Patient selection is still based on QRS duration and its morphology. Echocardiography and other imaging techniques for mechanical dyssynchrony assessment have failed to be a useful predictor for adequate patient selection. Therefore, we further want to test whether CRT itself does not only improve concomitant sleep apnea, but also if preexisting sleep apnea predicts the response to CRT in patients with previously conventional right-ventricular pacing undergoing an upgrade to CRT by additional implantation of a left ventricular lead.