• Cardiac remodelling
• cardiac resynchronisation therapy
• heart failure
• normal QRS duration
• pacemakers

Cardiac resynchronisation therapy (CRT) is now a standard class I indication for symptomatic heart failure patients in New York Heart Association (NYHA) functional class III or ambulatory class IV on optimal medical therapy, with left ventricular ejection fraction (LVEF) of 35% or less and QRS duration of 120 ms or greater.1 However, these account for approximately 20% of all heart failure patients, leaving a majority of those with narrow QRS complex not amenable for CRT.2 The prolonged QRS duration of 120 ms or greater has been arbitrarily used to identify dyssynchronous intra and/or interventricular contraction. Nevertheless, QRS duration is dynamic and may show reversible prolongation during periods of decompensation.3 It is often dissociated from its mechanical dyssynchrony, which was demonstrated in approximately 36–51% of heart failure patients with narrow QRS complex using echocardiography, radionuclide phase analysis or cardiac magnetic resonance tissue synchronisation imaging.4–6 The echocardiographic dyssynchrony assessed by tissue Doppler imaging (TDI) has been shown to predict mortality in these heart failure patients.7 Intraventricular dyssynchrony as evidenced by cardiac magnetic resonance also predicts mortality and morbidity after CRT.6 Furthermore, mechanical dyssynchrony, either independently or in combination with other clinical parameters, is associated with a superior survival outcome after CRT.8 9 Intuitively, therefore, the key benefit of CRT in restoring left ventricular synchrony should also extend to this group of narrow QRS patients, in an attempt to avert progressive dilation and electrical remodelling and possibly to translate into better long-term survival and a reduction in hospitalisations due to heart failure. Various imaging methods for mechanical dyssynchrony, in particular TDI and strain echocardiography, have emerged and flourished with numerous publications aiming to refine patient selection and outcome for CRT.