Double pulse transthoracic defibrillation in the calf using percent fibrillation cycle length as spacing determinate

Recent studies have found that when multiple pulses of energy are used for defibrillation with implantable electrodes, the spacing between these pulse s is better determined as a percentage of the fibrillation cycle length (CL ), rather than as a fixed function of time. Here, this concept is further t ested in the transthoracic defibrillation of calves, which are approximatel y the size of heavy humans. Eleven 90-110 kg calves (101 +/- 6 kg) were use d in evaluating the effectiveness in achieving transthoracic ventricular de fibrillation of ten double pulse waveforms (two 50 A 4-ms rectangular monop ulses) having leading edge-to-edge spacings of 4 ms (a 50 A 8-ms rectangula r monopulse) and 50, 60, 70, 80, 90, 100, 110, 120, 130 percent fibrillatio n CL, respectively. In each of these waveforms, the total time when 50 A cu rrent was flowing ton time) was 8 ms. Our results show an unequivocal adver se interaction between the pulses, when the spacing is around 60%-70% fibri llation CL; but that the two pulses combined to defibrillate as effectively as a single 8-ms pulse when the spacing is around 110%-130% fibrillation C L. Electrocardiographic analysis suggests that the adverse interaction is d ue to a refibrillation phenomenon. This study confirms that double pulses c an interact and have a negative effect on defibrillation efficacy. Our data suggests that the mechanism of this interaction involves the second pulse reinitiating fibrillation when the pulse separation is in a critical range of values. Our results are also compatible with the hypothesis that the spa cing of multiple pulses is better determined as a percentage of the fibrill ation CL than as absolute time, although more study is necessary to fully t est this hypothesis.