REGIONAL EFFECTS AND MECHANISM OF POSITIVE END-EXPIRATORY PRESSURE IN EARLY ADULT RESPIRATORY-DISTRESS SYNDROME

Objective.-To test the hypothesis that positive end-expiratory pressure (PEEP) prevents the collapse of a given lung region when it is equal to or greater than the hydrostatic pressure superimposed (SPL) to that region. Design.-Intervention study with sequential levels of PEEP applied in random order to a cohort of patients with adult respiratory distress syndrome (ARDS). Setting.-Referral center for ARDS in a university hospital. Patients.-Ten ARDS patients (with Murray scores >2.5). Intervention.-Basal computed tomographic (CT) section taken at 0, 2, 4, 5, 6, 8, 10, 12, 14, 15, and 20 cm H2O PEEP. Main Outcome Measures.-Basal lung CT sections were divided into 1 0 equal levels from the ventral to dorsal surfaces. In each level SPL was measured from density and height. The inflation of the level was measured as the gas/tissue ratio (g/t); g/t changes with PEEP were defined as the g/t-P curve. The slope of the g/t-P curve was defined as level compliance (ie, the ratio of change in the g/t of the level to the change in pressure). A linear g/t-P curve was the criterion to detect inflation without recruitment (ie, new pulmonary units opening at a given pressure). A biphasic g/t-P curve (change of compliance after an inflection point) was the criterion to detect recruitment. Pflex was defined as the pressure at which the inflection point occurred. Results.-The SPL increased from level 1 (ventral) to level 10 (dorsal) (r=.91; P<.01). The number of linear g/t-P curves decreased from level 1 to level 10 (r=.98; P<.01), while the number of biphasic g/t-P curves increased (r=.95; P<.01). The Pflex increased from level 1 to 10 (r=.97; P<.01) and Pflex was similar to SPL (Pflex=1.05+0.9 SPL; r=.75; P<0.01). Conclusions.-The increased SPL causes compression atelectasis; this is prevented when PEEP to a given lung region is equal to or greater than the SPL.