Measurement of diffusing capacity in pulmonary embolism.

Pulmonary function tests were carried out in 20 consecutive patients with pulmonary embolism (PE), diagnosed on the basis of a positive ventilation-perfusion lung scan carried out within 72 h of admission. Changes in forced expiratory volume in one second (FEV1), forced vital capacity (FVC) and arterial blood gas tensions were too variable to be helpful diagnostically. In contrast, transfer factor (diffusing capacity) of the lung (TLCO) was significantly reduced in all cases and, in spite of a period of anticoagulation, tended to remain subnormal during a follow-up period of up to 3 years. Lung scans, however, tended to return to normal within 3 months of the incident. Thus, a reduction of TLCO to below 75% of the predicted normal was found in all cases with abnormal lung scans and such measurements provide a useful and simple screening test for PE; a normal TLCO would effectively exclude such a diagnosis. The failure of TLCO to return to normal in the majority of cases suggests persistence of the underlying physiological defect, in spite of normalization of symptoms and lung scans following anticoagulation.