Mechanisms Underlying Poststimulation Block Induced by High-Frequency Biphasic Stimulation

Objective To reveal the possible mechanisms underlying poststimulation block induced by high-frequency biphasic stimulation (HFBS). Materials and Methods A new axonal conduction model is developed for unmyelinated axons. This new model is different from the classical axonal conduction model by including both ion concentrations and membrane ion pumps to allow analysis of axonal responses to long-duration stimulation. Using the new model, the post-HFBS block phenomenon reported in animal studies is simulated and analyzed for a wide range of stimulation frequencies (100 Hz-10 kHz). Results HFBS can significantly change the Na+ and K+ concentrations inside and outside the axon to produce a post-HFBS block of either short-duration (<500 msec) or long-duration (>3 sec) depending on the duration of HFBS. The short-duration block is due to the fast recovery of the Na+ and K+ concentrations outside the axon in periaxonal space by diffusion of ions into and from the large extracellular space, while the long-duration block is due to the slow restoration of the normal Na+ concentration inside the axon by membrane ion pumps. The 100 Hz HFBS requires the minimal electrical energy to achieve the post-HFBS block, while the 10 kHz stimulation is the least effective frequency requiring high intensity and long duration to achieve the block. Conclusion This study reveals two possible ionic mechanisms underlying post-HFBS block of axonal conduction. Understanding these mechanisms is important for improving clinical applications of HFBS block and for developing new nerve block methods employing HFBS.