A comparison of intracellular lidocaine and bupivacaine concentrations producing nerve conduction block in the giant axon of crayfish in vitro.

Clinically, lidocaine requires a larger concentration than bupivacaine to block nerves. Bupivacaine has a higher lipid solubility, tissue permeability, and affinity for sodium channels than lidocaine, resulting in greater anesthetic potency. Local anesthetics require access to the sodium channel from the intracellular milieu. In this study, we sought to determine the intracellular concentration of lidocaine and bupivacaine when a nerve was blocked in the giant axon of a crayfish. A solution of lidocaine or bupivacaine was perfused, and a nerve block was determined as the absence of an evoked action potential after tonic or phasic electrical stimulation. The intracellular lidocaine or bupivacaine concentration was measured using a lidocaine- or bupivacaine-sensitive glass micro-electrode. A phasic block was more effectively and rapidly achieved with a smaller concentration of bupivacaine than with lidocaine. The intracellular concentration and intra- to extracellular ratios were significantly larger with lidocaine than with bupivacaine when nerve conduction was blocked. These findings suggest that bupivacaine has a higher potency than lidocaine, at least in the giant axon of a crayfish in vitro. The implications of the present results are that bupivacaine is a more potent nerve block and produces a use-dependent (phasic) block at smaller concentrations than lidocaine.