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

Abstract

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(SEM 1)%, p=0.035 and aortic VTI by 4(SEM 1)%, 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 Δexternal work (ΔLV developed pressure × Δaortic VTI) against ΔMVO2, 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 ~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".

Keywords: Biventricular pacing; Myocardial oxygen consumption; Optimization.

Fig. 1

Experimental setup depicting the typical position of the three pacing wires (in the standard high right atrium, right ventricular apex and posterior/posterolateral coronary sinus branch). The Finometer was used for non-invasive haemodynamic AV delay optimization during stage I. In stage IIa, invasive measurements of aortic flow velocity and left ventricular pressure measurements were recorded at the 4 pacing states (AV delay of 40 ms, 120 ms, and the predetermined optimal AV delay; and LBBB) and their product was used for estimation of external cardiac work.

Fig. 2

Protocol of AV optimization using the alternations concept. The data are presented as mean and standard error of the mean (SEM). In this example alternations were performed between the reference AV delay (AV 120 ms) and the tested AV delay (AV 80 ms). A number of alternations are performed for each tested AV delay and this generates a number of replicates of data (six in this example) which are then averaged and the mean change in non-invasive systolic blood pressure from the reference AV delay can be calculated. The more replicates averaged (at the expense of more time required) the better the signal-to-noise ratio and therefore higher precision in identifying the true optimal AV delay. We have found that reliable, reproducible optimization can be achieved in approximately 12 min.

Fig. 3

The haemodynamic response measured by left ventricular (LV) developed pressure and aortic velocity-time integral (VTI) (an index of stroke volume) when pacing from LBBB to the 3 biventricular AV delays (with VV delay kept at 0 ms) of 40 ms (AV-40), 120 ms (AV-120) and the non-invasively predetermined haemodynamic AV optimum (AV-Opt). The data are presented as mean and standard error of the mean (SEM). The AV-Opt produced the highest pressure and aortic VTI, statistically significantly higher than the AV-120 (nominal AV setting). The changes in pressure and aortic VTI were in similar proportions at all pacing states, i.e. 1:1. Biventricular pacing at a physiologically ‘too short’ AV delay (AV-40) resulted in the same haemodynamic profile as LBBB. Therefore, changing the AV delay of biventricular pacing can affect haemodynamics dramatically; at one extreme AV-Opt provides better haemodynamics than for example AV-120 (a commonly programmed AV delay when AV optimization is not performed) and on the other extreme (i.e. AV-40) any benefit from ventricular resynchronization is offset.

Fig. 4

Effect of biventricular pacing with three different AV delays on myocardial oxygen consumption (top figure) and cardiac work (indexed by the product of LV developed pressure and aortic velocity time integral – bottom figure), compared to intrinsic conduction (LBBB). The data are presented as mean and standard error of the mean (SEM). Myocardial oxygen consumption increases when paced with an AV delay of 120 ms (AV-120) and at the optimal AV delay (AV-Opt), compared to LBBB. Similarly, but proportionally more, the cardiac work rises at AV-120 ms and AV-Opt; this rise is approximately one-third higher at AV-Opt than at AV-120. AV-40 was not significantly different compared with LBBB.

Fig. 5

Relationship between the change in cardiac work (indexed by the product of LV developed pressure and aortic velocity time integral) and change in myocardial oxygen consumption with biventricular pacing at each of the three AV delays (40 ms (AV-40), 120 ms (AV-120) and optimal AV (AV-Opt), from LBBB. The data are presented as mean and standard error of the mean (SEM). The slope (∆cardiac work ÷∆myocardial oxygen consumption) of the regression line is 1.80 (significantly higher than 1, p = 0.02), implying a more efficient myocardial state with biventricular pacing, at AV-120 or AV-Opt, than in LBBB. Of the two higher-efficiency states, the AV-Opt generates one-third more ‘useful’ cardiac work than AV-120, p = 0.012.