Mechanisms of optimized biventricular pacing in pulmonary stenosis: Effects on left ventricular geometry in swine

We tested the hypothesis that optimized biventricular pacing (BiVP) enhances cardiac output (CO) during critical pulmonary stenosis (PS) by attenuating distortions in left ventricular (LV) geometry. Following median sternotomy in six anesthetized pigs, heart block was induced by ethanol ablation. During epicardial, DDD BiVP, atrioventricular delay (AVD) was varied from 60 ms to 180 ms in 30 ms increments. At the AVD with the highest CO right-left delay (RLD) was varied from (+) 80 ms (RV first) to (-) 80 ms (LV first) in 20 ms increments. At each pacing setting, aortic flow, ECG, and LV diameter were measured in the control state (CON) and during PS, created by snaring the pulmonary artery until CO decreased 50%. Short axis LV echocardiograms were obtained at (+) and (-) 80 ms. In CON, RLD had no effect on function or geometry. During PS optimum BiVP resulted in significant increases in CO (1.12 L/min ± 0.13 SEM at RLD = + 40 ms versus 0.92 ± 0.12 at RLD = 0 and 0.73 ± 0.08 at RLD = -80), and LV fractional shortening (8.97% ± 0.51% at RLD = + 40 ms versus 7.34% ± 0.58% at RLD = 0 and 6.21% ± 0.66% at RLD = -80). In addition, LV eccentricity with (-) RLD was significantly different versus CON at both end-diastole (0.79 ± 0.07 vs 1.02 ± 0.03, P = 0.011 Student's t-test) and end-systole (0.83 ± 0.05 vs 1.00 ± 0.02, P = 0.017). However, with (+) RLD differences versus CON were not significant at either end-diastole (0.88 ± 0.06 vs 0.99 ± 0.03) or end-systole (0.92 ± 0.03 vs 1.01 ± 0.03). In swine hearts with PS, optimized BiVP increases CO, fractional shortening, and LV symmetry. BiVP warrants further study as treatment for acute postoperative heart failure.