A number of factors effect 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

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 of the perineum 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.

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.

Although the level of injection will obviously effect which dermatomes are blocked, spinal injections tend to be performed only in the lower lumbar region. The extent of the block is influenced more by the volume injected and the position of the patient than the actual interspace at which the injection occurs.

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 2 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 are more reliable when trying for a mid-thoracic block.

 
Preparation for Lumbar Puncture.

Assemble the necessary equipment on a sterile surface. It will include:

A spinal needle. The ideal would be 24-25 gauge with a pencil point tip to minimise the risk of the patient developing a post-spinal headache.

An introducer, if using a fine gauge needle as they are thin and flexible, and therefore difficult to direct accurately. A standard 19 gauge (white) disposable needle is suitable for use as an introducer.

A 5ml syringe for the spinal anaesthetic solution.

A 2 ml syringe for local anaesthetic to be used for skin infiltration.

A selection of needles for drawing up the local anaesthetic solutions and for infiltrating the skin.

A gallipot with a suitable antiseptic for cleaning the skin, eg chlorhexidine, iodine, or methyl alcohol.

Sterile gauze swabs for skin cleansing.

A sticking plaster to cover the puncture site.

The local anaesthetic to be injected intrathecally should be in a single use ampoule. Never use local anaesthetic from a multi-dose vial for intrathecal injection. Spare equipment and drugs should be readily available if needed.

(Continued...)

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