Electroporation of Cardiac Cell Membranes with Monophasic or Biphasic Rectangular Pulses

During defibrillation, cardioversion, and electrocution trauma, heart cells are exposed to potential gradients that increase the transmembrane potential (Vm). At sufficiently high Vm, pathological increases in cell permeability can occur. With enzymatically isolated frog heart cells (n = 29) we investigated the voltage and time sufficient for electroporation or cardiac cell membranes with rectangular voltage pulses, particularly with 5-msec monophasic, and 5- or 10-msec biphasic pulses. The rectangular voltage pulse (monophasic 0.1-1.5 V, 0.1-100 msec or symmetric biphasic 0.1-1 V, 0.4-10 msec total duration) was applied to the cell membrane using the cell-attached patch clamp technique, and a low voltage pulse train was added so that membrane conductance could be monitored continuously. Step increases in membrane conductance (breakdown) were observed, indicative of electroporation, and occurred with different combinations of pulse amplitude and duration; for example, for monophasic square pulses: (1 V, 0.2 msec) or (0.5 V, 0.5 msec), and for biphasic pulses: (1 V, 0.4 msec total duration) or (0.5 V, 0.8 msec). Using 5- or 10-msec rectangular pulses, breakdown occurred at a voltage around 0.4 V independent of polarity or waveform. The recovery of the permeabilized cell membrane after the voltage pulse was highly variable, in some cases not recovering at all while in other cases recovering after a lapse of seconds to minutes. These results suggest that monophasic and biphasic pulses of approximately 1 V, 0.2-0.4 msec and approximately 0.4 V, 5 msec can permeabilize the heart cell membrane even for minutes, time enough to cause an alteration in the cellular ionic composition leading to depressed or unexcitable tissue, a precursor for cardiac arrhythmia.