Atrial ectopic foci in the canine heart: hierarchy of pacemaker automaticity

The sinoatrial node (SAN) and adjacent tissue of the sulcus terminalis were surgically excised in sequential multiple sections to study the location and intrinsic rate of emerging pacemakers in the acute open-chest dog. Under chloralose anesthesia total of 17 ectopic atrial pacemakers emerged in 12 dogs ranging in location from the anterior interatrial band, midsulcus terminalis, and junction of the inferior vena cava-inferior right atrium. Four AV junctional and one idioventricular pacemakers also emerged in these experiments. Each successively inferior focus had a slower rate than the SAN. The average heart rate of the atrial ectopic pacemakers was 73 +/- 4% of the SAN but that the AV junctional pacemakers was 56 +/- 4% of the SAN (P less than 0.05). The greatest reductions in rate were achieved with total excision of the SAN and entire sulcus terminalis, but in only 50% of the animals did this result in a junctional rhythm. The location and rate of atrial ectopic foci support the notion that the AV junctional region is not the fastest pacemaker in the absence of the classically defined SAN and suggest that atrial ectopic pacemakers are intermediate in the hierarchy of cardiac pacemakers in the dog.