COMPARISON OF UPPER LIMIT OF VULNERABILITY AND DEFIBRILLATION PROBABILITY OF SUCCESS CURVES USING A NONTHORACOTOMY LEAD SYSTEM

Background An upper limit to the strength of shocks that induce fibril lation during the vulnerable period, the upper limit of vulnerability (ULV), has been shown to exist in both humans and animals. The purpose of this study was to compare ULV and defibrillation (DF) probability of success curves for a clinically useful nonthoracotomy lead system. Methods and Results Sixteen pentobarbital-anesthetized pigs were studi ed. Single-capacitor biphasic waveforms with both phases 5.5 ms in dur ation were used for ULV and DF testing. A right ventricular catheter e lectrode served as first-phase cathode and a superior vena cava cathet er electrode coupled with a cutaneous R2 patch electrode served as com mon first-phase anodes. A pacing catheter was placed in the right vent ricle to deliver a train of 15 S-1 stimuli at a pacing interval of 250 to 300 ms. A ULV shock was delivered on the peak of the T wave as mea sured from the surface ECG; if ventricular fibrillation was induced, a DF shock was delivered after 10 seconds of fibrillation. Shock voltag es were determined by an up-down protocol. Ventricular fibrillation wa s induced an average of 53 times in each animal. The composite data in dicate that below V97, that is, the voltage that leaves the animal in normal sinus rhythm 97% of the time when delivered on the peak of the T wave or the voltage that defibrillates 97% of the time, ULV is lower than DF. ULV and DF became significantly correlated at V80 and maxima lly correlated at V97. Even at V97, however, ULV and DF differed by mo re than 100 V in 2 of the 16 animals. Conclusions ULV approximately eq ualed DF at V97. This is fortunate because it is clinically important to set the device voltage at the uppermost portion of the probability of success curve. Estimating DF V97 from ULV V97 would reduce the numb er of fibrillation inductions needed to establish defibrillation shock strength requirements. However, the large difference between ULV V97 and DF in a few animals indicates that further improvement and testing of algorithms for determining ULV V97 must be developed before the te chnique is used clinically.