Acute respiratory distress syndrome caused by pulmonary and extrapulmonary disease - Different syndromes?

To assess the possible differences in respiratory mechanics between the acute respiratory distress syndrome (ARDS) originating from pulmonary disease (ARDS(p)) and that originating from extrapulmonary disease (ARDS(exp)) we measured the total respiratory system (Est, rs), chest wall (Est, w) and lung (Est, L) elastance, the intra-abdominal pressure (IAP), and the end-expiratory lung volume (EELV) at 0, 5, 10, and 15 cm H2O positive end-expiratory pressure (PEEP) in 12 patients with ARDS(P) and nine with ARDS(exp). At zero end-expiratory pressure (ZEEP), Est,rs and EELV were similar in both groups of patients. The Est, L, however, was markedly higher in the ARDS, group than in the ARDS(exp) group (20.2 +/- 5.4 versus 13.8 +/- 5.0 cm H2O/L, p < 0.05), whereas Est, w was abnormally increased in the ARDS(exp) group (12.1 +/- 3.8 versus 5.2 +/- 1.9 cm H2O/L, p < 0.05). The IAP was higher in ARDS(exp) than in ARDS(p) (22.2 +/- 6.0 versus 8.5 +/- 2.9 cm H2O, p < 0.01), and it significantly correlated with Est, w (p < 0.01). Increasing PEEP to 15 cm H2O caused an increase of Est, rs in ARDS(p) (from 25.4 +/- 6.2 to 31.2 +/- 11.3 cm H2O/L, p < 0.01) and a decrease in ARDS(exp), (from 25.9 +/- 5.4 to 21.4 +/- 55.5 cm H2O/L, p < 0.01). The estimated recruitment at 15 cm H2O PEEP was -0.031 +/- 0.092 versus 0.293 +/- 0.241 L in ARDS(p) and ARDS(exp), respectively (p < 0.01). The different respiratory mechanics and response to PEEP observed are consistent with a prevalence of consolidation in ARDS(p) as opposed to prevalent edema and alveolar collapse in ARDS(exp).