Group 3: Pulmonary hypertension due to lung diseases and/or hypoxia
In this group, the predominant cause of PH is alveolar hypoxia as a result of either chronic lung disease, impaired control of breathing, or residence at high altitude. However, the precise prevalence of PH in all these conditions remains largely unknown. In the revised classification, the heading has been modified to reinforce the link with the development of PH. A category of lung disease characterized by a mixed obstructive and restrictive pattern was added, including chronic bronchiectasis, cystic fibrosis and the recently described syndrome of combined pulmonary fibrosis and emphysema in which the prevalence of PH is almost 50% [66,67]. In PAH associated with parenchymal lung disease, the increase of pulmonary arterial pressure is usually modest (mean PAP lower than 35 mmHg) [68]. Interestingly, in some patients, increase of PAP is out of proportion and be higher than 35 mmHg [69]. In a retrospective study of 998 patients with chronic obstructive pulmonary disease who underwent RHC, only 1% had severe PH [70]. These patients with more severe PH were characterized by mild-to-moderate airway obstruction, severe hypoxemia, hypocapnia, and a very low diffusing capacity for carbon monoxide.