Large sample test of defibrillation waveform sensitivity

An unknown mechanism causes defibrillation efficacy to be sensitive to the temporal pattern (waveform) of the delivered energy. Using a guinea pig model, we tested hypotheses in 140 defibrillation waveforms.

Two hundred seven male guinea pigs (950 +/- 100 g) were instrumented to continuously monitor the ECG and an optical plethysmographic signal from a forepaw. Two amplifiers served as a voltage-based defibrillator with a maximum output of 400 V at 2 A. Defibrillation electrodes (12-mm diameter) were placed 40 mm apart on the thorax. Thirty ventricular fibrillation episodes were induced where the first 10 episodes were used to estimate ED50 for a biphasic pulse (7/2 msec) and the remaining episodes were defibrillated with 18 test waveforms and two control waveforms all at the ED50 energy. Seven groups of 20 waveforms were tested. We directly tested hypotheses based on charge banking/burping, frequency concentration, and stimulus strength/duration. Of the hypotheses tested, nine are able to predict at least a 10% change in efficacy (P < 0.05): parabolic fit to duration; maximum, minimum, and remaining delivered charge; power at peak frequency; stimulus charge; and maximum, minimum, and maximum of the absolute value of stimulus strength. However, of these, only three are independent predictors of waveform efficacy (P < 0.05, near-minimum residual variance): power at peak frequency; parabolic fit to the stimulus duration; and minimum stimulus strength.

Stimulus strength and duration are the main determinants of the efficacy of a defibrillation waveform.