Protocols 1 and 2: Pneumothorax, degenerative neuromuscular disease, bleeding disorders, cardiovascular instability (defined as hypotension or need for treatment with volume expansion or inotropes in the previous 24 hours), cyanotic congenital cardiovascular disease, treatment with narcotics, phrenic nerve damage/diaphragm paralysis, esophageal perforation, use of high frequency oscillatory ventilation