Alveolar Gas Equation Calculator

How does this alveolar gas equation calculator work?

This is a health tool that calculates the partial alveolar pressure of oxygen during the alveolar ventilation exchange based on the alveolar gas equation released in 1946.

There are five variables belonging to the respiratory process that are taken into account and should be filled in this alveolar gas equation calculator:

■ F_IO₂ – fraction of inspired gas that is oxygen, either expressed in percentage or decimal, usually 21%, 0.21 in normal air;

■ P_ATM – atmospheric pressure, reference value 760 mmHg;

■ PH₂O – saturated vapor pressure of water at body temperature and P_ATM;

■ p_aCO₂ – arterial partial pressure of carbon dioxide during the alveolar exchange;

■ RQ – respiratory exchange ratio or the respiratory quotient, meaning the ratio between pO₂ and pCO₂;

■ And the resultant is p_AO₂ which is the partial pressure of alveolar oxygen also written as pO₂.

The AGE formula is:

if this applies

then AGE can be simplified as:

The partial pressure of oxygen (pO₂) in the pulmonary alveoli is a very useful variable to compute because of its usage in the alveolar-arterial gradient of oxygen and in the right-to-left cardiac shunt.

The p_AO₂ can only be measured indirectly through the above formula. There are several assumptions that need to be made in order to consider the result from alveolar ventilation equation:

■ The inspired gas needs to be “pure” and not contain carbon dioxide or water;

■ The other gases in the inspired gas i.e. nitrogen are in equilibrium with their dissolved states in the blood;

■ Alveolar carbon dioxide is in equilibrium with the arterial blood, meaning that the alveolar and arterial pCO₂s are equal;

■ The equation respects the ideal gas law;

■ The alveolar gas is saturated with water.

The partial alveolar pressure of oxygen is determined by two opposing processes which are balanced in order to produce the movement of the oxygen in the alveolus and the extraction of carbon dioxide.

The first is created by the entry of oxygen and influenced by the rate of alveolar ventilation and by the starting partial pressure of oxygen in the external environment.

The second refers to the pulmonary diffusion rate of the oxygen in the capillaries.

When this fine balance is disturbed, arterial oxygenation has to suffer leading to serious pulmonary conditions.

Example of a calculation

Taking the following variables:

F_IO₂ = 0.21

P_ATM = 760 mmHg

PH₂O = 47 mmHg

p_aCO₂ = 40 mmHg

RQ = 0.8

It means that p_AO₂ is equal to 99.73 mmHg.

References

1) Curran-Everett D. (2006) A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation. Adv Physiol Educ; 30(2):58-62.

2) Cruickshank S, Hirschauer N. (2004) The alveolar gas equation. Oxford Journals Medicine & Health BJA: CEACCP Volume 4, Issue 1Pp. 24-27.

3) Conkin J. (2016) Equivalent Air Altitude and the Alveolar Gas Equation. Aerosp Med Hum Perform; 87(1):61-4.

4) Carroll GC. (1985) Misapplication of alveolar gas equation. N Engl J Med; 312(9):586.