CRÉTEIL, FRANCE--Silent pulmonary embolism (PE) is widespread in patients with deep venous thrombosis (DVT), and lung scans can help detect asymptomatic disease in many patients, according to the results of a recent international study of more than 600 patients.[1] However, baseline lung scans are less useful in predicting early thromboembolic recurrences during therapy. These findings underscore the importance of proper prophylaxis against and treatment for venous thromboembolism (VTE).

"Regardless of what interpretive criteria are used for assessing lung scans … the frequency of silent PE is 40% to 50% in patients with DVT," concluded the study authors, who were led by Michel Meignan, MD, of Henri Mondor Hospital, Créteil, France. However, they added, "Routine baseline lung scans for diagnosing asymptomatic PE in patients with DVT may not be necessary. The abnormalities may be of no clinical significance, as they seem nonpredictive of PE recurrence, at least after three months of therapy."

STUDY METHODOLOGY

As part of a multicenter prospective trial of low-molecular-weight heparin in DVT patients, Dr. Meignan and colleagues used perfusion lung scans to estimate the frequency of PE in DVT patients. Subjects included 622 outpatients who had a primary diagnosis of proximal DVT confirmed by venography but no clinical indication of PE. All patients underwent perfusion lung scanning, and 379 of these patients also had ventilation scans. As part of the three-month follow-up, patients with suspected PE received a second perfusion or perfusion-ventilation lung scan, and PE was diagnosed if the scan showed a new segmental perfusion defect. When the diagnosis was uncertain, pulmonary angiography was performed.

PROBABILITIES AND FREQUENCIES

Lung scans were categorized as normal, low probability, intermediate probability, or high probability, depending on the abnormalities present. Scans were considered normal when perfusion was normal, and very low probability if the abnormalities were limited to no more than three small perfusion defects.

High-probability scans were defined by the presence of segmental defects associated with either normal ventilation and normal chest films (ventilation-perfusion mismatched) or with normal chest films alone (chest radiograph/perfusion mismatched). Intermediate-probability scans were those that did not fit into any of the other three categories. PE frequency was then approximated by combining the lung scan probabilities with the positive predictive values for PE that had previously been reported for those scan results.

FINDINGS

The researchers found that the frequency of PE was 39.5% when high-probability scans were defined by at least two mismatched ventilation-perfusion defects. When one mismatched defect was the criterion, PE frequency was 49.5%. When perfusion scans were compared to chest films rather than ventilation scans, the PE frequency was calculated at 45.3%.

From these calculations of PE frequency, the researchers concluded that "a high percentage of patients [with DVT] have lung scans that fulfill the criteria for a high probability of PE regardless of the criteria used for scan analysis." They further suggested that the frequency of silent PE was probably closer to the highest value (49.5%), because of the deliberate exclusion of moderate mismatched defects in the intermediate probability category.

The highest proportion of normal scans was found in patients younger than age 30. However, when lung scans were abnormal, these young patients were much more likely than older patients to have high-probability findings.

Despite the high frequency of asymptomatic PE detected in their study, Meignan et al acknowledged that the cost-effectiveness of performing lung scans in all patients with DVT is unclear. The researchers also pointed out that submassive PE can be treated with the same dose of a low-molecular-weight heparin that is used for managing DVT. In their study, in which all patients were given treatment for DVT, the rate of recurrent PE (1.3% overall during three months' follow-up) was no higher in the patients with high-probability scans than in those with normal scans.

Thus, is it necessary to diagnose silent PE if outpatient treatment of DVT is being considered? "No," wrote Paul Stein, MD,[2] of the Henry Ford Heart and Vascular Institute in Detroit, Mich, in an accompanying editorial. He noted that in this study, the presence of PE did not appear to affect the frequency of recurrent thromboembolism or patient mortality.

However, he noted that even in many cases in which PE causes or contributes to death, it is unsuspected ante mortem. Thus, it remains unclear how extensively physicians should search for embolic disease--and exactly what they should do when asymptomatic PE is detected.

--Robert McCarthy

References
1. Meignan M, Rosso J, Gauthier H, et al. Systematic lung scans reveal a high frequency of silent pulmonary embolism in patients with proximal deep venous thrombosis. Arch Intern Med. 2000;160:159-164.
2. Stein PD. Silent pulmonary embolism. Arch Intern Med. 2000;160:145-146.

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