Fick principle

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For the diffusion law, see Fick's law of diffusion

Developed by Adolf Eugen Fick (1829 - 1901), the Fick principle is a technique for measuring cardiac output.

1 Variables
2 Equation
3 Assumed fick determination
4 Underlying principles
5 Use in renal physiology
6 References
7 External links

[edit] Variables

The following variables are measured:^[1]

VO₂ consumption per minute using a spirometer (with the subject re-breathing air) and a CO₂ absorber
Cv, the oxygen content of blood taken from the pulmonary artery (representing venous blood)
Ca, the oxygen content of blood from a cannula in a peripheral artery (representing arterial blood)

[edit] Equation

From these values, we know that:

$VO_2 = (CO \times\ C_A) - (CO \times\ C_V)$

where CO = Cardiac Output, C_A = Oxygen concentration of arterial blood and C_V = Oxygen concentration of venous blood.

This allows us to say

$CO = \frac{VO_2}{C_A - C_V}$

and hence calculate cardiac output.

[edit] Assumed fick determination

In reality, this method is rarely used these days due to the difficulty of collecting and analysing the gas concentrations. However, by using an assumed value for oxygen consumption, cardiac output can be closely approximated without the cumbersome and time-consuming oxygen consumption measurement. This is sometimes called an assumed Fick determination.

A commonly-used value for O₂ consumption at rest is 125 ml O₂ per minute per square meter of body surface area.

[edit] Underlying principles

The Fick principle relies on the observation that the total uptake of (or release of) a substance by the peripheral tissues is equal to the product of the blood flow to the peripheral tissues and the arterial-venous concentration difference (gradient) of the substance. In the determination of cardiac output, the substance most commonly measured is the oxygen content of blood, and the flow calculated is the flow across the pulmonary system. This gives a simple way to calculate the cardiac output:

$Cardiac\ Output = \frac{Oxygen\ consumption}{ArterioVenous\ Oxygen\ difference}$

Assuming there are no shunts across the pulmonary system, the pulmonary blood flow equals the systemic blood flow. Measurement of the arterial and venous oxygen content of blood involves the sampling of blood from the pulmonary artery (low oxygen content) and from the pulmonary vein (high oxygen content). In practice, sampling of peripheral arterial blood is a surrogate for pulmonary venous blood. Determination of the oxygen consumption of the peripheral tissues is more complex.

The calculation of the arterial and venous oxygen content of the blood is a simple process. Most oxygen in the blood is bound to hemoglobin molecules in the red blood cells. Measuring the content of hemoglobin in the blood and the percentage of saturation of hemoglobin (the oxygen saturation of the blood) is a simple process and is readily available to physicians. Using the fact that each gram of hemoglobin can carry 1.36 ml of O₂, the oxygen content of the blood (either arterial or venous) can be estimated by the following formula:

$Oxygen\ content\ of\ blood = \left [Hemoglobin \right] \left ( g/dl \right ) \ \times\ 1.36 \left ( ml\ O_2 /g\ of\ hemoglobin \right ) \times\ 10\ \times\ percentage\ saturation\ of\ blood$

[edit] Use in renal physiology

The principle can also be used in renal physiology to calculate renal blood flow.^[2]

In this context, it is not oxygen which is measured, but a particular solute whose plasma concentration and excretion is of interest. However, the equations are essentially the same.

[edit] References

^ Physiology at MCG 3/3ch5/s3ch5_3 - "Indirect Measurement of Cardiac Output"
^ Physiology at MCG 7/7ch04/7ch04p27 - "Measuring Renal Blood Flow: Fick Principle"