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 (FIO(2)s) in anesthetized sheep undergoing Flung or 2-lung ventilation. Animals-6 healthy adult female sheep. PROCEDURES: Sheep were anesthetized and administered 5 different FIO(2)s (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 FIO(2) after a 15-minute stabilization period. Vital capacity alveolar recruitment maneuvers were performed after blood collection. The previously used FIO(2) sequence was reversed for sample collection during Flung 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 PAO(2):FIO(2), alveolar-arterial difference in partial pressure of oxygen [PAO(2) - PAO(2)], [PAO(2) - Pao(2)]:FIO(2), [PAO(2) - PAO(2)]:PAO(2), and PAO(2):PAO(2)) were calculated at each FIO(2); 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 FIO(2)s, regardless of the magnitude of Qs/Qt, and could be potentially used as a surrogate for Qs/Qt measurements in healthy sheep.