• Indications include: previous CS; pre-existing maternal morbidity (e.g. cardiac disease); complications of pregnancy (e.g. pre-eclampsia); obstetric disorders (e.g. placenta praevia); fetal compromise; failure to progress.

• Specific anaesthetic considerations:

 physiological changes of pregnancy.

 difficult tracheal intubation and aspiration of gastric contents associated with general anaesthesia (GA) have been major anaesthetic causes of maternal mortality. Mortality is higher for emergency CS than for elective CS.

 aortocaval compression must be minimised by avoiding the supine position at all times.

 uterine tone is decreased by volatile anaesthetic agents; however, consideration of this should not result in the administration of inadequate anaesthesia.

 placental perfusion may be reduced by severe and/or prolonged hypotension and/or reduction in cardiac output.

 contraction of the uterus upon delivery results in autotransfusion of about 500 ml blood, helping to offset the average blood loss of 500–1500 ml (GA) or 300–700 ml (regional anaesthesia).

 neonatal depression should be avoided, but maternal awareness may occur if depth of anaesthesia is inadequate.

• The usual preoperative assessment should be made, with particular attention to:

 the indication for CS (e.g. pre-eclampsia) and other obstetric or medical conditions.

 whether CS is elective or emergency, and whether the fetus is compromised.

 assessment of the airway.

 maternal preference for general or regional anaesthesia.

 if used in labour, the quality of existing epidural analgesia should be assessed.

 assessment of the lumbar spine and any contraindications to regional anaesthesia.

The above points inform the decision to employ general or regional anaesthesia.

• Regimens used to reduce the risk of aspiration pneumonitis:

 fasting during labour: this practice has been questioned, and many centres allow clear fluids or certain foods (low fat and sugar content) for low-risk women in labour (if not receiving opioids), although this is still controversial.

 antacid therapy: 0.3 M sodium citrate (30 ml) directly neutralises gastric acidity; it has a short duration of action and should be given immediately before induction of anaesthesia.

 H₂ receptor antagonists: different regimens are employed in different units, with most reserving prophylaxis, e.g. with ranitidine, for women at risk of operative intervention or receiving pethidine (causing reduced gastric emptying). For elective CS, ranitidine 150 mg orally the night before and 2 h before surgery is often given. For emergency CS, ranitidine 50 mg iv may be given; cimetidine 200 mg im is an alternative as it has a more rapid onset than ranitidine.

 omeprazole and metoclopramide have also been used, to reduce gastric acidity and increase gastric emptying, respectively.

• Anaesthetic techniques:

 for all types of anaesthesia:

– wide-bore iv access (16G or larger).

– cross-matched blood available within 30 min.

– routine monitoring and skilled anaesthetic assistance, with all necessary equipment and resuscitative drugs available and checked.

– aortocaval compression is reduced by positioning the patient laterally during transport to theatre and surgery.

– ergometrine has been superseded by oxytocin as the first-line uterotonic following delivery because of the former’s side effects (e.g. hypertension and vomiting).

 general anaesthesia:

– rapid sequence induction with an adequate dose of anaesthetic agent is required to prevent awareness; a ‘maximal allowed dose’ based on weight may be insufficient. Neonatal depression due to iv induction agents is minimal. Suxamethonium is still considered the neuromuscular blocking drug of choice by most authorities, although rocuronium (especially with the advent of sugammadex) is an alternative.

– difficult and failed intubation (the latter ~1 : 300–500) is more likely in obstetric patients than in the general population. Contributory factors include a full set of teeth, increased fat deposition, enlarged breasts and the hand applying cricoid pressure hindering insertion of the laryngoscope blade into the mouth. Laryngeal oedema may be present in pre-eclampsia. Apnoea rapidly results in hypoxaemia because of reduced FRC and increased O₂ demand.

– IPPV with 50% O₂ in N₂O is commonly recommended until delivery; however, in the absence of fetal distress, 33% O₂ is associated with similar neonatal outcomes.

– 0.5–0.6 MAC of volatile agents (with 50% N₂O) reduces the incidence of awareness to 1% without increasing uterine atony and bleeding, or neonatal depression; however, at 0.8–1.0 MAC, awareness falls to 0.2% and the uterus remains responsive to uterotonics. Placental blood flow is thought to be maintained by vasodilatation caused by the volatile agents, and high levels of vasoconstricting catecholamines associated with awareness are avoided. Delivery of higher concentrations of volatile agents (‘overpressure’) in 66% N₂O has been suggested for the first 3–5 min whilst alveolar concentrations are low, to reduce awareness. Factors increasing the likelihood of awareness include lack of premedication, reduced concentrations of N₂O and volatile agents, and withholding opioid analgesic drugs until delivery. Up to 26% awareness was reported in early studies using 50% N₂O and no volatile anaesthetic agent.

– normocapnia (4 kPa/30 mmHg in pregnancy) is considered ideal; excessive hyperventilation may be associated with fetal hypoxaemia and acidosis due to placental vasoconstriction, impairment of O₂ transfer associated with low PCO₂, and reduced venous return caused by excessive IPPV.

– all neuromuscular blocking drugs cross the placenta to a very small extent. Shorter-acting drugs (e.g. vecuronium and atracurium), are usually employed, since postoperative residual paralysis associated with longer-lasting drugs has been a significant factor in some maternal deaths.

– opioid analgesic drugs are withheld until delivery of the infant, to avoid neonatal respiratory depression.

– aspiration may also occur during recovery from anaesthesia (when the effect of sodium citrate may have worn off). Routine gastric aspiration during anaesthesia has been suggested, especially in emergency cases. Before tracheal extubation, the patient should be awake and on her side.

– advantages: usually quicker to perform in emergencies. May be used when regional techniques are inadequate or contraindicated, e.g. in coagulation disorders. Safer in hypovolaemia.

– disadvantages: risk of aspiration, difficult intubation, awareness and neonatal depression. Hypotension and reduced cardiac output may result from anaesthetic drugs and IPPV. Postoperative pain, drowsiness, PONV, blood loss and DVT risk all tend to be greater.

 epidural anaesthesia (EA):

– facilities for GA should be available.

– bupivacaine 0.5% or lidocaine 2% (the latter with adrenaline 1 : 200 000) is most commonly used in the UK, often in combination. Addition of opioids (e.g. fentanyl) is common (though may not be necessary if many epidural doses have been given during labour). Bicarbonate has also been added to shorten the onset time, (e.g. 2 ml 8.4% added to 20 ml lidocaine or lidocaine/bupivacaine mixture), but risks errors from mixing up to 4–5 different drugs, especially in an emergency. Commercial premixed solutions of lidocaine/bupivacaine and adrenaline may be unsuitable as they have a low pH (3.5–5.5) and contain preservatives. Ropivacaine 0.5–0.75% or levobupivacaine 0.5% is also used. 0.75% bupivacaine is associated with a high incidence of toxicity, and has been withdrawn from obstetric use. Chloroprocaine is available in the USA and a small number of European countries (but not the UK); it has a rapid onset and offset. Etidocaine has also been used in the USA.

– L3–4 interspace is usually chosen.

– on average, 15–20 ml solution is required for adequate blockade (from S5 to T4–6). Loss of normal light touch sensation is a stronger predictor of intraoperative comfort than loss of cold sensation, although either may be used to assess the block. Smaller volumes are required for a specified level of block than in non-pregnant patients, as dilated epidural veins reduce the available volume in the epidural space.

– injection of solution in 5-ml increments at 5-min intervals reduces the risk of hypotension and extensive blockade (e.g. due to accidental spinal injection), but increases anaesthetic time and total dose. A single injection of 15–20 ml is advocated by some as producing a more rapid block. A catheter technique is used most frequently, although injection through the epidural needle may be performed.

– opioids (e.g. diamorphine 2–4 mg or fentanyl 50–100 µg) are routinely given epidurally for peri- and postoperative analgesia; maternal respiratory depression has been reported, albeit rarely.

– hypotension associated with blockade of sympathetic tone may be reduced by preloading with iv fluid; however, the use of vasopressors to prevent and treat hypotension in the euvolaemic patient is thought to be a more effective and rational approach. Ephedrine 3–6 mg or phenylephrine 50–100 µg is commonly given by intermittent bolus; continuous infusions are also effective. Phenylephrine appears to be associated with a better neonatal acid–base profile than large doses of ephedrine (thought to be due to ephedrine crossing the placenta and causing increased anaerobic glycolysis in the fetus via β-adrenergic stimulation).

– nausea and vomiting may be caused by hypotension and/or bradycardia.

– routine administration of O₂ by facemask has been questioned on the grounds of being ineffective and even possibly harmful to the fetus, due to generation of free radicals.

– during surgery many women feel pressure and movement, which may be unpleasant; all women should be warned of this possibility and of the potential requirement for general anaesthesia. Inhaled N₂O, further epidural top-ups, small doses of iv opioid drugs (e.g. fentanyl or alfentanil), iv paracetamol and ketamine may be useful. Shoulder tip pain may result from blood tracking up to the diaphragm and may be reduced by head-up tilt. Good communication between mother, anaesthetist and obstetrician is especially important when performing CS under regional anaesthesia, and the obstetrician should warn the anaesthetist before exteriorising the uterus or swabbing the paracolic gutters.

– advantages: the risks of GA are avoided. Onset of hypotension is usually slow, the mother may be able to warn of it early and it may be corrected before becoming severe. Minimal neonatal depression occurs compared with GA. The mother is not drowsy and is able to hold the baby soon after delivery. Her partner is able to be present during the procedure. Epidural analgesia provided in labour can be extended for operative delivery. The catheter can be used for further ‘top-up’ during surgery if required, and for postoperative analgesia.

– disadvantages: risk of dural tap, total spinal blockade, local anaesthetic toxicity, severe hypotension. It may be slow to achieve adequate blockade with a risk of patchy block. Inability to move the legs may be disturbing.

 spinal anaesthesia (SA):

– general considerations as for EA.

– 0.5% bupivacaine is used in the UK; plain bupivacaine (e.g. 3 ml) produces a more variable block than hyperbaric bupivacaine (e.g. 1.8–2.8 ml), which is the only form licensed for spinal anaesthesia. Plain levobupivacaine is also licensed in the UK. Hyperbaric tetracaine (amethocaine) 0.5% (1.2–1.6 ml) is used in the USA.

– injection is usually at the L3–4/L4–5 interspace.

– advantages: as for EA, but of quicker onset. Blockade is more intense and not patchy. Smaller doses of local anaesthetic drug are used.

– disadvantages: single shot; i.e. may not last long enough if surgery is prolonged. Spinal catheter techniques allow more control over spread and duration but are technically more difficult. Risk of post-dural puncture headache (reduced to under 1% if 25–27 G pencil-point needles are used). Hypotension is of faster onset and thus may be more difficult to control; has been associated with poorer neonatal acid–base status compared with EA/GA. Remains the most popular anaesthetic technique for CS in the UK.

 combined spinal–epidural anaesthesia (CSE):

– general considerations as for EA and SA.

– allows the fast onset and dense block of SA but with the flexibility of EA. Also allows a small subarachnoid injection to be extended by epidural injection of either saline (thought to act via a volume effect) or local anaesthetic, with greater cardiovascular stability.

– usually performed at a single vertebral interspace (needle-through-needle method)

• Dosage: up to 30 mg in compound oral preparations. 150–300 mg orally tds/qds for post-dural puncture headache; 250 mg iv (with 250 mg sodium benzoate).

• Side effects: as for aminophylline, especially CNS and cardiac stimulation.

• Regulation:

 vitamin D: group of related sterols. Cholecalciferol is formed in the skin by the action of ultraviolet light, and is converted in the liver to 25-hydroxycholecalciferol (in turn converted to 1,25-dihydroxycholecalciferol in the proximal renal tubules). Formation is increased by parathyroid hormone and decreased by hyperphosphataemia. Actions:

– increases intestinal calcium absorption.

– increases renal calcium reabsorption.

– increases bone mineralisation.

 parathyroid hormone: secretion is increased by hypocalcaemia and hypomagnesaemia, and decreased by hypercalcaemia and hypermagnesaemia. Actions:

– mobilises bone calcium.

– increases renal calcium reabsorption.

– increases renal phosphate excretion.

– increases formation of 1,25-dihydroxycholecalciferol.

 calcitonin: secreted by the parafollicular cells of the thyroid gland. Secretion is increased by hypercalcaemia, catecholamines and gastrin. Actions:

– decreases intestinal absorption of dietary calcium.

– inhibits mobilisation of bone calcium.

– increases renal calcium and phosphate excretion.

• Classified according to pharmacological effects in vitro and in vivo:

 class I: potent negative inotropic and chronotropic effects, e.g. verapamil. Acts mainly on the myocardium and conduction system; reduces myocardial contractility and O₂ consumption and slows conduction of the action potential at the SA/AV nodes. Thus mainly used to treat angina and SVT (less useful in hypertension). Severe myocardial depression may occur, especially in combination with β-adrenergic receptor antagonists.

 class II: acts on vascular smooth muscle, reducing vascular tone; minimal direct myocardial activity (although may cause reflex tachycardia):

– nifedipine: acts mainly on coronary and peripheral arteries, with little myocardial depression. Used in angina, hypertension and Reynaud’s syndrome. Systemic vasodilatation may cause flushing and headache, especially for the first few days of treatment.

– nicardipine: as nifedipine, but with even less myocardial depression.

– amlodipine and felodipine: similar to nifedipine and nicardipine, but with longer duration of action and therefore taken once daily.

– nimodipine: particularly active on cerebral vascular smooth muscle, it is used to relieve cerebral vasospasm following subarachnoid haemorrhage.

 class III: slight negative inotropic effects, without reflex tachycardia: diltiazem is used in angina and hypertension.