Effects of arterial oxygen tension and cardiac output on venous saturation: a mathematical modeling approach

OBJECTIVES: Hemodynamic support is aimed at providing adequate O(2) delivery to the tissues; most interventions target O(2) delivery increase. Mixed venous O(2) saturation is a frequently used parameter to evaluate the adequacy of O(2) delivery. METHODS: We describe a mathematical model to compare the effects of increasing O(2) delivery on venous oxygen saturation through increases in the inspired O(2) fraction versus increases in cardiac output. The model was created based on the lungs, which were divided into shunted and non-shunted areas, and on seven peripheral compartments, each with normal values of perfusion, optimal oxygen consumption, and critical O(2) extraction rate. O(2) delivery was increased by changing the inspired fraction of oxygen from 0.21 to 1.0 in steps of 0.1 under conditions of low (2.0 L.min(-1)) or normal (6.5 L.min(-1)) cardiac output. The same O(2) delivery values were also obtained by maintaining a fixed O(2) inspired fraction value of 0.21 while changing cardiac output. RESULTS: Venous oxygen saturation was higher when produced through increases in inspired O(2) fraction versus increases in cardiac output, even at the same O(2) delivery and consumption values. Specifically, at high inspired O(2) fractions, the measured O(2) saturation values failed to detect conditions of low oxygen supply. CONCLUSIONS: The mode of O(2) delivery optimization, specifically increases in the fraction of inspired oxygen versus increases in cardiac output, can compromise the capability of the “venous O(2) saturation” parameter to measure the adequacy of oxygen supply. Consequently, venous saturation at high inspired O(2) fractions should be interpreted with caution.