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Inhalation Imaging with Oxygen-15 Labeled Carbon Dioxide for Detection and Quantitation of Left-to-Right Shunts

Quantitation of left-to-right shunts was determined noninvasively from the pulmonary clearance pattern of inhaled 15oxygen-labeled carbon dioxide (C'1502). After a single breath inhalation of C"O, counts over the lungs were obtained from sequential 0.5 sec positron camera images. In 21 patients without left-toright shunts, counts declined exponentially due to the washout of C'1502 by the pulmonary blood flow. In 22 patients with left-to-right shunts, this monoexponential pulmonary clearance pattern was in- terrupted by an abnormal upward deviation, indicating tracer recirculation through the shunt to the lungs. Following surgical shunt closure in 10 patients, pulmonary C1502 clearance patterns became normal in nine and showed a small residual left-to-right shunt in one. Shunt size was derived from the ratio of the height of the recirculation curve to the height of the initial inhalation peak. These values significantly correlated with shunt size as determined by oximetry (r 0.83).