Pathophysiology
Clinical meaning
Acute Respiratory Distress Syndrome (ARDS) is a syndrome of non-cardiogenic pulmonary edema and refractory hypoxemia caused by diffuse alveolar damage. The Berlin definition classifies ARDS by severity based on the PaO2/FiO2 ratio: mild (200 to 300 mmHg), moderate (100 to 200 mmHg), and severe (less than 100 mmHg), all with bilateral opacities on chest imaging not fully explained by cardiac failure or fluid overload. While the Berlin definition treats ARDS as a single entity, accumulating evidence demonstrates that ARDS comprises distinct phenotypes with different pathobiology, treatment responses, and outcomes, making phenotype recognition increasingly important for nursing care. The most clinically actionable phenotypic distinction is between focal and diffuse ARDS, identified by CT morphology. Focal ARDS (approximately 30% of cases) is characterized by heterogeneous lung involvement: areas of consolidation and atelectasis concentrated in the dependent (posterior in supine patients) lung regions, with relatively preserved aeration in non-dependent (anterior) regions. Diffuse ARDS (approximately 70% of cases) shows homogeneous bilateral involvement with widespread ground-glass opacities and consolidation throughout all lung regions. This distinction matters because the mechanical behavior of the lungs differs fundamentally...