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(Circulation. 1995;91:1247-1252.)
© 1995 American Heart Association, Inc.

Articles

Comparison of Upper Limit of Vulnerability and Defibrillation Probability of Success Curves Using a Nonthoracotomy Lead System

Joseph J. Souza, MD; Robert A. Malkin, PhD; Raymond E. Ideker, MD, PhD

From the Department of Medicine, University of North Carolina Hospitals, Chapel Hill, NC (J.J.S.); the Department of Electrical Engineering, Duke University (R.A.M.); and the Departments of Medicine and Pathology, Duke University Medical Center, and the Department of Biomedical Engineering, Duke University, Durham, NC (R.E.I.).

Correspondence to Raymond E. Ideker, MD, PhD, University of Alabama at Birmingham, Room G82A, Volker Hall, Box 201, UAB Station, Birmingham, AL 35294-0019.

Background An upper limit to the strength of shocks that induce fibrillation 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 studied. Single-capacitor biphasic waveforms with both phases 5.5 ms in duration were used for ULV and DF testing. A right ventricular catheter electrode served as first-phase cathode and a superior vena cava catheter electrode coupled with a cutaneous R2 patch electrode served as common first-phase anodes. A pacing catheter was placed in the right ventricle to deliver a train of 15 S₁ stimuli at a pacing interval of 250 to 300 ms. A ULV shock was delivered on the peak of the T wave as measured from the surface ECG; if ventricular fibrillation was induced, a DF shock was delivered after 10 seconds of fibrillation. Shock voltages were determined by an up-down protocol. Ventricular fibrillation was induced an average of 53 times in each animal. The composite data indicate 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 maximally correlated at V97. Even at V97, however, ULV and DF differed by more than 100 V in 2 of the 16 animals.

Conclusions ULV approximately equaled 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 number 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 technique is used clinically.

Key Words: defibrillation • waves • fibrillation

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