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). 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 Ins under ischemic conditions, P=0.799), whereas VFT was significantly lower during coronary Occlusion (10.7 +/- 15.4 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.