[Effect of nasal CPAP on human diaphragm position and lung volume]

The cephalic margin of the zone of apposition (ZOA) was observed with ultrasonography at ambient pressure and during nasal continuous positive airway pressure (nasal CPAP) in nine awake healthy males in a supine position. In a relaxed state at ambient pressure, there was a significant (p < 0.001) linear relationship between lung volume and the movement of the cephalic margin of the ZOA over the range from maximum expiratory position (MEP) to maximum inspiratory position (MIP). With nasal CPAP, functional residual capacity increased significantly (p < 0.01) in proportion to the increase in CPAP. At 20 cmH2O CPAP, the mean increase in volume at end expiration was 36% of the vital capacity measured at ambient pressure. The cephalic margin of the ZOA moved significantly (p < 0.01) in a caudal direction as CPAP was increased. At 20 cmH2O CPAP, the cephalic margin of the ZOA at end expiratory position (EEP) had moved 55% of the difference from MIP to MEP measured at ambient pressure. The end expiratory diaphragm position during nasal CPAP was lower than the diaphragm position at ambient pressure when lung volumes were equal. These results suggest that during nasal CPAP the chest wall is distorted from its relaxed configuration, with a decrease in rib cage expansion and an increase in outward displacement of the abdominal wall.