Do we understand who benefits from resynchronisation therapy?

Interest in ventricular dyssynchrony in heart failure patients has been renewed recently by the introduction of cardiac resynchronisation therapy (CRT). CRT is currently accepted as an adjunct to the medical treatment of symptomatic heart failure in patients with severe left ventricular systolic dysfunction and ECG manifestations of ventricular conduction delay.1 It aims to reduce the electrical conduction delay by pre-excitation of lateactivated regions with leftor biventricular pacing to restore a more synchronous contraction pattern. Although this strategy has been proven to be effective at group level in large clinical trials, it is clear that up to 30% of such patients do not benefit from CRT. Part of the explanation for the failure of patient response may lie in the current suboptimal criteria used for patient selection. The clinically important question is to identify ventricular segments with delayed onset of contraction (active force development), as this represents regional function that can potentially be recruited by changing the sequence of electrical activation. There is growing evidence that in patients with left bundle-branch block (LBBB), the timing and extent of mechanical dyssynchrony are poorly related to QRS duration and that assessment of mechanical dyssynchrony may help to better identify responders.2–4 The study of Ghio et al. reported in this issue highlights the poor predictive value of QRS duration for the identification of mechanical dyssynchrony. Moreover, it has been suggested, somewhat provocatively, that even patients with a normal QRS duration might benefit from CRT, provided that a correctable mechanical dyssynchrony can be identified. However, there remain many unsolved issues