A defibrillator system employs an external ECG monitor (40) and a defibrillator (20). In operation, external ECG monitor (40) generates a synchronized cardioversion input signal as one of an external ECG waveform (50) of a heart (11) of a patient (10) or an external synchronized pulse (51) indicative of a detection by the external ECG monitor (40) of at least one QRS complex of the external ECG waveform (50). Defibrillator (40) includes a synchronized cardioversion input channel (29) for receiving the synchronized cardioversion input signal from external ECG monitor (40), and controls a conditional delivery of a defibrillation shock synchronized with the synchronized cardioversion input signal to the patient (10) in response to the defibrillator (20) receiving the synchronized cardioversion input signal. One condition for shock delivery is a measured time delay between an internal ECG waveform (30) and the synchronized cardioversion input signal being less than a baseline time delay.