Cardiac resynchronisation therapy (CRT) is a pacing treatment for heart failure. It was first proposed in the 1990s for selected patients with drug refractory heart failure, on the basis of a series of pioneering experiences performed in France.1 ,2\ud \ud The pathophysiological background for CRT is the presence, in a substantial subset of heart failure patients, of a left intraventricular conduction delay (wide QRS at 12 lead ECG) with associated left ventricular mechanical dyssynchrony. Stimulation of the left ventricle through an electrode, inserted into the venous system until its tip reaches a posterolateral or lateral branch of the coronary sinus, is able to correct the electrical dyssynchrony, thus improving haemodynamics, exercise performance and the patient's symptoms.\ud \ud In patients with symptomatic improvement, the correction of electrical dyssynchrony (ie, resynchronisation) is usually associated with a reduction in left ventricular volumes (‘reverse remodelling’), a reduction of mitral regurgitation and an improvement in left ventricular ejection fraction