Objectives: The effects of subthreshold stimulation performed at various sites in the perinodal and posteroseptal space on atrioventricular (AV) nodal conduction were investigated.
Background: The identification of specific or preferential sites of parasympathetic innervation to the AV node is suggested by observations made in both the animal and clinical laboratories. Pathologic studies of the parasympathetic innervation to the AV node show it is made up of serpiginous fibers traveling at highly specific sites within the myocardium endocardially towards the compact AV node.
Methods: We utilized endocardial subthreshold stimulation to selectively identify and characterize AV nodal inputs. Fourteen patients (age: 56 +/- 4 years) undergoing electrophysiologic testing with or without radiofrequency ablation for supraventricular tachycardia were studied. A steerable quadripolar catheter was positioned in 3 to 9 locations in the region between the site recording the His bundle electrogram and the coronary sinus (CS) os under flouroscopic and electroanatomic guidance. We mapped anterosuperior sites at or near sites with His potential recordings, and up to 2 mm inferior to the His bundle recording, posteroseptal sites included the CS os and sites along the posterior, superior, and inferior border of the CS. Atrial pacing was performed at a cycle length 50 ms longer than Wenckebach cycle length. Subthreshold stimulation was applied at a frequency of 10 Hz and 20 Hz delivered to the distal electrode pair. AH and HV intervals were recorded before and during subthreshold stimulation. AH prolongation was defined as a reproducible increase in AH interval by >10 ms from a stable baseline AH interval.
Results: Eight of 14 patients demonstrated prolongation of AV conduction at a mean of 1.75 +/- 0.2 sites. Mean AH prolongation was 56.4 +/- 13.0 ms (p = 0.02) from baseline. AH prolongation was achieved 15.4 +/- 1.8 mm below the His bundle recording in 7 patients, at the site of the His bundle recording in 3 patients, and along the posterior CS os border in 3 patients.
Conclusion: Subthreshold stimulation prolongs AV nodal conduction only at specific sites within the triangle of Koch, suggesting discrete parasympathetic endocardial inputs into the AV nodal region.