Measuring cardiac output (Fick principle)

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Measuring cardiac output (Fick principle)

Cardiovascular system


Measuring cardiac output (Fick principle)


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Fick principle p. 292

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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.


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.


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

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