Electrophysiological effect of overdrive suppression and combined autonomic blockade with propranolol and atropine in patients with sinus node dysfunction

The phenomenon of sinus node depression and recovery after rapid atrial pacing was studied in 36 patients before and after the combined administration of atropine and propranolol. Sinus node recovery time was measured, and the first 10 cycles following cessation of atrial pacing (‘secondary phase’) were evaluated. During the pharmacological test, the observed intrinsic heart rate was compared with theoretical values. Altered electrophysiological parameters were observed during the post-stimulation sinus node depression in all patients having an abnormal intrinsic heart rate. In patients with a normal intrinsic heart rate the electrophysiological study of the sinus node gave either normal or abnormal results. In the latter case, the possibility that altered electrophysiological parameters could result from functional' sinus node dysfunction is to be considered. Although the intrinsic heart rate may be normal, pacemaker cells could have disorders of their intrinsic electrophysiological mechanisms. Abnormalities observed during the secondary phase after autonomic blockade in patients with a previously unaltered post-stimulation recovery phase strongly support such a view. This study emphasises the diagnostic value of the behaviour of the secondary phase to distinguish between functional sinus node disorders and organic changes of the pacemaker cells.