[Reduction of the threshold of myocardial excitability after an extremely long asystolic pause: late phase of "supernormal excitability?].

A patient with a malfunctioning pacemaker due to lead insulation defect is reported. High voltage stimuli were always effective, while when the pulse amplitude was reduced to 3.8 volt stimuli were uneffective except when occurring after extremely long asystolic pauses. An advanced exit block (up to 7:1) thus occurred. Late lowering of the myocardial threshold of excitability could be explained by slow spontaneous diastolic depolarization occurring in myocardial fibers surrounding the tip of the electrode.