Defibrillation of 100 kg calves with asymmetrical, bidirectional, rectangular pulses.

The effectiveness in reversing ventricular fibrillation of 30 s duration of asymmetrical, bidirectional, rectangular waveforms in which the lagging half-cycle has the same duration but lower amplitude than the leading portion of the waveform was evaluated in a 2160-episode study involving anaesthetised calves. An additional 480-episode auxiliary study involved the interlacing of unidirectional and bidirectional wave episodes. The leading half-cycles of the 18 bidirectional waveforms evaluated were 35 A at 8 and 16 ms, 50 A at 4 and 8 ms, and 70 A at 2 and 4 ms. Associated with each of the six leading half-cycle configurations were lagging half-cycles having reverse current levels of 1/8th, 1/4th, and 1/2 of the leading half-cycle current amplitudes. Six waveforms were successful in 97% or more of the transthoracic episodes. Of these, three were 100% successful. Our data, when combined with those from earlier unidirectional and symmetrical, bidirectional, rectangular waveform studies, suggest that a broad category of bidirectional rectangular shocks are superior to the most favourable unidirectional rectangular shock.