Sinoatrial node artery distribution and its relation to hierarchy of cardiac automaticity

The sinoatrial node (SAN) artery (SANA) was cannulated in dogs anesthetized with chloralose (7 dogs) or pentobarbital (1 dog). Intra-SANA injection of acetylcholine (ACh) with edrophonium (EPh) produced a transient junctional rhythm (JR) when the cannula tip lay near SANA origin. However, when ACh-EPh was injected while the cannula tip lay within SAN region, a slower P wave rhythm, rather than a JR, developed. To determine visually the injected ACh-EPh distribution, an equal volume of dye was injected under similar conditions. Dye-distribution patterns were consistent in all atria. These averaged 44 +/- 2% of the total endocardial surface, indicating that SANA distribution is not discrete, but widespread. Dye extended cranially, caudally, and laterally and included documented subsidiary pacemaker sites. Dye approached, but never trespassed, the coronary sinus ostium. Results suggest that cholinergic drug injection into the proximal SANA may suppress not only the SAN, but subsidiary atrial pacemakers as well. In this situation, junctional pacemakers probably emerge, not because of inherently greater automaticity, but after default of subsidiary atrial pacemakers.