Pulmonary Arterial Hypertension (PAH)

Pulmonary hypertension (PH) is classified into 5 types, one of which is pulmonary arterial hypertension (PAH) or Group 1 PH, characterized by symptoms such as dyspnea, fatigue, chest pain, syncope, or edema. However, PH is more commonly caused by other cardiopulmonary diseases as compared to PAH. Thus, PAH must be distinguished from other causes of PH before a diagnosis of PAH is established, often resulting in delayed diagnosis.

Other causes of PH include left heart disease (Group 2), lung disease or hypoxia (Group 3), chronic thromboembolic PH (Group 4), and miscellaneous causes such as metabolic disorders, systemic disorders (such as sarcoidosis), or hematological disorders, such as splenectomy (Group 5).1

PH Due to Left Heart Disease (Group 2)

PH can be caused by most cardiac conditions, including left ventricular dysfunction, cardiomyopathy, congenital heart disease, and valvular disease. Heart diseases are detected by physical examination, electrocardiogram, echocardiography, and cardiac catheterization.

Clinical factors, such as old age (older than 65 years), obesity, coronary heart disease, atrial fibrillation, and diabetes are characteristic features that suggest left ventricular dysfunction. Transthoracic echocardiography provides important information in these patients, including left atrial enlargement, left ventricular hypertrophy, concentric remodeling of the LV (relative wall thickness >0.45), and indicators of elevated left ventricle filling pressures. These characteristic features can be used to distinguish PH due to left heart disease from PAH.2

PH Due to Lung Disease or Hypoxia (Group 3)

The advanced stages of all lung diseases cause PH detected by abnormal lung sounds on physical examination, pulmonary function testing, arterial blood gas analysis, or high-resolution computed tomographic (HRCT) lung imaging. 

The underlying lung disease can be identified using pulmonary function tests and arterial blood gas measurements. Patients may have mild to moderate reduction of lung volumes, but the partial arterial oxygen tension is normal or slightly decreased and the partial arterial carbon dioxide tension is usually decreased from hyperventilation.

HRCT can be used to evaluate thromboembolic disease, pulmonary fibrosis, emphysema, interstitial lung disease, or significant lung parenchymal disease.3

Patients with combined pulmonary fibrosis and emphysema syndrome (CPFE), who may also develop PH, may be misdiagnosed with PAH due to the presence of near-normal airflow and lung volumes, and no visible CPFE characteristic features, ie upper-lobe emphysema, abnormalities of gas exchange, and lower-lobe fibrosis, on chest radiographs in earlier stages of the disease. However, severely reduced diffusing capacity of the lung for carbon monoxide (DLCO) suggests a disease other than PAH, necessitating further tests such as HRCT scanning.4,5

Pulmonary Artery Obstructions (Group 4)

Pulmonary artery obstructions or embolism are detected by imaging procedures, screened by lung ventilation/perfusion (V/Q) scanning, and confirmed by CT and/or pulmonary arteriography. 

Chronic thromboembolic pulmonary hypertension (CTEPH), a disorder in which pulmonary emboli do not get dissolved by fibrinolysis, is preferably screened by V/Q scanning that typically shows at least one segmental-sized or larger perfusion defect. This is further confirmed by CT imaging or pulmonary angiography to distinguish CTEPH from PAH since the former can be surgically cured by pulmonary thromboendarterectomy.1,5,6


  1. Austin ED, Phillips JA III, Loyd JE. Heritable pulmonary arterial hypertension overview. In: Adam MP, Ardinger HH, Pagon RA, et al, eds. GeneReviews. University of Washington, Seattle; 2002. Updated December 23, 2020. Accessed July 7, 2021. 
  2. Galiè N, Hoeper MM, Humbert M, et al. Guidelines for the diagnosis and treatment of pulmonary hypertension: the task force for the diagnosis and treatment of pulmonary hypertension of the European Society of Cardiology (ESC) and the European Respiratory Society (ERS), endorsed by the International Society of Heart and Lung Transplantation (ISHLT) [published correction appears in Eur Heart J. 2011 Apr;32(8):926]. Eur Heart J. 2009;30(20):2493-2537. doi:10.1093/eurheartj/ehp297
  3. Stringham R, Shah NR. Pulmonary arterial hypertension: an update on diagnosis and treatment. Am Fam Physician. 2010;82(4):370-377.
  4. Jankowich MD, Rounds SIS. Combined pulmonary fibrosis and emphysema syndrome: a review. Chest. 2012;141(1):222-231. doi:10.1378/chest.11-1062
  5. Vachiéry JL, Gaine S. Challenges in the diagnosis and treatment of pulmonary arterial hypertension. Eur Respir Rev. 2012;21(126):313-320. doi:10.1183/09059180.00005412
  6. Barst RJ, McGoon M, Torbicki A, et al. Diagnosis and differential assessment of pulmonary arterial hypertension. J Am Coll Cardiol. 2004;43(12 Suppl S):40S-47S. doi:10.1016/j.jacc.2004.02.032

Reviewed by Debjyoti Talukdar, MD, on 7/1/2021.