Induction of ventricular fibrillation by T wave shocks: Observations from monophasic action potential recordings

Introduction: Shocks given during the vulnerable period of cardiac repolari zation may induce ventricular fibrillation (VF). However, the relationship of the vulnerable period and the monophasic action potential (MAP) has not yet been reported in humans. The purpose of this study was, therefore, to d etermine how the monophasic action potential recorded from the right ventri cle correlates with inducibility of VF using T wave shocks during ventricul ar pacing. Methods: Eleven patients undergoing implantable cardioverter defibrillator (ICD) implantation had a MAP catheter positioned in the right ventricle (RV ). The local monophasic action potential duration at 90% repolarization (MA P90) duration was measured during pacing at 400 ms. VF induction was attemp ted by pacing at 400 ms for 10 cycles and then giving a 1.0 joule monophasi c T wave shock at varying coupling intervals (CI) to the last paced stimulu s. The maximum and minimum CI that induced VF were determined and mapped in relation to the MAP90 recording. Results: The average paced MAP duration was 275 +/- 20ms. The minimum and m aximum CI to induce VF were 255 +/- 24ms and 325 +/- 36ms respectively. Thi s ranged from 93% to 118% of the MAP90 duration but because of delay in con duction time to the MAP catheter, shocks that induced ventricular fibrillat ion occurred between 74% and 99% of local repolarization time. Conclusion: VF is inducible with low energy T wave shocks falling during th e last 25% of the right ventricular MAP90 recording. This corresponds with VF initiation during phase III repolarization.