Myocardial bundles with slow conduction properties are present on the left interventricular septal surface of normal human hearts.

INTRODUCTION: The aim of this study was to examine the electrophysiologic characteristics of the normal left interventricular septum (LIVS). METHODS AND RESULTS: We explored the LIVS in search of endocardial potentials following ventricular electrograms during sinus rhythm in 28 patients without structural heart disease. In all patients, low-frequency (mean amplitude: 0.81 +/- 0.33 mV) systolic potentials (LA1) were detected in a basal-to-apical activation sequence (earliest to latest QRS-LA1 interval: 93.3 +/- 10.8 ms to 127.1 +/- 16.6 ms). Programmed stimulation demonstrated decremental conduction properties, and the effective refractory period of the tissue between the ventricular electrogram and LA1 at baseline (376.7 +/- 48.8 ms) was significantly prolonged by disopyramide (421.3 +/- 54.1 ms, P < 0.05 vs baseline). In 21 patients (75%), diastolic potentials (LA2) morphologically similar to LA1 (mean amplitude: 0.52 +/- 0.17 mV) were detected in an apical-to-basal activation sequence (earliest to latest QRS-LA2 interval: 368.9 +/- 32.4 ms to 440.7 +/- 45.8 ms). Para-Hisian pacing with capture of the His bundle showed shorter S-LA1 and S-LA2 intervals compared to the beats without His-bundle capture but with ventricular capture (156.3 +/- 11.2 ms vs 183.2 +/- 12.3 ms, and 385.7 +/- 21.6 ms vs 397.4 +/- 23.4 ms, respectively, P < 0.0001). At the same rate, the LA1-LA2 interval was significantly shorter during right ventricular apical than atrial overdrive pacing (220.4 +/- 23.1 ms vs 261.4 +/- 30.7 ms, P < 0.0001). CONCLUSION: Unique myocardial bundles with slow conduction properties and various electrical connections with the specialized conduction system may exist in the LIVS of normal human hearts.