Temperature‐Dependent Nerve‐Blocking Action of Lidocaine and Halothane

The effect of lidocaine and halothane on the compound action potential of rait sciatic nerves was studied under hypo‐ and hyperthermic conditions. In drug‐free desheathed nerves, a total but reversible cold block occurred at 10–11°C, and an irreversible heat block at 46°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 conductioh'octurred at 17°C with 100 μM lidocaine, at 20°C with 200 μM lidocaine and at 24°C with 400 μM lidocaine. In nerhes equilibrated in 200 μM lidocaine solution, the lidocaine concentrations in the nerves decreased as the temperature decreased; at 20°C, the lidocaine concentration was about half of that at 37°C. The nerve‐blocking effect of lidocaine was also potentiated by increasing the temperature above 37°C. At 30°C and 20°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°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.