Inclusion criteria:  Expected to be ventilated for > 48 hours Age > 50  Exclusion criteria:  Atrial fibrillation (at the admission or during intensive care unit stay); Neuromuscular disorders; Home ventilation prior to admission; Palliative intubation; Intubation for an indication to tracheostomy; Poor acoustic window (after the first assessment); Severe mitral and/or aortic valve stenosis or regurgitation