Oximetric detection of intracardiac left-to-right shunts.

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Abstract
To assess prevailing guidelines for the oximetric detection of intracardiac left-to-right shunts, the variability of right heart O2 saturation step-ups in 1121 catheterizations in children with aortic and pulmonary stenosis, who were studied as part of the joint study of the natural history of congenital heart defects, were examined. In the absence of intracardiac shunting, a step-up of more than 5.9% between the superior vena cava and the pulmonary artery, 3.5% between the right atrium and the pulmonary artery and 3.6% between the right ventricle and pulmonary artery, evidently occurs 5% of the time. By extrapolation from a normal distribution, a step-up of 8.7% between the superior vena cava and pulmonary artery, 5.6% between the right atrium and pulmonary artery and 5.2% between the right ventricle and the pulmonary artery would occur 1% of the time in the absence of intracardiac shunting. Assuming a normal Hb concentration of 15 g/dl, the 1% false positive rate corresponds to a step-up of 1.7 volumes percent from the superior vena cava to the pulmonary artery, 1.1 volumes percent from the right atrium to the pulmonary artery and 1.0 volumes percent from the right ventricle to the pulmonary artery. These data provide guidelines for the detection of abnormal shunts, but other independent methods such as indicator dilution or angiography should be employed in borderline situations.