Abstract: A device for preventing arrhythmias such as fibrillation in which a plurality of electrodes are placed at selected points on a heart to sense depolarizations. An astable multivibrator receives the depolarization signals from the electrodes and provides an output signal to a device which in turn provides cardiac stimulation signals to all of the electrodes on the heart for the purpose of preventing the arrhythmia. The multivibrator is preferably synchronized to the QRS complex of the heart with its refractory period. In a second embodiment of the device, separate sensing and stimulation electrodes are used.