Effect of waveform tilt on defibrillation thresholds in humans

Despite the common use of the implantable cardioverter defibrillator to treat patients with life-threatening ventricular arrhythmias, the mechanism of defibrillation and the optimal waveform for implanted devices are poorly understood. All of the currently available pulse generators deliver exponentially declining pulses that are either automatically or manually truncated to achieve tilts of about 50% to 65%. Although this value was chosen based on experimental animal data, several theoretical models have been developed to describe defibrillation, which raise into question this choice of waveform shape. Accordingly, the present study was designed to test the effect of waveform tilt on defibrillation efficacy in humans.

Twenty-three patients undergoing cardioverter defibrillator implantation were studied. Monophasic defibrillation thresholds (DFTs) were measured using a single reversal protocol at 35%, 50%, 65%, and 80% tilts by altering the pulse width of the shock. Mean defibrillation impedance was 41 +/- 6 omega. The DFT, measured by either leading-edge voltage or stored energy, was insensitive to altering the waveform tilt from 50% to 80%, only increasing when the tilt was reduced to 35%. A tilt of 65% yielded the lowest DFT voltage in only 8 of 23 patients. Significantly lower DFTs (> or = 40 V) were obtained using other tilts in seven patients. When the relationship between average current and pulse width was fit with a Weiss-Lapicque model, the data yielded a mean chronaxie of 4.6 +/- 3.0 msec and a rheobase of 4.2 +/- 1.7 A, but considerable patient variability was observed.

On average, DFTs in humans are insensitive to altering monophasic waveform tilts between 50% and 80%. There is, however, considerable patient variability, raising into question the premise that a single defibrillator waveform tilt is best for all patients.