Pulmonary Hypertension Diagnosis and Classification

Definition and Diagnosis

Pulmonary hypertension (PH) is defined as an increase in mean pulmonary arterial pressure (mPAP) ≥25 mmHg at rest as assessed by right heart catheterization, according to the European Respiratory Society, affecting approximately 1% of the global population 1, 2
PH is diagnosed through right heart catheterization, which remains the gold standard for confirming elevated pulmonary pressures, as stated by the European Respiratory Society 2
The most recent guidelines define PH as mPAP ≥25 mmHg at rest, with normal pulmonary capillary wedge pressure (PCWP) ≤15 mmHg in pre-capillary forms, as defined by the European Respiratory Society 1
The definition of PH on exercise (mPAP >30 mmHg) is not supported by sufficient published data, according to the European Respiratory Society 1, 3

PH is classified into five main groups based on etiology, pathophysiology, and therapeutic approaches, including Group 1: Pulmonary Arterial Hypertension (PAH), Group 2: PH Due to Left Heart Disease, Group 3: PH Due to Lung Diseases and/or Hypoxia, Group 4: Chronic Thromboembolic PH (CTEPH), and Group 5: PH with Unclear and/or Multifactorial Mechanisms, as classified by the European Respiratory Society 1, 3, 4, 5, 6
Group 1 PAH is characterized by pre-capillary PH with pulmonary vascular resistance >3 Wood units, as defined by the European Respiratory Society 1
Group 2 PH Due to Left Heart Disease is characterized by post-capillary PH with PCWP >15 mmHg, as defined by the European Respiratory Society 1, 4
Group 3 PH Due to Lung Diseases and/or Hypoxia includes conditions such as sleep-disordered breathing and alveolar hypoventilation disorders, as stated by the European Respiratory Society 3, 4, 6
Group 4 CTEPH results from persistent obstruction of pulmonary arteries following pulmonary embolism, as defined by the European Respiratory Society 4, 5
Group 5 PH with Unclear and/or Multifactorial Mechanisms includes conditions such as hematological disorders and systemic disorders, as stated by the European Respiratory Society 2, 5, 6

The European Respiratory Society defines pulmonary hypertension as a mean pulmonary arterial pressure (mPAP) ≥25 mmHg at rest measured by right heart catheterization 7
The American College of Radiology proposes lowering this threshold to >20 mmHg, though the 25 mmHg cutoff remains widely referenced in clinical practice 8

The European Respiratory Society classifies pulmonary hypertension into five distinct clinical groups based on etiology, pathophysiology, and treatment approaches 7
Group 1: Pulmonary Arterial Hypertension (PAH) is characterized by pre-capillary PH (mPAP ≥25 mmHg, pulmonary wedge pressure ≤15 mmHg, and pulmonary vascular resistance >3 Wood units) 7
Group 2: PH Due to Left Heart Disease is the most common form of PH and includes left ventricular systolic dysfunction, left ventricular diastolic dysfunction, valvular disease, and congenital/acquired left heart inflow/outflow obstruction 7, 9, 10

The European Respiratory Society recommends a comprehensive assessment algorithm for pulmonary hypertension, including clinical evaluation, initial diagnostic testing, pulmonary function testing, echocardiography, ventilation-perfusion scan, chest CT imaging, and right heart catheterization 7, 8, 11, 10, 12
Right heart catheterization is mandatory for definitive diagnosis and must be performed before initiating PAH-specific therapy 7, 10, 9

The European Heart Journal recommends referral to expert centers for patients with confirmed PAH, CTEPH, PH with severe pre-capillary component, NYHA functional class ≥III with intermediate to high-risk surgery planned, clinical or echocardiographic signs of severe PH and/or severe RV dysfunction, and patients with mild lung disease but severe PH 13, 10, 9
The European Respiratory Society emphasizes the importance of not delaying referral until after attempting empiric therapy, and PAH-specific medications should never be initiated without right heart catheterization confirmation and expert center involvement 7, 11

Pulmonary hypertension is defined by a mean pulmonary arterial pressure (mPAP) ≥ 25 mmHg at rest (with recent European Society of Cardiology/European Respiratory Society guidelines proposing >20 mmHg) measured by right‑heart catheterisation in the main pulmonary artery, which reflects a systemic (bilateral) pressure elevation throughout the pulmonary circulation. 14
Because the catheter samples pressure in the main pulmonary artery before it bifurcates, the measurement inherently represents the combined pressures of both lungs rather than isolated lung segments. 14
The haemodynamic definition therefore requires assessment of the entire pulmonary vascular bed; it does not permit evaluation of individual lung segments. 14

The only recognised form of “unilateral” pulmonary hypertension is segmental pulmonary hypertension, which is categorised under Group 5 (PH with unclear and/or multifactorial mechanisms) in the clinical classification. [14][15][16][17]
Segmental PH denotes a localized elevation of pulmonary pressures that involves only specific lung segments or an entire single lung, making it a distinct and rare entity that differs fundamentally from standard definitions of PH. 14

A common pitfall is confusing unilateral lung disease (e.g., unilateral chronic thromboembolic disease or unilateral lung pathology) with unilateral pulmonary hypertension. 14
Even when the underlying aetiology affects only one lung—such as unilateral pulmonary embolism or unilateral lung disease—if the mPAP measured in the main pulmonary artery is elevated, the condition is classified as bilateral PH by definition. 14
Chronic thromboembolic pulmonary hypertension (Group 4) may present with asymmetric or predominantly unilateral vascular obstruction, yet the resulting PH still involves the entire pulmonary circulation. 14

Computed‑tomography angiography and ventilation/perfusion (V/Q) scans can reveal unilateral perfusion defects or vascular abnormalities; these findings represent the underlying pathology rather than true unilateral pulmonary hypertension. 18