The most recent WHO classification system (with adaptations from the more recent ESC/ERS guidelines shown in italics) can be summarized as follows:WHO Group I – Pulmonary arterial hypertension (PAH)

Idiopathic
Heritable (BMPR2, ALK1, SMAD9, caveolin 1, KCNK3 mutations)
Drug- and toxin-induced (e.g., methamphetamine, amphetamine, or cocaine use )
Associated conditions:Connective tissue disease, HIV infection, Portal hypertension, Congenital heart diseases, SchistosomiasisWHO Group I – Pulmonary veno-occlusive disease (PVOD), pulmonary capillary hemangiomatosis (PCH)

Idiopathic
Heritable (EIF2AK4 mutations)
Drugs, toxins and radiation-induced
Associated conditions:connective tissue disease, HIV infectionWHO Group I" – Persistent pulmonary hypertension of the newborn
WHO Group II – Pulmonary hypertension secondary to left heart disease

Left ventricular systolic dysfunction
Left ventricular diastolic dysfunction
Valvular heart disease
Congenital/acquired left heart inflow/outflow tract obstruction and congenital cardiomyopathy
Congenital/acquired pulmonary venous stenosisWHO Group III – Pulmonary hypertension due to lung disease, chronic hypoxia

Chronic obstructive pulmonary disease (COPD)
Interstitial lung disease
Mixed restrictive and obstructive pattern pulmonary diseases
Sleep-disordered breathing
Alveolar hypoventilation disorders
Chronic exposure to high altitude
Developmental abnormalitiesWHO Group IV – chronic arterial obstruction

Chronic thromboembolic pulmonary hypertension (CTEPH)
Other pulmonary artery obstructions
Angiosarcoma or other tumor within the blood vessels
Arteritis
Congenital pulmonary artery stenosis
Parasitic infection (hydatidosis)WHO Group V – Pulmonary hypertension with unclear or multifactorial mechanisms

Hematologic diseases: chronic hemolytic anemia (including sickle cell disease)
Systemic diseases: sarcoidosis, pulmonary Langerhans cell histiocytosis: lymphangioleiomyomatosis, neurofibromatosis, vasculitis
Metabolic disorders: glycogen storage disease, Gaucher disease, thyroid diseases
Others: pulmonary tumoral thrombotic microangiopathy, fibrosing mediastinitis, chronic kidney failure, segmental pulmonary hypertension (pulmonary hypertension restricted to one or more lobes of the lungs)

Signs and symptoms
The symptoms of pulmonary hypertension include the following:

Less common signs/symptoms include non-productive cough and exercise-induced nausea and vomiting.
As such, the right ventricle cannot cope as well with higher pressures, and although right ventricular adaptations (hypertrophy and increased contractility of the heart muscle) initially help to preserve stroke volume, ultimately these compensatory mechanisms are insufficient; the right ventricular muscle cannot get enough oxygen to meet its needs and right heart failure follows. As the blood flowing through the lungs decreases, the left side of the heart receives less blood. This blood may also carry less oxygen than normal. Therefore, it becomes harder and harder for the left side of the heart to pump to supply sufficient oxygen to the rest of the body, especially during physical activity. During the end-systolic volume phase of the cardiac cycle, the Gaussian curvature and the mean curvature of right ventricular endocardial wall of PH patients was found to be significantly different as compared to controls.In PVOD (WHO Group I), pulmonary blood vessel narrowing occurs preferentially (though not exclusively) in post-capillary venous blood vessels. PVOD shares several characteristics with PAH, but there are also some important differences, for example differences in prognosis and response to medical therapy.Persistent pulmonary hypertension of the newborn occurs when the circulatory system of a newborn baby fails to adapt to life outside the womb; it is characterized by high resistance to blood flow through the lungs, right-to-left cardiac shunting and severe hypoxemia.Pathogenesis in pulmonary hypertension due to left heart disease (WHO Group II) is completely different in that constriction or damage to the pulmonary blood vessels is not the issue.