Differential use-dependent (frequency-dependent) effects in single mammalian axons: data and clinical considerations

The potential clinical scope of use-dependent block of conduction (UDB) was assessed by studying characteristics of UDB in vitro in individual mammalian axons. Single and repetitive stimulation was applied to rabbit cervical sympathetic and vagus nerves exposed to solutions containing lidocaine 0, 0.3, or 0.6 mmol/l (9.1 or 18.2 mg/dl) at 37 degrees C. Unit responses were recorded in dissected filaments or extracellularly in the vagus nodose ganglion. With lidocaine 0.3 mM, equilibrium conduction block, tested by single shocks, was rare. 40-Hz trains produced a significantly greater increase in latency (slowing of conduction) and a much greater incidence of UDB in the sympathetic units than in myelinated vagus axons of equivalent control conduction velocities or in unmyelinated axons. 10-Hz stimulation did not produce UDB. With lidocaine 0.6 mM, the incidence of equilibrium conduction block was too high among sympathetic axons to assess UDB, and significantly higher than among nonsympathetic myelinated and unmyelinated units. The observations support the hypothesis that the differential block of sympathetics observed clinically with spinal anesthesia may be, at least in part, a use-dependent (frequency-dependent) effect. UDB seems unlikely to contribute to local anesthetic block of pain impulses.