Role of activation mode of the implantable loop recorder in explaining the cause of unexplained syncope - both manual and automatic activation are needed for an optimal diagnostic yield

Purpose: A syncopal event is unexplained, if its cause in still unknown after an initial evaluation according to current guidelines. The cause of syncope in such patients is often structural heart disease, and most often arrhythmias. Continuous ECG recordings captured by an Implantable Loop Recorder (ILR) are well suited to disclose the exact arrhythmia mechanism of syncope. Methods: An ILR (Reveal) was implanted in 570 patients (pts) with unexplained syncope. Pts were followed until syncope recurrence with an ECG capture or for at least one year. All devices had auto-activation of preselected arrhythmias and the option of manual activation. Both modes could be switched ON or OFF at the discretion of the investigator. Details of the device programming are not available. However, in pts with an episode of recurrent syncope, the investigator indicated whether the episode was captured automatically or whether the patient or a bystander activated the device manually. Results: Of the 218 pts with recurrence of syncope, there was a device captured ECG episode in 175 (80%), 33 of which showed no arrhythmia. In the remaining 142 pts, ECG was captured with auto-activation in 59 pts (42%), with manual activation in 52 pts (37%) and with both capture modes in 64 pts (45%). Bradyarrhythmias were documented in 43, 14 and 38 pts with auto-activation only, manual activation only, and both activation modes, respectively. Tachyarrhythmias were captured in 17, 13 and 20 pts, respectively. The investigator indicated "other arrhythmia" in 2, 2 and 8 pts, and there was no arrhythmia in the captured ECG of 4, 24 and 5 pts, respectively. Ventricular tachycardias were found by auto-activation in 2, manual activation in 2 and both activation modes in 8 pts. Conclusion: Auto-activation and manual activation contributed equally to clarify the mechanism of syncope, but in 20% of recurrences there was no ECG capture at all, implying underuse of either or both of the activation modes. Auto-activation was more often associated with bradyarrhythmias, and manual activations more often with no arrhythmias, while the most serious arrhythmias, ventricular tachycardias, were found equally often with both capture modes. Thus both activation modes are necessary for an optimal diagnostic yield.