ARDS

increased permeability of the alveolar-capillary membrane and pulmonary edema

diffuse injury to the pulmonary capillary endothelium and alveolar epithelium that results in leakage of protein-rich fluid into the alveolar and interstitial spaces

aggregates of debris/ protein in alveolar spaces forming hyaline membranes predispose to alveolar collapse and impaired gas exchange

Diagnostic criteria

One consensus requires these 4 to exist:

1 Acute onset.

2 CXR: bilateral infiltrates.

3 Pulmonary capillary wedge pressure (PCWP) <19mmHg or a lack of clinical congestive heart failure.

4 Refractory hypoxaemia with P aO2: FiO2 <200 for ARDS.

Others include total thoracic compliance <30mL/cm H2O.

ABGs reveal decreased PaO2 and increased or normal PaCO2

Swan-Ganz catheter findings will reveal normal cardiac output and normal capillary wedge pressure but increased pulmonary artery pressure

For USMLE,

• ARDS is defined as having a pO2/FiO2 ratio below 300.
• If <200, moderately severe, <100 = severe
• Also wedge pressure is normal, (but it is not necessary to measure)