Scaling current and energy with body weight: requirements for the transthoracic ventricular defibrillation of calves as they grow from 50 to 150 kg.

To test the hypothesis that the effectiveness of a shock in achieving ventricular defibrillation is relatively independent of body weight if electrode diameter is proportional to the one-third power and current is proportional to the two-thirds power of weight, we studied defibrillation rates in 10 calves as they increased weight. At 50 kg, each calf was subjected to 20 fibrillation-defibrillation episodes using 10.3-cm diameter electrodes and 32-amp, 4-msec rectangular pulses for defibrillation. Two days after the original study, each calf underwent 20 additional episodes involving 44-amp pulses. With the specified scaling of electrode diameter and pulse amplitude, the two studies were repeated at weight intervals of 25 kg as the animals grew. Six calves survived. In the study that started with 32-amp pulses, first-shock success values of 28%, 49%, 66%, 51% and 23% were found in the six surviving calves at 50, 75, 100, 125 and 150 kg, respectively. The corresponding values were 93%, 96%, 93%, 94% and 91% in the study that started with 44-amp pulses. While the results of the 32-amp study fail to support our initial hypothesis, those obtained in the 44-amp current study appear compatible with the hypothesis.

Comparison of transthoracic square-wave defibrillation experience in the dog and calf.

Data from 5,280 fibrillation-defibrillation episodes in medium-size dogs, size scaled to a hypothetical 100-kg dog, and from 3,240 episodes in 100-kg calves, are used to derive families of curves showing percentage of success of unidirectional square-wave pulses in achieving defibrillation versus current amplitudes for energy levels in the range of 200 through 2,000 J. From the 3,240 episodes in the calves and 3,240 additional episodes in dogs, the incidence of post-defibrillation complete A-V block was found. In calves, the optimal 800-J curve peaks sharply at about the 93 percent success level for a current amplitude of 70 A. For the hypothetical dog, the 800-J curve has a broad peak at substantially the 100 percent success level for currents of 40 through 120 A. In general, the incidence of complete A-V block is very low in the dog and very high in the calf.

Transthoracic ventricular defibrillation in the 100 kg calf with unidirectional rectangular pulses.

The effectiveness in reversing ventricular fibrillation of 30 seconds duration of unidirectional rectangular-wave shocks having pulse widths of 0.5 through 64 msec, pulse amplitudes of 35, 50, 70, 100, and 140 amp, and pulse energies of 109 through 1,660 J was studied in 3,303 transthoracic fibrillation-defibrillation episodes in 100 kg calves. A total of 38 animals were used in the study. Postdefibrillation electrocardiograms were recorded. Families of curves of percent successful defibrillation vs pulse duration, percent successful defibrillation vs pulse energy, duration of postdefibrillation complete block or standstill vs energy, and time required for a return to normal sinus rhythm vs energy were derived. The most effective waveform studied (70 amp--8 msec--862 J) yielded defibrillation on the initial attempt in 93% of 120 episodes. In general, the duration of complete block or standstill and the time required for a return to normal sinus rhythm increased with increasing pulse current and pulse energy.