Double ventricular parasystole: intermittency and its relation to re-entrant premature beats.

Critical analysis of a case of double ventricular parasystole revealed that one parasystolic focus manifested complete protection, whereas the second focus appeared to be intermittent, protected from sinus and other impulse formation both early, as well as late, in the parasystolic cycle. The zone of protection failure, during which extrinsic impulses could penetrate into the parasystolic focus and discharge it, is observed between these early and late phases of protection. When the intermittent parasystolic focus is challenged by the other parasystolic beat, this zone is much wider and is located later in the cycle than the sinus beat. According to this observation, it is supposed that the intermittent parasystolic focus, in this case, is anatomically surrounded by two types of fibers, each showing different degrees of phase 3 and phase 4 block. The fibers located between the major fascicles and the intermittent parasystolic focus may have a shorter phase 3 and a less steep phase 4 depolarization than those located between two parasystolic foci. It is also demonstrated that during the phase 4 depolarization in these fibers around the focus, concealed conduction into this area by the sinus beat evoked exit delay of the parasystolic impulse, resulting in prolongation of the next parasystolic cycle. In this situation, a re-entry movement around the focus was predicted and, as expected, a re-entrant bigeminy coupled with the parasystolic rhythm was observed.