A defibrillator, comprising: a shock source structurally configured to store electric energy; an internal ECG monitor structurally configured to generate an internal ECG waveform of the heart of the patient; and a controller including a synchronized cardioversion input channel for receiving a synchronized cardioversion input signal from an external ECG monitor which is located external to the defibrillator, wherein the synchronized cardioversion input signal is one of an external ECG waveform of a heart of a patient or an external synchronized pulse indicative of a detection by the external ECG monitor of at least one QRS complex of the ECG waveform, and wherein the controller is structurally configured to: detect that the synchronized cardioversion input signal is the external ECG waveform in response to the synchronized cardioversion input signal being nonequivalent to a pulse profile; and detect that the synchronized cardioversion input signal is the external synchronized pulse in response to the synchronized cardioversion input signal being equivalent to the pulse profile; detect at least one QRS complex of the ECG waveform within the internal ECG waveform; measure a time delay between the internal ECG waveform and the external ECG waveform or the synchronized pulse; and either: control a delivery of a defibrillation shock synchronized with the synchronized cardioversion input signal by the shock source to the patient in response to the controller receiving the synchronized cardioversion input signal and in response to the measured time delay being less than a baseline time delay; or control at least one of a warning or a prohibition of a delivery by the shock source of the defibrillation shock synchronized with the synchronized cardioversion input signal to the patient in response to the measured time delay being greater than a baseline time delay. a shock source structurally configured to store electric energy; an internal ECG monitor structurally configured to generate an internal ECG waveform of the heart of the patient; and a controller including a synchronized cardioversion input channel for receiving a synchronized cardioversion input signal from an external ECG monitor which is located external to the defibrillator, wherein the synchronized cardioversion input signal is one of an external ECG waveform of a heart of a patient or an external synchronized pulse indicative of a detection by the external ECG monitor of at least one QRS complex of the ECG waveform, and wherein the controller is structurally configured to: detect that the synchronized cardioversion input signal is the external ECG waveform in response to the synchronized cardioversion input signal being nonequivalent to a pulse profile; and detect that the synchronized cardioversion input signal is the external synchronized pulse in response to the synchronized cardioversion input signal being equivalent to the pulse profile; detect that the synchronized cardioversion input signal is the external ECG waveform in response to the synchronized cardioversion input signal being nonequivalent to a pulse profile; and detect that the synchronized cardioversion input signal is the external synchronized pulse in response to the synchronized cardioversion input signal being equivalent to the pulse profile; detect at least one QRS complex of the ECG waveform within the internal ECG waveform; measure a time delay between the internal ECG waveform and the external ECG waveform or the synchronized pulse; and either: control a delivery of a defibrillation shock synchronized with the synchronized cardioversion input signal by the shock source to the patient in response to the controller receiving the synchronized cardioversion input signal and in response to the measured time delay being less than a baseline time delay; or control at least one of a warning or a prohibition of a delivery by the shock source of the defibrillation shock synchronized with the synchronized cardioversion input signal to the patient in response to the measured time delay being greater than a baseline time delay. control a delivery of a defibrillation shock synchronized with the synchronized cardioversion input signal by the shock source to the patient in response to the controller receiving the synchronized cardioversion input signal and in response to the measured time delay being less than a baseline time delay; or control at least one of a warning or a prohibition of a delivery by the shock source of the defibrillation shock synchronized with the synchronized cardioversion input signal to the patient in response to the measured time delay being greater than a baseline time delay.