Novel Noninvasive Estimation of Mixed Venous Oxygen Saturation by Echocardiography and Expired Gas Analysis.

Mixed venous oxygen (O2) saturation (SvO2) is an important measure to evaluate sufficiency of cardiac output (CO) relative to whole body O2 consumption (VO2), while clinical utilization is limited for required invasive catheterization. By Fick's equation, VO2 (ml/min) = CO (L/min) x Hb (g/dL) x 1.36 (ml/g) x (SaO2 - SvO2) / 10 [Hb = hemoglobin concentration, SaO2 = arterial blood O2 saturation]. Since VO2, CO, Hb and SaO2 can be measured noninvasively with expired gas analysis, echocardiography, simple blood test and percutaneous O2 saturation, SvO2 can be noninvasively calculated. We hypothesized that noninvasively calculated SvO2 shows significant correlation and agrees well with invasively measured SvO2. In 47 patients (29 men, mean age 70±12 years), SvO2 was directly measured by sampling pulmonary artery blood. Calculated SvO2 was also obtained by the method described above. Calculated SvO2 was significantly correlated with measured SvO2 (r = 0.79, P < 0.001) and significantly smaller than measured SvO2 (70±5.1 vs 72.1±4.9%, p < 0.001). Bias at SvO2 was -2.2% (95% confidence interval, -3.2 to -1.1%) with limits of agreement from -9.5 to 5.2%, demonstrating acceptable agreement. The optimal cut-offs values of calculated SvO2 was < 69% for reduced measured SvO2 < 70% with the area under the curve of 0.94. Reduced calculated SvO2 < 69% indicated a sensitivity of 92.9% and specificity of 90.9% for reduced measured SvO2. Noninvasively calculated SvO2 significantly correlated and agreed well with directly measured SvO2. This novel method allows practical evaluation of SvO2 to assess sufficiency of CO according to VO2.