Author + information
- Received August 17, 1995
- Revision received November 9, 1995
- Accepted November 30, 1995
- Published online April 1, 1996.
- Charles D. Swerdlow, MD, FACCa,*,
- Tom Ahern, MD, FACC*,
- Robert M. Kass, MDa,
- Scott Davie, MSa,
- William J. Mandel, MD, FACCa and
- Peng-Sheng Chen, MD, FACCa
- ↵*Address for correspondence: Dr. Charles D. Swerdlow, 8635 West Third Street, Los Angeles, California 90048.
Objectives. The goals of this study were to determine the probability of successful defibrillation at the upper limit of vulnerability and to evaluate a minimal safety margin for implantable cardioverter-defibrillator first shocks based solely on the upper limit of vulnerability.
Background. The upper limit of vulnerability is the strength at or above which ventricular fibrillation is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. It has been proposed as an estimate of defibrillation efficacy because it correlates with the defibrillation threshold and can be determined with a single episode of fibrillation.
Methods. We studied 40 patients prospectively at implantation of transvenous cardioverter-defibrillators. Defibrillation threshold was defined as the weakest biphasic shock that defibrillated after 10 s of ventricular fibrillation. The upper limit of vulnerability was defined as the weakest biphasic shock that did not induce ventricular fibrillation when given at 0, 20 and 40 ms before the peak of the T wave in ventricular paced rhythm at cycle length 500 ms. After determination of the upper limit of vulnerability and defibrillation threshold, patients underwent six additional fibrillation—defibrillation episodes. The strength of five of the defibrillation shocks was equal to the upper limit of vulnerability; the strength of one of the six shocks was randomly selected to be equal to the upper limit of vulnerability plus 3 J. The implantable cardioverter-defibrillator was tested at the upper limit of vulnerability plus 3 J in 28 patients.
Results. The defibrillation threshold was 8.8 ± 5.0 J (mean ± SD), and the upper limit of vulnerability was 11.3 ± 4.6 J; the defibrillation threshold and upper limit of vulnerability were highly correlated (r = 0.89, p < 0.001). The success rate for the 200 defibrillation shocks with strength equal to the upper limit of vulnerability was 90% (95% confidence intervals based on proportion of successes in 40 patients: 86% to 94%). All five defibrillation test shocks at the upper limit of vulnerability were successful in 24 patients (60%); four of five were successful in 12 patients (30%); and three of five were successful in 4 patients (10%). All 40 test shocks and 28 implantable cardioverter-defibrillator shocks with a strength equal to the upper limit of vulnerability plus 3 J were successful.
Conclusions. The upper limit of vulnerability is a good estimator of the shock strength associated with 90% probability of successful defibrillation (DFT90). A strength of 3 J above the upper limit of vulnerability is a good estimate of the minimal acute safety margin for implantable cardioverter-defibrillator first shocks.
This study was supported in part by a grant from the PM Foundation (Dr. Swerdlow), Los Angeles and by a FIRST award (Grant HL50259) from the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland. It was performed during the tenure of an Established Investigator Award (Dr. Chen) from the American Heart Association, Dallas, Texas and Wyeth-Ayerst, Philadelphia, Pennsylvania.
- Received August 17, 1995.
- Revision received November 9, 1995.
- Accepted November 30, 1995.
- American College of Cardiology