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Incidence of Pulmonary Embolism in Single Segmental Mismatch on Lung Scanning

Department of Nuclear Medicine, St. George Hospital, Sydney, Australia

Correspondence: For correspondence or reprints contact: Dr. R. J. Quinn, Department of Nuclear Medicine, St. George Hospital, Belgrave Street, Kogarah N.S.W. 2217, Australia.

ABSTRACT

Controversy exists as to whether patients with single segmental mismatch (SSM) on a ventilation/perfusion (VQ) lung scan should be given a low or an intermediate probability of pulmonary embolism (PE). Methods: Pulmonary angiography was used to evaluate the incidence of PE in SSM at the authors' institution. From January 1991 to January 1993, 1449 VQ scans were performed. Results: With modified Biello criteria, 283 were high probability; 628, low probability; 273, normal; and 273, intermediate probability. Of the intermediate probability scans, 61 had SSM. Forty of these patients underwent pulmonary angiography. Twelve patients had PE in the area of the SSM, giving an incidence of PE of 30%. The risk of PE in SSM in the different lung regions was also analyzed. Twenty-three SSM were in the bases of the lung with a 22% incidence of PE; 17 SSM were either in the midzone or apex with a 41% incidence of PE (p = not significant). Conclusion: SSM carries a 30% risk of PE. Accordingly, SSM should be given an intermediate probability of PE and not a low probability of PE.

Key Words: single segmental mismatch • ventilation-perfusion scan • pulmonary embolism