Effect of shock timing on defibrillation success

The goal of this study was to determine whether delivering transvenous defibrillation shocks, coordinated with the up/down-slope VF waveform patterns in the shocking lead, would improve the probability of successful defibrillation. Anesthetized swine (32-38 kg, n = 8) were implanted with an RV-->SVC + SQArray transvenous system to measure VF waveform patterns and to deliver shocks. The shocks were generated by a Cardiac Pacemakers Inc. biphasic waveform generator. Energy required for 50% success probability (E50) was determined using the multishock up-down protocol. VF was repeatedly induced and defibrillation shocks at E50 were given after 10 seconds. The defibrillation outcome, delivered energy (Ed), peak voltage (V), peak current (I), system impedance (Z) and VF waveform pattern at the time of shock were recorded and measured. Out of a total of 685 shocks, 324 (47%) succeeded and 361 (53%) failed. The Ed, V, I, and Z were similar for the two defibrillation outcome groups (success or failure). VF patterns were classified as high or low amplitude at the time of the shock based on the peak-to-peak amplitude of signals recorded between the shocking electrodes. Shocks that coincided with high amplitude VF patterns were further divided into shocks that occurred on the up-slope or on the down-slope. The probability of success when the E50 shocks were coincident with high or low amplitude fibrillation did not differ significantly (Student's t-test: 46% vs 48%. P = NS). However, during high amplitude fibrillation, shocks delivered on the up-slope were significantly more successful than those delivered on the down-slope (Chi-square: 67% vs 39%; P < 0.001). These results suggest that delivering defibrillation shocks during the up-slope of the high amplitude signal in the shocking lead may improve the probability of successful defibrillation of ICDs.