Heart failure remains a significant health problem in the Western countries despite the evolution achieved in terms of heart disease prevention and medical treatment. Its incidence and its prevalence reach 550,000/year and 5 million respectively in the United States. As a consequence, heart failure causes about 287,000 deaths in the US each year and puts a financial burden to the health system of $29.6 billion dollars due to the increasing number of hospitalizations especially in elderly patients.Cardiac resynchronization therapy (CRT) is a relatively recent advance in the management of heart failure patients, which has been very hopeful since its development. Large trials have established the efficacy of CRT in improving functional capacity (MUSTIC, MIRACLE) and reducing mortality (COMPANION, CARE HF) in patients with advanced heart failure (NYHA III-IV), reduced ejection fraction (EF<35%) and wide QRS complex >120 - 130 ms (MIRACLE). Data from meta-analyses also confirm the beneficial effect of CRT with respect to mortality from worsening heart failure and all cause mortality in patients with wide QRS complex.Nevetheless, not all patients suffering from heart failure have a prolonged QRS. On the contrary, it has been reported that nearly 50% of heart failure patients have a QRS < 120 ms. Moreover, CRT has been shown to improve haemodynamic variables in patients with heart failure and narrow QRS. Achilli et al, who studied 52 patients, reported that CRT produced similar clinical and functional benefit in patients with wide or narrow QRS, the cut-off point being 120 ms provided they had mechanical dyssynchrony. Bleeker at al came up with similar results when they compared 33 patients with normal QRS duration with an equal number of subjects with wide QRS. All participants ought to have mechanical dyssynchrony >65 ms on tissue Doppler imaging (TDI) study... (excerpt
