Influence of electrical axis of stimulation on excitation of cardiac muscle cells

Orthogonal sequential shock can defibrillate the heart with greater efficacy compared with single shock defibrillation. In this study we tested the hypothesis that cardiac cells have a preferred orientation in their response to excitatory extracellular electric fields, so that orthogonal shocks may stimulate distinct populations of cells. A micropaddle electrode system was used to deliver rectangular pulses for extracellular field stimulation of individual heart cells. We found that single frog and guinea pig ventricular myocytes are excitable with rectangular pulse field stimulation over a wide range of pulse durations, ranging from 10 msec to as little as 20 μsec. The excitation field strength varies inversely with pulse duration as described by the Weiss-Lapicque equation, although the frog myocytes show a significant ''notch'' at pulse durations of ~1-2 msec, and the guinea pig myocytes are more excitable than predicted for pulse durations of <0.2 msec. Every myocyte tested was more excitable when the long axis of the cell was oriented parallel to the stimulating field than when perpendicular to the field. For 2-msec pulses, the difference in field strength was a factor of 5.8±2.0 (n=30) for frog and 2.6±0.5 (n=23) for guinea pig myocytes. Complete excitation strength-duration curves were obtained in seven frog and 14 guinea pig cells for both parallel and perpendicular cell orientations. The increase in electric field required for stimulation of cells in a perpendicular orientation over a parallel orientation was approximately constant for pulse durations between 0.2 and 10 msec, by a factor of 5.5±2.2 (seven cells, six durations) for frog and 2.5±0.7 (14 cells, six durations) for guinea pig, but it was less at shorter durations. These results suggest that the efficacy of orthogonal sequential shock may be regulated in part by tissue excitability at the cellular level.