Pathophysiology of adult respiratory distress syndrome

We have studied 45 patients with adult respiratory distress syndrome (ARDS). Pathological studies were performed on 30 open lung biopsies and 15 autopsies. Concomitantly the physiologic characteristics of the hypoxemia in response to different inspired oxygen fraction (FIO2) and Positive End Expiratory Pressure (PEEP) were determined. These studies allowed categorization of ARDS into 3 groups. Group I had the most severe hypoxemia and a severe fixed shunt at all PEEP and FIO2. Pathologically, there was extensive acute edema, hemorrhage, and exudation to the point of consolidation. Group II had less severe hypoxemia which improved moderately but slowly in response to PEEP and shunt increased with decrease in FIO2 indicating diffusion disturbances or severe ventilation-perfusion abnormalities. Pathology showed severe fribrosis. Group III had least hypoxia which rapidly and markedly improved with PEEP. Shunt also increased with decrease in FIO2. Pathology showed acute changes similar to but less severe than group I. Prognosis was best for group III with 10 of 21 long term survivors (one after ECMO or extracorporeal membrane exygenation). In group I only 2 of 11 survived after prolonged periods of ECMO. In group II, 3 of 13 survived (one required ECMO) with satisfactory pulmonary function despite biopsy evidence of severe fibrosis during the acute illness. These pathological and physiologic studies are useful to better understand ARDS, to optimize respiratory care and for prognosis.