31. SPINAL ANAESTHESIA FOR OBSTETRIC PATIENTS

There are several important issues to consider when preparing anaesthesia for obstetric patients, including the physiological changes of pregnancy, the effect of anaesthesia and drugs on the newborn, and the risks and benefit of different anaesthetic techniques for the mother.

Advantages

Peripheral nerve blocks (pudendal and paracervical) are satisfactory for some obstetric procedures but spinal or epidural anaesthesia provide the best conditions for all obstetric procedures.

Spinal anaesthesia for caesarean section has several benefits compared to general anaesthesia, however spinal anaesthesia must be performed with care in the obstetric patient.

The main advantage of spinal anaesthesia is that the mother remains awake. This means she does not require endotracheal intubation (which is more difficult in the obstetric patient and has a higher rate of failure), and she can protect herself against aspiration of gastric contents (Mendelson’s syndrome). Spinal anaesthesia also means the mother can see her baby immediately; there is less blood loss than with general anaesthesia and reduced postoperative morbidity including fatigue, depression, fever and cough.

Studies of the advantages to the newborn are conflicting. Some studies have shown no difference between general and spinal anaesthesia while other studies have shown better newborn heart rate, less respiratory depression and better APGAR scores.

Physiological Changes of Pregnancy

Respiratory system

There are several normal changes of physiology in the obstetric patient that have major implications for anaesthesia.

Oxygen consumption increases during pregnancy by approximately 20% at term. This increase is compensated for by an increase in ventilation of 50% however the upward movement of the diaphagm by the uterus reduces the functional residual capacity. The increase in oxygen consumption and decrease in oxygen storage means that the mother can rapidly become hypoxic. Obesity, lying down and the lithotomy position increases the risk of rapid hypoxia. If the anaesthetist chooses general anaesthesia for caesarean section then the mother is at risk of developing hypoxia.

There are also changes in the mother’s airway that may make intubation more difficult including swelling of the airway, and large breasts. Difficulties with endotracheal intubation occur more commonly in obstetric patients (1:300) than in general surgical patients (1:3000). Inability to secure an airway is the leading cause of anaesthetic related maternal death.

Gastrointestinal System

During pregnancy the secretion of gastric acid increases and, in the last months of pregnancy, gastric emptying is delayed. (The enlarging uterus displaces the pylorus of the stomach). Labour pain will also delay gastric emptying. Non-pregnant patients will usually empty their stomachs of food within 6 hours, however a labouring patients may not empty her stomach for 8 to 24 hours. As pregnancy progresses, the lower oesophageal sphincter becomes less efficient at preventing oesophageal reflux. All these changes increase the risk of respiratory aspiration of gastric contents.

Cardiovascular System

There are many cardiovascular changes with pregnancy.

Of concern to the anaesthetist planning spinal anaesthesia is aorto-caval compression. After 28 weeks the pregnant uterus will obstruct the inferior vena cava when the mother is supine. Most mothers (90%) compensate for the vena caval obstruction by increased vasoconstriction and increased heart rate. With spinal or epidural anaesthesia the blockade of sympathetic nerves will reduce the mother’s ability to compensate for aorto-caval compression. The mother will become hypotensive. The supine position must be avoided in all obstetric patients with epidural or spinal anaesthesia. These patients must be cared for in the lateral position or with a minimum of 15 degrees of left lateral tilt. Uterine blood flow is largely pressure dependent so maternal hypotension must be treated immediately. The lateral tilt should be increased, intravenous fluids given and vasoconstrictors given if the blood pressure remains low (less than 100 mmHg systolic). Ephedrine is recommended, as it is less likely to constrict uterine vessels. However, as the uterine vessels become less sensitive to vasoconstrictors in late pregnancy and as uterine blood flow is largely pressure dependent, metaraminol or phenylephine may be considered as an alternative if ephedrine is ineffective. In some patients right lateral tilt is more effective. The whole patient may be tilted or a wedge placed under the patient’s hip to tilt the pelvis and abdomen.

Another consequence of the pregnant uterus compressing the inferior vena cava is that blood returning to the heart from the lower limbs is diverted in part though the epidural veins. This has two effects. It reduces the volume of the epidural and spinal space, which in part explains why obstetric patients need less local anaesthetic for spinal and epidural anaesthesia. (With pregnancy there is also an increase in sensitivity of nerve fibres to local anaesthetics). It also increases the risk of epidural haematoma.

Local Anaesthetic Alternatives

Caesarean section (anaesthesia should extend to T6)

0.5% bupivacaine plain / heavy 2.5 ml or

0.5% bupivacaine plain / heavy 2.2 to 2.5 ml and 10 to 20 µg of fentanyl or

2% lignocaine 2.0 to 2.5 ml or

5% heavy lignocaine 1.4 to 1.6 ml.

Forceps delivery

Lift out (low) forceps: 1.5 ml of plain or 0.5% heavy bupivacaine.

High or rotational forceps: 2.5 ml of plain or 0.5% heavy bupivacaine.

(Heavy is the same as hyperbaric).

Recommended Technique for Spinal Anaesthesia for Caesarean Section

Preoperative visit. Explain the spinal anaesthetic to the mother, perform a full preoperative assessment especially checking the patient’s airway.

Premedication. Give a non-particulate antacid (e.g. sodium citrate) when leaving the ward. Ideally a H-2 antagonist (e.g. ranitidine or cimetidine) should also be given orally 2 hours prior to surgery.

Check the anaesthetic machine and resuscitation equipment and drugs. Check that suction is available. Check the oxygen delivery system. Prepare emergency drugs and equipment (ephedrine, suxamethonium, thiopentone, laryngoscopes, endotracheal tubes).

Transport the patient to the operating theatre in the lateral position.

Check the mother’s heart rate and blood pressure and foetal heart rate.

Place a large intravenous cannula and give 500 to1000 ml of intravenousfluid.

Perform the spinal. Use the smallest needle possible. A non-cutting point will produce fewer headaches. It may be easier to perform the spinal with the mother sitting up.

Position the mother supine with at least 15 degrees of left lateral tilt and administer oxygen though a face mask.

Monitor the mother’s blood pressure and heart rate.

Treat hypotension with further lateral tilt, intravenous fluids and 10 mg intravenous ephedrine. Repeat ephedrine if required. Consider using metaraminol 0.5 mg if not responding.

After delivery of the baby, 5.0 international units of syntocinon should be given by slow intravenous injection.

The recommended dose of syntocinon is 5.0 international units by slow intravenous administration. It can cause hypotension tachycardia and arrhythmias. Syntocinon can cause cardiac arrest in severely hypovolaemic patients.