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EXPERIMENTAL STUDY

Biphasic waveform external defibrillation thresholds for spontaneous ventricular fibrillation secondary to acute ischemia

Gregory P. Walcott, MD^*,*, Cheryl R. Killingsworth, MD, PhD^*, William M. Smith, PhD and Raymond E. Ideker, MD, PhD, FACC^*

^* Cardiac Rhythm Management Laboratory, Division of Cardiovascular Diseases, Department of Medicine, University of Alabama at Birmingham, Birmingham, Alabama, USA
Cardiac Rhythm Management Laboratory, Division of Cardiovascular Diseases, Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, Alabama, USA
Cardiac Rhythm Management Laboratory, Division of Cardiovascular Diseases, Department of Physiology, University of Alabama at Birmingham, Birmingham, Alabama, USA

Manuscript received January 31, 2001; revised manuscript received August 23, 2001, accepted October 17, 2001.

^* Reprint requests and correspondence: Dr. Gregory P. Walcott, Cardiac Rhythm Management Laboratory, 1670 University Boulevard, B140 Volker Hall, Birmingham, Alabama 35294-0019, USA.
gpw{at}crml.uab.edu

OBJECTIVES: The goal of this study was to determine if the defibrillation threshold (DFT) after spontaneous ventricular fibrillation (VF) secondary to acute ischemia differs from the DFT for electrically induced VF in the absence of ischemia in anesthetized, closed-chest dogs and pigs.

BACKGROUND: The efficacy of external defibrillators has been tested mainly in animals and humans using E-VF, yet external defibrillators are often used in patients to halt S-VF.

METHODS: Protocol 1: biphasic truncated exponential (BTE) waveform shocks were delivered through electrodes placed in an anterior-anterior (A-A) position (left and right lateral thorax) in nine dogs. After measuring the E-VF DFT, acute ischemia was induced with an angioplasty balloon in either the left anterior descending or left circumflex coronary artery, and the S-VF DFT was determined. Protocol 2: in a group of 12 pigs, the E-VF DFT and S-VF DFT were determined for electrodes in the A-A position and in the anterior-posterior position (A-P). Protocol 3: the E-VF DFT was determined in seven pigs. Then up to three shocks 1.5x the E-VF DFT were delivered to S-VF. If defibrillation did not occur, a step-up protocol was used until defibrillation occurred.

RESULTS: Protocol 1: the DFT for E-VF was 65 ± 28 J (mean ± SD) compared with 226 ± 97 J for S-VF, p < 0.05. Protocol 2: the DFT was 152 ± 58 J for E-VF and 315 ± 123 J for S-VF for A-A electrodes. The DFT was 100 ± 43 J for E-VF and 206 ± 114 J for S-VF for A-P electrodes. Protocol 3: 11/37 shocks of strength 1.5x E-VF DFT (182 ± 40 J) stopped the arrhythmia. The episodes of S-VF not halted by these shocks required energy levels of up to 400 J for defibrillation.

CONCLUSIONS: External defibrillation of S-VF induced by acute ischemia requires significantly more energy than VF induced by 60-Hz current in the absence of ischemia. A safety margin >1.5x the DFT for electrically induced VF may be necessary in BTE external defibrillators to defibrillate S-VF.

Abbreviations and Acronyms   A-A   anterior-anterior   A-P   anterior-posterior   BTE   biphasic truncated exponential   DFT   defibrillation threshold   ECG   electrocardiogram   LAD   left anterior descending   LCX   left circumflex   VF   ventricular fibrillation

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