Characterization of sinoatrial parasympathetic innervation in humans

Sinoatrial Parasympathetic Innervation. Introduction: The response to sinoa trial parasympathetic nerve stimulation (shortened atrial refractoriness) w as used to determine the atrial distribution of these nerve fibers in human s. We hypothesized that, in humans, parasympathetic nerves that innervate t he sinoatrial node also innervate the right atrium and that the greatest de nsity of innervation is near the sinoatrial nodal fat pad. Methods and Results: Temporary epicardial wire electrodes were sutured in p airs in the sinoatrial nodal fat pad, high right atrium, and right ventricl e by direct visualization during coronary artery bypass surgery in nine pat ients. Appropriate electrode placement was confirmed by electrically stimul ating the fat pad in the operating room to prolong sinus cycle length by 50 %. Experiments were performed in the electrophysiology laboratory 1 to 5 da ys after surgery. Programmed atrial stimulation was performed via an endoca rdial electrode catheter advanced to the right atrium, The catheter tip ele ctrode was moved in 1-cm concentric zones around the epicardial wires by fl uoroscopic guidance. Atrial refractoriness was determined in the presence a nd absence of sinoatrial parasympathetic nerve stimulation at each catheter site. In 8 of 9 patients, parasympathetic nerve stimulation reproducibly p rolonged sinus cycle length by 50%, There was no effect on AV nodal conduct ion (no prolongation of PR interval) and no change in AV nodal refractorine ss. Atrial effective refractory periods reproducibly shortened in response to parasympathetic nerve stimulation in 1-cm zones up to 3 cm surrounding t he fat pad, by a mean (+/- SEM) of 26.6 +/- 4.3 msec (zone 1), 11.4 +/- 1.8 msec (zone 2), and 10.0 +/- 2.5 msec (zone 3), respectively (P = 0.0001), At distances > 3 cm from the fat pad, the effective refractory period did n ot shorten. Conclusion: Stimulation of parasympathetic nerves that innervate the sinoat rial node shortened atrial refractoriness in humans.