- Increase in true shunt. Studies have demonstrated a reduction in atelectasis formation by application of CPAP during pre-oxygenation and PEEP during anaesthesia. Also, 25º inclination of the patient in the supine position, on the operating table, has demonstrated prolongation of time of apnea in a significant amount. Outside the operating room, PEEP of 5 cmH2O has been demonstrated useful in prolonging time of apnea when used during pre-oxygenation in hypoxic patients requiring tracheal intubation in the intensive care.