Flecainide administration produced a lengthening of the PR interval and disappearance of the abnormal Q waves observed in V1-V3. This occurred because the drug interrupted conduction through a right free wall accessory pathway that was causing the pre-excitation seen on the baseline electrocardiogram, leaving a right bundle branch block pattern. Nonetheless, the finding of this accessory pathway likely does not explain the mechanism causing the syncopes, which appear to be situational.