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Home > Cardiac output

So if there are 70 beats per minute, and 70 ml blood is ejected with each beat of the heart, the cardiac output is 4900 ml/minute. This value is typical for an average adult at rest, although cardiac output may reach up to 30 liters/minute during extreme exercise.

When cardiac output increases in a healthy but untrained individual, most of the increase can be attributed to increase in heart rate. Change of posture, increased sympathetic nervous system activity, and decreased parasympathetic nervous system activity can also increase cardiac output. Heart rate can vary by a factor of approximately 3, between 60 and 180 beats per minute, whilst stroke volume can vary between 70 and 120 ml, a factor of only 1.5.

1 Measuring Cardiac Output

There are many invasive and several non-invasive methods for measuring cardiac output in mammals.

An extremely crude non-invasive method, often used in the teaching of physiology to under-graduates, reasons as follows:

• The pressure in the heart rises as blood is forced into the aorta
• The more stretched the aorta, the greater the pulse pressure
• In healthy young subjects, each additional 2ml of blood results in a 1 mmHg rise in pressure (This is the crude assumption, which even if the subject did have a model aorta, the elastic properties of the aorta mean that it offers increased resistance to expansion at increased pressures)
• Therefore Stroke volume = 2ml x Pulse pressure
• Cardiac Output is therefore 2ml x Pulse Pressure x Heart Rate

1.1 The Fick principle

Developed by Adolf Eugen Fick ( 1829 - 1921), this involves measuring:

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

From these values, we know that:

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

This allows us to say

and hence calculate cardiac output. In reality, this method is rarely used these days due to the difficulty of collecting and analysing the gas concentrations.

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:

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 serrogate 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 moleculeIn science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of multiple atoms joined by shared pairs of electrons in a covalent bond''. It may consist of atoms in the red blood cellRed blood cells are the most common type of blood cell and are the vertebrate body's principal means of delivering oxygen to body tissues via the blood. Red blood cells are also known as erythrocytes from Greek erythros for "red" and kytos for "hollow", ns. 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 gramFor other meanings of gram see gram (disambiguation). The gram (also spelt gramme is a unit of measurement of mass, and is defined in the SI system of units as one thousandth of a kilogram. See 1 E -3 kg for comparisons with other masses. The symbol for g 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:

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Topics: Cardiac Output Blood Oxygen Heart Content Pulmonary Arterial...

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