| PH subtype | Pathological changes | Contributory mechanisms |
| PAH | Pathological lesions in distal pulmonary arteries Medial hypertrophy Intimal proliferative and fibrotic changes Adventitial thickening with perivascular inflammatory infiltrates and thrombotic lesions | Endothelial dysfunction, leading to changes in vasoactive and growth regulatory factors Vascular cell proliferation Inflammation Thrombosis |
| PH-LHD | Enlarged and thickened pulmonary veins Pulmonary capillary dilatation Interstitial oedema Alveolar haemorrhage Lymphatic vessel and lymph node enlargement Medial hypertrophy and intimal fibrosis of distal pulmonary arteries | Vasoconstrictive reflexes arising from stretch receptors localised in the left atrium and pulmonary veins Endothelial dysfunction Vasoconstriction Proliferative remodelling of the pulmonary vessel wall |
| PH-ILD | Medial hypertrophy Intimal obstructive proliferation of the distal pulmonary arteries Destruction of vascular bed in areas subject to emphysema or fibrosis | Hypoxic vasoconstriction Endothelial dysfunction leading to imbalance in vasoactive signalling molecules Mechanical stress of hyperinflated lungs Capillary loss Inflammation Toxic effects of cigarette smoke |
| CTEPH | Persistent organised thrombi in the medial layer of the pulmonary vasculature Vessel occlusion Remodelling of the major pulmonary vasculature Small-vessel pulmonary arteriopathy (indistinguishable from changes seen in PAH) | Non-resolution of emboli Endothelial dysfunction from shear stress, pressure and inflammation Cytokine release In situ thrombosis Vasculotrophic mediator release |
PAH: pulmonary arterial hypertension; LHD: left heart disease; ILD: interstitial lung disease; CTEPH: chronic thromboembolic PH. Modified from [1].