The principle:

" 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."

It is the blood flow we are interested in: this is cardiac output. This method is the purest and most accurate means of estimating the cardiac output. It is not confused by low output states, valvular regurgitation, shunts or arrhythmias. The major source of error is the act of measuring the amount of exhaled oxygen, and the change in cardiac output over the period of measurement.

In practice, it is too fiddly to apply outside the laboratory. Also, ICU patients have inflamed lungs whose parenchyma consumes oxygen and confuses the issue further.

The principle in detail

VO2, the oxygen consumption, is simply the difference between the inspired and expired O2. You can measure it with an exhaled gas collection bag.

You can also estimate it. Conventionally, resting metabolic consumption of oxygen is

3.5 ml of O2 per kg per minute, or

125ml O2 per square meter of body surface area per minute.

Lets say the meaty pinkish lump below is the patient.

Illustration of Ficks principle

Fick teaches us that VO2 (oxygen extraction) is determined by the following equation:

VO2 equation

We can rearrange that to form an equation which calculates cardiac output on the basis of oxygen extraction:

Relationship of cardiac output to oxygen extraction

So, in a normal person, with a body surface area of 2m2 and thus with a VO2 of 250ml per minute,

CO =  250ml / (200ml – 150ml)

      = 250 / 50

      = 5 L/min

And there you have it. That is the "direct" Ficks method for measuring cardiac output.


Whom shall I reference here?...

Fick A. Uber die messung des Blutquantums in den Hertzvent rikeln.Sitzber Physik Med Ges Wurzburg July 9th: 36, 1870

That is not a link to a pdf file or article, but rather to a site which is selling the original printed article, bound together with the rest of that years issues of Verhandlungen der Physikalisch-Medicinischen Gesellschaft in Würzburg.