Alveolar gas equation

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Alveolar air equation
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The alveolar gas equation is the method for calculating

lungs are properly transferring oxygen into the blood
. The alveolar air equation is not widely used in clinical medicine, probably because of the complicated appearance of its classic forms. The
alveolar-arterial gradient of oxygen and the amount of right-to-left cardiac shunt, which are both clinically useful quantities. However, it is not practical to take a sample of gas from the alveoli in order to directly measure the partial pressure of oxygen. The alveolar gas equation allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable. It was first characterized in 1946.[1]

Assumptions

The equation relies on the following assumptions:

Equation

If is small, or more specifically if then the equation can be simplified to:

where:

Quantity Description Sample value
The alveolar partial pressure of oxygen () 107 mmHg (14.2 kPa)
The fraction of inspired gas that is oxygen (expressed as a decimal). 0.21
PATM The prevailing atmospheric pressure 760 mmHg (101 kPa)
The saturated vapour pressure of water at body temperature and the prevailing atmospheric pressure 47 mmHg (6.25 kPa)
The arterial partial pressure of carbon dioxide ( ) 40 mmHg (5.33 kPa)
RER The respiratory exchange ratio 0.8

Sample Values given for air at sea level at 37 °C.

Doubling will double .

Other possible equations exist to calculate the alveolar air.[2][3][4][5][6][7][8]

Abbreviated alveolar air equation

PAO2, PEO2, and PiO2 are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and VD/VT is the ratio of physiologic dead space over tidal volume.[9]

Respiratory quotient (R)

Physiologic dead space over tidal volume (VD/VT)

See also

References

  1. S2CID 42010762
    .
  2. ^ Raymond L, Dolan W, Dutton R, et al: Pulmonary function and gas exchange during altitude hypoxia (abstract). Clin Res 19:147, 1971
  3. Undersea Biomed Res
    4:183-198, 1977
  4. ^ Rossier P-H, Blickenstorfer E: Espace mort et hyperventilation. Helv Med Acta 13:328-332, 1946
  5. ^ Riley RL, Lilienthal JL Jr, Proemmel DD, et al: On the determination of the physiologically effective pressures of oxygen and carbon dioxide in alveolar air.
    Am J Physiol
    147:191-198, 1946
  6. ^ McNicol MW, Campbell EJM: Severity of respiratory failure: arterial blood gases in untreated patients. Lancet 1:336-338, 1965
  7. Respir Care
    22:491-500, 1977
  8. ^ Suwa K, Geffin B, Pontoppidan H, et al: A nomogram for deadspace requirement during prolonged artificial ventilation. Anesthesiology 29:1206-1210, 1968
  9. Am J Physiol
    146:637-653, 1946

External links