VO2 for Dummies, Chapter 1:

VO2 means oxygen consumption and usually refers to
the volume of oxygen consumed in 1 minute.

The following is written in the context of VO2 measurements for fitness or athletic evaluation. Clinical VO2 testing most often involves the monitoring of EKG and other physiologic indices,
and the reason for the test most likely will be to assist in diagnosis of disease or measurement
of disability or progress of rehabilitation.

The air we breathe contains roughly 21% oxygen (O2), 78% Nitrogen (N2) and various trace gases, such as Argon, Carbon Dioxide (CO2) and a few more.
We can ignore N2 in our discussion, since it does not partake in metabolism.

Note: Wondering why the "2" as in O2 and CO2? Perhaps you had the measles when this subject was discussed in school. The reason
is that some gas molecules get lonesome and always hang around in pairs.
Of course, your chemistry teacher would have a more elegant explanation, but would we Dummies understand it?

So we inhale 21% O2 and exhale, typically, 17%O2 and about 4% CO2. We have consumed, "burnt", metabolized roughly 4% oxygen. Of course, nothing is ever that simple, because the first part of your exhalation is the very last part of your inhalation. So it is still nearly 21% O2, but then as the source of exhaled gas comes from deeper in the lungs, it contains more and more CO2, and less and less O2. This change in gas concentration occurs rapidly, and the precise measurement requires some sophisticated instruments.

Much easier to measure are mixed exhaled gases. Assume you exhale into a plastic bag, close the bag quickly and massage it so that you are sure the gases are mixed. Now a single measurement of this mixture will give you mixed exhaled O2 and CO2, again, ignoring N2. As I said above, the typical mixture will contain from 16% to 17% O2 and 3% to 4% CO2. That in itself tells us only that we are alive, it does not tell us how much O2 we consumed. For that we need to know or measure the volume of the bag. Exhaled volume is mostly expressed as "VE" (Volume, exhaled) or "minute ventilation". You guessed it, it means the volume exhaled in 1 minute.

In practice, we want to measure not just one breath, but the gas concentrations and volumes of many breaths over longer time periods. This early method of gas collection in large bags, known as the "Douglas Bag Method", is still practiced by some purists and for teaching purposes, but modern, computerized instruments greatly simplify those measurements. Computers also take care of necessary corrections for temperature, humidity and barometric pressure.

Now to calculate VO2, a simplified equation might look like this:

VO2 = VE x (O2inspired - O2expired)

In plain language, this means VO2 equals the total volume of gas exhaled in 1 minute (VE) multiplied by the difference between inhaled and exhaled oxygen.
So if you exhaled a total of 10 liters of gas during 1 minute, and your mixed exhaled O2 is measured at 17%, then

VO2 = 10 x (0.21 - 0.17)

(Note that 21% and 17% must be written as a fraction,
which is 0.21 and 0.17)

simplified to: VO2 = 10 x 0.04 = 0.4

so your VO2 in this example would be 0.400 liters (or 400 milliliters) per minute, which is close to typical for a person at rest.

But that is usually not the end of the calculation. Surely, it is obvious that the VO2 of a mouse will be so much less than that of a human. Just the same, the basic VO2 of a small person will be less than that of a much larger person. It is conceivable, therefore, that a maximum oxygen consumption (VO2max) of 2 liters of a 5 foot tall person may actually make him or her more fit that the VO2max of 2.5 liters of a 6 foot tall person.

How do we take care of that?

Well, we report VO2 in relation to body weight in kilogram. For this calculation liters are converted to milliliters by multiplying liters x 1000.

Example: The 5-footer has a body weight of 60kg, so her VO2max of 2 liters divided by her body weight equals to 33 milliliters per kg (ml/kg). (2000 divided by 60)

The 6-footer weighs 90kg, so 2500 divided by 90 = 27.7 milliliters per kg.
You see, the 5-footer really has a higher VO2max when expressed in milliliters per kilogram of body weight. It's not only fair, there is just no other way to compare the mouse to the elephant (or the human).

Naturally, all this is still a bit of oversimplification, but good enough for us Dummies.

To continue to chapter 2, click here:
Why measure VO2

Copyright (C) John Hoppe 2008
12-12-08