Sinus nodal echoes. Clinical case report and canine studies

Sinus nodal echoes are illustrated in (1) a case report, and (2) a study of the effects of atrial premature beats after atrial drive in dogs. When atrial premature beats confront the sinus node while it is still refractory, 3 types of response may be seen: (1) Complete interpolation—the subsequent sinus beat (or escape) comes precisely at the expected time; (2) incomplete interpolation—the subsequent sinus beat is delayed; and (3) sinus echoes—the sinus beat appears earlier than expected. In all 3 instances the node is entered, but the pacemaker fails to be reset. Although the echo has the form of a sinus beat, it is followed by a pause, presumably as a result of repenetration of the sinus node through pathways unused during exit. The curves characterizing the expansion by vagal stimulation of the nodal refractory period and total echo circuit time are defined, together with the latency of cholinergic effect on nodal refractoriness, sinus automaticity and exit conduction of the echo. The secondary concealment zone of a completely interpolated atrial premature beat is established. Atrial preexcitation (before the echo) sometimes evokes a second echo. The limiting factor on sustained sinoatrial reciprocation thus appears to be total echo circuit time rather than refractoriness of atrium or echo entrance pathways. The repetitive echoes seen in this study may be the basis for some clinical cases of sinus or atrial tachycardia.