The mean pressure in the pulmonary artery (mPAP) in a normal resting adult is ∼16 mmHg. Pulmonary hypertension (PH) is defined as a mPAP exceeding 25 mmHg at rest. The increased PAP can be due to a rise in pulmonary vascular resistance (PVR), increased pulmonary blood flow due to a systemic to pulmonary shunt (Eisenmenger’s syndrome; see Chapter 55) or back pressure from the left heart. PH increases right ventricular afterload, eventually leading to right heart failure.

Types of Pulmonary Hypertension

PH was initially (in 1973) classified as primary if it was idiopathic (without a known cause) and secondary if a cause could be identified. More complex classification schemes designed to group the various manifestations of PH according to their pathological and/or clinical features and management options were then created in 1998, 2003, and most recently at the 4th World Symposium on PH held in Dana Point in 2008 (Figure 52.1, top). Together, the various forms of PH affect ∼100 million people worldwide.

Group 1 PH, also termed pulmonary arterial hypertension (PAH) comprises heritable (hPAH) and idiopathic PAH (iPAH) and also PH associated with a number of other conditions (aPAH). Patients demonstrate a clinical syndrome indicative of severe PH and an increased PVR associated with a unique set of pulmonary vascular abnormalities (see below). Both hPAH and iPAH are characterized by a decreased expression of bone morphogenetic protein receptor type 2 (BMPR2) which, usually in hPAH and sometimes in iPAH, is associated with mutations in BMPR2, its cognate gene. Group 1′ PH, associated with pulmonary veno-occlusive disease and pulmonary capillary haemangiomatosis, has features resembling those of PAH separate clinical entity.

Groups 2–5 are forms of secondary PH. Group 2 PH is due to left heart disease, chiefly ventricular failure or mitral and/or aortic valve disease, which results in increased left atrial pressure that backs up into the pulmonary artery. Group 3 PH is associated with lung diseases such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis and other conditions such as sleep apnoea, the common factor being the presence of alveolar hypoxia. Group 4 PH is associated with chronic thromboembolic disease, with a persistent blockage of pulmonary arteries arising from venous thromboembolism. Group 5

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