How much oxygen is extracted per minute?

  • Tissue oxygen extraction is calculated by subtracting mixed venous oxygen content from arterial oxygen content.

The Fick equation is used to calculate the VO2, the oxygen consumption. This is computed by figuring out how much oxygen has been lost between the arterial side and the venous side of the circulation and multiplying the result by the cardiac output. In the following equation, VO2 is the oxygen consumption per minute, CaO2 is the content of oxygen in arterial blood, and CvO2 is the content of oxygen in venous blood:

VO2 = Q x (CaO2-CvO2) mlO2/min

The CnO2 is (1.34 x Hb x SnO2/100) + 0.003 x PnO2, where n = a or v

The major difference between the two is obviously the hemoglobin saturation, which is roughly 100% on the arterial side and 75% on the venous side.

Substituting inwards, where hemoglobin is 15g/dl: CaO2 is approx 20ml/100ml, CvO2 is 15ml/100ml: the difference is 5ml/100ml = 50 ml/l multiplied by a cardiac output of 5L = O2 consumption per minute is 250ml.

So the mixed venous O2 saturation can be used to calculate the oxygen consumption: if SvO2 is decreasing, the O2 consumption is increasing.


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