Pulmonary hypertension is said to occur when the mean pulmonary artery pressure exceeds 25 mmHg at rest or 30 mmHg with exercise. The term pulmonary arterial hypertension (PAH) denotes a series of apparently unrelated disorders that share the histopathological entity known as plexogenic pulmonary arteriopathy (PPA).
There are many conditions that can lead to the development of pulmonary hypertension and these have traditionally been classified into five groups (World Conference on Pulmonary Hypertension, Venice 2003; Dana Point meeting on Pulmonary Hypertension 2008).
Those disorders listed in Group 1 share the histopathology of PPA although why such apparently diverse disorders should do so remains unclear. Examples of Group 1 disorders include idiopathic PAH, PAH associated with genetic factors (e.g bone morphogenetic protein receptor 2 mutations), connective tissue disorders (e.g systemic lupus erythematosis), portal hypertension, HIV and anorexigens, congenital heart diseases, pulmonary veno occlusive disease and pulmonary capillary haemangiosis. Advanced pulmonary vasodilator therapy is given to patients who have pulmonary hypertension associated with these conditions.
Group 2 disorders comprise left heart disorders e.g valvular disease and atrial or ventricular dysfunction while Group 3 diseases reflect pulmonary hypertension in association with lung disease or hypoxemia e.g chronic obstructive pulmonary disease (COPD), interstitial lung disease or sleep disordered breathing.
Pulmonary hypertension associated with chronic pulmonary thremboembolism is included in Group 4 and Group 5 and comprises miscellaneous conditions with unclear or multifactorial aetiologies such as histiocytosis, lymphangioleiomyomatosis, glycogen storage disease, Gaucher disease and post splenectomy.