The cardiac output can be estimated by dividing oxygen consumption by the difference in oxygen content between arterial and venous blood. The difference between the arterial and mixed venous blood oxygen concentration correlates with oxygen uptake per unit of blood as it flows through the lungs (Fick Principle). The method is cumbersome due to the need to collect expired air and arterial blood gases.

oxygen content in blood =

= (hemoglobin in g/dL) * (1.34 mL oxygen per g hemoglobin) * (oxygen saturation in per cent)



• 1.34 mL per g is the amount of oxygen that a gram of hemoglobin can carry if 100% saturated

• Oxygen saturation is expressed in volume per cent (vol%), or the mL of oxygen per 100 mL of blood.


cardiac output in liters per minute =

= (oxygen consumption) / ((difference in oxygen content between mixed venous and arterial blood in vol%) * 10) =

= (oxygen consumption) / (((oxygen content arterial blood in vol%) - (oxygen content mixed venous blood in vol%)) * 10)



• Oxygen consumption can be estimated by calculating the patient's BSA and multiplying by the basal oxygen consumption of 125 mL oxygen per square meter BSA.

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