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

OBJECTIVES Hemodynamic support is aimed at providing adequate O2 delivery to the tissues; most interventions target O2 delivery increase. Mixed venous O2 saturation is a frequently used parameter to evaluate the adequacy of O2 delivery. METHODS We describe a mathematical model to compare the effects of increasing O2 delivery on venous oxygen saturation through increases in the inspired O2 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 O2 extraction rate. O2 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 O2 delivery values were also obtained by maintaining a fixed O2 inspired fraction value of 0.21 while changing cardiac output. RESULTS Venous oxygen saturation was higher when produced through increases in inspired O2 fraction versus increases in cardiac output, even at the same O2 delivery and consumption values. Specifically, at high inspired O2 fractions, the measured O2 saturation values failed to detect conditions of low oxygen supply. CONCLUSIONS The mode of O2 delivery optimization, specifically increases in the fraction of inspired oxygen versus increases in cardiac output, can compromise the capability of the "venous O2 saturation" parameter to measure the adequacy of oxygen supply. Consequently, venous saturation at high inspired O2 fractions should be interpreted with caution.