There are several reasons for preferring spinal anaesthesia to general anaesthesia for caesarean sections. Babies born to mothers having spinal (or epidural) anaesthesia may be more alert and less sedated as they have not received any general anaesthestic agents through the placental circulation. As the mother's airway is not compromised, there is a reduced risk of aspiration of gastric contents causing chemical pneumonitis (Mendelson's syndrome).

Many mothers also welcome the opportunity of being awake during the delivery and being able to feed their child as soon as the operation is complete. There are, however, also disadvantages. It may be difficult to perform the spinal injection as lumbar flexion may be impeded by the pregnant uterus and, if labour has started, the mother may be unable to remain still when having contractions. Unless small gauge needles (25 gauge) are used, the incidence of post-spinal headache may be unacceptably high. Spinal anaesthetics for caesarean section should not be performed until the anaesthetist has accumulated sufficient experience in their performance with non-pregnant patients.

In the absence of hypovolaemia due to bleeding, spinal anaesthesia is a simple and safe alternative to general anaesthesia for manual removal of a retained placenta. It does not produce uterine relaxation and if this is required, a general anaesthetic with a volatile agent may be preferred.

Technique Spinal anaesthesia is performed and managed in pregnant patients in the same way as in non-pregnant patients but with a number of special considerations.

It is generally recommended that obstetric patients should be pre-loaded with not less than 1500 mls of a crystalloid solution before the dural puncture is performed.

Although spinal anaesthesia is not contra-indicated in the presence of mild pre-eclampsia, remember that such patients may have altered clotting function and are relatively hypovolaemic. There is always a chance that a pre-eclamptic patient may suddenly fit and anticonvulsant drugs (diazepam or thiopentone) must be immediately available. The advantages and disadvantages of spinal versus general anaesthesia will have to be carefully considered for each patient.

Pregnant women need smaller volumes of spinal anaesthetic solution than non-pregnant women in order to obtain a given height of block. For a caesarean section, anaesthesia should extend to T6 (about the bottom of the sternum) to be completely successful. This can usually be achieved with the following regimes, although the hyperbaric agents are more predictable:

• 2.0-2.5 ml of a hyperbaric solution of 0.5% bupivacaine or

• 2.0-2.5 ml of an isobaric solution of 0.5% bupivacaine or

• 1.4-1.6 ml of a hyperbaric solution of 5% lignocaine or

• 2.0-2.5 ml of an isobaric solution of 2% lignocaine with added adrenaline (0.2 ml of 1:1000)

Anaesthesia to T10 is needed for removal of a retained placenta. This can be obtained by injecting 1.5mls of a hyperbaric solution with the patient sitting and then lying her down.

Positioning of the Pregnant Patient Pregnant patients should never lie supine as the gravid uterus will compress the vena cava and, to a lesser extent the aorta (aorto-caval compression) resulting in hypotension. They should, instead, always lie with a lateral tilt. This can be achieved either by tilting the whole table or by inserting a wedge under the patients right hip. The uterus is displaced slightly to the left and the vena cava is not compressed (see Update No. 2).

As with all patients undergoing surgery under spinal anaesthesia, oxygen should be given during the operation. As hypotension commonly occurs despite fluid preloading, many anaesthetists routinely give a dose of vasoconstrictor intravenously. Ephedrine is the favoured vasoconstrictor as it does not cause constriction of the uterine blood vessels. If it is not available, one of the other vasoconstrictors discussed previously should be used as untreated hypotension can seriously damage the unborn infant.

After delivery of the baby, syntocinon is the oxytocic of choice as it is less likely to produce maternal nausea and vomiting than ergometrine.

This article contained links to the following additional information:

Table 1 - Dermatome Levels for Different Surgical Procedures.
Table 2 - Quantities of Local Anaesthetics for Spinals.

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