Effect of biphasic shock duration on defibrillation threshold with different electrode configurations and phase 2 capacitances - Prediction by upper-limit-of-vulnerability determination

Background-The defibrillation threshold (DFT) may be affected by biphasic s hock duration (BSD), electrode configuration, and capacitance. The upper li mit of vulnerability (ULV) may be used to estimate the DFT. For different l end configurations and phase 2 capacitances, we investigated in 18 pigs whe ther the use of ULV may predict waveforms with lowest DFT. Methods and Results-DFT and ULV were determined by up-down protocols for 10 BSDs. ULVs were measured by T-wave scanning during ventricular pacing (cyc le length 500 ms). In protocol 1 (n=6), a pectoral "active can" was combine d with an electrode in the superior vena cava as common cathode and a right ventricle electrode as anode (AC+SVC). In protocol 2 and protocol 3 (n=6 e ach), only the ''active can" was used as proximal electrode (AC). Capacitan ce was 150 mu F during both phases in protocol 1 and protocol 3 but 150 mu F (phase I) and 300 mu F (phase 2) in protocol 2. ULV and DFT demonstrated a linear correlation in each protocol (r=0.78 to 0.84). Lowest DFTs were fo und at 10 ms for AC+SVC and at 14 ms for AC (P<0.001). At optimal BSDs, vol tage DFTs did not differ significantly between AC (527+/-57 V) and AC+SVC ( 520+/-70 V), Switching capacitors for phase 2 in a way that reduced leading -edge voltage by 50% while doubling capacity did not change BSD for optimal voltage DFT but increased minimum DFT: from 527+/-57 V to 653+/-133 V (P=0 .04). Conclusions The BSD with lowest DFT is shorter for AC+SVC than for AC. Ther e is no significant difference in voltage DFT between both at optimal BSD. A lower phase 2 capacitance reduces DFTs irrespective of BSD. Because stren gth-duration curves for DFT and ULV correlate for different BSDs, lead syst ems, and phase 2 capacitances, ULV determination may allow the prediction o f waveforms with lowest DFT.