Spinal Anaesthesia - A Practical Guide (page 3)

Issue 3 (1993) Article 2: Page 3 of 8

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Spinal Anaesthesia - A Practical Guide (Continued)

Local Anaesthetics for Spinal Anaesthesia.

Local anaesthetic agents are either heavier (hyperbaric), lighter (hypobaric), or have the same specific gravity (isobaric) as the CSF. Hyperbaric solutions tend to spread below the level of the injection, while isobaric solutions are not influenced in this way. It is easier to predict the spread of spinal anaesthesia when using a hyperbaric agent. Isobaric preparations may be made hyperbaric by the addition of dextrose. Hypobaric agents are not generally available. The other factors affecting the spread of local anaesthetic agents when used for spinal blocks are described later.

Bupivacaine (Marcaine). 0.5% hyperbaric (heavy) bupivacaine is the best agent to use if it is available. 0.5% plain bupivacaine is also popular. Bupivacaine lasts longer than most other spinal anaesthetics: usually 2-3 hours.

Lignocaine (Lidocaine/Xylocaine). Best results are obtained with 5% hyperbaric (heavy) lignocaine which lasts 45-90 minutes. 2% lignocaine can also be used but it has a much shorter duration of action. If 0.2ml of adrenaline 1:1000 is added to the lignocaine, it will usefully prolong its duration of action. Lignocaine from multi-dose vials should not be used for intrathecal injection as it contains potentially harmful preservatives.

Cinchocaine (Nupercaine, Dibucaine, Percaine, Sovcaine). 0.5% hyperbaric (heavy) solution is similar to bupivacaine.

Amethocaine (Tetracaine, Pantocaine, Pontocaine, Decicain, Butethanol, Anethaine, Dikain). A 1% solution can be prepared with dextrose, saline or water for injection.

Mepivacaine (Scandicaine, Carbocaine, Meaverin). A 4% hyperbaric (heavy) solution is similar to lignocaine.

Spinal Anaesthesia and Common Medical Conditions.

Respiratory Disease. A low spinal block has no effect on the respiratory system and is therefore ideal for patients with respiratory disease unless they cough a lot. Frequent coughing results in less than ideal conditions for the surgeon. A high spinal block can produce intercostal muscle paralysis, but this does not usually create any problems, unless the patient is very limited by his respiratory disease.

Hypertension. Hypertension is not a contra-indication to spinal anaesthesia but, ideally, it should be controlled before any anaesthetic is administered. Hypertensive patients should have their blood pressure closely monitored during the anaesthetic and any episode of hypotension vigorously treated.

Sickle cell disease/trait. Spinal anaesthesia may be advantageous for patients with sickle cell disease. Follow the same rules as for general anaesthesia: ensure that the patient is well oxygenated, well hydrated and not allowed to become hypotensive. Consider warming the intravenous fluids and do not allow the patient to become cold. Avoid the use of tourniquets.

Pre-operative Visit.

Patients should be told about their anaesthetic during the pre-operative 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 and is quite normal. 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, if they feel pain they will be given a general anaesthetic.

Premedication is not always necessary, but if a patient is apprehensive, a benzodiazepine such as 5-10 mg 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 unnecessary.

Pre-loading.

All patients having spinal anaesthesia must have a large intravenous cannula inserted and be given intravenous fluids immediately before the spinal. 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 500 mls. 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 1500 mls.

The fluid should preferably be normal saline or Hartmann's solution. 5% dextrose is readily metabolised and so is not effective in maintaining the blood pressure.

Positioning the Patient for Lumbar Puncture.

Lumbar puncture is most easily performed when there is maximum flexion of the lumbar spine (fig 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. The anaesthetist can either sit or kneel whilst performing the block.

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