




OXYGEN 



How much oxygen is extracted per minute?
The Fick equation is used to calculate the VO_{2}, 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, VO_{2} is the oxygen consumption per minute, CaO_{2} is the content of oxygen in arterial blood, and CvO_{2} is the content of oxygen in venous blood: VO2 = Q x (CaO_{2}CvO_{2}) mlO_{2}/minThe CnO_{2} is (1.34 x Hb x SnO_{2}/100) + 0.003 x PnO_{2}, 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: CaO_{2} is approx 20ml/100ml, CvO_{2} 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 O_{2} saturation can be used to calculate the oxygen consumption: if SvO_{2} is decreasing, the O_{2} consumption is increasing. 

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