Alternate methods for the determination of atrial capture threshold utilizing the telemetered intracardiac electrogram.

Periodic determination of pacemaker capture threshold is important to ensure appropriate pacemaker function. During dual chamber pacing, it is sometimes difficult to identify evidence of atrial depolarization on surface electrocardiography (ECG), and this can interfere with the ability to ascertain atrial capture. We describe new methods for determining atrial capture threshold using a standard telemetered endocardial atrial electrogram (AEGM). For the first method, the atrial output is decremented until loss of atrial capture is demonstrated by the appearance of native P wave activity on the AEGM. The atrial capture threshold can then be accurately determined as the point at which a stepwise increase in atrial output results in extinction of the native P wave activity. The second method uses the direct visualization of the AEGM recorded between the ring electrode and pacemaker generator during unipolar (lead tip electrode) pacing. This requires the presence of a bipolar lead. Using this method of recording, it is possible to identify a signal after the atrial stimulus artifact during atrial capture, which disappears with loss of capture. This signal is consistent with a paced "evoked atrial potential" and allows verification of atrial capture. After validating the methods in two sets of test patients with clearly identifiable atrial depolarization on surface ECG, one method was successfully applied to a patient in whom atrial depolarization could not be reliably ascertained on surface ECG. These methods promise to be useful in selected patients in whom confirmation of atrial capture would otherwise be difficult.