Perspectives on the Evolution of Diagnosis and Management of Acute Pulmonary Embolism

Commentary

In this issue of the Canadian Association of Radiologists’ Journal, Nguyen et al. present a two-part guidance document on the imaging assessment of APE. In Part 1, CTPA protocol optimization, safety measures, and alternative diagnostic imaging modalities are reviewed. ¹ For the diagnostic radiologist frequently confronted with requests for CTPA examinations, Part 1 provides important information on the “when” and “how” questions of APE evaluation. In following the recommendations presented in Part 1, diagnostic radiologists will be well positioned to direct the acquisition of imaging examinations that can be used to confirm a diagnosis of APE. Part 2 focuses on interpretation pitfalls in the evaluation of CTPA. ² This information takes the diagnostic radiologist beyond the basics of CTPA interpretation and allows for the identification of technically limited examinations, frequently encountered artifacts, and differential diagnoses. Importantly, understanding the information presented in Part 2 will help prevent overdiagnosis of APE on CTPA examinations, a common problem which can lead to unnecessary or incorrect treatment.

The information provided in this guidance document will be best applied when considered in the context of recent developments in the description and management of patients with APE. In the past decade, considerable effort has been directed at risk stratifying patients presenting with acute PE. Both the American Heart Association (AHA) and European Society of Cardiology (ESC) have presented schemes for describing patients with APE. The most severe category, massive (AHA) or high risk (ESC) APE, includes patients presenting with systemic hypotension or need for vasopressor support. This uncommon group of patients are at the highest risk for death and have an average mortality of approximately 30% within 1 month. ³

The second category, submassive (AHA) or intermediate risk (ESC) APE, is characterized by patients presenting with evidence of right ventricular (RV) strain without systemic hypotension. Diagnostic radiologists have an important role in identifying these patients, as CTPA examinations allow for an assessment of RV strain that can complement or replace assessment by echocardiography. The most common and best studied CTPA imaging finding is an RV/left ventricular (LV) ratio of >0.9. ⁴ This ratio is best determined on reconstructed four-chamber long axis images with simple measurements of cavity size. There are other supportive CTPA imaging features of RV strain which include deviation of the intraventricular septum to the left, dilatation of the base of the right ventricle with preserved contractility of the apex (CT equivalent of McConnell’s sign), and the distension of venous collateral pathways. In addition to imaging features, RV strain can also be determined via increases in cardiac biomarkers such as brain natriuretic peptide and troponins. Patients with submassive/intermediate risk APE account for 35-55% of hospitalized patients with APE and have short term mortality rates of 2-15% despite treatment with anticoagulation. ³

Patients with APE who do not have systemic hypotension or evidence of RV strain fall into the third category of low-risk APE (AHA and ESC). Patients in this group account for 40-60% of hospitalized APE patients and carry an average one month mortality of approximately 1%. ³ It is important to note that categorization of patients into these groups is not based on clot burden. While it may be tempting to describe patients with large central pulmonary emboli as having “massive” APE, this is not recommended as patients with normal cardiopulmonary reserve can absorb large amounts of APE without developing RV strain.

The importance of stratifying patients into these groups goes beyond prognosis and has implications for placement and management. Patients with massive and submassive APE will require hospitalization, and those with massive APE or submassive APE with a perceived high risk for decompensation will typically be placed in intensive care units. For patients with low-risk APE, inpatient or outpatient management is determined by patient specific factors. In addition, while systemic anticoagulation is the cornerstone of treatment for patients without contraindications, more aggressive therapies including systemic thrombolysis, surgical embolectomy, and endovascular techniques such as catheter directed fibrinolysis, ultrasound-assisted catheter directed thrombolysis, and percutaneous thrombectomy have advanced in the last several decades. These more aggressive therapies carry life threatening risks including massive hemorrhage and acute hemodynamic or respiratory decompensation. ³

Given improved risk stratification and available therapies, management of patients with APE has become increasingly complex. This has led to the development of APE response teams (PERTs). The PERT model shares similarities with other rapid response teams (stroke, coronary, trauma) in that it offers a multidisciplinary approach bringing together clinicians with different areas of expertise with the goal of real-time review of massive/high-risk and submassive/intermediate-risk APE patients. PERTs have the potential for several benefits, including more rapid and appropriate use of aggressive therapies in patients with massive/high-risk APE, a group which data has suggested is currently undertreated. For patients with submassive/intermediate-risk APE, PERTs can help weigh the risks and benefits of aggressive therapies which may be of value in those patients with a perceived high risk of decompensation. Input from diagnostic radiologists is important for PERTs as data from CTPA is integral to patient management decisions. In addition to determining whether there are findings of RV strain, CTPA examinations can also be used to accurately describe the anatomic distribution of the clot burden, features of which can be used to determine which aggressive therapy will be of best benefit. For example, patients with large central saddle APE may be best served with percutaneous thrombectomy while those with segmental emboli will be better treated with systemic or catheter directed thrombolysis. Another important CTPA finding is the presence of a clot in the inferior vena cava, right atrium, or right ventricle (clot-in-transit). When present, a clot-in-transit carries a 5-fold increase in death and may support a more aggressive approach to thrombus removal. ³

At present, the management of APE remains in evolution with several questions requiring further investigation. These include increased safety and efficacy data regarding more aggressive therapies as well as the determination of imaging and clinical findings that are predictive of rapid decompensation in submassive/intermediate-risk patients. As work in this field progresses, more refined diagnostic and treatment recommendations are to be expected. Diagnostic radiologists will need to be aware of these new advances to continue to contribute to the best possible care of patients with APE.