Fick's principle

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The Fick principle describes the classic physiological method developed by Adolf Fick to determine the cardiac output . The term is not identical to Fick's laws of diffusion ( also formulated by Adolf Fick) (Fick's laws) . The procedure is based on the principle of the indicator dilution method:

The flow of an indicator taken up by an organ or released by it corresponds to the difference in the indicator concentrations in the inflow tract and in the outflow tract. The indicator used here is oxygen, the formula results:

where HMV is the cardiac output, the amount of oxygen absorbed per time and avD O 2 is the arteriovenous oxygen difference . It is therefore necessary to measure the breathing gases and to determine the oxygen concentrations in the arterialized and mixed venous blood.

Derivation

To determine the cardiac output , you need the amount of oxygen ( ) absorbed in the lungs per time , which can be determined by means of a spirometric measurement of the respiratory gases, as well as the oxygen concentration in the arterial ( ) and mixed venous ( ) blood, which can be determined via a Blood gas analysis can determine. With pulmonary blood flow per time ( ) is the amount of oxygen absorbed in the lungs

Since practically all blood that flows through the lungs also flows through the heart, one can equate with the HMV. The formula for the cardiac output is obtained by changing it

where also called arteriovenous oxygen difference (avD O 2 ).

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