Use of the oxygen content-based index, fshunt, as an indicator of pulmonary venous admixture at various inspired oxygen fractions in anesthetized sheep

Objective-To evaluate the use of the oxygen content-based index, Fshunt, as an indicator of venous admixture (Qs/Qt) at various fractions of inspired oxygen (FIO2s) in anesthetized sheep undergoing 1-lung or 2-lung ventilation. Animals-6 healthy adult female sheep. Procedures-Sheep were anesthetized and administered 5 different FIO2s (0.21, 0.40, 0.60, 0.80, and 1.00) in random order during 2-lung mechanical ventilation. Arterial and mixed venous blood samples were obtained at each FIO2 after a 15-minute stabilization period. Vital capacity alveolar recruitment maneuvers were performed after blood collection. The previously used FIO2 sequence was reversed for sample collection during 1-lung ventilation. Blood samples were analyzed for arterial, pulmonary end-capillary, and mixed venous oxygen content and partial pressure and for hemoglobin concentration. Oxygen hemoglobin saturation, Qs/Qt, Fshunt, and oxygen tension-based indices (OTIs; including PaO2:FIO2, alveolar-arterial difference in partial pressure of oxygen [PAO2 -PaO2], [PAO2 -PaO2]:FIO2, [PAO2 -PaO2]:PaO2, and PaO2:PAO2) were calculated at each FIO2; associations were evaluated with linear regression analysis, concordance, and correlation tests. Intermethod agreement between Qs/Qt and Fshunt was tested via Bland-Altman analysis. Results-Strong and significant associations and substantial agreement were detected between Fshunt and Qs/Qt. Relationships between OTIs and Qs/Qt varied, but overall correlations were weak. Conclusions and Clinical Relevance-Whereas OTIs were generally poor indicators of Qs/Qt, Fshunt was a good indicator of Qs/Qt at various FIO2s, regardless of the magnitude of Qs/Qt, and could be potentially used as a surrogate for Qs/Qt measurements in healthy sheep