Fick principle

Developed by Adolf Eugen Fick (1829–1901), the Fick principle has been applied to the measurement of cardiac output. Its underlying principles may also be applied in a variety of clinical situations.

The essence of the Fick principle is that blood flow to an organ can be calculated using a marker substance if the following information is known:

In Fick's original method, the "organ" was the entire human body and the marker substance was oxygen.

The principle may be applied in different ways. For example, if the blood flow to an organ is known, together with the arterial and venous concentrations of the marker substance, the uptake of marker substance by the organ may then be calculated.

In Fick's original method, the following variables are measured:

From these values, we know that:

where CO = Cardiac Output, C_a = Oxygen concentration of arterial blood and C_v = Oxygen concentration of mixed venous blood.

This allows us to say

and hence calculate cardiac output.

Note that (C_a – C_v) is also known as the arteriovenous oxygen difference.

In reality, this method is rarely used 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 125ml O₂ per minute per square meter of body surface area.

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 thus giving the arteriovenous oxygen difference, and the flow calculated is the flow across the pulmonary system. This gives a simple way to calculate the cardiac output:

Assuming there is no intracardiac shunt, 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.

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