Dr Patricia Coyle,
Black Lion Hospital, Addis Ababa, Ethiopia.

Introduction		Maintenance of anaesthesia
Physiological changes in late pregnancy		In extremely difficult circumstances
Preparation for anaesthesia		Failed intubation drill
Induction of anaesthesia		Other measures

Caesarean section is a surgical procedure commonly performed in rural hospitals in developing countries. Those responsible for providing anaesthesia for such surgery must be fully aware of the basic principles involved.

Caesarean sections can be performed under general anaesthesia, regional anaesthesia or local infiltration. This article will consider only general anaesthesia: regional and local techniques will be described in a subsequent article.

When selecting the type of anaesthetic to be used, the anaesthetist must take into consideration his or her own experience and that of the surgeon, the condition of the patient, the degree of urgency and the availability of equipment and drugs. General anaesthesia has the advantage of speed, but introduces the risk of airway complications including aspiration of gastric contents or failed intubation. Regional anaesthesia, such as spinal or epidural block, avoids these risks, but takes longer to perform, and may cause undesirable hypotension secondary to peripheral vasodilation.

 
Physiological changes in late pregnancy.

1. When the patient lies on her back the pregnant uterus compresses the vena cava and the aorta and obstructs blood flow. Compression of the former leads to a diminished venous return and a fall in maternal cardiac output, and this together with the compression of the aorta will reduce the blood flow to the uterus, with undesirable effects on the fetus. In some women this "aorto-caval" compression may lead to hypotension (supine hypotensive syndrome).

   The conscious patient can respond by improving her position, but under general anaesthesia this is impossible. Spinal or epidural anaesthesia considerably worsens the problem due to the sympathetic blockade produced.

   By tilting the patient to the left by about 15 degrees the pressure from the uterus on the vena cava is reduced. This can be achieved by tilting the operating table or by placing a wedge under the patient's right buttock (figure 1).

2. There is diminished tone in the lower oesophageal sphincter and in later pregnancy the raised intra-abdominal pressure and altered gastro-oesophageal angle make gastric reflux more likely. In labour the administration of opioids markedly slows gastric emptying.

   During induction of anaesthesia passive regurgitation of stomach contents into the pharynx may occur, and lead to aspiration pneumonia. This is likely if the pH of the stomach contents is less than 3 (very acidic) and more that 30 mls of fluid is aspirated. The mother may be protected from this complication by using local anaesthesia instead of general anaesthesia when possible, by reducing food intake in labour to minimise stomach contents and by a careful rapid sequence induction of anaesthesia using cricoid pressure (see figs 2 & 3). The stomach acid should also be neutralised if a Caesarean section is planned. An intravenous H₂ blocker such as ranitidine or cimetidine is effective but takes an hour to work. An antacid such as 30mls of 0.3 Molar sodium citrate is reliable if given immediately before induction of anaesthesia and will last about 1 hour. A combination of both methods will protect for several hours.

   Some anaesthetists pass stomach tubes prior to induction of anaesthesia to empty the stomach. Although this may remove some fluid it is rare for them to empty the stomach efrectively, as solid material cannot generally be removed.

3. There is a greater increase in plasma volume than red cell mass causing dilutional anaemia. Cardiac output is increased. Patients with cardiac disease (either congenital or valvular) are at particular danger during pregnancy, due to their inability to adapt to these changes.

4. There is a decrease in the resting lung volume caused by pressure from the enlarged uterus, and there is an increase in the basal metabolic rate. Oxygen reserves are therefore diminished and hypoxia develops rapidly if airway problems occur.

5. Many drugs used in anaesthesia cross the placental barrier and may affect the fetus, particularly opioids such as morphine and sedatives such as diazepam. During anaesthesia these drugs should be avoided until the umbilical cord has been clamped.

(Continued...)

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