Premature Systoles Originating in the Sino-Auricular Node

Abstract

Premature systoles arising in the sino-auricular node are characterized in the electrocardiogram by premature P waves that are identical in contour with other sinus P waves and by the absence of a prolongation of the returning cycle. Actually the returning cycle may be shorter than the normal cycle because of prolonged sino-auricular conduction of the premature beat (Wenckebach, 1908). According to Wenckebach and Winterberg (1927) slight changes in contour of the premature P wave do not rule out sinus origin of the premature impulse; these aberrant P waves are ascribed to aberrant conduction in the auricles. Varying intra-auricular conduction has also been suggested (Scherf, 1945) to explain the altered contour of the sinus P wave that occurs in second degree auriculo-ventricular block, accompanied by shortening of the P-P intervals, in those P-P intervals containing a ventricular complex. It would appear that a contour of the premature P wave identical with that of the sinus P wave should be postulated for the diagnosis of sinus premature systoles in order to avoid confusion with auricular premature systoles. In exceptional cases, however, a diagnosis of sinus premature systoles seems justified in spite of a slightly different P wave contour when sinus arrhythmia is absent and when the returning cycle is equal to or shorter than the normal cycle. Because of their rarity, two more cases are being added in this paper to the few instances of sinus premature systoles reported (Wenckebach, 1908; Lewis, 1925; Clerc et al., 1938; Simon and Langendorf, 1944; and Geiger and Goerner, 1945). Case 1. A man, aged 60, was admitted to the hospital because of neurological complaints. A cardiogram (Fig. 1) was taken to determine the nature of his irregular heart action. The patient had received no medication. Lead I shows an undisturbed regular sinus rhythm with a P-P interval of0-98 sec.; all the other leads show bigeminal rhythm with a P-P interval of 0-98 sec. alternating with a P-P interval of 0-80 sec. There is a first degree A-V block (P-R equals 030 sec.) with variations in the P-R (0-32 and 034 sec.) in the leads with bigeminal rhythm. The P waves show only minute differences in contour. The absence of a compensatory pause (the P-P interval in lead I equals the post-extrasystolic P-P intervals in the other leads) identifies the premature systoles as arising in the sino-auricular node. Comparison of lead I with the other leads permits exclusion of other possibilities to be considered in cases of persistent sinus bigeminy, such as non-conducted sinus premature systoles occurring after every second beat, 3: 2 S-A block with the Wenckebach phenomenon, and alternans of the S-A conduction time.Case 2. A woman, aged 80, having hypertensive cardiovascular disease, was admitted with a provisional diagnosis of a recent myocardial infarct. No drugs were administered. The cardiogram is shown in Fig. 2. The first seven beats in lead I are regularly spaced and have a P-P interval of 0-64 sec. They are followed by a bigeminal rhyth...

Modulated parasystole originating in the sinoatrial node.

“…(6) The parasystolic (i.e., slower) pacemaker center, SN2, is composed of a population of electrically coupled pacemaker cells surrounded by an area of depressed excitability (double ring in figure 3, A). SN1 (and therefore atrial) discharges modulate the activity of SN2 according to a biphasic PRC (figure 3, C).…”