Inclusion Criteria:  Patient invasively ventilated for ≥ 72 hours Have an endotracheal tube or tracheostomy in situ Respiratory rate of ≤ 35 breaths/min Fraction Inspired Oxygen (Fi02) ≤ 0.50 Co-operative and able to participate in physical rehabilitation and inspiratory muscle training Age ≥ 18 years Patient consent or, in the case that the patient is unable, advice from the next of kin and acceptance of oral and written information describing the study.  Exclusion Criteria:  Undrained pneumothorax/pneumomediastinum The absence of an arterial catheter for blood sampling at study start Unlikely to survive Pregnancy Consultant discretion that patient is not appropriate