Right to left shunt measurement in patients suspect for pulmonary embolism

31 patients suspect for acute pulmonary emboli were studied by measuring arterial PO2 (room air) and right to left shunt (100% oxygen breathing) to determine if measurement of the right to left shunt aided in the diagnosis of pulmonary embolism. Ventilation/perfusion or serial perfusion lung scans were performed on each patient. All patients exhibited arterial hypoxemia (PaO2 less than or equal to 80) and 27 had a shunt greater than 7%. The shunt was 15.1 +/- 3.6% in 9 patients with lung scans highly suggestive of emboli. 20 patients with lung scans of low probability for emboli had significantly less shunting averaging 10.9 +/- 3.7%. 5 patients of the latter group had shunts averaging 15.2 +/- 3.8% attributable to occult pneumonia not initially apparent. An increased right to left shunt fraction is not diagnostic of pulmonary emboli, but a normal one makes the diagnosis unlikely.