Noninvasive estimation of mixed venous oxygen content

抄録

Mixed venous and arterial oxygen content (CvO₂ and CaO₂) are fundamental factors to connect cardiac output and oxygen uptake by the Fick principle. Direct measurements of CaO₂ and CvO₂ require blood samplings and quantifications of the oxygen contents. Arterial blood can be sampled from a peripheral artery, but a right cardiac catheter is necessary for mixed venous blood sampling. Moreover, quantifications of the blood oxygen contents are complicated compared with those of blood oxygen partial pressures. Accordingly, indirect methods to estimate CaO₂ and CvO₂ have been developed. CaO₂ can be evaluated as the sum of chemically and physically dissolved oxygen in arterial blood. If mixed venous blood is sampled, CvO₂ can be obtained from mixed venous oxygen saturation and partial pressure as well. As for noninvasive methods without blood samplings, arteriovenous oxygen difference (CaO₂ – CvO₂) was obtained from a relationship between respiratory exchange ratio and carbon dioxide partial pressure during rebreathing. Recently, we developed another noninvasive method to estimate CvO₂ from SpO₂ and heart rate with a pulse oximeter, and obtained an equation CaO₂ – CvO₂ (vol%) = –0.265×10^-6h² + 0.289×10^-3h + 7.74 with altitude h in meters. In this review, these two noninvasive methods, rebreathing and pulse oximeter, are discussed.