ABSTRACT
Acute respiratory distress syndrome (ARDS) is usually viewed as the functional and
morphological expression of a similar underlying lung injury caused by a variety of
insults. However, the distinction between ARDS due to a direct (ARDSp) versus an indirect (ARDSexp) lung injury is gaining more attention as a means of better comprehending
the pathophysiology of ARDS and for modifying ventilatory management. From the few
published studies, we can summarize that: (1) the prevalent damage in early stages
of a direct insult is intra-alveolar, whereas in indirect injury it is the interstitial
edema. It is possible that the two insults may coexist (i.e., one lung with direct
injury (as in pneumonia) and the other with indirect injury, through mediator release
from the contralateral pneumonia); (2) the radiological pattern, by chest x-ray or
computed tomography (CT), is different in ARDSp (characterized by prominent consolidation)
and ARDSexp (characterized by prominent ground-glass opacification); (3) in ARDSp
lung elastance is more markedly increased than in ARDSexp, where the main abnormality
is the increase in chest wall elastance, due to abnormally high intra-abdominal pressure;
(4) positive end-expiratory pressure (PEEP), inspiratory recruitment, and prone position
are more effective to improve respiratory mechanics, alveolar recruitment, and gas-exchange
in ARDSexp. Further studies are warranted to better define if the distinction between
ARDS of different origins can improve clinical management and survival.
KEYWORD
Acute respiratory distress syndrome - pulmonary pneumonia - extrapulmonary pneumonia
