Vagal postganglionic innervation of the canine sinoatrial node

Differential, selective distribution of parasympathetic, postganglionic innervation to the atrioventricular nodal (AVN) region of the canine heart was recently described. Ablation of parasympathetic pathways to the AVN by disruption of the epicardial fat pad at the junction of the inferior vena cava and inferior left atrium did not interfere with normal vagal control of the sinoatrial node (SAN) function. In sharp contrast, surgical dissection of the fat pad overlying the right pulmonary vein-left atrial junction interrupted the major right and left vagal inputs to the SAN region. The pulmonary vein fat pad (PVFP) in the dog heart is triangular in shape with roughly equilateral dimensions of approximately 1 cm, its base extending from superior to inferior veins, and its apex extending nearly to the sinus nodal artery as it courses rostrally in the sulcus terminalis. Careful dissection of smaller fat pads around the circumference of the pulmonary veins and particularly over the rostral-dorsal surfaces of the right superior pulmonary vein and adjacent right atrium, completed SAN parasympathetic denervation. Care in making these dissections left the vagal supply to the AVN region essentially intact, and preserved the sympathetic supplies to both SAN and AVN regions. Autonomic ganglia, varying in size from 1 or 2 cells to 80-100 cells, were found scattered throughout the ventral PVFP (overlying and surrounding the right pulmonary vein-left atrial junction). The ganglia were generally imbedded in fatty connective tissue, although they commonly rested very close to, or were loosely surrounded by epicardial muscle. Ganglia were also found in smaller fat pads on the dorsal surfaces of the atrium between the azygos and the right superior pulmonary vein.