Synchronized and desynchronized sinus node activity.

In certain cases because of sinus arrhythmia and/or anarchic return and postreturn cycles caused by premature atrial stimulation, the sinoatrial conduction time cannot be calculated since a chaotic postextrasystolic pattern appears. We examined 49 patients with a history and ECG signs suggesting sinus node dysfunction by programmed extrastimulation method as proposed by Strauss et al. prior to and after pharmacologic autonomic blockade (with propranolol 0.2 mg/kg body weight and atropine sulfate 0.04 mg/kg intravenously). Patients with normal intrinsic heart rate (IHR) (n = 31) showed chaotic postextrasystolic responses in 38%, which in every case could be eliminated by autonomic blockade, resulting in a clear I and II phase with a well estimated intrinsic conduction time. Patients with pathologic IHR (n = 18) gave chaotic responses in 22% which after drug testing increased up to 72%. The chaotic postextrasystolic patterns are to be interpreted as the desynchronization of the sinus potentials, while plateau (phase II) as functional integrity, synchronized activity of the sinus node, dependent on the momentaneous balance of the autonomous tone, and on the integrity of the pacemaker cells. The desynchronized chaotic responses are considered on one hand as a dystonic neurovegetative electrophysiologic characteristic, on the other hand as a primary extrastimulation parameter of the intrinsic sinus node dysfunction.