Blood oxygen measurements in the assessment of intracardiac left to right shunts: A critical appraisal of methodology

The proper use of sequential diagnostic blood oxygen determinations for assessing left to right shunts requires a thorough understanding of the normal variability of blood oxygen measurements among the various right heart chambers and the influence of alterations in circulating hemoglobin levels and systemic blood flow. Oximetric measurements were obtained in 23 patients without a left to right shunt and compared with measurements obtained in 42 patients with a proved left to right shunt to examine the normal range of oxygen variability in blood samples from right heart chambers. The effect of fluctuations in hemoglobin levels was evaluated by comparing increases in percent oxygen saturation and oxygen content. Differences in percent saturation were found to be more useful than differences in oxygen content for detection of cardiac shunts. A mean difference of at least 7 percent oxygen saturation was found to be required for a firm diagnosis of a shunt at the atrial level, and 5 percent oxygen saturation for a shunt at the ventricular or great vessel level.The curvilinear relation between systemic blood flow and oxygen step-up in determining the pulmonary to systemic flow ($\text{Q}{}_{\mathrm{P}}\text{Q}{}_{\mathrm{S}}$) ratio was expressed in a series of equations and depicted by a three dimensional surface. Interventions such as exercise augment both systemic blood flow and oxygen step-up, resulting in a shift to a more steeply rising portion of the surface and a dramatic increase in shunt flow. The minimal left to right shunt detectable with oxlmetry is largely dependent on the level of systemic blood flow.