What is pulmonary edema?

What is Pulmonary Edema

Pulmonary edema is the abnormal accumulation of fluid in the pulmonary interstitium and alveolar spaces of the lungs, resulting from either elevated hydrostatic pressure (cardiogenic) or increased capillary permeability (non-cardiogenic). 1, 2

Fundamental Pathophysiology

Pulmonary edema develops when the rate of fluid filtration into the lung exceeds the capacity of lymphatic drainage to remove it. 2 The normal lung continuously filters fluid across the pulmonary capillary endothelium, but this filtrate is confined to interstitial spaces and removed through lymphatic channels. 2 Edema only manifests when filtration overwhelms this lymphatic clearance capacity, which can increase several-fold before decompensation occurs. 2

Two Main Categories

Cardiogenic (Hydrostatic) Pulmonary Edema

This results from elevated pulmonary capillary pressure secondary to left-sided heart failure, causing fluid to be forced from capillaries into the interstitium and alveoli. 1, 3, 2

Key mechanisms include: 1, 4, 5

• Increased left ventricular diastolic pressure from systolic or diastolic dysfunction
• Marked increase in systemic vascular resistance superimposed on insufficient myocardial functional reserve
• Elevated left atrial pressure reflected back into pulmonary venous circulation
• Fluid redistribution directed into the lungs rather than simple fluid accumulation

Common precipitating causes: 1, 4, 3

• Acute myocardial infarction or acute myocardial injury
• Decompensated chronic heart failure
• Hypertensive crisis with sudden blood pressure elevation
• Acute valvular disorders (severe mitral or aortic insufficiency, papillary muscle rupture, ventricular septal defect)
• Atrial fibrillation with rapid ventricular response
• Aortic dissection with valvular or coronary involvement
• Prosthetic valve thrombosis

Non-Cardiogenic (Permeability) Pulmonary Edema

This results from injury to the endothelial and epithelial barriers, allowing fluid and protein to leak into the alveolar space despite normal hydrostatic pressures. 1, 2, 6

Key mechanisms include: 1, 7

• Increased pulmonary capillary permeability from inflammatory mediators
• Breakdown of the alveolar-epithelial barrier from dysregulated inflammation, leukocyte infiltration, procoagulant processes, cell death, and mechanical stretch
• Reactive oxygen and nitrogen species damaging ion channels and altering fluid balance
• Accumulation of extravascular lung water and plasma protein exudation into alveolar spaces

Common causes: 1, 4, 3

• Acute respiratory distress syndrome (ARDS) from diffuse pulmonary infections, acute interstitial lung diseases, or drug/chemical-induced lung injury
• High-altitude pulmonary edema (HAPE) in susceptible individuals at altitudes typically above 2,500 meters
• Iatrogenic fluid overload
• Massive blood transfusions
• Injurious mechanical ventilation

Clinical Presentation

Patients typically present with: 1

• Severe respiratory distress with dyspnea and orthopnea
• Tachypnea with respiratory rates often exceeding normal limits
• Rales (crackles) over lung fields on auscultation
• Arterial oxygen saturation typically <90% on room air prior to oxygen therapy
• In cardiogenic cases: peripheral edema, jugular venous distension, S3 gallop rhythm, hepatomegaly

Important clinical distinction: Cardiogenic pulmonary edema is characterized by a history of cardiac disease, physical examination findings of heart failure, and radiographic evidence of cardiomegaly and vascular congestion, whereas non-cardiogenic edema lacks these cardiac features. 4

Diagnostic Considerations

The distinction between cardiogenic and non-cardiogenic pulmonary edema is crucial because treatment strategies differ fundamentally. 4, 2 Cardiogenic edema requires management of cardiac dysfunction and reduction of hydrostatic pressures, while non-cardiogenic edema requires addressing the underlying cause of increased permeability and supportive care. 2

Key diagnostic features on imaging: 1

• Chest X-ray shows peri-bronchial cuffing, cardiomegaly (in cardiogenic cases), pulmonary venous congestion, and pleural effusion
• Lung ultrasound demonstrates B-line artifacts with 94% sensitivity and 92% specificity for pulmonary edema diagnosis
• Echocardiography assesses volume status, left ventricular filling pressures (E/e' ratio), and structural cardiac abnormalities

Prognostic Implications

The presence of pulmonary edema carries significant morbidity and mortality implications. 1 In cardiogenic cases, low blood pressure on admission is associated with poor prognosis, and estimates of death or rehospitalization within 60 days range from 30-50%. 1 The development of pulmonary hypertension secondary to chronic pulmonary edema worsens prognosis, particularly when related to fibrotic changes in the pulmonary vasculature. 1