Fick principle

Last updated

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

Contents

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.

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; [1] 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.[ citation needed ]

Variables

In Fick's original method, the following variables are measured: [2]

Equation

From these values, we know that:

where

This allows us to say

and hence calculate cardiac output.

Note that (CaCv) is also known as the arteriovenous oxygen difference.[ citation needed ]

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.[ citation needed ]

A commonly used value for O2 consumption at rest is 125  mL O2 per minute per square meter of body surface area.[ citation needed ]

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 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:[ citation needed ]

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 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 1.34  mL of O2, the oxygen content of the blood (either arterial or venous) can be estimated by the following formula:

Assuming a hemoglobin concentration of 15  g/dL and an oxygen saturation of 99%, the oxygen concentration of arterial blood is approximately 200  mL of O2 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 150  mL of O2 per L.

Therefore, using the assumed Fick determination, the approximated cardiac output for an average man (1.9 m3) is:

Cardiac Output = (125  mL O2/minute × 1.9) / (200  mL O2/L − 150  mL O2/L) = 4.75  L/min

Cardiac output may also be estimated with the Fick principle using production of carbon dioxide as a marker substance. [3]

Use in renal physiology

The principle can also be used in renal physiology to calculate renal blood flow. [4]

In this context, it is not oxygen which is measured, but a marker such as para-aminohippurate. However, the principles are essentially the same.

Related Research Articles

<span class="mw-page-title-main">Hypoxia (medicine)</span> Medical condition of lack of oxygen in the tissues

Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body. Although hypoxia is often a pathological condition, variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise.

<span class="mw-page-title-main">Cardiac output</span> Measurement of blood pumped by the heart

In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols , , or , is the volumetric flow rate of the heart's pumping output: that is, the volume of blood being pumped by a single ventricle of the heart, per unit time. Cardiac output (CO) is the product of the heart rate (HR), i.e. the number of heartbeats per minute (bpm), and the stroke volume (SV), which is the volume of blood pumped from the left ventricle per beat; thus giving the formula:

<span class="mw-page-title-main">Arterial blood gas test</span> A test of blood taken from an artery that measures the amounts of certain dissolved gases

An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe and a thin needle, but sometimes the femoral artery in the groin or another site is used. The blood can also be drawn from an arterial catheter.

<span class="mw-page-title-main">Cyanosis</span> Decreased oxygen in the blood

Cyanosis is the change of body tissue color to a bluish-purple hue, as a result of decrease in the amount of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Cyanosis is apparent usually in the body tissues covered with thin skin, including the mucous membranes, lips, nail beds, and ear lobes. Some medications may cause discoloration such as medications containing amiodarone or silver. Furthermore, mongolian spots, large birthmarks, and the consumption of food products with blue or purple dyes can also result in the bluish skin tissue discoloration and may be mistaken for cyanosis. Appropriate physical examination and history taking is a crucial part to diagnose cyanosis. Management of cyanosis involves treating the main cause, as cyanosis isn’t a disease, it is a symptom.

<span class="mw-page-title-main">Venous blood</span> Deoxygenated blood

Venous blood is deoxygenated blood which travels from the peripheral blood vessels, through the venous system into the right atrium of the heart. Deoxygenated blood is then pumped by the right ventricle to the lungs via the pulmonary artery which is divided in two branches, left and right to the left and right lungs respectively. Blood is oxygenated in the lungs and returns to the left atrium through the pulmonary veins.

<span class="mw-page-title-main">Pulse oximetry</span> Measurement of blood oxygen saturation

Pulse oximetry is a noninvasive method for monitoring blood oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy of the more accurate reading of arterial oxygen saturation (SaO2) from arterial blood gas analysis.

<span class="mw-page-title-main">Cardiac catheterization</span> Insertion of a catheter into a chamber or vessel of the heart

Cardiac catheterization is the insertion of a catheter into a chamber or vessel of the heart. This is done both for diagnostic and interventional purposes.

<span class="mw-page-title-main">Pulmonary artery catheter</span> Catheter for insertion into a pulmonary artery

A pulmonary artery catheter (PAC), also known as a Swan-Ganz catheter or right heart catheter, is a balloon-tipped catheter that is inserted into a pulmonary artery in a procedure known as pulmonary artery catheterization or right heart catheterization. Pulmonary artery catheterization is a useful measure of the overall function of the heart particularly in those with complications from heart failure, heart attack, arrythmias or pulmonary embolism. It is also a good measure for those needing intravenous fluid therapy, for instance post heart surgery, shock, and severe burns. The procedure can also be used to measure pressures in the heart chambers.

<span class="mw-page-title-main">Hypoxemia</span> Abnormally low level of oxygen in the blood

Hypoxemia is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia has many causes, and often causes hypoxia as the blood is not supplying enough oxygen to the tissues of the body.

<span class="mw-page-title-main">Oxygen–hemoglobin dissociation curve</span> Visual tool used to understand how human blood carries and releases oxygen

The oxygen–hemoglobin dissociation curve, also called the oxyhemoglobin dissociation curve or oxygen dissociation curve (ODC), is a curve that plots the proportion of hemoglobin in its saturated (oxygen-laden) form on the vertical axis against the prevailing oxygen tension on the horizontal axis. This curve is an important tool for understanding how our blood carries and releases oxygen. Specifically, the oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2), and is determined by what is called "hemoglobin affinity for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it.

A right-to-left shunt is a cardiac shunt which allows blood to flow from the right heart to the left heart. This terminology is used both for the abnormal state in humans and for normal physiological shunts in reptiles.

The Haldane effect is a property of hemoglobin first described by John Scott Haldane, within which oxygenation of blood in the lungs displaces carbon dioxide from hemoglobin, increasing the removal of carbon dioxide. Consequently, oxygenated blood has a reduced affinity for carbon dioxide. Thus, the Haldane effect describes the ability of hemoglobin to carry increased amounts of carbon dioxide (CO2) in the deoxygenated state as opposed to the oxygenated state. Vice versa, it is true that a high concentration of CO2 facilitates dissociation of oxyhemoglobin, though this is the result of two distinct processes (Bohr effect and Margaria-Green effect) and should be distinguished from Haldane effect.

A cardiac shunt is a pattern of blood flow in the heart that deviates from the normal circuit of the circulatory system. It may be described as right-left, left-right or bidirectional, or as systemic-to-pulmonary or pulmonary-to-systemic. The direction may be controlled by left and/or right heart pressure, a biological or artificial heart valve or both. The presence of a shunt may also affect left and/or right heart pressure either beneficially or detrimentally.

A pulmonary shunt is the passage of deoxygenated blood from the right side of the heart to the left without participation in gas exchange in the pulmonary capillaries. It is a pathological condition that results when the alveoli of parts of the lungs are perfused with blood as normal, but ventilation fails to supply the perfused region. In other words, the ventilation/perfusion ratio of those areas is zero.

The Shunt equation quantifies the extent to which venous blood bypasses oxygenation in the capillaries of the lung. “Shunt” and “dead space“ are terms used to describe conditions where either blood flow or ventilation do not interact with each other in the lung, as they should for efficient gas exchange to take place. These terms can also be used to describe areas or effects where blood flow and ventilation are not properly matched, though both may be present to varying degrees. Some references refer to “shunt-effect” or “dead space-effect” to designate the ventilation/perfusion mismatch states that are less extreme than absolute shunt or dead space.

<span class="mw-page-title-main">Oxygen saturation (medicine)</span> Medical measurement

Oxygen saturation is the fraction of oxygen-saturated haemoglobin relative to total haemoglobin in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal arterial blood oxygen saturation levels in humans are 96–100 percent. If the level is below 90 percent, it is considered low and called hypoxemia. Arterial blood oxygen levels below 80 percent may compromise organ function, such as the brain and heart, and should be promptly addressed. Continued low oxygen levels may lead to respiratory or cardiac arrest. Oxygen therapy may be used to assist in raising blood oxygen levels. Oxygenation occurs when oxygen molecules enter the tissues of the body. For example, blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood. Oxygenation is commonly used to refer to medical oxygen saturation.

The near-infrared (NIR) window defines the range of wavelengths from 650 to 1350 nanometre (nm) where light has its maximum depth of penetration in tissue. Within the NIR window, scattering is the most dominant light-tissue interaction, and therefore the propagating light becomes diffused rapidly. Since scattering increases the distance travelled by photons within tissue, the probability of photon absorption also increases. Because scattering has weak dependence on wavelength, the NIR window is primarily limited by the light absorption of blood at short wavelengths and water at long wavelengths. The technique using this window is called NIRS. Medical imaging techniques such as fluorescence image-guided surgery often make use of the NIR window to detect deep structures.

<span class="mw-page-title-main">Arterial blood</span> Blood in an artery

Arterial blood is the oxygenated blood in the circulatory system found in the pulmonary vein, the left chambers of the heart, and in the arteries. It is bright red in color, while venous blood is dark red in color. It is the contralateral term to venous blood.

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 circulates in the body. The a-vO2 diff and cardiac output are the main factors that allow variation in the body's total oxygen consumption, and are important in measuring VO2. The a-vO2 diff is usually measured in millilitres of oxygen per 100 millilitres of blood (mL/100 mL).

Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monoxide tension (PxCO). The subscript x in each symbol represents the source of the gas being measured: "a" meaning arterial, "A" being alveolar, "v" being venous, and "c" being capillary. Blood gas tests (such as arterial blood gas tests) measure these partial pressures.

References

  1. Fick, Adolf (9 July 1870). "Ueber die Messung dea Blutquantums in den Herzventrikela". Verhandlungen der Physikalisch-medizinische Gesellschaft zu Würzburg (in German). 2: XVI–XVII. hdl:2027/mdp.39015076673493 . Retrieved 24 Oct 2017. NB: summary of his principle is under point (4) of the proceedings.
  2. Nosek, Thomas M. "Section 3/3ch5/s3ch5_3". Essentials of Human Physiology. Archived from the original on 2016-03-24. - "Indirect Measurement of Cardiac Output"
  3. Cuschieri, J; Rivers, EP; Donnino, MW; Katilius, M; Jacobsen, G; Nguyen, HB; Pamukov, N; Horst, HM (June 2005). "Central venous-arterial carbon dioxide difference as an indicator of cardiac index". Intensive Care Medicine. 31 (6): 818–22. doi:10.1007/s00134-005-2602-8. PMID   15803301. S2CID   8311073.
  4. Nosek, Thomas M. "Section 7/7ch04/7ch04p27". Essentials of Human Physiology. Archived from the original on 2016-03-24. - "Measuring Renal Blood Flow: Fick Principle"