EFFECTS OF POSITIVE END-EXPIRATORY PRESSURE ON REGIONAL DISTRIBUTION OF TIDAL VOLUME AND RECRUITMENT IN ADULT-RESPIRATORY-DISTRESS-SYNDROME

The distribution of tidal volume (VT) and recruitment was investigated by chest computed tomography (CT in eight sedated-paralyzed patients with the adult respiratory distress syndrome (ARDS). A CT section was obtained in the supine position at 0, 5, 10, 15, and 20 cm H2O positive end-expiratory pressure (PEEP) and at the corresponding inspiratory plateau pressure (21 +/- 1.8, 26 +/- 1.4, 31 +/- 1.8, 38 +/- 2.1, and 46 +/- 3.2 cm H2O [mean +/- SE]), keeping VT constant. Each CT section was divided along its ventral-dorsal height into 10 equally spaced intervals (levels). V-i(insp) and V-i(exp) were defined as the gas volume for level i(i = 1 to 10) at end-inspiration and at end-expiration, respectively. The following variables were computed at each lung level: (1) distribution of CT section tidal volume (VTct), i.e., the fraction of VT that inflates a given lung level; (2) the plateau-induced and PEEP-induced recruitment, i.e, the amount of lung tissue previously collapsed that inflates at plateau pressure and at PEEP, respectively; (3)the reopening-collapsing tissue, i.e., the amount of lung tissue that regains inflation at plateau pressure and collapses at PEEP. With increasing PEEP from 0 to 20 cm H2O, the VTct distribution decreased significantly (p < 0.01) in the upper levels, did not change in the middle levels, and increased significantly (p < 0.01) in the lower levels. The plateau-induced recruitment increased from upper to lower levels (p < 0.01) but was similar at plateau pressures from 21 to 46 cm H2O. PEEP-induced recruitment significantly increased (p < 0.01) from 0 to 20 cm H2O and from upper to lower levels. Consequently, the reopening-collapsing tissue decreased significantly (p < 0.01) with PEEP. We conclude that (PEEP makes the gas distribution more homogeneous in sedated-paralyzed ARDS patients, stretching the upper levels and recruiting the lower ones. PEEP reduces the reopening-collapsing tissue, keeping open the tissue recruited by plateau pressure.