Abstract: The present invention involves an apparatus and method for detecting and treating fibrillation in the heart. Electrodes sense electrical activity in two distinct locations of the heart. The electrocardiac activations are monitored and the interval between activations at the two locations is measured and checked for variation. Upon detection of significant variation in the length of the intervals, the fibrillation condition is determined and a defibrillator is activated, so that a defibrillating shock may be delivered to the heart. For atrial fibrillation, the first defibrillating shock is relatively low and is increased until the fibrillation is no longer detected.