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A proposal for a new temperature-corrected formula for the oxygen content of blood

To the Editor,

Oxygen (O2) is essential for normal aerobic metabolism in mammals. O2 delivery is the process of transport of oxygenated blood from the alveolar capillaries to the tissues and is dependent on two factors: arterial O2 content (CaO2) and cardiac output. The carriage in the blood occurs as O2 bound to hemoglobin and O2 dissolved in the plasma and is expressed as the sum of these components with the following equation: CaO2 = 1.39 × Hb (g/dl) × SaO2 + 0.0031 × PaO2 (mmHg) (1). CaO2 is the milliliters of O2 per 100 mL of blood, SaO2 is the fraction of hemoglobin (Hb) that is saturated with O2, the O2-combining capacity of Hb is 1.39 mL of O2 per gram of Hb, Hb is grams of Hb per 100 mL of blood, PaO2 is the O2 tension, and solubility of O2 in plasma is 0.003 mL of O2 per 100 mL plasma for each mm Hg of PaO2 [1, 2]. The pressure is 1 atm (101.325 kPa). However, the temperature is not clearly specified. In fact, the coefficient of 1.39 in the previous item of the equation was obtained under the standard condition of 0 °C (273 K). It is obtained as 22.4 (volume of 1 mol of gas at 273 K) × 4 (molecular number bound to 1 molecule of hemoglobin)/64,458 (molecular weight of hemoglobin)/1000 (correction coefficient) [3, 4]. In the case of 37 °C (310 K), the coefficient should be 1.58 due to Boyle-Charles law or combined gas law (273 + 37)/273 × 1.39. On the contrary, the coefficient 0.0031 from the last item of the equation is derived from evidence on O2 solubility at 37 °C (310 K). It is obtained as 0.0239 (Bunsen absorption coefficient α of oxygen at 37 °C) × 100/760 [5]. These items from the same equation are derived at different temperatures. Although equation (1) is established and is described in almost all published textbooks, the equation should be revised so that it is more clinically relevant. In lieu of equation (1), we propose a new temperature correction formula: CaO2 (ml/dl) = 1.58 × Hb (g/dl) × SaO2 + 0.0031 × PaO2 (mmHg) at 37 °C and 1 atm. Lack of experimental results is one of limitations of our proposal, and we need to confirm them in clinical settings.

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K.H, M.M, and K.S. conceived and designed the study. Y.F performed the analysis. K.H, M.M, and K.S. wrote the paper. All authors read and approved the final manuscript.

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Correspondence to Kiichi Hirota.

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Hirota, K., Murata, M. & Shingu, K. A proposal for a new temperature-corrected formula for the oxygen content of blood. JA Clin Rep 6, 62 (2020).

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