Parasystole with Fixed Coupling

Four examples of spontaneous and six of artificial ventricular parasystole exhibiting fixed coupling of the premature beats were presented. In five cases, one of the pacemakers was located in the S-A node, in four in the A-V junction, and in one at first in the A-V junction and later in the ventricles. The material was divided according to the presence either of mutual protection of the two pacemakers (group I, four cases) or of unilateral protection of the parasystolic pacemaker (group II, five cases). When the rates of both pacemakers are regular and the conditions for impulse propagation remain stable, fixed (forced) coupling of parasystolic beats develops under the following conditions: in group I (with mutual protection) (a) if there is-by chance or by experimental adjustment-a simple numerical relation between the two pacemaker rates, or (b) if subthreshold stimuli become effective only during a short super-normal period of excitability in the early portion of the ventricular cycle; and in group II (with unilateral protection) as a result of a fixed relationship that develops secondarily to the discharge of the dominant pacemaker by the parasystolic pacemaker, a mechanism referred to as "reversed coupling." This mechanism may be combined with that of group Ib.