Temperature-dependent nerve-blocking action of lidocaine and halothane

The effect of lidocaine and halothane on the compound action potential of rat sciatic nerves was studied under hypo-hyperthermic conditions. In drug-free desheathed nerves, a total but reversible cold block occurred at 10-11 degrees C, and an irreversible heat block at 46 degrees C. Cooling potentiated the dose-dependent blocking action of lidocaine (decreased amplitude, and increased duration and latency of the compound action potential). Total block of conduction occurred at 17 degrees C with 100 microM lidocaine, at 20 degrees C with 200 microM lidocaine and at 24 degrees microM with 400 microM lidocaine. In nerves equilibrated in 200 microM lidocaine solution, the lidocaine concentrations in the nerves decreased as the temperature decreased; at 20 degrees C, the lidocaine concentration was about half of that at 37 degrees C. The nerve-blocking effect of lidocaine was also potentiated by increasing the temperature above 37 degrees C. At 30 degrees C and 20 degrees C, 1 vol. % halothane caused a slight time-dependent inhibition of the compound action potential. When the nerves were exposed to 2.5 vol. % halothane, the decrease in amplitude and increase in duration and latency were potentiated by hypothermia and were also time dependent. Interaction of halothane and temperature of 40 degrees C was not significant. Although thermodynamic principles suggest similarities between high pressure and cooling in reversing anaesthesia, cooling was found to potentiate the anaesthetic effect of lidocaine and halothane in this study. Temperature may thus be an interesting physical variable in studying nerve-blocking mechanisms.