PH is categorized into 5 groups by the World Health Organization WHO:
WHO Group 1: Pulmonary arterial hypertension
WHO Group 2: PH from left heart disease
WHO Group 3: PH from chronic hypoxic lung disease
WHO Group 4: PH from chronic thromboembolic lung disease
WHO Group 5: PH from unclear multifactorial mechanisms
(Rose-Jones 2015)