Preoperative Visit

Patients should be told about their anaesthetic during the preoperative visit. It is important to explain that although spinal anaesthesia abolishes pain, they may be aware of some sensation in the relevant area, but it will not be uncomfortable. It should also be explained that their legs will become weak or feel as if they don't belong to them any more. They must be reassured that these sensations are perfectly normal and that if, by any chance, they feel pain they will be given a general anaesthetic.

Premedication is often unnecessary, but if a patient is apprehensive, a benzodiazepine such as 5-10mg of diazepam may be given orally 1 hour before the operation. Other sedative or narcotic agents may also be used. Anticholinergics such as atropine or scopolamine (hyoscine) are not routinely required. 

Intravenous Pre-loading

All patients having spinal anaesthesia must have a large intravenous cannula inserted and be given intravenous fluids immediately before the spinal. This helps prevent hypotension following the vasodilation which is produced. The volume of fluid given will vary with the age of the patient and the extent of the proposed block. A young, fit man having a hernia repair may only need 500mls. Older patients are not able to compensate as efficiently as the young for spinal-induced vasodilation and hypotension and may need 1000mls for a similar procedure. If a high block is planned, at least a 1000mls should be given to all patients. Caesarean section patients need at least 1500mls. Crystalloids such as 0.9% Normal Saline or Hartmans are most commonly used. Dextrose 5% should be avoided as it is not effective for maintaining the blood volume. 

Positioning the Patient for Lumbar Puncture

Lumbar puncture is most easily performed when there is maximum flexion of the lumbar spine (figure 2). This can best be achieved by sitting the patient on the operating table and placing their feet on a stool. If they then rest their forearms on their thighs, they can maintain a stable and comfortable position. Alternatively, the procedure can be performed with the patient lying on their side with their hips and knees maximally flexed.

An assistant may help to maintain the patient in a comfortable curled position. The sitting position is preferable in the obese whereas the lateral is better for uncooperative or sedated patients. Consider the consequences of sudden hypotension or a vaso-vagal attack for a sitting patient. The anaesthetist can either sit or kneel whilst performing the block.

Factors Affecting the Spread of the Local Anaesthetic Solution

A number of factors affect the spread of the injected local anaesthetic solution within the CSF and the ultimate extent of the block obtained.

Among these are:

• the baricity of the local anaesthetic solution
• the position of the patient
• the concentration and volume injected
• the level of injection
• the speed of injection

The specific gravity of the local anaesthetic solution can be altered by the addition of dextrose. Concentrations of 7.5% dextrose make the local anaesthetic hyperbaric (heavy) relative to CSF and also reduce the rate at which it diffuses and mixes with the CSF. Isobaric and hyperbaric solutions both produce reliable blocks. Injecting hyperbaric solutions and then altering the patient's position probably produces the most controllable blocks.

If a patient is kept sitting for several minutes after the injection of a small volume of a hyperbaric solution of local anaesthetic, a classical "saddle block" affecting only the sacral nerve roots will result.

The spinal column of patients lying on their side is rarely truly horizontal. Males tend to have wider shoulders than hips and so are in a slight "head up" position when lying on their sides, whilst for females with their wider hips, the opposite is true. Regardless of the position of the patient at the time of injection and whatever the initial extent of the block obtained, the level of the block may change if the patient's position is altered within twenty minutes of the injection of a hyperbaric agent.

The quantity of local anaesthetic (in milligrams) injected will determine the quality of the block obtained whilst its extent will also be determined by the volume in which it is injected. Large volumes of concentrated solutions will, thus, produce dense blockade over a large area. As spinal anaesthetics are generally only injected in the lumbar region, the extent of the block is influenced more by the volume and concentration injected and the position of the patient than the actual interspace at which the injection occurs.

The speed of injection has a slight effect on the eventual extent of the block. Slow injections result in a more predictable spread while rapid injections produce eddy currents within the CSF and a somewhat less predictable outcome.

Finally, increased abdominal pressure from whatever cause (pregnancy, ascites etc.) can lead to engorgement of the epidural veins, compression of the dura and hence a reduction in the volume of the CSF. A given quantity of local anaesthetic injected into the CSF might then be expected to produce a more extensive block. 

Quantities of Local Anaesthetics to Use

The degree of spinal blockade needed, as measured by the height of the block, will depend on the operation to be performed (see Table 1). For certain blocks, less local anaesthetic is needed when hyperbaric rather than plain solutions are used. Special considerations apply to obstetric patients and so the following chart does not apply to them (see later section).

The volumes of local anaesthetic shown in Table 1 should be considered only as a guideline. The lower volumes suggested should generally be injected in particularly small people. More may have to be given if the resultant block is not high enough for the proposed operation. Hyperbaric agents and appropriate positioning are the most reliable way of obtaining a mid-thoracic block. 

(Continued ...)

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