The invention relates to an arrhythmia detection system and method for defibrillating the heart of a patient experiencing abnormal cardiac rhythm, wherein the abnormal cardiac rhythm (comprising one of fibrillation, high rate tachycardia, and low rate tachycardia) is first detected, the heart rate is sensed so as to distinguish between fibrillation and high rate tachycardia, on the one hand, and low rate tachycardia, on the other hand, and automatic defibrillation of the heart of the patient is implemented when one of fibrillation and high rate tachycardia is determined. In one embodiment, base and apical electrodes are connected to a probability density function (PDF) circuit and a rate circuit. When both abnormal cardiac rhythm and excessively high heart rate are detected, defibrillation of the heart of the patient is implemented. In a second embodiment, a sensing button is also connected to the heart, and a switch is interposed between the electrodes and sensing button, on the one hand, and the interface, on the other hand. During monitoring of cardiac rhythm by the PDF circuit, the switch automatically connects the electrodes to the PDF circuit, while, during sensing of the heart rate, the switch connects the sensing button to the heart rate circuit. Upon detection of the need for defibrillation, the switch automatically connects the defibrillation pulse generator to the electrodes. Further features include a timed reset capability.