Can pulmonary hypertension be unilateral?

Can Pulmonary Hypertension Be Unilateral?

No, pulmonary hypertension cannot be truly unilateral in the conventional sense, as PH is defined by mean pulmonary arterial pressure measured in the main pulmonary artery, which reflects a systemic (bilateral) elevation of pressure throughout the pulmonary circulation. 1, 2, 3

Understanding the Definition of Pulmonary Hypertension

• Pulmonary hypertension is defined as a mean pulmonary arterial pressure (mPAP) ≥25 mmHg at rest (with recent guidelines proposing >20 mmHg) measured by right heart catheterization in the main pulmonary artery. 1, 2, 4
• This measurement inherently reflects pressures from both lungs combined, as the catheter samples from the main pulmonary artery before it bifurcates. 3
• The hemodynamic definition requires assessment of the entire pulmonary vascular bed, not individual lung segments. 1, 4

The Exception: Segmental Pulmonary Hypertension

• The only recognized form of "unilateral" pulmonary hypertension is termed "segmental pulmonary hypertension," which is listed under Group 5 (PH with unclear and/or multifactorial mechanisms) in the clinical classification. 1
• Segmental PH represents localized elevation of pulmonary pressures affecting only specific segments or one lung, rather than the entire pulmonary circulation. 1
• This is a distinct and rare entity that differs fundamentally from standard PH definitions. 1

Clinical Implications and Pitfalls

• A critical pitfall is confusing unilateral lung disease (such as unilateral chronic thromboembolic disease or unilateral lung pathology) with unilateral pulmonary hypertension. 1
• Even when the underlying cause affects only one lung (e.g., unilateral pulmonary embolism, unilateral lung disease), if the mean pulmonary arterial pressure in the main pulmonary artery is elevated, this constitutes bilateral PH by definition. 1, 4
• Chronic thromboembolic pulmonary hypertension (CTEPH), classified as Group 4 PH, may have asymmetric or predominantly unilateral vascular obstruction, but the resulting PH affects the entire pulmonary circulation. 1, 3

Diagnostic Considerations

• Right heart catheterization measures pressure in the main pulmonary artery before bifurcation, making it impossible to diagnose "unilateral PH" using standard diagnostic criteria. 3, 4
• Imaging studies (CT angiography, V/Q scans) may reveal unilateral perfusion defects or vascular abnormalities, but these represent the underlying pathology, not unilateral PH per se. 1
• If segmental PH is suspected based on imaging showing dramatically asymmetric pulmonary vascular disease with localized pressure elevation, this rare entity should prompt referral to a specialized PH center for comprehensive evaluation. 3