Regional distribution of gas and tissue in acute respiratory distress syndrome. I. Consequences for lung morphology

Objective: To compare the computed tomographic (CT) analysis of the distrib ution of gas and tissue in the lungs of patients with ARDS with that in hea lthy volunteers. Design: Prospective study over a 53-month period. Setting: Fourteen-bed surgical intensive care unit of a university hospital. Patien ts and participants: Seventy-one consecutive patients with early ARDS and 1 1 healthy volunteers. Measurements and results: A lung CT was performed at end-expiration in patients with ARDS (at zero PEEP) and healthy volunteers. In patients with ARDS, end-expiratory lung volume (gas + tissue) and funct ional residual capacity (FRC) were reduced by 17 % and 58 % respectively, a nd an excess lung tissue of 701 +/- 321 ml was observed. The loss of gas wa s more pronounced in the lower than in the upper lobes. The lower lobes of 27 % of the patients were characterized by "compression atelectasis," defin ed as a massive loss of aeration with no concomitant excess in lung tissue, and "inflammatory atelectasis," defined as a massive loss of aeration asso ciated with an excess lung tissue, was observed in 73 % of the patients. Th ree groups of patients were differentiated according to the appearance of t heir CT: 23 % had diffuse attenuations evenly distributed in the two lungs, 36 % had lobar attenuations predominating in the lower lobes, and 41 % had patchy attenuations unevenly distributed in the two lungs. The three group s were similar regarding excess lung tissue in the upper and lower lobes an d reduction in FRC in the lower lobes. In contrast, the FRC of the upper lo bes was markedly lower in patients with diffuse or patchy attenuations than in healthy volunteers or patients with lobar attenuations. Conclusions: Th ese results demonstrate that striking differences in lung morphology, corre sponding to different distributions of gas within the lungs, are observed i n patients whose respiratory condition fulfills the definition criteria of ARDS.