Effects of cardiac resynchronization therapy on myocardial contractile reserve during exercise

Myocardial contractile reserve (MCR) is a marker of myocardial viability. The impact of cardiac resynchronization therapy (CRT) on MCR during exercise has been little studied. Our aim was to investigate the effects of CRT on global and regional MCR by exercise echocardiography.

Twenty-seven CRT responders (56% ischaemic) were assessed by echocardiography during rest and bicycle exercise. Images were acquired during spontaneous rhythm ('off') and active CRT ('on'). Global MCR was expressed as the change (Δ) in left ventricular ejection fraction (LVEF) and aortic velocity-time integral (VTI) from rest to exercise. Regional MCR was expressed as the change in peak systolic tissue velocity (Sm) of the septum and lateral wall. Left ventricular ejection fraction and aortic VTI increased at 'on' during rest and exercise, and the increase in global MCR during exercise was higher during 'on' than 'off' (Δ LVEF 6 ± 6 vs. 3 ± 3%; P = 0.009). Septum Sm increased during 'on' at rest, and it was still higher during exercise (P ≤ 0.01), although the absolute change from rest to exercise was similar during 'on' and 'off'. Lateral wall Sm did not change at rest during 'on', but basal lateral wall MCR during exercise was higher at 'on' than 'off' (P = 0.036).

In CRT responders, there is a pacing-dependent increase in global MCR during exercise. The changes in regional MCR of the septum and lateral wall show different patterns. These factors may help to understand the determinants of improved exercise tolerance in CRT responders.