The end result of a nerve block is predictable once you understand where the nerve goes, what it connects to, what type of nerve it is, and what has been injected around it.
The factors that determine the effect of a nerve block are:-
- Type of local anaesthetic - the duration of action of these differ considerably:-
- Lignocaine (Lidocaine, xylocaine) - 2 hours
- Prilocaine (Citanest) - 4 hours
- Bupivacaine (Marcaine) - 6 hours
- Levobupivacaine (Chirocaine) - 6 hours
- Dose of Local Anaesthetic - the duration of action is related to the total dose (mg) injected. The larger the volume and the higher the % concentration, the higher the dose and the longer the length of action.
- Dose (mg) = Drug % x Volume (ml) x 10
- Therefore 10 ml of bupivacaine 0.5% = 0.5 x 10 x 10 = 50 mg
- Block Extenders - a nerve block tends to wear off because the local anaesthetic is absorbed into the local tissues / circulation, reducing the amount available to block the nerve. Adding a drug which reduces blood flow (vasoconstrictor) to the area has the effect of reducing tissue / circulation uptake, and therefore makes the block last longer. Weak doses vasoconstricting drugs like adrenaline and clonidine are often added to nerve blocks to prolong them.
- Acidity of the solution - how easily a local anaesthetic penetrates a nerve depends on the acidity of the solution injected. In general making a solution less acidic by adding a small amount of alkali increases the ease of penetration into the nerve, and therefore improves the onset time. Sodium bicarbonate (alkali) can be added to an epidural injection during a caesarian section to speed up the onset time of the block and the delivery of the baby. Local anaesthetics do not work as effectively in the presence of infection, because the tissues in the infected area are more acidic, making it more difficult for the drug to penetrate the nerve.
- Type of Nerve - blocking a sensory nerve will only result in loss of sensation, whereas blocking a mixed sensory / motor nerve can result in loss of sensation and loss of motor power (weakness).
- Size of Nerve - smaller diameter nerves are more susceptible to being blocked when compared to larger diameter ones. Smaller nerves therefore have a faster onset time and vice versa for larger ones.
Degree of Nerve Myelination - within each main nerve there are individual nerves fibres which have differing characteristics (see table below). In general fibres with the smallest diameter and least amount of myelin, are the most susceptible to being blocked with local anaesthetic. These characteristics also explain why the order of onset of the block is Pain block 1st, Sensory block 2nd, and Motor block 3rd. The recovery of the blockade is in the reverse order ie. movement recovers first, followed by sensation, followed by pain.
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