Although electrocardiographic monitoring during syncope is the most unambiguous method of diagnosing or excluding an arrhythmia, it requires recurrence of syncope with potential morbidity. We hypothesized that long-term monitoring of patients with syncope would yield significant asymptomatic abnormalities that might preclude waiting for recurrence of actual syncope.

Sixty patients (age, 67±16 years; 27 men) with recurrent unexplained syncope, aged ≥30 years, with a left ventricular ejection fraction ≥35% and negative results on conventional monitoring were enrolled in a prospective study involving long-term automatic arrhythmia detection monitoring with an implantable loop recorder. Pre-specified significant asymptomatic arrhythmias were a pause >5 seconds, 3° atrioventricular block >10 seconds, heart rate (HR) <30 beats/min for >10 seconds while awake, wide complex tachycardia >10 beats, and narrow complex tachycardia >180 beats/min for >30 beats. Borderline asymptomatic arrhythmias included 3- to 5-second pauses, HR <30 beats/min for <10 seconds while awake, HR <30 beats/min for >10 seconds while asleep, and nonsustained wide complex tachycardia or narrow complex tachycardia.

Recurrent symptoms developed in 30 patients during the 1-year follow-up period (47%), with arrhythmias detected in 14 patients (23%). Pre-specified significant asymptomatic arrhythmias developed in 9 patients (15%), with bradycardia in 7 patients who underwent pacemaker implantation. Twenty patients (33%) had borderline asymptomatic arrhythmias. Five of these patients (25%) went on to have more pronounced diagnostic arrhythmias of the same mechanism during further follow-up, including pauses of 6 to 17 seconds duration in 3 patients.

Long-term monitoring of patients with unexplained syncope with automatic arrhythmia detection demonstrated that significant asymptomatic arrhythmias were seen more frequently than anticipated, leading to a change in patient treatment. Automatic arrhythmia detection provides incremental diagnostic usefulness in long-term monitoring of patients with syncope.