphere, and as triatomic oxygen (O₃, ozone) and monatomic oxygen (O) in the upper atmosphere. The most plentiful element in the Earth’s crust (as opposed to nitrogen, the most plentiful in the atmosphere), it makes up 21% of air by volume. Discovered independently in 1771 by Scheele and Priestley, the latter calling it ‘dephlogisticated air’. Recognised as a gas by Lavoisier, who named it and explained the process of combustion. Combines with many other elements and molecules, and most abundant as water.

Essential for cellular respiration in animals and lower plants; higher plants take in CO₂ and release O₂ during photosynthesis. Boiling point is −183°C; melting point is −218°C; critical temperature is −118°C. Atomic weight is 16; specific gravity is 1.1 for liquid O₂ and 1.4 for gaseous O₂.

Commercial O₂ is supplied in liquid form, manufactured by the fractional distillation of air. Available in hospitals by piped gas supply, O₂ concentrators or in cylinders at 137 bar.

[Carl W Scheele (1742–1786), Swedish chemist]

Oxygen cascade. Series of steps of PO₂ from atmospheric air to mitochondria in cells:

ent dry atmospheric gas: 21 kPa (160 mmHg): influenced by barometric pressure and inspired O₂ concentration.

ent humidified tracheal gas: 19.8 kPa (150 mmHg).

ent alveolar gas: 14 kPa (106 mmHg): influenced by alveolar ventilation and O₂ consumption.

ent arterial blood: 13.3 kPa (100 mmHg): influenced by mismatch.

ent capillary blood: 6–7 kPa (45–55 mmHg): influenced by blood flow and haemoglobin concentration.

ent mitochondria: 1–5 kPa (7.5–40 mmHg).

Reduction in PO₂ at any stage, e.g. due to hypoventilation, lung disease, causes reduction in subsequent steps, risking inadequate mitochondrial PO₂ for aerobic metabolism (below the Pasteur point).

Oxygen concentrator. Device for extracting O₂ from atmospheric air. Air is passed under pressure through a column of zeolite which acts as a molecular sieve, trapping nitrogen and water vapour whilst leaving O₂ and trace gases. Nitrogen is removed by depressurising the column. Two columns are used, each alternatively adsorbing or expelling nitrogen. Produces a continuous supply of over 90% O₂, suitable for most medical uses. Range in size from small units for home use to large ones supplying whole hospitals.

Oxygen delivery (). Calculated O₂ flux. Used with O₂ consumption () to optimise treatment in critical illness.

May be used to supplement other measured cardiovascular variables, e.g. BP, CVP, CO. As falls, a critical point is reached, after which also falls, representing tissue anaerobic respiration. Maintenance of above 600 ml/min/m², and above 170 ml/min/m², was previously suggested to increase survival in critical illness, but this is no longer considered to be the case.

O₂ extraction ratio has also been used (normally 22–30%):

Vincent JL (1991). Can J Anaesth; 38: R44–7

Oxygen extraction ratio. Ratio of oxygen uptake () to oxygen delivery (), expressed as a percentage. Normally about 25%, it increases during periods of increased tissue demand, e.g. exercise. Also varies according to the tissue involved; thus myocardial extraction ratio is about 70%.

Oxygen failure warning device. Device attached to (or incorporated into) the anaesthetic machine; designed to alert anaesthetists to failure of the O₂ supply. Earlier models were often unreliable, e.g. Bosun device (required batteries to power a warning light, which could be switched off, and only operated when the N₂O supply was connected).

• Ideal features of mechanical devices:

ent audible warning activated when O₂ pressure falls below a certain value; powered by O₂ itself.

ent warning continues when O₂ is exhausted; powered by N₂O.

ent delivery of N₂O turned off.

ent breathing system opened to atmosphere, allowing inhalation of air.

ent cannot be switched off.

Most consist of a spindle, kept at one end of its casing by the normal working O₂ pressure; a spring moves the spindle towards the other end as O₂ pressure falls, allowing O₂ to pass to a whistle via a port previously blocked by the spindle. Further movement as O₂ pressure continues to fall allows N₂O to flow to a whistle, stops N₂O delivery to the patient and opens the system to air. Many have a visual indicator too, e.g. producing a colour change.

More recent devices are electronic rather than gas-powered.

Oxygen flux. Amount of O₂ delivered to the tissues per unit time.

Equals:

where CO = cardiac output in l/min

Hb = haemoglobin concentration in g/dl

S_aO₂ = arterial O₂ saturation of haemoglobin

1.34 = Hüfner’s constant

P_aO₂ = arterial PO₂ in kPa

0.0225 = ml of O₂ dissolved per 100 ml plasma per kPa (0.003 ml per mmHg)

Normally 850–1200 ml/min, or 500–700 ml/min/m² if cardiac index is used.

The tissues cannot utilise all of the transported O₂: the last 20–25% remains bound to haemoglobin. Tissue O₂ supply can increase during times of extra demand via increases in cardiac output, e.g. during exercise. If the O₂ carrying capacity of blood is reduced (e.g. in anaemia) cardiac output must increase at rest in order to maintain O₂ flux, and reserves are less. Myocardial depression during anaesthesia in this situation is thus particularly hazardous.

Oxygen, hyperbaric. O₂ therapy at greater than atmospheric pressure, usually 2–3 atmospheres. Increases the amount of dissolved O₂ in blood, according to Henry’s law. In 100 ml blood, 0.3 ml O₂ dissolves at PO₂ of 13.3 kPa (100 mmHg). Thus for 100% O₂ at 3 atmospheres, dissolved O₂ = 5.7 ml. Since haemoglobin is always saturated, even in venous blood, its binding capacity for CO₂ and buffering capacity are reduced, and pH falls. The resultant hyperventilation may result in hypocapnia.

Used in the treatment of carbon monoxide poisoning, air embolism, gas gangrene, decompression sickness and chronic wounds, and has been investigated as an adjunct to radiotherapy and in multiple sclerosis. Single-patient chambers filled with 100% O₂ may be used, or large pressurised chambers containing patient and attendants, with a tightly fitting mask applied to the patient.

Gill AL, Bell CNA (2004). QJM; 97: 385–95

See also, Oxygen transport

Oxygen measurement. Methods include:

ent gas analysis:

– chemical, e.g. conversion to non-gaseous compounds, with reduction in overall volume of gas mixture (Haldane apparatus).

– physical:

– O₂ electrode (Clark electrode; polarographic cell): silver/silver chloride anode and platinum cathode in potassium chloride solution inside a cylinder, with a gas-permeable plastic membrane covering its end. 0.6 V potential is applied across the electrodes. O₂ diffuses to the cathode, reacting with electrons and water to form hydroxide ions and causing current flow proportional to the O₂ concentration. Thus:

The following reactions occur at the anode:

May be used with gas or liquid samples. Maintained at 37°C. Falsely high readings caused by halothane are prevented by using a membrane impermeable to halothane.

– paramagnetic cell: most gases are diamagnetic, i.e. repelled by magnetic fields. O₂ and nitric oxide are paramagnetic, i.e. attracted. The cell contains two nitrogen-filled glass spheres joined by a bar which is suspended on a vertical wire within a magnetic field. O₂ introduced into the cell is attracted into the magnetic field, displacing the spheres and rotating the bar against the torque of the wire. Degree of rotation is proportional to the number of O₂ molecules. It may be measured by observing the deflection of a beam of light reflected by a mirror mounted on the wire, or by measuring the current required to prevent rotation when passing through a coil mounted on the bar. A rapid response paramagnetic device employs an alternating magnetic field applied to two streams of gas, one sample and the other reference; the O₂ concentration in the sample gas is represented by a difference in pressure between the two streams. Alternatively, an alternating magnetic field is applied to the gas, producing a sound wave; its amplitude is proportional to the concentration of O₂ (magnetoacoustic technique). Accuracy of these techniques is high.

– non-specific methods, e.g. mass spectrometer, ultraviolet light absorption.

ent measurement of arterial PO₂:

– O₂ electrode as above. Tiny intravascular probes have been developed for continuous arterial measurement.

– transcutaneous electrode: similar to the O₂ electrode, but with a heating coil to cause vasodilatation, increase rate of O₂ diffusion, and reduce the difference between arterial and skin PO₂. Inaccurate, especially in adults, and with slow response time; they may also cause burns.

– fibreoptic sensor placed intravascularly; measures intensity or wavelength of reflected light.

ent measurement of arterial O₂ content: e.g. liberation of gas from blood with chemical analysis (van Slyke apparatus) or use of an O₂ electrode.

ent measurement of oxygen saturation of haemoglobin.

[Leland C Clark (1918–2005), US biochemist]

Oxygen radicals, see Free radicals

Oxygen saturation. Refers to percentage saturation of haemoglobin with O₂; equals

May be calculated for whole blood, and the dissolved O₂ component subtracted, or measured using oximetry. Normal range in arterial blood at 37°C, pH 7.40 and normal barometric pressure is 97–100%. Reduced in cardiac or respiratory disease.

Oxygen therapy. Used to:

ent correct hypoxaemia due to mismatch, hypoventilation or impaired alveolar gas diffusion. Only partially corrects hypoxaemia due to shunt.

ent increase pulmonary O₂ reserves, e.g. in case of apnoea, hypoventilation.

ent increase the amount of dissolved oxygen, e.g. in anaemia, cyanide poisoning and carbon monoxide poisoning (also increases rate of carboxyhaemoglobin dissociation).

ent other uses include reduction of pulmonary hypertension, reduction of air-filled cavities (e.g. subcutaneous emphysema, pneumothorax, air embolism, intestinal distension), and special uses of hyperbaric O₂ (see Oxygen, hyperbaric).

The effects of breathing 100% O₂ compared with air are shown in Table 33.

• Methods of administration:

ent fixed performance devices; i.e. F_IO₂ is constant, despite changes in inspiratory flow rate:

– O₂ tent.

– anaesthetic breathing system.

– Venturi devices or high air flow O₂ enrichers (HAFOE): the facemask feed connector incorporates holes designed to allow entrainment of atmospheric air into the O₂ stream by jet mixing.

Specific connectors produce set F_IO₂ values at certain O₂ flow rates, assuming the patient’s peak inspiratory flow rate does not exceed the total gas flow rate (if this occurs, air will be entrained via the side-holes in the facemask, and the F_IO₂ will fall). Devices that deliver lower F_IO₂ entrain more air and deliver higher total gas flow, and so deliver the stated F_IO₂ more reliably; e.g. the 0.28 F_IO₂ device delivers a total flow of 45 l/min to the patient, while the 0.6 F_IO₂ delivers 30 l/min total flow.

ent variable performance devices; i.e. F_IO₂ depends on inspiratory flow rates:

– nasal cannulae. A nasal catheter, with a foam cuff to aid placement in the nostril, is also available.

– plastic masks, e.g.:

– moulded hard plastic.

– Edinburgh: soft plastic.

– MC: soft plastic with foam-padded edges.

– All perform similarly, delivering approximately 25–30% O₂ at 2 l/min O₂ flow, and 30–40% at 4 l/min flow.

ent other means of administration include IPPV and its variations, CPAP, apnoeic oxygenation and hyperbaric therapy. Transtracheal administration has also been used in chronic lung disease requiring continuous O₂ therapy, via a narrow-bore catheter inserted above the sternal notch.

• Problems of O₂ therapy:

ent reduction of hypoxic ventilatory drive in a small group of patients who have chronic CO₂ retention, e.g. COPD. Apnoea may result if chronic hypoxaemia is reversed, thus necessitating controlled O₂ therapy. 24% O₂ is administered initially; if arterial PCO₂ has not risen by more than 1–1.5 kPa (7.5–10 mmHg), and PO₂ has not improved adequately, 28% O₂ is administered, then 30%, etc., until satisfactory PO₂ and PCO₂ have been achieved. It has recently been suggested that this phenomenon is related to an acute decrease in pulmonary vascular resistance following O₂ administration, rather than a reduction in hypoxic ventilatory drive itself.

ent pulmonary and CNS O₂ toxicity.

ent absorption atelectasis.

ent increased risk of explosions and fires.

ent retinopathy of prematurity.

[MC: Mary Catterall (1939–2009), London physician]

Table 33 Effects of breathing air or 100% O₂

	Air	100% O₂
Alveolar PO₂ (kPa [mmHg] )	14 (106)	88 (667)
Arterial blood
PO₂ (kPa [mmHg] )	13.3 (100)	84 (638)
O₂ saturation (%)	99	100
O₂ content (ml/100 ml blood):
bound to haemoglobin	19.7	20.1
dissolved	0.3	1.9
Venous blood
PO₂ (kPa [mmHg] )	5.3 (40)	7 (53.2)
O₂ saturation (%)	75	85
O₂ content (ml/100 ml blood):
bound to haemoglobin	14.9	17.2
dissolved	0.1	0.2

Oxygen, tissue tension (P_TO₂). Partial pressure of O₂ in tissues; represents the balance between local supply and consumption of O₂. Most often measured in subcutaneous tissue because of its ease of measurement and relative stability (normally 1 ml/kg/min). Measured using an O₂ electrode mounted on a microcatheter or a fibreoptic probe placed under the skin. Has been used to indicate tissue perfusion; thought to be important in wound healing and susceptibility to infection.

Ragheb J, Buggy DJ (2004). Br J Anaesth; 92: 464–8

Oxygen toxicity. May be:

ent respiratory: pulmonary toxicity is related to actual PO₂, not concentration. Tracheobronchial irritation and substernal discomfort are noticed by healthy volunteers after 12–24 h breathing 100% O₂. Reduced vital capacity, compliance and diffusing capacity, and increased arteriovenous shunt and dead space may occur after 24–36 h. Changes include endothelial damage, reduced mucus clearance and infiltration by inflammatory cells. Surfactant levels may decrease and capillary permeability increase. Eventually fibrosis may occur, although maximal safe concentrations and duration of O₂ therapy are unclear. Up to 48 h breathing 100% O₂ is thought not to be associated with permanent damage; up to 50% is considered safe for any period. Certain cytotoxic drugs increase the incidence and severity of fibrosis, e.g. bleomycin. Free radical formation is the most likely mechanism. Free radical scavengers, surfactant and leukotriene blocking drugs have been investigated as protective or therapeutic agents, but prevention is considered more effective. The lowest F_IO₂ that produces an acceptable arterial PO₂ should be used whenever O₂ is administered.

ent neurological: at above 2–3 atmospheres, convulsions may occur (Bert effect).

ent ocular: exposure to high arterial PO₂ for long periods may lead to retinopathy of prematurity.

Oxygen transport. In a normal person with a haemoglobin concentration of 15 g/dl breathing air, arterial blood carries approximately 20 ml O₂ per 100 ml:

ent 19.7 ml combined with haemoglobin.

ent 0.3 ml dissolved in plasma.

In venous blood, 15 ml is carried per 100 ml blood:

ent 14.9 ml combined with haemoglobin.

ent 0.1 ml dissolved.

Normally, the amount carried by haemoglobin is only slightly increased with O₂ therapy, since haemoglobin is already over 97% saturated; the dissolved O₂ is increased in proportion to the arterial PO₂: 0.0226 ml per kPa per 100 ml blood (0.3 ml per 100 mmHg).

See also, Oxygen flux

Oxygenation index. Indicator of the degree of impairment of oxygenation, often used in studies of neonatal respiratory support as a means of assessing severity of respiratory failure when comparing different respiratory therapies:

Values above 40 are associated with about 80% mortality with conventional treatment.

Oxygenators, see Cardiopulmonary bypass

Oxyhaemoglobin dissociation curve. Plot of oxygen saturation of haemoglobin against PO₂, for normal haemoglobin and PCO₂ at 37°C (Fig. 125a). The curve is sigmoid-shaped because of the increasing affinity of haemoglobin for successive O₂ molecules after the first.

• Important points on the curve:

ent P₅₀: PO₂ at which saturation is 50%; normally about 3.5 kPa (27 mmHg).

ent venous blood: normally corresponds to 75% saturation and PO₂ of 5.3 kPa (40 mmHg).

ent arterial blood: normally corresponds to 97% saturation and PO₂ of 13.3 kPa (100 mmHg).

Thus a small drop in PO₂ from normal levels causes only slight reduction in arterial saturation, because the curve is flat at this point. If PO₂ is already reduced, e.g. in lung disease, the same small drop may cause significant desaturation, corresponding to the steep part of the curve.

The curve is shifted to the right (i.e. P₅₀ > 3.5 kPa) by acidosis, hyperthermia, hypercapnia (Bohr effect) and increased 2,3-DPG levels (Fig. 125b). Saturation becomes lower for any given PO₂; i.e. O₂ is bound less avidly. Thus O₂ unloading to the tissues is favoured.

The curve is shifted to the left (i.e. P₅₀ < 3.5 kPa) in the opposite situations, and in fetal haemoglobin, methaemoglobinaemia and carbon monoxide poisoning. Saturation becomes greater for any given PO₂; i.e. O₂ is bound more avidly. Thus fetal haemoglobin can bind O₂ from maternal haemoglobin.

See also, Myoglobin

Fig. 125 Oxyhaemoglobin dissociation curve: (a) normal; (b) shift to right or left

Oxymorphone hydrochloride. Opioid analgesic drug, derived from morphine and 6–8 times as potent. Developed in 1955. Available in the USA for parenteral and rectal use. Action lasts 4–5 h.

Oxytocin. Hormone secreted by the posterior pituitary gland, causing smooth muscle contraction in the uterus and milk ducts. Used to stimulate uterine contraction, e.g. during labour, postpartum or postabortion. Less effective in early pregnancy. Acts within 2–3 min of iv injection. Synthetic preparation (Syntocinon) is free of vasopressin, and thus preferable to pituitary extract.

Causes less nausea/vomiting than ergometrine and does not cause hypertension (but see below). Available as 5 U and 10 U, and as 5 U in combination with ergometrine 500 µg.

• Dosage:

ent to stimulate labour: 1–4 mU/min iv, increased in at least 20-min steps up to 20 mU/min.

ent at caesarean section or after vaginal delivery: 2.5–5 U slowly iv (may also be given im, although not licensed by this route). Often given as an infusion after caesarean section (typically 40 U/500 ml, given over 4 h).

ent to treat postpartum haemorrhage: 5–10 U slowly iv followed by 5–40 U/500 ml crystalloid, infused as required.

• Side effects:

ent uterine hyperstimulation and fetal distress.

ent reduction in SVR and BP and increased cardiac output results from vasodilatation (thought to be due at least partly to the preservative/vehicle rather than to the drug itself), together with autotransfusion from the uterus to the systemic circulation. Tachycardia may also occur. These changes may be severe, especially when a single bolus of oxytocin is given rapidly, in doses > 5 U and following ephedrine administration. Severe hypotension may occur when oxytocin is given to patients with cardiac disease.

ent severe hyponatraemia has followed prolonged infusion if diluted in dextrose solutions, exacerbated by a direct antidiuretic effect of the hormone itself.