Obstetrics

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CHAPTER 8 Obstetrics

Anaesthesia for non-obstetric surgery during pregnancy

A total of 1% of patients require GA during pregnancy. In the first trimester, there is a risk of organogenesis; in the third trimester there is a risk of premature labour. Therefore, the second trimester is the safest.

In descending order of preference:

• spinal: method of choice

• epidural: higher doses of LA required and block less effective

• other regional blocks

• combined epidural and GA: lower doses of inhalational agents and narcotics, good postoperative analgesia and can avoid N₂O

• GA (NB delayed gastric emptying present from first trimester onwards).

Regional anaesthesia

Epidurals give excellent/satisfactory analgesia in 91% of mothers. Increased use of regional techniques probably accounts for the continuing reduction in maternal mortality by avoiding risks of failed intubation and aspiration. Comparative Obstetric Mobile Epidural Trial (COMET) 2001 showed that women requesting analgesia for pain relief were more likely to require instrumental delivery if receiving 0.25% bupivacaine boluses rather than low dose bupivacaine infusion or combined spinal/epidural, but no significant difference in LSCS rates between groups (Table 8.1).

Table 8.1 Advantages and disadvantages of epidural anaesthesia

Advantages	Disadvantages
Maternal participation at delivery	May take too long to perform if there is fetal distress
Avoids risk of failed intubation	Hypotension
Reduced risk of aspiration	Risk of patchy, incomplete block
Avoids morbidity from GA drugs	Backache
Avoids risk of awareness	Urinary retention
Earlier breast-feeding
Good postoperative analgesia
Less postnatal depression

Indications for epidural

• Slow or painful labour

• Pregnancy-induced hypertension

• Cardiac or respiratory disease

• Premature or high risk fetus

• Multiple pregnancy

• Breech delivery

• Trial of labour.

Epidural test dose

Recommended to avoid complications of inadvertent intravenous injection of bupivacaine. Usually use a dose insufficient to cause total spinal anaesthesia if injected into the intrathecal space.

Catecholamines

Addition of adrenaline (>15 μg) or isoprenaline to the test dose may cause tachycardia within 1 min following intravenous injection. However, this sign may be unreliable because pregnancy may alter physiological changes to catecholamines and the pain of contraction may also cause sudden tachycardia. Catecholamines have also been shown to reduce placental blood flow in animal studies. Therefore, their use is controversial. If used, give between contractions and avoid in hypertensive or pre-eclamptic patients.

Opioids

The addition of 100 μg fentanyl to a test dose has been reported as a reliable indicator of intravascular injection by causing sedation and altered perception.

Air

Air (1 mL) injected intravenously and detected by parasternal Doppler has been proposed as an effective marker of inadvertent intravenous injection. There appear to be no adverse maternal or fetal effects from this small dose of air.

Regional anaesthesia

Platelet count <100 000, abnormal clotting or prolonged bleeding time is a relative contraindication to regional anaesthesia.

Epidurals are best established early and generally unsuitable for emergency procedures. Provide less effective anaesthesia than subarachnoid block and are more likely to require conversion to GA. Epidurals are the technique of choice for labour or LSCS in pre-eclamptic toxaemia (PET), where they provide better haemodynamic stability and improved uteroplacental flow, with avoidance of intubation risks from laryngeal oedema. l-bupivacaine 0.5% is as effective as bupivacaine 0.5% with less risk of CNS or CVS toxicity. Ropivacaine 0.75% is equipotent with bupivacaine 0.5%.

Spinals are the commonest form of anaesthesia for LSCS. Pencil point needles now reduce incidence of post-dural puncture headache requiring blood patch to <0.5%. Colloid fluid preload is more effective than crystalloid in preventing hypotension, but greater risk of anaphylaxis. Avoidance of aortocaval compression and use of vasopressor (e.g. phenylephrine 50–100 μg) significantly reduces hypotension. 2.25 mL 0.5% heavy bupivacaine achieves an adequate block to T4 at term, but larger volumes may be required earlier in pregnancy because of less venous congestion reducing the volume of the epidural space. Addition of opioids (fentanyl/morphine/diamorphine) reduces the incidence of intraoperative visceral pain with little risk of respiratory depression.

Combined spinal-epidural anaesthesia (CSE) may provide better anaesthesia than epidural alone. In a needle-through-needle technique, the spinal needle is advanced <15 mm beyond tip of Tuohy. Alternatively, puncture the subarachnoid space with spinal needle and replace the stylette immediately, then site epidural catheter via Tuohy needle at difference space. Give epidural test dose and then follow with subarachnoid injection. A smaller intrathecal dose (e.g. 1.0 mL 0.5% bupivacaine) followed by epidural increments improves haemodynamic stability; 0.25 mg intrathecal diamorphine ≡ 5 mg epidural diamorphine. There is some concern regarding breaching of the dura increasing infection risk and the complexity of the technique.

Epidural volume extension (EVE) A low subarachnoid block can be extended in a cephalad direction by an epidural injection of 10 mL of normal saline given within 5 min of the initial subarachnoid block. Probably related to compression of the subarachnoid space by the epidural saline, resulting in cephalad spread of local anaesthetic within the subarachnoid space. EVE allows CSE to be performed with small initial intrathecal doses of local anaesthetic and, as saline is used for the epidural ‘top-ups’, the total dose of local anaesthetic used is reduced.

Continuous spinal anaesthesia using an ultra fine bore catheter threaded through a spinal needle has been associated with cauda equine syndrome and is little used in the UK.

General anaesthesia

Technique of choice for emergency LSCS. Probably safer than an epidural if there is severe hypertension, uncorrected hypovolaemia, fetal distress, coagulopathy or risk of convulsions. GA morbidity most commonly due to anaphylaxis or airway crises. Short-acting antihypertensive drugs may be necessary to prevent intubation-induced hypertension, which causes CVA, pulmonary oedema and reduced placental blood flow. (Unlike thiopentone induction or lidocaine, remifentanil 1 μg.kg⁻¹ prevents the haemodynamic changes at intubation). Beware of laryngeal oedema. Magnesium increases sensitivity to non-depolarizing neuromuscular blockers by inhibiting presynaptic calcium-facilitated neurotransmitter release.

Postoperative. Pre-eclampsia may not begin to resolve until 3–4 days post-delivery. Therefore, monitor BP and urine output carefully; 60% of patients who develop pulmonary oedema do so >48 h after delivery.

Failed intubation

Difficult intubation occurs in 1:300 (1:2000 in normal population). More common in Africans and Afro-Caribbeans.

It is the commonest cause of anaesthetic-related maternal deaths (it is not failure of intubation but subsequent failure of oxygenation that kills). Difficult intubation is due to left lateral tilt, increased weight, increased breast size, full dentition, laryngeal oedema, incorrect application of cricoid pressure, minimum dose of induction agent and attempted intubation before onset of neuromuscular blockade.

Perform no more than three attempts at intubation. Know the failed intubation drill which is being modified in many hospitals to use the laryngeal mask. Maintain cricoid pressure (if it does not prevent LMA insertion) which reduces aspiration risk and stops gastric distension if the patient is being bagged).

Aspiration

Incidence

In 1946, Mendelson described an asthma-like syndrome in 0.15% of deliveries using GA by mask due to aspiration of gastric contents. In more recent studies, the incidence of pulmonary aspiration syndrome is reported as 0.01%. More common with coughing during direct laryngoscopy and emergency surgery, even with cricoid pressure.

Symptoms

Of patients who aspirate, 63% are asymptomatic. Commonest symptoms are cough, bronchospasm, hypoxaemia and X-ray changes.

Gastric volumes

It is traditionally taught that >0.4 mL/kg aspirate of pH <2.5 is needed to cause symptoms, but these figures were based on a study from a single Rhesus monkey. Now thought to be a larger volume of at least 0.8 mL/kg.

Prophylaxis

A dose of 30 mL 0.3 M sodium citrate has replaced magnesium-based antacids, which did not mix with gastric contents and caused pneumonitis themselves if aspirated. Also give H₂ antagonist, e.g. ranitidine 150 mg p.o. (avoid cimetidine, which increases blood levels of bupivacaine), together with a gastric prokinetic, e.g. metoclopramide. Avoid prolonged fasting which is associated with increasing gastric volume and decreased pH. Anticholinergics only inhibit vagal stimulation and have little effect on gastric pH.

Anaesthetic manoeuvres reducing risk

Pre-oxygenation, rapid sequence induction with cricoid pressure (44 N), avoiding bagging following suxamethonium, avoiding attempts at intubation before patient is fully paralysed and extubating awake.

Postpartum

Assuming no opioids are present, rate of emptying, gastric pH and volume of stomach contents rapidly return to normal values within about 6–8 h. However, reflux (80% of women at term) may persist for up to 48 h. Therefore, anti-aspiration measures necessary for 48 h postpartum.

Oaa/Aagbi Guidelines for Obstetric Anaesthesia Services

Association of Anaesthetists of Great Britain and Ireland and the Obstetric Anaesthetists Association 2005

1. A duty anaesthetist should be immediately available for the Delivery Suite 24 h/day.

2. There should be a nominated consultant in charge of obstetric anaesthesia.

3. There should be a clear line of communication from the duty anaesthetist to the supervising consultant at all times.

4. Women should have antenatal access to information about the availability and provision of all types of analgesia and anaesthesia.

5. There should be an agreed system whereby the anaesthetist is given sufficient advanced notice of all potential high-risk patients.

6. Where a 24-h epidural service is offered, the time from the anaesthetist being informed about an epidural until being able to attend the mother should not normally exceed 30 min, and must be within 1 hour except in exceptional circumstances.

7. Separate staffing and resources should be allocated to elective caesarean section lists to prevent delays due to emergency procedures and provision of regional analgesia in labour.

8. The assistant to the anaesthetist must have no other conflicting duties, must be trained to a recognized national standard and must work regularly in the obstetric unit.

9. The training undergone by staff in the maternity recovery unit and the facilities provided must be to the same standard as for general recovery facilities.

10. Appropriate facilities should be available for the antenatal and peripartum management of the sick obstetric patient.

Amniotic fluid embolus

Amniotic fluid may cause amniotic fluid embolus characterized by sudden cardiorespiratory collapse and coagulatory abnormalities. It is thought that the amniotic fluid itself is relatively non-toxic, even in volumes as large as 500 mL. Toxicity appears to be determined by the meconium content. In a pig model, 10 mL.kg⁻¹ of meconium-free amniotic fluid caused few problems, whereas just 3 mL.kg⁻¹ of meconium-contaminated amniotic fluid caused severe cardiorespiratory and coagulatory abnormalities.

American Society of Anesthesiologists. Practice guidelines for obstetrical anesthesia. A report by the American Society of Anesthesiologists. Task Force on Obstetrical Anesthesia. Anesthesiology. 1999;90:600-611.

Association of Anaesthetists of Great Britain and Ireland and the Obstetric Anaesthetists Association. Revised Guidelines, 2005. OAA/AAGBI guidelines for obstetric anaesthesia services. Reproduced with the kind permission of the Association of anaesthetists of Great Britain and Ireland

Bogod D.G. The postpartum stomach – when is it safe? Editorial. Anaesthesia. 1994;49:1-2.

Comparative Obstetric Mobile Epidural Trial (COMET) Study Group UK. Effect of low-dose mobile vs traditional epidural techniques on mode of delivery: a randomized controlled trial. Lancet. 2001;358:19-23.

Duley L., Neilson J.P. Magnesium sulphate and pre-eclampsia. BMJ. 1999;319:3-4.

Gomar C., Fernandez C. Epidural analgesia–anaesthesia in obstetrics. Eur J Anaesth. 2000;17:542-558.

Levy D.M. Emergency caesarean section: best practice. Anaesthesia. 2006;61:786-791.

McDonnell N.J., Keating M.L., Muchatuta N.A., et al. Analgesia after caesarean delivery. Anaesth Intensive Care. 2009;37:539-551.

McGlennan A., Mustafa A. General anaesthesia for caesarean section. Contin Edu Anaesth, Crit Care Pain. 2009;9:148-151.

Ong K.-B., Sashidharan R. Combined spinal–epidural techniques. Contin Edu Anaesth, Crit Care Pain. 2007;7:38-41.

Levy D.M. Anaesthesia for caesarian section. BJA CEPD Rev. 2001;1:171-176.

Macarthur A.J., Gerard W. Ostheimer ‘What’s new in obstetric anesthesia’ lecture,. Anesthesiology. 2008;108:777-785.

McGrady E., Litchfield K. Epidural analgesia in labour. Contin Edu Anaesth, Crit Care Pain. 2004;4:14-17.

May A. Obstetric anaesthesia and analgesia. Anaesthesia. 2003;58:1186-1189.

Morris S. Management of difficult and failed intubation in obstetrics. BJA CEPD Rev. 2001;1:117-121.

Turner J.A. Severe preeclampsia: anesthetic implications of the disease and its management. Am J Ther. 2009;16:284-288.

Maternal and fetal physiology

Maternal physiology

Cardiovascular

• Cardiac output 30–40% above normal by 32 weeks. Aortocaval compression is sufficient to reduce cardiac output from 20 weeks

• ↑ heart rate of 15%, ↑ stroke volume of 30%. Fall in SVR results in unchanged BP

• Cardiac hypertrophy and dilation cause ECG changes of left axis deviation, ST depression and flattening/inversion of T wave in III

• Albumin is diluted, reducing plasma oncotic pressure and predisposing to pulmonary oedema at lower pressures

• ↑ progesterone due to pregnancy and reduced protein binding increases myocardial sensitivity to bupivacaine

• Aortocaval compression significant from mid-pregnancy.

Respiratory

• Reduction in lung volume by a 4 cm elevation of diaphragm compensated for by increased transverse and AP diameter of the chest due to hormonal effects that loosen ligaments

• Increased minute volume by 40%, ↑ tidal volume and 15% ↑ respiratory rate. Causes respiratory alkalosis and shifts O₂ dissociation curve to the left. Increase in P₅₀ from 3.5 to 4.0 facilitates oxygen unloading across the placenta

• ↑ tidal volume with ↑ FRC results in faster pre-oxygenation in 2 min of tidal breathing (more effective than four vital capacity breaths) and quick gas induction, accelerated by reduced MAC

• Increased CC which may exceed FRC

• Increased O₂ consumption.

Gastrointestinal

• Uterine pressure increases intragastric pressure and distorts lower oesophageal sphincter, causing incompetence

• Delayed gastric emptying and increased acid production.

Blood

• Increased red cell mass by 20–30% and increased plasma volume by 40–50% at term, causing dilutional anaemia

• Hypercoagulable state with increased fibrinogen and factors VII, VIII, X, and XIII. Platelet, clotting and fibrinolytic systems all activated.

Renal

• Increase in GFR of 60% reduces plasma urea and creatinine by 40%.

Epidural

• Volume of epidural space is decreased due to distended venous plexus, reducing the volume of LA needed for epidurals by 30% and increasing the risk of epidural vein catheterization.

Metabolism

• Increased volume of distribution of intravenous agents prolongs their elimination half-lives. Elimination t_1/2 for thiopentone is doubled at term

• Serum cholinesterase levels fall by 25% during the first trimester and fall further by 33% during the first 7 postpartum days. The decreased levels of enzymes are adequate for normal hydrolysis of suxamethonium during gestation, but may prolong duration of action in 10% of patients in the postpartum period.

Fetal physiology

• Fetus exists in a hypoxic environment. Oxygenation enhanced by left shift of O₂ dissociation curve with fetal P₅₀ = 2.6 compared with maternal value of 4.0.

• Uterine vascular bed is maximally dilated at term but remains responsive to the effects of catecholamines. Ephedrine acts on β > α receptors and has less effect on placental flow than other vasopressors which act by α stimulation, e.g. methoxamine.

• Fetal acidosis results in more local anaesthetic taken up by the fetus with increased risk of toxicity.

Effect of anaesthesia and surgery on the fetus during LSCS

• Maternal catecholamine levels are lower with epidural/spinal than with GA.

• IPPV lowers cardiac output and reduces placental flow, especially with hypovolaemia. Minimize effects by reducing mean intrathoracic pressure.

• Hyperventilation shifts maternal O₂ dissociation curve to the left, reducing placental O₂ transfer and umbilical blood flow, and causes fetal acidosis. Slight maternal hypercapnia may benefit the fetus by improving oxygen delivery.

• Maternal P_ao₂ <13 kPa reduces fetal oxygenation and delays onset of spontaneous ventilation in the newborn. A maternal F_io₂ >0.65 was shown to improve Apgar scores and reduce fetal acidosis at delivery, but the study did not control for aortocaval compression or F_in₂o.

• Aortocaval compression occurs in 15% of women at term. When lying supine, the pregnant uterus at term almost completely obstructs the inferior vena cava. Reduced with lateral tilt, more effective to the left.

• Degree of fetal acidosis on delivery is related to the time from uterine incision to delivery. Induction–delivery time has little effect on acidosis if using left lateral tilt.

Effect of anaesthetic drugs on the fetus during LSCS

• Placental transfer of drugs is influenced by protein binding, high lipid solubility, low ionization and kPa.

• Fetus is not anaesthetized by i.v. induction agents because hepatic extraction and dilution of drugs with blood from the lower limbs and upper body reduce the concentration of drug delivered to the fetal CNS.

• Injection of induction agent at the onset of a contraction reduces the dose delivered to the fetus. Fetal plasma levels of induction agents follow the same pattern as in adults by 2–3 min.

• Thiopentone reduces placental blood flow and may reduce fetal O₂ delivery. Ketamine increases placental flow, but increased force of uterine contraction may worsen cord prolapse or abruptio placentae.

• N₂O may reach equilibrium in the fetus with a possible risk of diffusion hypoxia in the fetus upon delivery which should therefore have O₂. There is no evidence of methionine synthetase depression when N₂O is used for LSCS. Less than 1.0 MAC halothane, enflurane or isoflurane does not increase perioperative blood loss and is not detrimental to the fetus. Minimal fetal fluoride levels with enflurane.

• Neuromuscular blocking drugs are large highly ionized molecules with minimal placental transfer. However, fetal paralysis has been reported following extremely high doses of non-depolarizing drugs, e.g. when given in error. Suxamethonium administration to patients with homozygous plasma cholinesterase deficiency may result in fetal paralysis.

• Opioids rapidly cross the placenta to achieve fetal levels equal to those of the mother. Fentanyl 1 μg/kg at induction does not affect fetal Apgar score, neurobehavioural scores or acid–base status. Opioids may reduce the maternal stress response and lessen catecholamine-induced placental vasoconstriction.

Fetal and maternal effects of epidurals

• Improve coordination of uterine contraction by reducing noradrenaline levels

• Relief of pain reduces maternal hyperventilation

• Improve placental blood flow in pre-eclampsia and reduce fetal acidosis in the second stage of labour

• Fetal acidosis accelerates transfer of LA across placenta

• General anaesthesia is associated with lower Apgar scores at 1 min compared with regional techniques, but both techniques have similar scores by 5 min. Therefore, use a regional technique if possible when delivering a baby with fetal distress, but remember further fetal deterioration may occur while epidural is inserted.

Cardiotocograph

• Type I decelerations (early) – head compression causing vagal reflex

• Type II decelerations (late) – uteroplacental insufficiency

• Type III decelerations (variable) – umbilical cord compression.

Intraoperative Blood Cell Salvage in Obstetrics

National Institute for Clinical Excellence 2005(www.nice.org.uk/IPG144distributionlist)

Guidance

Intraoperative blood cell salvage is an efficacious technique for blood replacement and its use is well established in other areas of medicine, but there are theoretical safety concerns when it is used in obstetric practice.

Procedure

It has not been routinely adopted in obstetrics because of specific concerns about amniotic fluid embolism and about haemolytic disease in future pregnancies as a result of re-infusing amniotic fluid or fetal red blood cells.

Data collection is therefore important and clinicians should report all complications to the Medicines and Healthcare Products Regulatory Agency (www.mhra.gov.uk).

Pregnancy-induced hypertension

Incidence

• Pre-eclampsia – 5% all pregnancies

• Eclampsia – 0.05% all pregnancies.

Associations

Associated with positive family history, young primiparous, elderly, multiparous, diabetes, pre-existing hypertension, renal disease and collagen vascular disease. There is some evidence to suggest that there is an autosomal recessive predisposition to an immune response within the placenta.

Characterized by a triad of:

• hypertension – ↑ BP >30/15 mmHg; ↑ systolic above 160 mmHg, ↑ diastolic >15 mmHg

• proteinuria – >0.3 mg/kg; severe >5 g/day.

Pathophysiology

Thought to be triggered by an autoimmune reaction against the placenta (Fig. 8.2).

Figure 8.2 Pathophysiology of pre-eclampsia.

Airway. Facial and laryngeal oedema may make intubation difficult. Consider awake fibreoptic intubation in elective patients undergoing GA.

Cardiovascular. In untreated patients, cardiac index (CI) is low/normal, SVR is normal/high and PCWP is low/normal due to contraction of intravascular volume by as much as 30–40% in severe cases. Fluid challenge may improve cardiovascular stability, increase CI and reduce SVR to more normal levels. PCWP becomes normal/high, but CVP correlates poorly. Consider invasive monitoring if oliguria persists following 500 mL fluid challenge.

Treat hypertension with hydralazine 5–10 mg i.v. boluses or 5–40 mg/h infusion. Causes headache, tremor and vomiting, mimicking symptoms of eclampsia. Also consider labetalol up to 1 mg/kg (reports of fetal bradycardia if used in the presence of fetal distress) or sodium nitroprusside 0.3–8 μg/kg/min. Nifedipine may cause severe hypotension if used with magnesium.

Central nervous system. Cerebral vasospasm, microinfarcts, petechial haemorrhage and oedema cause CNS irritability, visual disturbance and headache. May be worsened by hypertension following pressor response to intubation. Fits are more common in teenage mothers and multiple pregnancies. CNS haemorrhage is a major cause of maternal deaths.

Treat CNS irritability with magnesium sulphate 4 g loading dose then 1–3 g/h which suppresses EEG excitatory activity, aiming for therapeutic blood level of 2–4 mmol/L. Titrate against deep tendon reflexes. Also vasodilates uterine vessels and attenuates uterine vascular response to catecholamines. May accumulate in renal failure. Excess (>4 mmol/L) causes respiratory paralysis, heart block and fetal weakness. Treat with calcium gluconate 1 g i.v.

Coagulation. Thrombocytopenia <100 000 is common. Normal platelet counts are associated with prolonged bleeding times in 10–25% of pre-eclamptics; 34% of patients with severe eclampsia have prolonged bleeding times. Low-grade DIC is common.

Recent Collaborative Low Dose Aspirin Study (CLASP) cast doubt on the efficacy of aspirin in reducing the incidence of pre-eclampsia.

Hepatic. Abnormal LFTs due to oedema and hepatic congestion. Associated with HELLP syndrome (Haemolysis, Elevated Liver enzymes, Low Platelets).

Renal. Decreased glomerular filtration, acute tubular necrosis and increased permeability to proteins causing proteinuria.

Gaiser R.R. Old concepts applied to new problems: the fetus as a patient. Curr Opin Anaesth. 1998;11:251-253.

Heidemann B.H., McClure J.H. Changes in maternal physiology during pregnancy. BJA CEPD Rev. 2003;3:65-68.

NICE. Intraoperative blood cell salvage in obstetrics. Interventional Procedures Guideline, 144. London: National Institute for Health and Clinical Excellence, 2005.

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