Overdrive Suppression

THE RELATIONSHIP among cardiac pacemakers is characterized by the fact that usually one pacemaker is dominant and all the others are subsidiary. The sinoatrial node acts as the dominant pacemaker and all other pacemaker tissues are discharged by a conducted impulse before their diastolic depolarization can attain threshold. These pacemakers are called subsidiary to emphasize the fact that under normal circumstances they are engaged in conducting impulses but under abnormal circumstances they may become actual pacemakers. Subsidiary in this sense means that one or the other of these pacemaker tissues assumes the function of impulse formation when the dominant pacemaker fails to initiate the activation of the heart. On the basis of these recognized facts, a dominant pacemaker can be defined as the pacemaker tissue which first succeeds in attaining threshold and in initiating a propagated impulse. The other pacemakers are subsidiary because their diastolic depolarization fails to attain the threshold before a propagated impulse initiates local excitation. Although these concepts are undisputed, certain observations seem to be at variance with the idea that the relationship among pacemakers involves only a successful race to the threshold. This is clearly shown by the fact that a sudden suppression of a dominant pacemaker is followed by a period of quiescence before a subsidiary pacemaker begins to discharge rhythmically. The period of quiescence is far longer than the interval between two beats of the newly established subsidiary rhythm. This suggests that subsidiary pacemakers are somehow kept suppressed. This form of inhibition will be referred to as overdrive suppression.