Quantitative Assessment of RV Strain Using cMRI Following Catheter Intervention on PE

Overview

Acute pulmonary embolism (PE) is a condition in which the vessels carrying blood to the lungs become suddenly blocked, usually by a blood clot. There are a number of adverse consequences that result, with one of the most significant being strain on the right side of the heart (which must push blood through the blocked arteries to the lungs). Although this strain on the right heart is very important, current methods for measuring it are flawed. The standard practice is to obtain an echocardiogram (ultrasound of the heart), from which indirect measurements of the size of the heart are used to make inferences about right heart strain. This method can help guide management in some patients, but it in not a sensitive test and does not provide detailed information. Patients with PE are treated with blood thinning medications. Some patients may be referred to the Interventional Radiology (IR) team for endovascular intervention, in which catheters are placed into the patient's vessels under radiologic guidance and advanced to the lungs to remove the clot entirely. Cardiac magnetic resonance imaging (MRI) is a well-established imaging technique that produces highly detailed images of the heart's structure and function, with no risks to patients of ionizing radiation or intravenous contrast. Cardiac MRI is far superior to echocardiogram in evaluation of the right side of the heart, however it has not been widely used in the evaluation of patients with PE. We propose that by using a fast MRI protocol, we will be able to detect right heart strain with more accuracy than echocardiogram. Furthermore, we hypothesize that MRI images obtained before and after IR catheter-directed therapy will demonstrate the degree to which strain is relieved with this treatment. Finally, we believe that using MRI may help to guide management of patients with PE by detecting early or mild heart strain before it progresses. In order to test these hypotheses, we plan to image PE patients who have been referred to the IR team with MRI. Patients recruited for this study will undergo two short MRI scans - one immediately before treatment, and one after completion of IR treatment (which lasts approximately 12-24 hours).

Pulmonary embolism occurs when embolic material (usually clot in the venous system) travels to the heart and lodges in the pulmonary vasculature. This obstruction, if significant, can lead to pulmonary arterial hypertension, which places increased demand on the right ventricle (RV), which must pump against the blockage. This can manifest as right ventricular dysfunction (RVD), which is characterized by dilatation of the RV, wall motion abnormalities and other structural and functional changes. There is a significant body of data demonstrating that function of the RV is an important prognostic indicator in patients with acute PE. In these studies, RVD allowed identification of patients who, although clinically stable on presentation, were at risk for hemodynamic instability and high morbidity/mortality. As a result, many authors have suggested that RVD represents an important indication for more aggressive therapy in patients with submassive PE. Currently, echocardiography is the most commonly used method for evaluating the RV in acute PE. However, obtaining quality imaging of the RV using echocardiography is technically difficult, and determining a reliable indicator of RV function has proved challenging. A number of markers of right ventricular function have been described, including size criteria, ejection fraction, wall motion, tricuspid regurgitation, paradoxical septal motion, and others. However, there is no general consensus on which methodology or measurements produce the most clinically meaningful data. The American Heart Association guidelines for submassive pulmonary embolism use the ratio of the right ventricle to left ventricle at end diastole (RV:LV ratio), which is defined as greater than 0.9 in patients with RVD. However, there is a large degree of heterogeneity in echocardiographic criteria for RVD used in the literature. Additionally, the quantitative data provided by echocardiography is suspect, as they show only modest correlation with cardiac magnetic resonance imaging (cMRI) or computed tomography (CT) in evaluation of the RV. Even when specifically utilizing the American Society of Echocardiography guidelines, echocardiography proves to be significantly less accurate than cMRI for evaluation of the RV, especially in patients with a dilated ventricle. Cardiac magnetic resonance imaging is considered the reference standard for accurate evaluation of ventricular structure and function. It has proven its accuracy, reliability, and prognostic value in the setting of other pathologies that result in RV dysfunction, such as pulmonary hypertension. However, to date, this modality has not been used to provide detailed information about the structure and function of the RV in patients with acute PE. Recently, new therapies for acute PE have come into use that allow more precise treatment of the embolus itself through the use of endovascular catheters. These catheter-directed therapies (CDTs) deliver thrombolytic medication or other treatment strategies directly to the pulmonary circulation. There is a strong body of evidence supporting the use of these therapies, and some devices have been FDA-approved for this indication. Despite this, there is a lack of definitive markers for the patients who would benefit most from this therapy. Additionally, techniques currently used to attempt to quantify response to therapy (such as RV:LV ratio) are not ideal. There remains a need for a quantitative method for evaluating the structure and function of the RV in patients with acute PE in order to determine their risk for hemodynamic compromise, the need for CDT, and response after therapy is completed.