ARDS Caused by Pulmonary and Extrapulmonary Disease: Different Syndromes?
ARDS Caused by Pulmonary and Extrapulmonary Disease: Different Syndromes?
Abstract & Commentary
Synopsis: Pulmonary mechanics at zero PEEP and with incremental increases in PEEP differ in patients with ARDS of pulmonary (RDSp) vs. extrapulmonary (ARDSexp) origin. Elastance of the lung at zero PEEP was higher in pulmonary ARDS. With application of PEEP, total respiratory system elastance decreased in extrapulmonary ARDS.
Source: Gattinoni L, et al. Am J Respir Crit Care Med 1998;158:3-11.
Gattinoni’s group in milan systemically evalu-ated pulmonary mechanics at 0, 5, 10, and 15 cm H2O of positive end-expiratory pressure (PEEP) in 12 patients with the acute respiratory distress syndrome caused by pulmonary processes (ARDSp) and in nine patients with ARDS caused by extrapulmonary processes (ARDSexp) within two days of the onset of the syndrome.
At zero PEEP, total respiratory system static elastance (Est RS) and end-expiratory lung volumes (EELV) were similar in both groups. However, in patients with ARDSp, lung static elastance (Est L) was higher than that in ARDSexp (20.2 ± 5.4 vs 13.8 ± 5.0 cm H2O/L; P < 0.05), whereas chest wall static elastance (Est W) was higher in ARDSexp patients than in patients with ARDSp (12.1 ± 3.8 vs 5.2 ± 1.9 cm H2O/L; P < 0.05). In addition, intra-abdominal pressure (IAP) was higher as measured by transurethral catheter in ARDSexp than in ARDSp (22.2 ± 6.0 vs 8.5 ± 2.9 cm H2O; P < 0.01), and this pressure was significantly correlated with chest wall elastic resistance (P < 0.01).
With the application of PEEP in ARDSp, both Est RS (25.4 ± 6.2 to 31.2 ± 11.3 cm H2O/L; P < 0.01) and Est L increased, while with ARDSexp there were decreases in Est RS (25.9 ± 5.4 to 21.2 ± 5.5 cm H2O/L; P < 0.01), Est W and Est L at 15 cm H2O. In ARDSp, EELV increased from 0.56 ± 0.25 L at 0 PEEP to 1.15 ± 0.36 L (P < 0.05) at 15 cm H2O PEEP. However, it was estimated that recruited lung volume was -0.031 ± 0.09 L. In contrast, in ARDSexp, EELV increased from 0.60 ± 0.29 L at 0 PEEP to 1.49 ± 0.34 L (P < 0.05) at 15 cm H2O PEEP, estimated recruited lung volume being 0.29 ± 0.24 L.
Comment by Robert M. Kacmarek, PhD, RRT
Pathologic descriptions of ARDS have not differentiated features of the syndrome based on the etiology of the ARDS. The results of this study would indicate, at least from the standpoint of a potential therapeutic response in the early phase of ARDS, that manifestations of pulmonary mechanics are different dependent upon the etiology of the process. In patients with extrapulmonary ARDS, recruitment of lung is possible, whereas in pulmonary ARDS, recruitment may not be possible. As a result, the application of PEEP in pulmonary ARDS may cause overdistension of already open lung units. As a result, patients with pulmonary-originating ARDS may be more prone to ventilator-induced lung injury than patients with ARDS caused by extrapulmonary processes. On the other hand, in extrapulmonary ARDS, lung volume is recruitable, and because of the elevated Est W the likelihood of overdistension is decreased, but the probability of cardiovascular compromise with the application of PEEP is increased.
These data, if verified by other studies, may help to explain the diverse results of clinical trials in ARDS. The mixture of ARDSp and ARDSexp patients in a given study may contribute to the equivocal or detrimental results obtained in various trials. However, a number of concerns regarding the present study must be raised. First, almost all recruited patients had either pneumonia or abdominal sepsis. Other causes of either ARDSp or ARDSexp were not evaluated. More important, only 15 cm H2O PEEP was applied, and no "lung-recruitment" maneuvers were used in either group. Specifically, in ARDSp a recruitment maneuver and higher PEEP levels (up to 20-25 cm H2O) may be necessary to demonstrate lung recruitment (Amato et al. N Engl J Med 1998;338:347). In addition, all ventilator settings were constant during the application of PEEP and, as a result, overdistension may have been present at 15 cm H2O PEEP in patients with ARDSp, accounting for the increased elastance.
Based on these data I would not change my clinical management of ARDS. However, these findings may help to explain some of the limited responses to PEEP we have observed in ARDSp. However, Gattinoni et al may have simply missed the mark by not evaluating high enough PEEP levels or using recruitment maneuvers.
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