Making Physiologic Sense of the Diagnosis of Pulmonary Embolism: Potential for a
Pulmonary embolism (pe), a potentially fatal disease that is all too common, is one of the most difficult conditions to diagnose in the ED. The clinical diagnosis of PE is unreliable as demonstrated in numerous studies. Studies of PE in ambulatory patients, such as those who present to EDs, have been lacking. The traditional method of diagnosis by imaging techniques involves extensive use of ancillary personnel and consultants, and the patient is out of the control of the ED for a prolonged period of time. It would be most useful if a test could be developed that could be performed at the bedside, or at least while the patient remains in the ED.
Kline and colleagues propose a novel approach to the diagnosis of PE using the combination of alveolar dead space and plasma D-dimer as a screen to exclude acute PE in ambulatory patients. Obstruction of blood flow to normally ventilated alveoli increases alveolar dead space. In this study, the alveolar dead space fraction was determined by a simple equation using peak end-tidal CO2, measured at the bedside by capnometer, and the arterial PCO2 from a blood gas. Plasma D-dimer has been shown to be a sensitive test for deep venous thrombosis and PE but lacks specificity for PE. Thus, combining the two tests should retain sensitivity but should confer higher specificity than either alone.
Kline et al studied 170 ambulatory patients presenting to the ED with a suspected diagnosis of PE. PE was excluded in 144 and confirmed in 26, based on diagnostic imaging, autopsy, and follow-up. For every patient with a confirmed PE, at least one of the tests was abnormal. In other words, there were no false-negatives when both tests were considered togetherfor a sensitivity of 100%. In patients without PE, there were frequent false-positives for either test. When both tests were positive, PE was confirmed in 19 cases (sensitivity = 73%; positive predictive value = 86%). Kline et al also compared the diagnostic accuracy of the alveolar dead space index compared with the commonly used A-a gradient and found it to be significantly better.
They conclude that, in symptomatic ambulatory patients, when the alveolar dead space fraction is normal with a normal plasma D-dimer, further testing for PE may not be necessary. When either test is positive, the diagnosis should be pursued with further testing. (Kline JA, et al. Use of the alveolar dead space fraction [Vd/Vt] and plasma D-dimers to exclude acute pulmonary embolism in ambulatory patients. Acad Emerg Med 1997;4:856-863.)
COMMENT BY JEFFREY W. RUNGE, MD
The value of a diagnostic screening test rises dramatically with the severity of a disease. PE is a diagnosis that emergency physicians simply cannot afford to miss, being a treatable disease with dire consequences. Unfortunately, excluding the diagnosis at this point requires expensive ancillary testing. In response to that problem, Kline et al propose a solution that can be performed at the bedside by emergency physicians or respiratory therapists coupled with a rapid qualitative laboratory test. If reliable, such an approach could save the significant time and money required to evaluate patients who present with clinical suspicion for PE. It would also save the lives of patients with an atypical presentation in which the diagnosis was considered, but the wrong educated guess was made.
The use of dead-space measurements to help in the diagnosis of a PE is not a new idea. The concept was proposed 30 years ago before the advent of new, computer-driven technology for end-tidal CO2 measurement. This new technology, with the advent of the D-dimer assay, justifies a fresh look at this approach. Furthermore, most of the work done in the diagnosis of PE has been performed in hospitalized patients whose differential diagnosis is quite different from that of ambulatory patients presenting to the ED. The consequences of a decision by a hospital clinician to start heparin before or after the diagnostic test is hardly comparable to the emergency physician’s decision of whether to send a patient home with chest pain. Thus, any useful screening test for PE in the ED must have a negative predictive value that approaches 100%.
In this small sample studied by Kline et al, the negative predictive value of both tests together was 100%. But before this approach can receive wide acceptance by emergency physicians, those numbers must stand the test of a large sample of patients at different sites. The fact that there were no false-negatives in 26 patients does not guarantee the same result in 260 or 2600. Its value as a screening tool to exclude PE depends upon the true negative predictive value that must be discovered over time. We can only hope that a large multi-hospital evaluation of this technique will be on the way soon. But until then, those who currently have access to these tests may find it at least as useful as a chest radiograph, an A-a gradient, or an ECG.
Kline et al should be commended for applying original thought to an old problem, but more work in this area is needed. As for now, the diagnosis of the patient with unexplained dyspnea or pleuritic pain remains an expensive and time-consuming workup.
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