Alterations of lung and chest wall mechanics in patients with acute lung injury: effects of positive end-expiratory pressure

16 mechanically ventilated patients with acute lung injury (ALI) (eight patients with moderate ALI [moderate group], eight patients with severe ALI [adult respiratory distress syndrome, ARDS group]) and in eight normal anesthetized-paralyzed subjects (control group), we partitioned the total respiratory system mechanics into the lung (L) and chest wall (w) mechanics using the esophageal balloon technique together with the airway occlusion technique during constant flow inflation. We measured lung elastance (Est, L), chest wall elastance (Est,w), and total lung (Rmax,L) and chest wall (Rmax,w) resistance. Rmax,L includes airway (Rmin,L) and ''additional'' lung resistance (DR,L). DR,L represents the ''additional'' component due to the viscoelastic phenomena of the lung tissues and time-constant inequalities (pendelluft). Measurements were repeated at 0, 5, and 10 cm H2O of positive end-expiratory pressure (PEEP) in the control group and at 0, 5, 10, and 15 cm H2O PEEP in patients with ALI. The end-expiratory lung volume (EELV) was measured at each level of PEEP. Specific total lung (sRmax, L), airway (sRmin,L), and ''additional'' lung (sDR,L) resistances were obtained as Rmax,L x EELV, Rmin,L x EELV, and DR,L x EELV, respectively. At PEEP 0 cm H2O, we found that both Est,L (23.7 +/- 5.5 and 13.8 +/- 3.3 versus 9.3 +/- 1.7 cm H2O/L; p < 0.01) and Est,w (13.2 +/- 5.4 and 9.9 +/- 2.1 versus 5.6 +/- 2.3 cm H2O/L; p < 0.01) were markedly increased in patients with ARDS and moderate ALI compared with control subjects, with a significant (p < 0.01) effect of the severity of the disease on Est,L (p < 0.01). Rmax,L was significantly (p < 0.01) higher in patients with ARDS and moderate ALI compared with control subjects, because of an increase in Rmin,L (4.4 +/- 1.9 and 2.7 +/- 1.3 versus 2.1 +/- 0.9 cm H2O/L/s; p < 0.01), and DR,L (3.2 +/- 0.8 and 1.5 +/- 1.1 versus 1.1 +/- 0.6 cm H2O/L/s; p < 0.01), with a significant effect of the severity of the disease (p < 0.01). Nevertheless, SRmax,L, sRmin,L, and sDR,L were not significantly different between groups. In patients with ALI, PEEP higher than 10 cm H2O significantly (p < 0.01) increased Rmax,L, DR, L, and sDR,L while it did not affect sRmin, L. in conclusion, we have shown that in mechanically ventilated patients with ALI: 1) not only lung but also chest wall elastance is increased; 2) increased total, airway, and ''additional'' lung resistance probably reflects, at PEEP 0 cm H2O, a reduction in lung volume; 3) the severity of the disease significantly influenced lung mechanics; 4) PEEP higher than 10 cm H2O significantly increased both total and ''additional'' lung resistance