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) (eigh t patients with moderate ALI [moderate group], eight patients with sev ere ALI [adult respiratory distress syndrome, ARDS group]) and in eigh t normal anesthetized-paralyzed subjects (control group), we partition ed the total respiratory system mechanics into the lung (L) and chest wall (w) mechanics using the esophageal balloon technique together wit h the airway occlusion technique during constant flow inflation. We me asured lung elastance (Est, L), chest wall elastance (Est,w), and tota l lung (Rmax,L) and chest wall (Rmax,w) resistance. Rmax,L includes ai rway (Rmin,L) and ''additional'' lung resistance (DR,L). DR,L represen ts the ''additional'' component due to the viscoelastic phenomena of t he lung tissues and time-constant inequalities (pendelluft). Measureme nts were repeated at 0, 5, and 10 cm H2O of positive end-expiratory pr essure (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 measu red at each level of PEEP. Specific total lung (sRmax, L), airway (sRm in,L), and ''additional'' lung (sDR,L) resistances were obtained as Rm ax,L x EELV, Rmin,L x EELV, and DR,L x EELV, respectively. At PEEP 0 c m 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 pa tients 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 seve rity of the disease (p < 0.01). Nevertheless, SRmax,L, sRmin,L, and sD R,L were not significantly different between groups. In patients with ALI, PEEP higher than 10 cm H2O significantly (p < 0.01) increased Rma x,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) n ot only lung but also chest wall elastance is increased; 2) increased total, airway, and ''additional'' lung resistance probably reflects, a t PEEP 0 cm H2O, a reduction in lung volume; 3) the severity of the di sease significantly influenced lung mechanics; 4) PEEP higher than 10 cm H2O significantly increased both total and ''additional'' lung resi stance.