Transcript Related Guidelines

Imaging Studies in the Evaluation of PAH

Jeffrey S. Sager, MD, MSc · University of Southern California

Disclosures

May 12, 2021

Key Takeaways:

  • Images in pulmonary arterial hypertension (PAH) are used to support or refute its presence, determine etiology, and evaluate the prognosis and response to therapies 

  • Noninvasive imaging is used before proceeding with right heart catheterization (RHC) to determine if the patient’s clinical presentation supports the presence of pulmonary hypertension (PH)

  • PH can be evaluated with a multitude of imaging techniques, including echocardiogram, chest X-rays, computed tomography (CT) scans, and ventilation-perfusion (V/Q) scintigraphy. Novel imaging techniques, such as single-photon emission computed tomography (SPECT) and dual-emission CT, continue to be investigated

This transcript has been edited for clarity.   

Pulmonary hypertension results from a diverse array of complex conditions resulting in elevated pulmonary pressures found in the pulmonary vasculature. The World Symposium on Pulmonary Hypertension classifies pulmonary hypertension into five groups, according to disease manifestation, pathophysiology, hemodynamic changes, and therapeutic approaches. Group one, [is] also known as pulmonary arterial hypertension; group 2, pulmonary hypertension due to left heart disease; group 3, pulmonary hypertension due to lung disease; group 4, chronic thromboembolic pulmonary hypertension, or CTEPH; and group 5, a miscellaneous group of pulmonary hypertension with unclear or multifactorial mechanisms. 

We define pulmonary hypertension by measuring the pulmonary pressures using a right heart catheterization tool. This is the gold standard for diagnosis of pulmonary hypertension. The diagnosis of pulmonary hypertension is based on a comprehensive evaluation of clinical symptoms, physical examination, and imaging modalities. Unfortunately, diagnosis of pulmonary hypertension, and thus its treatment, is often delayed, as clinical symptoms overlap with more prevalent respiratory and cardiovascular diseases. 

Familiarity with imaging techniques, knowledge of key findings, and awareness of nuances of the imaging studies in pulmonary hypertension are imperative, since they have the potential for accelerating diagnosis and initiation of pulmonary hypertension–specific therapies, an approach that is likely to improve the prognosis of this deadly disease. We use images in pulmonary arterial hypertension for three main purposes: One is to help support the presence of pulmonary hypertension; another reason is to determine the possible etiology of why somebody would have elevated pulmonary pressures; and a third reason is to evaluate the prognosis and response to therapies that may be implemented for pulmonary hypertension. 

We typically use noninvasive imaging before proceeding with right heart catheterization to help figure out whether the patient’s clinical presentation supports the presence of pulmonary hypertension. Once we have a clinical suspicion, we then proceed with right heart catheterization. A number of imaging tools are utilized, including echocardiogram, chest X-ray, computed tomography, and ventilation and perfusion imaging. Additional imaging, such as cardiac MRI, and newer tools, such as dual-energy CT and single-photon emission computed tomography—or SPECT—scan have also emerged. Their places in the diagnostic evaluation of pulmonary hypertension today remain to be determined. In general, each imaging technique provides incremental information with varying degrees of sensitivity and specificity, which helps us fix the presence and identify the etiology of pulmonary hypertension. 

To start with the echocardiogram, the transthoracic Doppler echocardiogram is typically performed as the initial workup in a patient with suspected pulmonary hypertension. Echocardiogram is advantageous in that it is widely available, it’s inexpensive, and it’s a safe test. Echo can estimate the systolic pulmonary arterial pressure from the peak tricuspid regurgitant jet velocity, and it can give estimates of right ventricular size and function with significant limitations. The echo can assist IVC distensibility, an important marker of volume overload. Echocardiogram is critical in determining the most common causes of pulmonary hypertension in the United States, namely pulmonary hypertension due to left heart disease, or group 2 pulmonary hypertension. 

Next would be a chest X-ray. A chest X-ray is usually obtained as a baseline test. A normal chest X-ray does not exclude pulmonary hypertension. However, a chest X-ray is commonly abnormal in established disease. Chest X-ray findings that support the presence of pulmonary arterial hypertension include cardiomegaly due to right atrial enlargement and right ventricular enlargement. This can be seen by reduced retrosternal air spacing on chest X-ray. Other important findings include a dilation of the central pulmonary arteries and pruning of the peripheral blood vessels. In addition to vascular findings related to pulmonary hypertension, chest X-ray can be helpful in narrowing the differential diagnosis of its etiology. Chest X-ray can be helpful in supporting the diagnosis of left heart disease—for example, patients who present with pulmonary edema or bilateral pleural effusions. 

Chest X-ray could be helpful in evaluating lung parenchymal diseases such as interstitial lung disease and emphysema. Signs of pulmonary embolism, such as Westermark sign or Hampton’s hump, may denote the need for further workup for chronic thromboembolic pulmonary hypertension, since approximately 2% to 3% of acute pulmonary emboli lead to chronic thromboembolic pulmonary hypertension. 

While chest X-ray can help steer the future diagnostic path, it suffers from significant limitations. Some of the major limitations are the nonspecific nature of the findings and lack of correlation to disease severity with the extent of radiographic abnormalities. Thus, a normal chest X-ray does not exclude pulmonary hypertension. 

The next modality that is commonly used is a CT scan. CT scans have gained acceptance as one of the frontline tests for the evaluation of pulmonary hypertension. Far-standing excellent spatial and temporal resolution and the ability to comprehensively evaluate the cardiopulmonary structures are some of the distinct advantages that CT scan offers. 

A major role of CT scan is the assessment of the cause of the underlying pulmonary hypertension. CT scan allows comprehensive evaluation of the pulmonary vasculature and lung parenchyma. When evaluating the lung parenchyma, the CT scan can detect emphysema. It can also detect pulmonary fibrosis. The CT scan can reveal abnormalities in the lung parenchyma of patients with pulmonary hypertension. Chronic progressive processes such as emphysema and idiopathic pulmonary fibrosis have characteristic CT findings. Centrilobular nodules, ground glass opacities, smooth thickening of the interlobular septa, enlarged mediastinum lymph nodes, and pleural effusions can be seen in a rare form of pulmonary hypertension known as pulmonary veno-occlusive disease. Additionally, a dilated esophagus may suggest scleroderma and help point us in the direction of the cause of the pulmonary arterial hypertension. Evaluation of a dilated pulmonary artery and reduced pulmonary artery distensibility, which is the percent change in cross-sectional area between diastole and systole of the pulmonary artery measured by a certain type of CT pulmonary angiogram, are markers of pulmonary hypertension. 

A ratio of CT-measured main pulmonary artery transverse diameter, comparing it to the ascending aorta diameter, that is greater than one, is highly indicative of pulmonary hypertension, with 96% positive predictive value. Several studies have suggested a strong association between the ratio of the main pulmonary artery to ascending aorta diameter and its correlation to the pulmonary pressures. An enlarged pulmonary artery can compress the left recurrent laryngeal nerve, resulting in a cardiovocal syndrome known as Ortner's syndrome, characterized by vocal cord palsy and hoarseness. 

CT pulmonary angiogram is also an important assessment of patients who have chronic thromboembolic pulmonary hypertension who are being considered for surgical candidacy. In CTEPH patients, the ventilation-perfusion study is still considered the initial imaging test of choice. However, once the diagnosis of CTEPH is suggested, CT pulmonary angiogram is commonly used to assess the extent of disease and the secondary cardiopulmonary changes. 

CT pulmonary angiogram can also identify distal obstructions, narrowing of the pulmonary arteries and their branches, and distal stenosis. Other findings include patch-like defects, intimal irregularities, pulmonary artery webs or bands, as well as enlargement of the bronchial arteries due to collateral blood supply. The presence of bronchial artery collaterals also has prognostic significance in patients with chronic thromboembolic pulmonary hypertension, since these patients tend to do better postoperatively. 

Like all imaging modalities, CT scan is not without limitations. Pulmonary artery size on CT angiogram is a poor predictor of pulmonary hypertension. CT imaging also provides limited hemodynamic information. It also exposes patients to radiation, which is especially concerning in patients who are undergoing serial examinations. 

The next important imaging modality is the V/Q scan, or the lung-ventilation perfusion scan. The V/Q scan is the imaging modality of choice for screening patients with suspected chronic thromboembolic pulmonary hypertension because of its high sensitivity. Imaging in chronic thromboembolic pulmonary hypertension reveals wedge-shaped perfusion defects with normal ventilation. Meanwhile, complete absence of perfusion to one lung may indicate other conditions, such as a malignancy, vasculitis, and fibrosing mediastinitis. 

A normal or low probability V/Q scan effectively excludes chronic thromboembolic pulmonary hypertension, with a sensitivity of approximately 90% to 98% and a specificity of 94% to 98%, compared to CT pulmonary angiography, which yields the sensitivity of 50% to 98% with similar specificity.[1] In the adequate setting, the presence of one or more segmental mismatched perfusion defects warrants further investigation, and we recommend patients undergo pulmonary angiogram. Limitations of the V/Q scan include its potential for underestimating the extent of central vascular obstruction. 

Additionally, despite the high sensitivity of the V/Q scan in detecting chronic thromboembolic pulmonary hypertension and the recommendation to be obtained as part of the diagnostic workup for all patients with pulmonary hypertension, it is, unfortunately, underutilized. High-resolution CT imaging of the chest is often used instead, in part because it is more readily available. Analysis of pulmonary hypertension registries reveals that 43% of patients diagnosed with pulmonary arterial hypertension never receive a V/Q scan.[2] 

The last imaging [modality] we will talk about is the cardiac MRI. While cardiac MRI remains underutilized, it may overcome some of the major limitations of echocardiogram in the assessment of a right ventricular function. A wide range of sequences can provide comprehensive structural and functional information in these patients. 

Cardiac magnetic resonance imaging is safe and does not expose patients to ionizing radiation. However, it is expensive, not widely available, and requires operated expertise. Patient claustrophobia can sometimes present difficulty with acquiring the test. 

So, to conclude, pulmonary hypertension can be evaluated with a multitude of imaging techniques. Chest X-rays are commonly used in the initial evaluation of pulmonary hypertension and can provide information regarding changes in cardiac morphology, underlying lung disease, and loss of peripheral blood vessels. CT pulmonary angiogram evaluates vascular structures, cardiac chambers, and lung parenchyma. The V/Q scan is most useful in identifying chronic thromboembolic pulmonary hypertension patients, highlighting unmatched perfusion defects. The cardiac MRI provides useful information of RV and LV anatomy and function. The novel imaging techniques, such as the SPECT and the dual-emission CT, continue to be investigated, and their role in the diagnostic evaluation of pulmonary hypertension remains to be elucidated.

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