Background The origin of 40% of syncope cases remains unknown even after a complete diagnostic workup.Previous studies have suggested that ATP testing has value in selecting successful therapy.This patient-blinded,multicenter,randomized superiority trial tested whether,in patients with syncope of unknown origin,selecting cardiac pacing in those with a positive ATP test leads to fewer recurrences than those who do not receive pacing.Methods and Results From 2000 to 2005,80 consenting patients (mean age,75.9 ± 7.7 years;81% women;56% without diagnosed structural heart disease) with syncope of unknown origin and atrioventricular or sinoatrial block lasting > 10 seconds (average,17.9 ± 6.8 seconds) under ATP administration (20-mg IV bolus) were recruited from 10 hospitals,implanted with programmable pacemakers,and randomized to either active pacing (dual-chamber pacing at 70 bpm) or backup pacing (atrial pacing at 30 bpm).Patients were followed up regularly for up to 5 years for any syncope recurrence,the primary outcome.Mean follow-up was 16 months.Syncope recurred in 8 of 39 patients (21%) randomized to active pacing and in 27 of 41 (66%) randomized to backup pacing (control),yielding a hazard ratio of 0.25 (95% confidence interval,0.12-0.56).After recurrence,the 27 recurrent control patients were reprogrammed to active pacing,and only 1 reported subsequent syncope.Conclusion his study suggests that,in elderly patients with syncope of unknown origin and positive ATP tests,active dual-chamber pacing reduces syncope recurrence risk by 75% (95% confidence interval,44-88). Background The origin of 40% of syncope cases remains unknown even after a complete diagnostic work work. The Previous studies have suggested that ATP testing has value in usted successful therapy. T. patient-blinded, multicenter, randomized superiority trial tested whether, in patients with syncope of unknown origin, selecting cardiac pacing in those with a positive ATP test leads to fewer recurrences than those who do not receive pacing. Methods and Results from 2000 to 2005, 80 consenting patients (mean age, 75.9 ± 7.7 years; 81% women; 56 % without diagnosed structural heart disease) with syncope of unknown origin and atrioventricular or sinoatrial block lasting> 10 seconds (average, 17.9 ± 6.8 seconds) under ATP administration (20-mg IV bolus) were recruited from 10 hospitals, implanted with programmable pacemakers, and randomized to either active pacing (dual-chamber pacing at 70 bpm) or backup pacing (atrial pacing at 30 bpm). Patients were followed up regularly for up to 5 years for any sync recur recurrence, the primary outcome. Article follow-up was 16 months. Syncope recurred in 8 of 39 patients (21%) randomized to active pacing and in 27 of 41 (66%) randomized to backup pacing (control), yielding a hazard The ratio of 0.25 (95% confidence interval, 0.12-0.56). After recurrence, the 27 recurrent control patients were reprogrammed to active pacing, and only 1 reported subsequent syncope. Confc his study suggesting that, in elderly patients with syncope of unknown origin and positive ATP tests, active dual-chamber pacing reduces syncope recurrence risk by 75% (95% confidence interval, 44-88).