Excitation of the Intrinsic Conduction System Through His and Interventricular Septal Pacing

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Direct His bundle pacing results in rapid synchronous ventricular activation. However, clinical experiences with such pacing have been associated with long procedure times and compromised pacing and sensing performance. We evaluated myocardial activation sequences (AS) for pacing of the His bundle and peri-His region and assessed acute pacing performance using custom-designed plunge electrodes. Unipolar pacing was performed in isolated swine hearts (n = 10) using four quadripolar stimulation/sensing electrodes implanted into the interventricular septum and equally spaced between the membranous septum and the coronary sinus ostium (zones 1-4, respectively; electrode depth (ED) 1 = most distal, ED 4 = most proximal). Optimal pacing sites were defined as: pacing thresholds < or = 1.5 V, a P-R ratio of < or = 0.5, and > or = 50% occurrence of an intrinsic midseptal left ventricular (LV) endocardial electrical breakout (BO) and activation pattern. Pacing thresholds improved with greater depth of electrode location within the septum (ED 1: 1.51 +/- 0.8 V vs ED 4: 5.2 +/- 3.8 V, P < 0.001), as did the P-R ratio (0.34 +/- 0.6 vs 0.78 +/- 1.0, P < 0.05). His potentials were only observed in zone 1 and 2 electrodes (0.12 and 0.02 mV, respectively). Only electrodes in zones 1 and 2 produced LV endocardial electrical BOs in the midseptal region that demonstrated an intrinsic-like endocardial AS. Depth 1 and 2 electrodes (11.75 and 8.75 mm, respectively) in zone 1 satisfied all three optimal pacing site requirements. This study has shown that LV activation patterns similar to sinus rhythm may be achieved without direct activation of the His bundle, while maintaining acceptable pacing and sensing performance. These data indicate that pacing systems designed to stimulate the tissues below the point at which the His bundle penetrates the central fibrous body may provide improved system efficiency and LV performance in comparison to both direct His bundle pacing and traditional pacing sites.