• Effects:

 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.

 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.

 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.

• Anaesthetic considerations:

 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.

 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.

 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.

 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.

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

 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.

• Methods used:

 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.

 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.

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

 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.

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

 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:

 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.

 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.

 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.

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

 paracervical block: rarely performed because of fetal arrhythmias.

 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.

 local infiltration of the abdomen for caesarean section.

• Particular problems in obstetric anaesthetic practice:

 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.

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

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

 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.

 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.

• Dosage:

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

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

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

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

 hypoproteinaemia and decreased plasma oncotic pressure.

 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).

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

 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.

• Normal pattern of contractions:

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

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

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

• Altered by:

 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.

 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.

• Two main types are described:

 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.

 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.

• Management:

 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.

 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.