Quantitation of right to left shunting by double indicator and oxygen techniques

An improved double indicator technique for quantitating right to left shunting has been validated in a canine right heart bypass shunt model and compared to standard O₂ shunt measurements in the same preparation. A bolus of dissolved sulfur hexafluoride (SF₆) and indocyanine green dye (ICG) is injected into systemic venous return and a single, time averaged arterial blood sample is collected during the initial circulation of indicators. Because of its low solubility, SF₆ is eliminated essentially quantitatively from blood traversing gas filled alveoli; correction for volatile tracer in arterial blood derived from nonshunt pathways is therefore unnecessary. ICG remains confined to the vascular space and SF₆ is not lost in shunt pathways. Ratios of SF₆ ICG shunt to directly measured shunt averaged 0.99 ± 0.27 (SD) in 55 comparisons of shunts ranging from 2 to 25% of cardiac output; differences between actual and measured shunt averaged 0.5 ± 2.9% of cardiac output. Simultaneously determined ratios of O₂ shunt to directly measured shunt averaged 0.98 ± 0.48 in 34 comparisons; differences between actual and measured shunt were 0.7 ± 3.4% of cardiac output.