Noninvasive detection and quantification of left-to-right shunts in children using oxygen-15 labeled carbon dioxide

A method for the detection and quantification of left-to-right intracardiac shunts is described which utilizes a single breath inhalation of oxygen-15 labeled carbon dioxide (C15O2). The inhaled gas rapidly crosses the alveolar membrane and the oxygen-15 label is exchanged through the carbonate cycle to form oxygen-15 labeled water within the pulmonary capillary blood. Pulmonary indicator clearance curves are measured by external scintillation probes. A simplified method of shunt flow quantification was developed from indicator dilution principles and used for the analysis of the clearance curves. Inhalation studies were performed with 62 children on the day prior to cardiac catheterization. The presence or absence of left-to-right shunt was confirmed by contrast angiography in all cases. Twenty-six children were found to have no shunts by C15O2 inhalation, oximetry or angiography. Of the 36 with shunts, 34 were detected by C15O2. Two of these were designated as equivocal because they were considered to be less than the threshold of definitive detection by C15O2 (having Qp/Qs less than 1.2); 32 were positive and there were two false negatives with small ventricular septal defects. There were no false positives by C15O2. The correlation coefficient between C15O2 and oximetry values of shunt flow for those patients with proven shunts was 0.82.