Effects of Transient Myocardial Ischemia on the Ventricular Defibrillation Threshold

Background: Acute myocardial ischemia and the mode of ventricular fibrillation (VF) induction influence the ventricular defibrillation threshold (DFT).

Objectives: The purpose of this study was to determine the effects of transient regional left ventricular (LV) ischemia on the DFT.

Methods: Ventricular effective refractory period (ERP), ventricular fibrillation threshold (VFT), and DFT were measured under nonischemic conditions (control) in 26 pigs weighing 25–35 kg. Myocardial ischemia was then induced by occlusion of the mid left anterior descending coronary artery, and measurements of ERP and VFT were repeated after 2 minutes of occlusion. The coronary artery ligation was released immediately after the onset of VF and DFT was measured.

Results: LV ERP was unchanged by ischemia (199 ± 19 ms at control vs. 200 ± 22 ms under ischemic conditions, P = 0.799), whereas VFT was significantly lower during coronary occlusion (10.7 ± 5.4 mA vs. 37.7 ± 13 mA, P = 0.000). Brief myocardial ischemia caused a significant increase in DFT (13.5 ± 12.6 J after coronary occlusion vs. 6.8 ± 6.8 J at control, P = 0.023). The duration of coronary occlusion was not correlated with the amounts of energy required to defibrillate (P = 0.526).

Conclusions: This experimental study shows that transient myocardial ischemia markedly increases the DFT, suggesting that specific defibrillation algorithms should be designed for recipients of implantable defibrillators at risk of myocardial ischemia.