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Abstract

Introduction: The Biventricular Pacing after Cardiac Surgery (BiPACS) trial demonstrated a 29% increase in cardiac index after aortic valve surgery in 14 patients undergoing temporary biventricular pacing (BiVP) vs. 11 Standard of Care controls (p=0.138). The mechanism of this benefit is examined in the current study. Methods: BiVP effects were compared one hour (Phase I), and 18 hours (Phase III) after separation from CPB. BiVP was optimized using aortic flow probe determination of CO in Phase I and using mean arterial pressure and thermal dilution CO in phase III. CO was subsequently compared during 30-second periods of no pacing (NoP), atrial pacing (AAI) or optimized BiVP in random sequence. Heart rate (HR) for AAI and BiVP was ninety or else ten beats per minute above HR during NoP. For post hoc analysis, the effect of HR on was determined by comparing CO during NoP with CO during AAI. The effect of combined HR and CRT on CO was determined by comparing NoP vs. BiVP. The effect of CRT alone on CO was determined by comparing AAI vs. BiVP. These constructs allowed the relative effects of HR and CRT on CO to be assessed over time. Results: Thirty-three patients were optimized in phase I and twenty-one in phase III. Overall, BiVP, including HR and CRT effects, was the optimum pacing mode, increasing CO 6.6% vs. NoP in phase I (4.38 ± 1.59 vs. 4.65 ± 1.67 L/min, p=<0.001) and 11.0% in phase III (4.94 ± 1.32 vs. 5.42 ± 1.36 L/min, p=<0.002). The contribution of CRT alone, determined by comparing AAI (4.34 ± 1.56) to BiVP (4.65 ± 1.67 L/min) was 95% of the total increase in CO with BiVP in phase I. In phase III, comparison of CRT (5.42 ± 1.36 L/min) with AAI (5.35 ± 1.40 L/min), demonstrates that the contribution of CRT to CO decreases to 11.8%. Thus, HR becomes the predominant driver of CO at 18 hours. Conclusions: Pacing augmentation of CO after CPB involves mechanisms that change over time. CRT is most important early, when ischemia-reperfusion, increased LV stiffness and myocardial edema may be prominent. HR effects become dominant 18 hours later. Effects of temporary pacing on CO after CPB merit further study, seeking to maximize clinical benefits vs. currently employed standards of care.