Imaging of pulmonary hypertension

Pulmonary hypertension (PH) is a rare and complex condition characterised by progressive worsening of pulmonary vascular resistance. The consensus definition of PH is an aggregate of haemodynamic factors that include a mean pulmonary artery pressure of greater than 25mmHg at rest or more than 30mmHg during exercise, normal pulmonary capillary wedge pressure, pulmonary vascular resistance >3 Wood units, and normal or reduced cardiac output. This article will highlight the latest classification of PH, its pathogenesis, multimodality imaging features and the various treatment options. Over the last 35 years, the classification of PH has undergone major reorganisation. For a long time, a simplistic approach was followed, dividing the condition into primary or secondary depending on the presence or absence of risk factors. Subsequent attempts have broadened the classification and cluster disease processes into those with similar pathophysiological mechanism and therapeutic options. The most recent revision to the classification followed the 4th world symposium on PH in Dana point, California held in 2008. This updated classification is listed in table 1. Even though multifactorial causes for PH have been identified and tremendous advances made in the field of therapeutics, the overall prognosis remains poor with approximately 15% mortality within one year. An important exception is chronic thromboembolic disease (CTEPH) for which definitive treatment is available in the form of pulmonary endarterectomy. The factors that determine prognosis include functional class, exercise capacity as measured by six-minute walk (6MW) test or cardiopulmonary exercise test, severity of right heart dysfunction and blood levels of brain natriuretic peptide, an increase in the latter is associated with a worse prognosis. The histopathological changes in all forms of pulmonary arterial hypertension (PAH) share many similar qualitative features but with differences in the disease distribution according to the site of insult. PAH is a panvasculopathy mainly affecting the small pulmonary arteries. General features of pulmonary vascular remodelling include medial hypertrophy, intimal and adventitial proliferation and right ventricular hypertrophy, while plexiform arteriopathy due to focal disruption of the internal elastic lamina and necrotising arteritis form more complex lesions. Recurrent inflammation can be complicated by formation of in-situ thrombosis and dissection of pulmonary arteries. In disease processes that predominantly affect the post capillary segments, the small veins and venules demonstrate the medial hypertrophy and intimal proliferation with extensive and diffuse occlusion in pulmonary veno-occlusive disease (PVOD). Alternatively, a patchy panlobular microvasculopathy can be seen with pulmonary capillary haemangiomatosis (PCH). PVOD and PCH can also be associated with pulmonary haemosiderosis and interstitial oedema. Symptoms of PH are generally insidious with non-specific clinical signs. Electrocardiography may show the presence of right ventricular strain but has low sensitivity (55%) and specificity (70%) to detect significant PH. Therefore, imaging plays a pivotal role in the diagnosis, assessment and management of patients with PH. Generic imaging features of PH include a dilated main pulmonary artery, enlargement of right-sided cardiac chambers, right ventricular hypertrophy, paradoxical septal motion and tricuspid regurgitation. Echocardiography is an important screening as well as a first line diagnostic tool in PH evaluation. This non-invasive test is inexpensive, widely available and can be used as a bedside test without any radiation burden. It provides morphological and functional assessment of right heart chambers and estimation of pulmonary artery pressure using Doppler echocardiography to demonstrate a tricuspid regurgitation jet. Pulsed wave tissue Doppler imaging and three-dimensional (3D) echocardiography can measure other variables such as right ventricular fractional area change (RVFAC), RV myocardial performance index (Tei index) and tricuspid annular plane systolic excursion (TAPSE). Of these, the Tei index assessing both systolic and diastolic function of the RV has been shown to be of prognostic relevance in PH. Another echocardiographic predictor of mortality is the presence of pericardial effusion. In-spite of its many advantages, limitations of echocardiography includes operator dependence and poor acoustic windows. Chest radiography, multi-detector computed tomography (MDCT), magnetic resonance imaging (MRI), ventilationperfusion scintigraphy and pulmonary angiography are other imaging modalities that can be used for diagnosing the aetiology, assessment of disease severity and follow up. The utility of each of these techniques is addressed in the forthcoming section. Right heart catheterisation (RHC) is the gold standard investigation for haemodynamic assessment in PH. Although invasive, it has a good safety profile with a reported proceTABLE 1 Dana point classification of PH. 1 Pulmonary arterial hypertension (PAH)