Figure 5. Recording of successfully functioning ICD system illustrates defibrillation testing during device implantation using the shock on T-wave protocol. Shown from top to bottom are a surface ECG recording, "markers" indicating the functions performed by the ICD device, or intracardiac signals being detected from moment to moment, and recorded intracardiac electrograms (IEGM). While the heart is in sinus rhythm (SR), eight pacing stimuli are delivered at a cycle length of 400-msec (V-Pace), after which a low-energy shock (1.2-J) is delivered during the T wave (T-shock). The T-shock induces ventricular fibrillation (VF) that is promptly detected and terminated by a 20-J shock, restoring SR. CD = shock (charge) delivered; FD = ventricular fibrillation detected; FS = ventricular fibrillation sense; VP = ventricular pace; VS = ventricular sense.