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Background: The effects of nasal continuous positive airway pressure (CPAP) on the lung parenchyma of patients with COPD, to our knowledge, have never been assessed by high-resolution CT (HRCT) scanning.Methods: HRCT scans were obtained at the apex, hilum, and basis of the lungs at functional residual capacity while on spontaneous respiration and at the end of CPAP trials of 5 cm water (H(2)O), 10 cm H(2)O, and 15 cm H(2)O in 11 stable patients with COPD and eight healthy volunteers. Lung aeration was assessed by quantitative density parameters and by qualitative analysis of each CT image after processing by means of a density-based color-mask computational algorithm. The quantitative parameters were density histograms, the relative area of the lungs with attenuation values < -950 Hounsfield units (percentage of hyperaerated areas) and the 15th percentile (the density value separating the 15% voxels of least density).Results: A CPAP of 5 cm H(2)O caused little increase in lung aeration in both groups, but in some patients with COPD, CPAP deflated some regions of the lungs. CPAP levels of 10 cm H(2)O and 15 cm H(2)O increased the emphysematous zones in all sectors of the lungs, including dorsal and apical regions in patients with COPD compared to little hyperaeration predominantly in the ventral areas in healthy volunteers.Conclusions: Nasal CPAP causes variable effects on regional lung aeration in relation to the applied pressure and the regional distribution of emphysema in patients with COPD. Low pressure levels may cause regional lung deflation in some patients. High levels increase the emphysematous areas wherever they are located inside the lungs. [ABSTRACT FROM AUTHOR]