30. CAESAREAN SECTION

The choice of anaesthesia for caesarean section depends on the experience of the anaesthetist, the wishes of the mother, the urgency of the procedure and the health of the mother and foetus.

The anaesthetist must understand the physiological changes of pregnancy, avoid aortocaval compression, avoid neonatal depression and be aware that difficult tracheal intubation and aspiration of gastric contents with general anaesthesia are major causes of maternal morbidity and mortality.

Preoperatively the anaesthetist must perform a full preoperative assessment with particular attention to assessment of the airway for possible difficult intubation, contraindications to regional anaesthesia, the reason for the caesarean section and determine if the patient is hypovolaemic. The average blood loss from caesarean section is 600 to 700 ml.

Choice of Anaesthesia

The advantages of regional anaesthesia (spinal or epidural) include an awake mother, minimal newborn depression, reduced blood loss and avoiding the risks of general anaesthesia. General anaesthesia may be necessary when regional anaesthesia is contraindicated (maternal preference, coagulopathy, infection, raised intracranial pressure), there is severe foetal distress or maternal haemorrhage. General anaesthesia has the advantages of less hypotension in the hypovolaemic patient, better control of the airway and ventilation and rapid onset. However there are potential problems including aspiration of gastric contents, failed intubation, difficult mask ventilation, uterine atony, neonatal depression and maternal awareness.

Aspiration Risk

The anaesthetist must try to reduce the risk of aspiration of gastric contents in all patients having a caesarean section (general anaesthesia and regional anaesthesia). The patient should be fasted if possible. For elective caesarean sections, an oral H-2 receptor (ranitidine or cimetidine) should be given the night before and two hours before surgery. For emergency caesarean section, a H-2 receptor antagonist may be given as soon as the decision to operate is made. All patients should receive a non-particulate oral antacid such as sodium citrate, within 1 hour of the start of anaesthesia. All patients should be positioned with a lateral tilt to reduce aortocaval compression and receive oxygen if available. 

Regional Anaesthesia

Spinal anaesthesia is a simple, rapid and reliable technique if there is no contraindication. The anaesthetist must be aware that spinal anaesthesia may be dangerous if the mother has untreated hypovolaemia or large blood loss.

Epidural anaesthesia is an alternative technique. It has a slower onset than spinal anaesthesia (20 minutes) and the anaesthesia may not be as effective but the dose of epidural anaesthetic can be titrated and repeated if required. The epidural can also be used for postoperative analgesia. A dose of 15 to 20 ml of 3% chloroprocaine or 0.5% bupivacaine or 2% lignocaine with adrenaline 1:200,000 is usually effective. The anaesthetist should inject 5 ml of local anaesthetic each 5 minutes and assess the level of the block. Giving increments of local anaesthetic will avoid hypotension and a high block.

General Anaesthesia

General anaesthesia may be the technique of choice for emergency caesarean section, when regional anaesthesia is refused or contraindicated, or when large blood loss is expected. It allows rapid anaesthesia, control of the patient’s airway and less hypotension. However, the risk of aspiration is increased and general anaesthesia may cause foetal depression. There is also a risk of awareness, and failure to intubate remains a major cause of maternal morbidity and mortality.

If general anaesthesia is chosen, the patient must breathe 100% oxygen for 3 minutes immediately before the induction of anaesthesia.

Position the patient with a lateral tilt to avoid aorto-caval compression.

The anaesthetist must use a rapid sequence induction with cricoid pressure, intravenous thiopentone 4 to 5 mg/kg or propofol 2 to 2.5 mg/kg and succinylcholine (suxamethonium) 1.5 mg/kg. The cricoid pressure should be maintained until the trachea is intubated.

The mother is ventilated with 50% mixture of oxygen and nitrous oxide with low amounts of an inhalation agent (enflurane 1%, isoflurane 0.75% or halothane 0.5%). Anaesthetic requirements are decreased during pregnancy. In animal experiments the minimum alveolar concentration (MAC) of halothane is reduced by 25 to 40%. High doses of inhalation agents can cause increased uterine bleeding. Low doses of inhalation agents do not increase uterine bleeding or neonatal depression and will reduce maternal awareness. Using 50% nitrous oxide without a volatile inhalation agent will cause awareness in more than 20% of mothers. Muscle relaxation may be achieved with a short-acting non-depolarising agent or repeated doses of suxamethonium. After delivery of the baby the anaesthetist can give the mother an intravenous opioid (pethidine 50 to 100 mg or morphine 5 to 10 mg).

Most anaesthetic agents apart from the muscle relaxants will cross the placenta and can cause neonatal depression.

5 international units (IU) of oxytocin should be given intravenously immediately after the delivery of the baby. It must be given slowly. One side-effect of oxytocin is relaxation of vascular smooth muscle that will cause a fall in diastolic and systolic blood pressure, and a reflex tachycardia. Hypovolaemic patients may have a serious fall in blood pressure.

At the end of anaesthesia, remember that the mother is still at risk of aspiration of gastric contents. She must be awake and in a lateral position before the endotracheal tube is removed.