Atrial parasystole with interpolation Observations on prolonged sinoatrial conduction

Abstract

1. 1. We have reported an unusual case of sinus rhythm with atrial premature systoles due to an independent ectopic pacemaker (atrial parasystole). 2. 2. Marked variations in the duration of the returning cycle (P′-P) after the ectopic atrial beats, in spite of a fairly regular rate of the sinus pacemaker, are accounted for in the following manner: (a) A P′-P interval longer than the P-P cycle indicates discharge of the sinus pacemaker by the ectopic impulse. This is seen with P′ occurring in the middle portion of the cycle. (b) P′-P intervals about equal to the P-P cycle indicate interference of the ectopic with the next sinus impulse (fully compensatory pause). This is seen with P′ occurring late in the cycle. (c) P′-P intervals shorter than the P-P cycle indicate interpolation of P′ with postextra-systolic prolongation of S-A conduction time. This is seen with P′ occurring early in the cycle. 3. 3. Events subsequent to interpolation of the ectopic atrial impulse indicate that the sinus impulse which follows the postextrasystolic sinus impulse is blocked rather than interfered with. Thus, the mechanism is that of a pseudointerpolation with postponed compensatory pause, analogous to the same phenomenon which occurs after nodal or ventricular premature systoles. 4. 4. Comparison of a single atrial cycle (P-P) with one which contains an interpolated atrial premature systole (P-P′-P) permits determination of the delay in S-A conduction of the postextrasystolic beat.