Right and left inferior extensions of the atrioventricular node may represent the anatomic substrate of the slow pathway in humans

OBJECTIVES The purpose of this study was to investigate the electrophysiologic characteristics of the inferior extensions of the human atrioventricular (AV) node and their possible relationship to slow pathway conduction. BACKGROUND The human heart contains right and left inferior extensions of the AV node that relate to right and left atrial inputs. METHODS Fourteen patients admitted for catheter ablation of left-sided accessory pathways were studied. Atrial pacing was performed from multiple sites in both atria, and simultaneous His-bundle recordings from right and left sides of the septum were made. RESULTS Significant differences of A-H and stimulus to His (St-H) intervals with pacing at various sites were found. St-H intervals were similar during constant pacing from the low fight atrium or the left inferoparaseptal area (112 +/- 28 ms vs 112 +/- 26 ms, P = .8, for right His recordings and 114 +/- 23 ms vs 111 +/- 25 ms, P = .9, for left His recordings). At maximum decrement, there were significantly shorter St-H intervals during left inferoparaseptal pacing compared to low fight atrial pacing (201 +/- 24 ms vs 218 +/- 44 ms, P = .02, for fight His recordings, and 200 +/- 24 ms vs 219 +/- 41 ms. P = .009, for left His recordings). Differences on right His recordings between St-H intervals at maximum decrement and at constant pacing from the low fight atrium were significantly higher than corresponding differences on left His recordings during pacing from the left inferoparaseptal area (P = .035). CONCLUSIONS Our findings support the concept that the right and left inferior extensions of the human AV node may represent the anatomic substrate of the slow pathway as defined electrophysiologically. (C) 2004 Heart Rhythm Society. All rights reserved.