Failure to oxygenate is caused by reduced diffusing capacity and ventilation perfusion mismatch. This can often be overcome by restoring FRC by increasing baseline airway pressure using CPAP. If the problem is atelectasis due, for example, to mucus plugging or diaphragmatic splinting following abdominal surgery, or moderated amounts of pulmonary edema, CPAP, as delivered by facemask or endotracheal tube, may sufficiently restore pulmonary mechanics to avoid addition inspiratory support. CPAP is easy to apply: all that is required is a PEEP valve and a flow generator. 

The flow generator is important as peak inspiratory flow in most patients is 30-60 liters per minute, and this flow rate is required to avoid a situation where the patient is attempting to breathe in against an expiratory (PEEP) valve. The magnitude of PEEP is determined by a spring loaded mechanism on the expiratory valve. When delivered through an endotracheal tube, CPAP can be administered by attaching a PEEP valve to the end of a T-piece, or through a "flow by circuit" within a mechanical ventilator.