ELECTRICAL-STIMULATION OF CARDIAC MYOCYTES

The influence of nonuniform cell shape and field orientation on the fi eld stimulation thresholds of cardiac myocytes was studied both experi mentally and computationally. The percent change in excitation thresho ld, which was studied with patch clamp technique, was found to be 182 +/- 83.1% (mean +/- SD) higher when the electric field (EF) was parall el to the transverse cell axis versus the longitudinal axis (p < 0.000 6). On reversing the polarity of the applied EF, the percentage change in threshold was observed to increase by 98.9 +/- 71.0% (p < 0.0002), implying asymmetry of the stimulation threshold of isolated myocytes. Finite element models were-made to investigate the distribution of th e transmembrane potential of these experimentally studied myocytes. A typical cell model showed that the maximum transmembrane potential ind uced on opposite ends of the cell was 39.1 mV and -46.5 mV for longitu dinal field (aligned with the long axis of the cell), but was 40.5 mV and -44.8 mV for transverse field (aligned with the shea axis of the c ell). More significantly, it was found that the maximum transmembrane potential occurred at discrete points or ''hot spots'' on the cell mem brane. It is hypothesized that the depolarization of the cell initiate s at the hot spot and then spreads over the entire cell. The different excitation thresholds for different polarities of the applied EF can be explained by the different maximum induced at the opposite ends of the cell.