Complex interactions between the sinoatrial node and atrium during reentrant arrhythmias in the canine heart

Background-Numerous studies implicate the sinoatrial node (SAN) as a participant in atrial arrhythmias, including atrial flutter (AFL) and atrial fibrillation (AF). However, the direct role of the SAN has never been described. Methods and Results-The SAN was optically mapped in coronary perfused preparations from normal canine hearts (n17). Optical action potentials were recorded during spontaneous rhythm, overdrive atrial pacing, and AF/AFL induced by acetylcholine (ACh; 0.3 to 3 μmol/L) and/or isoproterenol (Iso; 0.2 to 1 μmol/L). An optical action potential multiple component algorithm and dominant frequency analysis were used to reconstruct SAN activation and to identify specialized sinoatrial conduction pathways. Both ACh and Iso facilitated pacing-induced AF/AFL by shortening atrial repolarization. The entire SAN structure created a substrate for macroreentry with 9.6±1.7 Hz (69 episodes in all preparations). Atrial excitation waves could enter the SAN through the sinoatrial conduction pathways and overdrive suppress the node. The sinoatrial conduction pathways acted as a filter for atrial waves by slowing conduction and creating entrance block. ACh/Iso modulated filtering properties of the sinoatrial conduction pathways by increasing/decreasing the degree of the entrance block, respectively. Thus, the SAN could beat independently from AF/AFL reentrant activity during ACh (49±39%) and ACh/Iso (62±25%) (P=0.38). Without ACh, the AF/AFL waves captured the SAN and overdrive suppressed it. Spontaneous SAN activity could terminate or convert AFL to AF during cholinergic withdrawal. Conclusions-The specialized structure of the SAN can be a substrate for AF/AFL. Cholinergic stimulation not only can slow sinus rhythm and facilitate AF/AFL but also protects the intrinsic SAN function from the fast AF/AFL rhythm.