The defibrillator may include a heart rhythm detector to detect the heart rhythm of a patient, a manual mode controller structured to set the defibrillator in a synchronous shock operating mode or an asynchronous shock operating mode depending on an input from a human operator, a shock module to cause the defibrillator to deliver a shock to the patient according to the operating mode, and an automatic mode controller structured to, after the shock module has delivered the shock to the patient, set the external defibrillator to the synchronous shock operating mode or the asynchronous shock operating mode depending on the detected heart rhythm of the patient and without input from the human operator.