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 distribution of gas and tissue in the lungs of patients with ARDS with that in healthy volunteers. Design: Prospective study over a 53-month period. Setting: Fourteen-bed surgical intensive care unit of a university hospital. Patients and participants: Seventy-one consecutive patients with early ARDS and 11 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 functional residual capacity (FRC) were reduced by 17 % and 58 % respectively, and an excess lung tissue of 701 +/- 321 ml was observed. The loss of gas was more pronounced in the lower than in the upper lobes. The lower lobes of 27 % of the patients were characterized by "compression atelectasis," defined as a massive loss of aeration with no concomitant excess in lung tissue, and "inflammatory atelectasis," defined as a massive loss of aeration associated with an excess lung tissue, was observed in 73 % of the patients. Three groups of patients were differentiated according to the appearance of their 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 groups were similar regarding excess lung tissue in the upper and lower lobes and reduction in FRC in the lower lobes. In contrast, the FRC of the upper lobes was markedly lower in patients with diffuse or patchy attenuations than in healthy volunteers or patients with lobar attenuations. Conclusions: These results demonstrate that striking differences in lung morphology, corresponding to different distributions of gas within the lungs, are observed in patients whose respiratory condition fulfills the definition criteria of ARDS.