Introduction

Pulmonary hypertension (PH) describes the haemodynamic state when the mean pulmonary artery pressure measured during right heart catheterisation at rest is equal to or greater than 25 mmHg .

There are five groups of PH . In group 1 PH the raised pulmonary artery pressures are due to a disorder of the pulmonary arterioles themselves: a vasculopathy termed pulmonary arterial hypertension (PAH). Idiopathic, familial, connective tissue disease (CTD)-associated and congenital heart disease-associated PH have in common this vasculopathy. In addition, there are some much rarer causes of PAH, such as Human Immunodeficiency Virus (HIV) and schistosomiasis. In groups 2, 3 and 4 PH, the raised pulmonary artery pressures are not due to any inherent disorder of the pulmonary vasculature. Rather, left heart disease, lung disease and pulmonary thromboembolism cause PH in these groups respectively. Group 5 PH is caused by miscellaneous diseases where the exact causative mechanisms are poorly understood. Sickle cell disease-associated PH is in the process of moving from group 1 to group 5 due to its multifactorial nature, with anaemia and high cardiac output being important contributors to the raised pulmonary artery pressures.

PAH is largely due to three causes of roughly equivalent prevalence: idiopathic and familial disease, congenital heart disease and CTD. Of the CTDs, systemic sclerosis (SSc) is responsible for about 75% of cases and scleroderma-spectrum diseases including undifferentiated CTD and mixed CTD (MCTD) make up about a further 10%. Systemic lupus erythematosus (SLE) accounts for no more than about 10% of cases, but these rare patients are fascinating and – as we shall see – do seem to be clinically distinct from patients with SSc-associated PAH (SSc-PAH).