Physiology of Spinal Anaesthesia

Local anaesthetic solution injected into the subarachnoid space blocks conduction of impulses along all nerves with which it comes in contact, although some nerves are more easily blocked than others. There are three classes of nerve: motor, sensory and autonomic. Stimulation of the motor nerves causes muscles to contract and when they are blocked, muscle paralysis results. Sensory nerves transmit sensations such as touch and pain to the spinal cord and from there to the brain, whilst autonomic nerves control the calibre of blood vessels, heart rate, gut contraction and other functions not under conscious control.

Generally, autonomic and sensory fibres are blocked before motor fibres. This has several important consequences. For example, vasodilation and a drop in blood pressure may occur when the autonomic fibres are blocked and the patient may be aware of pressure or movement and yet feel no pain when surgery starts.

Practical implications of physiological changes. The patient should be well hydrated before the local anaesthetic is injected and should have an intravenous infusion in place so that further fluids or vasoconstrictors can be given if hypotension occurs. 

Anatomy

The spinal cord usually ends at the level of L2 in adults and L3 in children. Dural puncture above these levels is associated with a slight risk of damaging the spinal cord and is best avoided. An important landmark to remember is that a line joining the top of the iliac crests is at L4 to L5. Remember the structures that the needle will pierce before reaching the CSF (figure 1).

The skin. It is wise to inject a small bleb of local anaesthetic into the skin before inserting the spinal needle.

Subcutaneous fat. This, of course, is of variable thickness. Identifying the intervertebral spaces is far easier in thin patients.

The supraspinous ligament that joins the tips of the spinous processes together.

The interspinous ligament which is a thin flat band of ligament running between the spinous processes.

The ligamentum flavum is quite thick, up to about 1cm in the middle and is mostly composed of elastic tissue. It runs vertically from lamina to lamina. When the needle is within the ligaments it will feel gripped and a distinct "give" can often be felt as it passes through the ligament and into the epidural space.

The epidural space contains fat and blood vessels. If blood comes out of the spinal needle instead of CSF when the stylet is removed, it is likely that an epidural vein has been punctured. The needle should simply be advanced a little further.

The dura. After feeling a "give" as the needle passes through the ligamentum flavum, a similar sensation may be felt when the needle is advanced a further short distance and pierces the dural sac.

The subarachnoid space. This contains the spinal cord and nerve roots surrounded by CSF. An injection of local anaesthetic will mix with the CSF and rapidly block the nerve roots with which it comes in contact. 

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 down (due to gravity) from the level of the injection, while isobaric solutions are not influenced in this way. Hypobaric solutions are rarely used. 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. 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.

Lidocaine/Lignocaine (Xylocaine). Best results are said to be obtained with 5% hyperbaric (heavy) lidocaine, which lasts 45-90 minutes. 2% lignocaine can also be used but it has a shorter duration of action. If 0.2ml of adrenaline 1:1000 is added to the lignocaine, it will usefully prolong its duration of action. Recently concerns have been raised about the safety of 5% lidocaine (it is said to be potentially neurotoxic) despite it having been used uneventfully for over forty years. Lidocaine 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.

Tetracaine (Amethocaine, 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.

Pethidine/Meperidine. The 5% solution (50mg/ml) has local anaesthetic properties and is a versatile agent. The standard intravenous preparation is preservative-free and is isobaric. A dose of 0.5-1mg/kg is usually adequate for spinal anaesthesia.

Ropivacaine (Naropin) is a recently introduced long-acting local anaesthetic, similar to bupivacaine. It is not currently licensed for uses as a spinal anaesthetic.

It is generally thought that of the commonly used anaesthetic agents, lidocaine has a more rapid onset than bupivacaine, though some authors question this. Meperidine has a very rapid onset but can also wear off rapidly. It should also be remembered, especially when hyperbaric agents have been used, that patient movement, for example putting the patient "head-down" can cause the block to extend even some 20-30 minutes after it has been performed. 

Spinal Anaesthesia and Common Medical Conditions

Respiratory disease. A low spinal block (below the umbilicus) 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 has a very limited respiratory reserve and is, for example, unable to lie flat.

Uncontrolled hypertension or severe valvular disease. Although moderate hypertension is not a contra-indication to spinal anaesthesia, it should be remembered that there is an almost inevitable fall in blood pressure when spinal anaesthesia is induced. This can be particularly precipitous in patients with severe uncontrolled hypertension. Patients with aortic stenosis require a stable blood pressure (sustained after-load) to maintain their coronary perfusion. If they have a sudden fall in blood pressure, they may develop intractable cardiac arrest.

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. 

(Continued ...)

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