Fick's Principle for Cardiac Output (CO) Measurement - QS Study
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Fick’s Principle for Cardiac Output (CO) Measurement

Cardiac Output is simply the amount of blood pumped by the heart per minute. It is the quantity of blood pumped into aorta each minute by the heart. Necessarily, the CO is the product of the heart rate, which is the number of beats per minute, and the stroke volume, which is amount pumped per beat. It is an important measure of cardiovascular function.

Methods of measurements of Cardiac Output (CO)

  1. Direct method: For animal
  2. Indirect method: For humans. Two methods are applicable to humans.
  3. Fick’s principle method
  4. Indicator ditulation method.


Fick’s principle: The Fick’s principle states that the amount of a substance taken up by an organ (or by the whole body) per unit of time is equal to the arterial level of the substance minus the venous level.

Consumption of a substance (ml/min)

Arterio-venous ditten•nce (AO2 –VO2) of that substance x Blood flow (CO)

Method: The Fick’s principle can be used to determine CO by measuring the amount of oxygen consumed by the body in a given period and dividing this value by the A-V difference across the lung. Because systemic arterial blood has the same oxygen content in all parts of the body, the arterial oxygen content can be measured in a sample obtained from any convenient artery.

A sample of venous blood in the pulmonary artery is obtained by the mean of a cardiac catheter.


Lungs: Oxygen used = 250 ml /min.

Oxygen = 140 ml/L Rt heart  → CO = 5000 ml/min → Oxygen = 190 ml/L Rt heart

Output of Lt ventricle or

CO = [Oxygen Consumption (ml/min)] / [AO2.VO2]

= (250 ml/min) / (190-140) ml

= (250 ml/min) / 50 ml/L = 5 L/min.

Here, Oxygen consumption in lung = 250 ml/min

Arterial blood oxygen level = 190 ml/L

Venous blood Oxygen level = 140 ml/L in the pulmonary atery.