Two Cases of Supraventricular Parasystole

PARASYSTOLE is a mechanism resulting from rhythmic formation of impulses from two foci in the heart. In the case of atrial parasystole, one focus is usually in the sinoatrial node and the other somewhere in the atrial myocardium. The focus with the faster impulse formation serves as a dominant pacemaker for the heart. The parasystolic focus characteristically forms impulses with remarkable regularity and seems invulnerable to the effects of the sinus impulse. On some occasions the atrial depolarization wave is not visible with each fired parasystolic impulse due to refractoriness of the surrounding atrial muscle from a recently conducted sinus impulse. The parasystolic pacemaker, however, continues to operate in an almost perfectly rhythmic fashion as evidenced by the fact that each atrial depolarization wave of the ectopic form occurs at the time of a multiple of the expected interval if not at the expected interval. Thus, the idea of an abnormal protection or entrance block was proposed to explain the inability of the sinus impulse to discharge and render refractory the parasystolic focus.' Alternatively, Katz and Pick suggested the possibility that fibers in the adjacent area to the parasystolic focus might normally have the property of unidirectional conduction and provide a preferential pathway allowing impulses to leave but not to enter the area.'We readily admit that we do not have the final answer, nor do we know of any conclusive experimental evidence to explain the phenomenon. We do think, however, that it can be diagnosed by the criteria sug-