# Measuring cardiac output (Fick principle)

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### Measuring cardiac output (Fick principle)

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#### Content Reviewers:

Rishi Desai, MD, MPH

#### Contributors:

Cardiac output is defined as the volume of blood ejected by the left ventricle per unit time.

The volume of blood is called the stroke volume, and the unit used to measure cardiac output is one minute.

So cardiac output can be calculated as stroke volume which is the volume of blood pumped out of the left ventricle per beat, times the number of heartbeats per minute, which is around 70.

Another way to calculate cardiac output is by using the Fick Principle, which simply states that at rest, the oxygen going into an organ minus the oxygen left out of an organ must equal how much oxygen that organ has used.

Using the Fick Principle’s equation of (cardiac output = oxygen consumption/arterio-venous oxygen difference), cardiac output can be calculated.

Both methods render normal cardiac output to around 5 litres per minute on average.

To measure cardiac output, we first have to take a look at the left ventricle.

There’s a moment when the left ventricle is fully relaxed.

It occurs at the end of filling or diastole, also called the end-diastolic point, and the volume of blood within the left ventricle is called the end-diastolic volume, and it’s about 120 milliliters.

Then the left ventricle contracts, forcing blood through the aorta and into the whole arterial system.

After that is another moment when the left ventricle is fully contracted.

It occurs at the end of contraction or systole, also called the end-systolic point, and the volume of blood within the left ventricle is called end-systolic volume, and it’s about 50 milliliters.

So, end-diastolic volume minus end-systolic volume, gives us the stroke volume, which is the volume of blood that the left ventricle ejects with every heartbeat, or stroke.

In this case, the stroke volume is 120 minus 50, which equals 70 milliliters.

Every minute, though, our heart beats about 70 times, on average.

So if we multiply the stroke volume times the heart rate, or times our heart beats per minute, we’ll get the cardiac output, which is the total volume of blood that the left ventricle ejects in one minute.

Using the stroke volume of 70 ml/ beat and a resting heart rate of 70 beats/minute, cardiac output is 4900 ml/min, or 4.9 liters/min.

Remember that a normal adult body contains approximately 5 liters of blood, so this means that every last drop of blood travels through the left ventricle at least once every minute!

Another way to measure cardiac output which does not involve using the stroke volume or heart rate is called the Fick principle.

The Fick Principle simply states that at rest, the oxygen going into an organ minus the oxygen left out of an organ must equal how much oxygen that organ has used.

Using the Fick Principle’s equation of (cardiac output = oxygen consumption/arteriovenous oxygen difference), cardiac output can be calculated.

By convention, the “organ” used for calculation is actually the body.

So the Fick principle states that at rest the oxygen in the pulmonary veins going to the body (full of oxygen) minus the oxygen left in the pulmonary arteries coming back from the body (oxygen left after the body has used some) must equal how much oxygen the body has used.

The Fick principle also makes the assumption that at rest, the cardiac output for the right and left ventricle are the same.

Summary

The Fick principle is a mathematical equation that calculates cardiac output by multiplying the difference between arterial and venous oxygen partial pressures by the blood flow through the vascular bed. In other words, it takes into account how much oxygen is being delivered to and removed from the tissues. This makes it a more accurate measure of cardiac output than simply measuring heart rate or blood pressure.

Sources
1. "Medical Physiology" Elsevier (2016)
2. "Physiology" Elsevier (2017)
3. "Principles of Anatomy and Physiology" Wiley (2014)
4. "" Critical Care (2002)
5. "Methods in pharmacology: measurement of cardiac output" British Journal of Clinical Pharmacology (2011)
6. "Human Anatomy & Physiology" Pearson (2018)