Learning Objective: By the end of this section, the learner should be able to describe the mechanisms, causes, and clinical features of pulmonary edema, distinguish cardiogenic from non-cardiogenic forms, and understand the role of Starling forces, capillary permeability, and lymphatic drainage in pulmonary fluid balance.

Pulmonary Edema

Edema in the lung interstitium can have grave consequences because it interferes with gas exchange, causing hypoxemia and hypercapnia. Normally, low hydrostatic pressure in pulmonary capillaries and efficient lymphatic drainage protect the lungs against fluid accumulation. However, alterations in Starling forces, capillary permeability, or lymphatic function can lead to pulmonary edema.

Cardiogenic Pulmonary Edema 

Mechanism:

Increased left atrial pressure → elevated pulmonary venous and capillary hydrostatic pressure → fluid filtration into the interstitium.

Protective:

Initially, increased lymph flow removes interstitial proteins.

Clinical Signs:

Orthopnea (dyspnea when supine, relieved by sitting upright).

Diagnosis:

Elevated pulmonary wedge pressure confirms increased left atrial pressure.

Treatment:

Reduce left atrial pressure (e.g., diuretics).

Activity

Non-Cardiogenic Pulmonary Edema

Mechanism:

Direct alveolar epithelial injury or primary capillary endothelial injury → increased permeability.

Clinical Signs:

Rapid-onset severe dyspnea, hypoxemia, diffuse pulmonary infiltrates, and potential respiratory failure.

Common Causes:

Sepsis, bacterial pneumonia, trauma, and gastric aspiration.

Pathophysiology:

Protein-rich alveolar fluid inactivates surfactant → reducing lung compliance.

Diagnosis:

Pulmonary wedge pressure is normal or low, distinguishing it from cardiogenic edema.