External Validation of VVI

Failure of liberation from mechanical ventilation increases days spent being mechanically ventilated, the length of stay and the morbidity of patients. Thus it is crucial that those patients who are ready to be liberated are accurately detected. In this study the investigators want to externally validate an index (the volume-velocity index, VVI) which in a preliminary study was found to predict with high accuracy the outcome of ventilator liberation. In this study, in patients who are deemed ready to undergo weaning by their treating physician the investigators will measure the excursion of the diaphragm, the principal respiratory muscle, the inspiratory time and the tidal volume and the volume breathed by the patient at each respiration in three phases: 1. during pressure support ventilation with 5/5, 2. during pressure support ventilation with 8/0, 3. during t-tube or pressure suppor