Why there is a need to discuss pulmonary hypertension other than pulmonary arterial hypertension?

Table 1 The definitions of pulmonary hypertension groups I, II, III, IV[1,7,22]

Group	Definition
Group I: Pulmonary arterial hypertension	Is defined as: Mean pulmonary artery pressure ≥ 25 mmHg at rest, and end-expiratory pulmonary artery wedge pressure ≤ 15 mmHg, and pulmonary vascular resistance > 3 Wood units
Group II: PH due to left heart disease	Is defined as: mPAP ≥ 25 mmHg, and PAWP > 15 mmHg, and normal or reduced CO
Group III: PH due to chronic lung disease and/or hypoxia	Patients with confirmed COPD or DPLD, without chronic thromboembolic disease or left heart disease, who meet at least two of the following criteria: mPAP > 35 mmHg mPAP ≥ 25 mmHg AND cardiac index < 2 lt/min per square pulmonary vascular resistance > 6 Wood units
Group IV: Chronic thromboembolic pulmonary hypertension	CTEPH is defined as pre-capillary PH as assessed by right heart catheterization (mean PAP ≥ 25 mmHg, PCWP ≤ 15 mmHg) in the presence of multiple chronic/organized occlusive thrombi/emboli in the elastic pulmonary arteries (main, lobar, segmental, subsegmental) after at least three months of effective anticoagulation

PH: Pulmonary hypertension; CO: Cardiac output; COPD: Chronic obstructive pulmonary disease; DPLD: Diffuse parenchymal lung diseases; PAP: Pulmonary artery pressure.