Introduction

The pulmonary circulation receives the entire cardiac output and its vessels are uniquely structured to provide a low resistance system to accommodate the high flow. Thus the vessels are thin walled, comparatively wide, and acutely sensitive to raised pressure. The pulmonary arterial pressure for a normal resting adult at sea level is 14 ± 3 mmHg (1 mmHg ≈ 133.3 Pa) (mean ± standard error of the mean) and so pulmonary hypertension is defined as a level that would be expected to occur in fewer than 1% of a normal population; 25 mmHg or above at rest [1, 2]. This allows a clinical diagnosis to be made. At present, in line with the National Institutes of Health (NIH) Registry, exclusion of possible causative conditions, such as cardiac, respiratory, thromboembolic or connective tissue diseases, allows a diagnosis of idiopathic (or primary) rather than secondary pulmonary hypertension. For secondary disease, the predisposing condition is usually evident and patients can be monitored for possible hypertensive changes. The idiopathic form is insidious and, when it does finally present clinically, it is with nonspecific symptoms of dyspnoea and fatigue, so diagnosis is delayed, on average by two years [2]. The principal pathological feature of chronic pulmonary hypertension is pulmonary vascular remodelling, where both the cellular and extracellular components of the arteries undergo dramatic reorganization.

Our knowledge of the pathogenesis of pulmonary hypertension is limited and, consequently, despite major advances in recent years, pharmacological therapy is not always successful. For primary disease in particular, there is no real cure and lung or heart–lung transplantation is often the only option for survival [3].