Alveolar gas equation

The alveolar gas equation allows the oxygen partial pressure in the alveoli of the lungs (p _A O ₂ ) to be calculated. This in turn is required to calculate the alveolo-arterial oxygen pressure difference or the size of the right-left shunt , both of which are clinically interesting measures. Since you cannot take gas samples from the alveoli in order to measure the oxygen partial pressure directly, you have to calculate it indirectly. This type of calculation was first described in 1946.

requirements

The equation is based on the following assumptions:

• The inhaled gas does not contain carbon dioxide (CO ₂ ).
• Nitrogen (and all other gases except oxygen) are in equilibrium with their dissolved part in the blood.
• The inhaled and alveolar gases follow the ideal gas equation .
• The carbon dioxide (CO ₂ ) in the alveolar gas is in equilibrium with the arterial blood; H. the partial pressures in the alveolus and in the arterial blood are equal.
• The alveolar air is saturated with water vapor .

equation

${\displaystyle p_{A}O_{2}=F_{I}O_{2}(P_{ATM}-pH_{2}O)-{\frac {p_{a}CO_{2}(1-F_{I}O_{2}(1-RQ))}{RQ}}}$

If F _I O _{2 is} small, or more precisely if , then the equation can be simplified as follows: ${\displaystyle F_{I}O_{2}(1-RQ)\ll 1}$

${\displaystyle p_{A}O_{2}\approx F_{I}O_{2}(P_{ATM}-pH_{2}O)-{\frac {p_{a}CO_{2}}{RQ}}}$

size	description	Sample value
p A O 2	The alveolar oxygen partial pressure (pO 2 )	107 mmHg (14.2 kPa)
F I O 2	The percentage of oxygen in the inhaled gas	0.21
P ATM	The prevailing air pressure	760 mmHg (101 kPa)
pH 2 O	The water vapor pressure at body temperature	47 mmHg (6.25 kPa)
p a CO 2	The partial pressure of carbon dioxide in arterial blood (pCO 2 )	40 mmHg (5.33 kPa)
RQ	The respiratory quotient	0.8

(Example values ​​for air at sea level at 37 ° C.)

Web links

• Free interactive model of the simplified and complete versions of the alveolar gas equation (AGE)
• S. Cruickshank, N. Hirschauer: The alveolar gas equation in Continuing Education in Anaesthesia, Critical Care & Pain, Volume 4 Number 1 2004

Individual evidence

1. ^ Curran-Everett D: A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation Archived from the original on April 15, 2013. In: Adv Physiol Educ . 30, No. 2, June 2006, pp. 58-62. doi : 10.1152 / advan.00076.2005 . PMID 16709734 . Accessed December 31, 2017.