Clinical Validity of a Normal Perfusion Lung Scan in Patients: DISCUSSION
The results indicate that patients with suspected pulmonary embolism and a normal perfusion lung scan have an excellent prognosis without anticoagulant therapy. Symptomatic venous thromboembolism was rare on follow-up in our patients with normal perfusion scans (0.6 percent [three of 515 patients]). Further, two of the three patients with venous thromboembolism on follow-up had predisposing factors which placed them at continued risk for venous thromboembolism. The results on follow-up indicate that a normal perfusion scan excludes clinically important pulmonary embolism. Our findings support and extend the observations of Kipper et al who reported pulmonary embolism on follow-up in only one of 68 patients with suspected pulmonary embolism and normal perfusion scans.
Pulmonary embolism is strongly associated with proximal-vein thrombosis. Proximal-vein thrombosis was found at presentation in only 1 percent of our patients with normal perfusion scans. This low frequency of proximal-vein thrombosis further supports the conclusion that the perfusion scan is rarely falsely normal. It is possible that the patients in our study with proximal-vein thrombosis may have had pulmonary emboli that were missed by perfusion scanning. An alternative diagnosis was not made in four of the five patients with proximal-vein thrombosis. These patients may have had small (<2 mm in diameter) or incompletely occlusive emboli, which experimental studies indicate may remain undetected by perfusion scanning.
Previous studies have demonstrated that bilateral incomplete pulmonary artery stenosis may result in symmetrical lung perfusion and an apparently normal perfusion scan. It is possible that the perfusion scan may be normal in the presence of a saddle embolus which does not obstruct the main pulmonary outflow tract. If a normal perfusion scan in the presence of a saddle embolus can occur, our findings suggest that it does so rarely.
The outcome on follow-up in our patients with normal perfusion scans compares favorably with the 2 percent rate of pulmonary embolism reported by others on follow-up in patients with negative pulmonary angiograms. However, perfusion lung scanning is less invasive than pulmonary angiography.