Directional variability of stimulation threshold measurements in isolated guinea pig cardiomyocytes: relationship with orthogonal sequential defibrillating pulses.

Reports on delivery of separated orthogonal pulses markedly improving cardiac defibrillation have suggested that the stimulation threshold of heart fibers varies in accordance with their orientation within the electric field. The present work was aimed at investigating the directional variability of stimulation thresholds in isolated guinea pig cardiomyocytes. This variability was measured in 48 single myocytes by rotating each one through a theta (theta) angle between two-fixed parallel electrodes 1.1 cm apart, thus making theta vary between the electric field and the myocyte axis. For theta = 0 degrees, the mean longitudinal current stimulation threshold was 16.92 +/- 4.20 mA (n = 48). When theta was increased by increments of 10 degrees up to 90 degrees, the stimulation threshold increased in an exponential way. For theta = 90 degrees, the mean transverse stimulation threshold was 63.13 +/- 13.30 mA. These results clearly demonstrate the dependence of isolated cardiomyocyte stimulation thresholds on their orientation within the electric field and may account for the improved efficacy of defibrillation previously observed after delivery of orthogonal pulses.