Using the Fick equation to calculate the arteriovenous difference and cardiac output
The amount of oxygen extracted from each aliquot of blood—tissue oxygen extraction or consumption—also differs among organ systems.
Measurement of oxygen consumption per minute by a specific organ system would require determination of the oxygen content of venous blood draining that organ. In clinical practice, an overall measurement of oxygen utilization is determined by catheterization of the right heart and determination of mixed venous oxygen content (MVO2). The mixed venous oxygen content represents the weighted average of oxygen content in venous blood from all organ systems.
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The relationship among the variables related to oxygen consumption is expressed by the Fick principle: oxygen consumption is the product of cardiac output and the arteriovenous oxygen difference.
In the setting of a fixed cardiac output, increased oxygen demand results in an increased arteriovenous oxygen difference, and with reduced oxygen demand the arteriovenous oxygen difference declines.
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