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The observation that heart failure with reduced ejection fraction is often accompanied by asynchronous contraction of the left ventricle, leading to a loss of mechanical efficiency in ejection and an increase in mitral regurgitation, has prompted the development of cardiac resynchronisation therapy (CRT) as a therapeutic option in symptomatic heart failure. This technique, based on pacing the left ventricle, both ventricles, or both ventricles and the right atrium, has now been shown in randomised trials to reduce morbidity and hospitalisations1 and to decrease mortality2 in symptomatic patients with reduced ejection fraction and a QRS width of 120 ms or more. In these studies, the width of the QRS complex has been used as a surrogate for cardiac mechanical asynchrony. However, following current guidelines to identify candidates, about a quarter of patients treated by CRT will lack improvement or even deteriorate (“non-responders”). On the other hand, the proportion of heart failure patients who might benefit but who are not covered by current indications is unknown. Substantial left ventricular asynchrony has been documented in patients with congestive heart failure and normal QRS duration,3,4 suggesting that some of these patients might respond to CRT. Thus, a better way to select CRT candidates is needed to optimise the application of this costly therapy. While current guidelines base their criteria largely on clinical and ECG criteria,5 echocardiography is particularly promising, because this technique directly evaluates mechanical (as opposed to electrical) asynchrony with high temporal and spatial resolution.
Conceptually, cardiac asynchrony can be divided into atrioventricular, interventricular, and intraventricular asynchrony (of the left ventricle), the latter being widely regarded as the most important component.
A long atrioventricular interval (time interval from atrial to ventricular excitation or contraction) is detrimental in heart failure because: (1) early passive diastolic filling time is reduced; …