VERTICAL GRADIENT OF REGIONAL LUNG-INFLATION IN ADULT-RESPIRATORY-DISTRESS-SYNDROME

We obtained chest computed tomography (CT) sections in 12 normal subje cts (controls) and 17 patients with the adult respiratory distress syn drome (ARDS) to investigate regional lung inflation. A basal CT sectio n (just above the diaphragm) was obtained in the supine position at ze ro cm H2O end-expiratory pressure. In each CT section the distance fro m ventral to dorsal surface (hT) was divided into 10 equal intervals, and 10 lung levels from ventral (no. 1) to dorsal (no. 10) were define d. Knowing the average density and the volume of each level, we comput ed: (1) the tissue volume; (2) the gas/tissue (g/t) ratio (index of re gional inflation); (3) the hydrostatic pressure superimposed on each l evel (SPL), estimated as density x height. The total volume of the bas al CT section was 49 +/- 2.5 ml x m(-2) (mean +/- SE) in control subje cts and 43 +/- 2.3 ml x m(-2) in patients with ARDS (p = not significa nt [NS]). The tissue volume, however, was 16.7 +/- 0.8 ml x m(-2) in c ontrol subjects and 31.6 +/- 1.7 ml x m(-2) in patients with ARDS (p < 0.01). The g/t ratio in level 1 averaged 4.7 +/- 0.5 in control subje cts and 1.2 +/- 0.2 in patients with ARDS (p < 0.01), and this ratio d ecreased exponentially from level 1 to level 10, both in controls and patients with ARDS. The Kd constant of the exponential decrease was 13 .9 +/- 1.3 cm in control subjects and 7.8 +/- 0.8 cm in patients with ARDS (p < 0.01). This indicates a greater rate of change of g/t with h T in ARDS. The SPL in level 10 was 4.5 +/- 0.2 cm H2O in control subje cts and 10.5 +/- 0.4 cm H2O in patients with ARDS (p < 0.01). In concl usion: (1) the ARDS lung is characterized by decreased gas volume and increased tissue volume, whereas total lung volume is similar to contr ols; (2) along the vertical gradient the g/t ratio decreased and SPL i ncreased at a greater rate than controls; (3) The increased SPL, cause d by the increased tissue volume, may be one of the major factors resp onsible for the increased regional inflation gradient in ARDS.