The Fick principle states that blood flow to an organ can be calculated using a marker substance if the following information is known:
* Amount of marker substance taken up by the organ per unit time
* Concentration of marker substance in arterial blood supplying the organ
* Concentration of marker substance in venous blood leaving the organ
Developed by
Adolf Eugen Fick (1829–1901), the Fick principle has been applied to the measurement of
cardiac output
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: th ...
. Its underlying principles may also be applied in a variety of clinical situations.
In Fick's original method, the "organ" was the entire human body and the marker substance was oxygen. The first published mention was in conference proceedings from July 9, 1870 from a lecture he gave at that conference; it is this publishing that is most often used by articles to cite Fick's contribution.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.
Variables
In Fick's original method, the following variables are measured:
* V̇O
2, oxygen consumption in ml of pure gaseous oxygen per minute. This may be measured using a
spirometer
A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs. A spirometer measures ventilation, the movement of air into and out of the lungs. The spirogram will identify two different types of abnormal ventil ...
within a closed rebreathing circuit incorporating a
CO2 absorber
* ''C
a'', the oxygen content of blood taken from the
pulmonary vein
The pulmonary veins are the veins that transfer oxygenated blood from the lungs to the heart. The largest pulmonary veins are the four ''main pulmonary veins'', two from each lung that drain into the left atrium of the heart. The pulmonary v ...
(representing oxygenated blood)
* ''C
v'', the oxygen content of blood from an intravenous cannula (representing deoxygenated blood)
Equation
From these values, we know that:
:
where
* ''CO'' = Cardiac Output
* ''C
a'' = Oxygen content of arterial blood
* ''C
v'' = Oxygen content 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
The arteriovenous oxygen difference, or a-vO2 diff, is the difference in the oxygen content of the blood between the arterial blood and the venous blood. It is an indication of how much oxygen is removed from the blood in capillaries as the blood ...
.
Assumed Fick determination
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 O
2 consumption at rest is O
2 per minute per square meter of
body surface area
In physiology and medicine, the body surface area (BSA) is the measured or calculated surface area of a human body. For many clinical purposes, BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal a ...
.
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
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
content of
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...
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.
The calculation of the arterial and venous oxygen concentration of the blood is a straightforward process. Almost all oxygen in the blood is bound to
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
s in the
red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s. 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
The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to ...
of hemoglobin can carry of
O2, the oxygen content of the blood (either arterial or venous) can be estimated by the following formula:
:
Assuming a hemoglobin concentration of and an oxygen saturation of 99%, the oxygen concentration of arterial blood is approximately of O
2 per L.
The saturation of mixed venous blood is approximately 75% in health. Using this value in the above equation, the oxygen concentration of mixed venous blood is approximately of O
2 per L.
Therefore, using the assumed Fick determination, the approximated cardiac output for an average man (1.9 m²) is:
:Cardiac Output = ( O
2/minute × 1.9) / ( O
2/L − O
2/L) =
Cardiac output may also be estimated with the Fick principle using production of
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
as a marker substance.
Use in renal physiology
The principle can also be used in
renal physiology
Renal physiology (Latin ''rēnēs'', "kidneys") is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassiu ...
to calculate
renal blood flow
In the physiology of the kidney, renal blood flow (RBF) is the volume of blood delivered to the kidneys per unit time. In humans, the kidneys together receive roughly 25% of cardiac output, amounting to 1.2 - 1.3 L/min in a 70-kg adult male.
It ...
.
[ - "Measuring Renal Blood Flow: Fick Principle"]
In this context, it is not oxygen which is measured, but a marker such as
para-aminohippurate
Aminohippuric acid or ''para''-aminohippuric acid (PAH), a derivative of hippuric acid, is a diagnostic agent useful in medical tests involving the kidney used in the measurement of renal plasma flow. It is an amide derivative of the amino acid g ...
. However, the principles are essentially the same.
References
External links
Overview at cvphysiology.com
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