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Figure 2 (A) Simultaneous recording of endocardial and epicardial monophasic action potentials (MAPs) from the right ventricular outflow tract (RVOT) and electrocardiographic lead I during open chest surgery (tholacotomy implantable cardioverter defibrillator implantation) in Patient 1. In the epicardial MAP, an incomplete depolarization of phases 0 and 1, deep notch in phase 2 and delayed dome in phase 3 are observed. In contrast, a MAP obtained from an endocardial site of the RVOT exhibits a normal morphology. Because of the activation delay in the RVOT, a "notch" in the recording from the epicardium provokes a current flow from the endocardium to the epicardium at the end of QRS, which relates to J point (ST segment) elevation. The "dome" causes a rapidly attenuated or reversal transmural current, which results in a steep downslope of the ST-segment and T-wave inversion. The repolarization time in the epicardium was longer than those in the endocardium. (B) Epicardial MAPs recorded in a control patient during coronary artery bypass grafting. Despite detailed mapping around the epicardial sites of the RVOT, a "spike and dome" configuration was not observed in any of the patients. Endo = endocardial MAP; Epi = epicardial MAP; LAD = left anterior descending coronary artery; TV = tricuspid valve; star = pacing spike.