Differential nerve block by bupivacaine and 2-chloroprocaine. An experimental study

The differential blocking effect of bupivacaine and 2-chloroprocaine was studied on isolated rabbit cervical sympathetic trunks (B and C fibres) and phrenic nerves (A fibres). The B fibres (myelinated, preganglionic) were more sensitive than C fibres (unmyelinated, postganglionic) to nerve block by bupivacaine and 2-chloroprocaine, as assessed by changes in action potential amplitude and latency. The blocking action of both bupivacaine and 2-chloroprocaine caused a decrease in amplitude in the B fibres approximately twice as great as that produced in the C fibres. In similar experimental conditions, the fastest conducting A fibres (myelinated, motor) were least affected. Bupivacaine 200 mumol litre-1 completely blocked all B fibres in 7 min and C fibres in 10 min. The average action potential amplitude of A fibres at that time was 78%. 2-Chloroprocaine 300 mumol litre-1 completely blocked B fibres in 4 min and C fibres in 15 min. At 15 min the average action potential amplitude of the A fibres was still approximately 35%. Although these two local anaesthetics differ structurally and physico-chemically, the rates of block of the different fibres were in the same order in vitro.