How I Do It: Imaging Pulmonary Hypertension in Pediatric Patients Using CT Angiography
Pulmonary hypertension is a complex process affecting pulmonary and cardiac functions. It is defined as a pulmonary pressure of more than 30 mm Hg. Its etiologies can be categorized as preload, pulmonary, or afterload pathologies. Preload abnormalities include any processes that may lead to increased pulmonary blood flow, such as left-to-right shunts. Over time, increased flow leads to vasoconstriction and/or cellular proliferation, increasing pulmonary resistance and pressure. Pulmonary etiologies include any intrinsic lung diseases that lead to loss of cellular function and secondary regional changes in pulmonary blood flow and resistance. Afterload processes include cardiovascular diseases that increase left-sided systemic pressures. Progressive elevated systemic pressures lead to increased pulmonary resistance and pressure. Conventional evaluation of pediatric patients with pulmonary hypertension includes chest radiography, unenhanced CT, echocardiography, and right-heart/pulmonary-artery catheterizations. Low-dose CT angiography (CTA) is an alternative option that is being utilized with increased frequency. CTA provides a comprehensive cardiopulmonary evaluation of pediatric patients with pulmonary hypertension. Cardiac morphology is assessed, along with the pulmonary vasculature, thoracic aorta, airway, and lung parenchyma. The goal of the CTA evaluation is help guide therapy while reducing the need invasive catheterization. Even if a right heart catheterization is performed, a robust CTA can reduce fluoroscopy time and minimize patient risk. Therapy may include positive pressure oxygenation, steroids, and direct and indirect vasodilators.
Figure 1.Technically, the pulmonary-hypertension low-dose CTA is relatively easy to perform. Contrast delivery is targeted to the right heart. Thin sections with 80kV and low amperage are then acquired through the chest. Imaging should include the low neck and upper abdomen so that any potential pulmonary collateral flow will be imaged. When interpreting the study, there are five core areas to address. The first is evaluation of the caliber, contour, and patency of the central and peripheral pulmonary arteries. The classic description of the pulmonary arteries in patients with pulmonary hypertension is the presence of enlarged central pulmonary arteries with peripheral pruning. Peripheral pruning reflects a combination of arteriole vasoconstriction and/or cellular proliferation, which is the main source for the pulmonary hypertension. The presence of thickened peripheral pulmonary arteries indicates a component of cellular proliferation. The enlarged central pulmonary arteries reflect physiologic changes of elevated pulmonary pressure with or without volume overload. The ascending thoracic aorta can be used as a reference to help determine the enlargement of the main pulmonary artery. The abnormal pulmonary perfusion leads to mosaic perfusion in the lung parenchyma.
Figure 2.The second area to address is composed of the sequelae of pressure overload on the pulmonary arteries and right heart, with or without volume overload. Right-ventricle myocardial hypertrophy and chamber enlargement are key findings in this instance. The enlarged right ventricle with elevated pressures can cause left-ventricular diastolic dysfunction, evidenced by flattening of the interventricular septum and posterior displacement of the left ventricle. Over time, elevated right-ventricular pressures can lead to tricuspid insufficiency, with enlargement of the right atrium, further enlargement of the right ventricle, and enlargement of the central inferior vena cava and hepatic veins. The third point to evaluate is the presence of intracardiac or extracardiac shunts. Common intracardiac shunts include atrial and septal defects. Extracardiac shunts include anomalous pulmonary venous drainage and a patent ductus venosus. The third area to address is composed of the sequelae of pressure overload on the pulmonary arteries and right heart, with or without volume overload. Right-ventricle myocardial hypertrophy and chamber enlargement are key findings in this instance. The enlarged right ventricle with elevated pressures can cause left-ventricular diastolic dysfunction, evidenced by flattening of the interventricular septum and posterior displacement of the left ventricle. Over time, elevated right-ventricular pressures can lead to tricuspid insufficiency, with enlargement of the right atrium, further enlargement of the right ventricle, and enlargement of the central inferior vena cava and hepatic veins. The fourth area of assessment is the airway (central and peripheral). Airway disease such as tracheobronchomalacia may be present in pediatric patients with pulmonary hypertension as co-morbid pathology with underlying lung disease. Airway disease can lead to poorly aerated lung tissue, with dependent and non-dependent atelectasis and air trapping. The fifth area to address is the lung parenchyma. Evaluation addresses the degree of normal versus abnormal lung. Abnormal findings include chronic changes (cysts, septal lines, parenchymal bands), inflammation (ground glass opacities), mosaic perfusion, atelectasis, and air trapping.
Figure 3.The case pictured is a high resolution, low-dose CT angiogram performed in a two-month-old boy (born at 25 weeks’ gestation). The key findings highlight the comprehensive assessment CTA affords in pediatric patients with pulmonary hypertension. Figure 1 is a minimum intensity projection image showing mild narrowing the proximal right main bronchus with irregularity of the bronchi,, consistent with bronchomalacia. There are associated chronic lung changes of prematurity and regions of atelectasis. The areas of atelectasis can be targeted with positive end-expiratory pressure. A volume-rendered image shows an enlarged main pulmonary artery consistent with pulmonary hypertension (Figure 2, arrow). A four-chamber multiplanar reformation of the heart (Figure 3) demonstrates moderate right-ventricular hypertrophy (arrow) with mild to moderate right-ventricular and atrial enlargements.