Figure 49-31 The three essential components of a nondependent lung continuous positive airway pressure (CPAP) system consist of (1) an oxygen source, (2) a pressure relief valve, and (3) a pressure manometer to measure CPAP. CPAP is created by the free flow of oxygen into the lung versus the restricted outflow of oxygen from the lung by the pressure relief valve. PEEP, positive end-expiratory pressure; ZEEP, zero end-expiratory pressure. (From Benumof JL: Anesthesia for Thoracic Surgery. Philadelphia, WB Saunders, 1987.)