Changes in perfusion scintigraphy in the first days of heparin therapy in patients with acute pulmonary embolism

Patients with suspected pulmonary embolism often receive heparin therapy for hours to days before ventilation/perfusion scintigraphy is completed. We assessed to what extent the lung scan classification and pulmonary vascular perfusion changed over a period of 2-4 days of full anticoagulant therapy. In 312 consecutive patients with at least one segmental perfusion defect on the initial perfusion scan, classification of both the initial and the final lung scan allowed us to study alteration in scan classification. Changes in pulmonary perfusion were assessed scintigraphically in a subgroup of 64 patients with proven pulmonary embolism. Among 79 patients with an initial high-probability lung scan, the final scan remained high probability in 77 whereas it became non-diagnostic and normal in one patient each. The lung scan classification did not change in any of the 233 patients who initially had a non-diagnostic scan. Thus, a different lung scan category was observed in only 2 out of 312 patients (0.6%; 95% CI 0.1%-2.3%). The mean pulmonary perfusion at baseline in the subgroup of 64 patients with pulmonary embolism was 62% (SD +/-17%; range 26%-89%). A mean absolute improvement in pulmonary perfusion of only 4%+/-11% (NS) was observed in the 2-4 days of observation. It is concluded that intravenous heparin therapy for a period of 2-4 days has only a minimal influence on the diagnostic lung scan classification and induces only minor changes in pulmonary vascular obstruction in the majority of patients.