acute lung injury (ALI) are terms used to reflect what we think is a
relatively specific form of pathologic injury to the lung but occurring
from a wide diversity of causes or associated conditions. The
assumption underlying use of these terms is that the abnormality
reflects diffuse alveolar damage, involving both the endothelial and
epithelial layers. This damage is characterized pathophysiologically by
a breakdown in the barrier and gas exchange functions in the lung.
Initially this results in flooding of the alveolar spaces with
protein-rich edema fluid resulting in severe gas exchange
abnormalities. If the process is sustained, fibroproliferation occurs
with collagen deposition and lung remodeling. Since pathology
specimens are rarely available and no practical methods exist to
measure the barrier function or endothelial and epithelial injuries, we
are left with definitions using surrogates or clinical reflections of
these processes. Thus, we have defined ARDS and ALI in terms of their
associated gas exchange abnormalities and radiologic manifestations.
The specific definitions used have enormous effects on the outcomes of
epidemiologic studies. Therefore, whenever epidemiologic data are
evaluated, the specific definitions used must be kept in mind.