Cardiac resynchronization therapy and AV optimization increase myocardial oxygen consumption, but increase cardiac function more than proportionally

Background: The mechanoenergetic effects of atrioventricular delay optimization during biventricular pacing ("cardiac resynchronization therapy", CRT) are unknown. Methods: Eleven patients with heart failure and left bundle branch block (LBBB) underwent invasive measurements of left ventricular (LV) developed pressure, aortic flow velocity-time-integral (VTI) and myocardial oxygen consumption (MVO2) at 4 pacing states: biventricular pacing (with VV 0 ms) at AVD 40 ms (AV-40), AVD 120 ms (AV-120, a common nominal AV delay), at their pre-identified individualised haemodynamic optimum (AV-Opt); and intrinsic conduction (LBBB). Results: AV-120, relative to LBBB, increased LV developed pressure by a mean of 11(SEM 2)%, p= 0.001, and aortic VTI by 11(SEM 3)%, p= 0.002, but also increased MVO2 by 11(SEM 5)%, p= 0.04. AV-Opt further increased LV developed pressure by a mean of 2(SEM1)%, p= 0.035 and aortic VTI by 4(SEM1)%, p= 0.017. MVO2 trended further up by 7(SEM 5)%, p= 0.22. Mechanoenergetics at AV-40 were no different from LBBB. The 4 states lay on a straight line for Delta external work (Delta LV developed pressure x Delta aortic VTI) against Delta MVO2, with slope 1.80, significantly >1 (p= 0.02). Conclusions: Biventricular pacing and atrioventricular delay optimization increased external cardiac work done but also myocardial oxygen consumption. Nevertheless, the increase in cardiac work was similar to 80% greater than the increase in oxygen consumption, signifying an improvement in cardiac mechanoenergetics. Finally, the incremental effect of optimization on external work was approximately one-third beyond that of nominal AV pacing, along the same favourable efficiency trajectory, suggesting that AV delay dominates the biventricular pacing effect - which may therefore not be mainly "resynchronization". (C) 2013 The Authors. Published by Elsevier Ireland Ltd. All rights reserved.