Once it has been determined that a patient is at low, moderate, or high risk for pulmonary embolism, which tests should be ordered and how should they be interpreted?
The first part of this two-part “Point-of-Care Guide”1 discusses how to use two validated clinical decision rules to determine the likelihood of pulmonary embolism. The Wells decision rule uses only the history and physical examination,2 whereas the Wicki decision rule also requires the results of arterial blood gas measurements and a chest radiograph.3
An experienced clinician may make a reasonably accurate estimate of the likelihood of pulmonary embolism without using any clinical decision rule.3,4 Furthermore, if the risk assessment based on a clinical decision rule differs from the clinician's instinctual clinical assessment, it seems prudent to rely on the assessment that places the patient in the highest risk group. For example, if the Wells rule places a patient in the low-risk group but the clinician has a higher index of suspicion based on a global assessment of the patient, the patient should be treated as moderate risk. The second part of this guide discusses how to use this clinical information.
One strength of an evidence-based approach is that it tailors the diagnostic strategy to the patient. A “one size fits all” approach may overinvestigate patients who are at low risk and miss disease in patients who are at high risk. In contrast, an evidence-based approach uses the information from the clinical evaluation to guide the selection of tests and their interpretation. Several groups have developed and validated protocols for the diagnosis of pulmonary embolism that rely on the clinical assessment, d-dimer test, ventilation-perfusion (V/Q) scanning, ultrasonography of the proximal leg veins, and helical computed tomographic (CT) scanning.2,4–7
The recommended protocol in this Point-of-Care Guide is based on a validated protocol that was developed by Wells and associates2 and modified by Kearon6 to add the option of helical CT scanning instead of V/Q scanning. The protocol is intended to be used in the evaluation of patients in primary care and emergency department settings. Unlike the protocols proposed by Mussett and colleagues7 and Perrier and coworkers,4 this protocol does not require angiography, except as an option in a small percentage of patients with indeterminate findings. The protocol is based on the finding that patients with a low clinical probability and a negative noninvasive test almost certainly do not have pulmonary embolism, whereas patients with a high clinical probability and a positive noninvasive test almost certainly have pulmonary embolism. Patients with an intermediate clinical probability or an indeterminate noninvasive test require further testing and closer follow-up.
It is also important for clinicians to understand that many of the tests are “asymmetric,” in that they are helpful for ruling in disease when results are positive or ruling out disease when results are negative—but not both. For example, the d-dimer test is quite sensitive: when this test is negative in a low-risk patient, it is very good at ruling out pulmonary embolism; however, when the test is positive, it does not rule in the diagnosis, and further confirmatory testing is required. Conversely, helical CT scanning is very good at ruling in disease when intraluminal filling defects in segmental or larger pulmonary arteries are seen, but is not helpful when the findings are normal or indeterminate. Patients with non-diagnostic V/Q or helical CT scans require ultrasonography of the leg veins to help diagnose or exclude pulmonary embolism.
The protocol that is included in the accompanying evidence-based patient encounter form applies to adult patients presenting with new or worsening shortness of breath or chest pain in the emergency department or outpatient setting. Patients with symptoms for more than 30 days, patients with no symptoms for three days before presentation, patients with recent anticoagulation, inpatients, pregnant patients, and patients with suspected thrombosis of an upper extremity vein were excluded from the validation studies and are not appropriate for evaluation using this protocol.2 Patients who are critically ill and patients with limited cardiovascular reserve may require more extensive evaluation, because the consequences of a small missed pulmonary embolism are greater in these groups. The d-dimer test is much less specific in hospitalized patients, and V/Q scanning often is not diagnostic in patients with chronic pulmonary disease.6
Applying the Evidence:
Mr. Smith is a 62-year-old man who complains of increasing shortness of breath over the past 24 hours. He has no swelling of the legs, no pain in the calves on palpation, and no cough, fever, or other symptoms consistent with an alternate diagnosis such as pneumonia. He denies hemoptysis and has no history of malignancy or previous venous thromboembolism. However, he recently drove eight hours with his grandchildren. His heart rate is 104 beats per minute. Based on his clinical risk assessment, he scores 4.5 points using the Wells model (no alternative diagnosis and heart rate) and therefore has a moderate probability of pulmonary embolism (approximately 16 percent).
Mr. Smith's physician sent him to the hospital for an immediated-dimer test and helical CT scanning. The d-dimer test was negative, but because of the patient's intermediate clinical risk, the test result did not exclude pulmonary embolism. Helical CT scanning revealed an intraluminal filling defect in the segmental arteries, which confirmed the diagnosis of pulmonary embolism. Had the helical CT scan shown only a subsegmental intraluminal filling defect, further testing (beginning with ultrasonography of the leg veins) would have been indicated to help establish the diagnosis.