Alveolar air equation

The alveolar air equation is the method for calculating partial pressure of alveolar gas (P_AO₂). The equation is used in assessing if the 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.

Interpretation

To clinically interpret P_aO₂ one has to also know the patient's P_aCO₂, F_IO2 (fraction of inspired oxygen) and the P_B (barometric pressure), all components of the equation for P_AO₂.

Equations

Several possible equations exist to calculate the alveolar air.

$P_A O_2 = F_I O_2(PB-P H_2 0) - P_A CO_2 (F_I O_2 + \frac{1-F_I O_2}{R})$

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$P_A O_2 = P_I O_2 - P_A CO_2 (F_I O_2 + \frac{1-F_I O_2}{R})$

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$P_A O_2 = P_I O_2 - \frac{V_T}{V_T-V_D}(P_I O_2 - P_E O_2)$

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$P_A O_2 = \frac{P_E O_2 - P_I O_2 (\frac{V_D}{V_T})}{1- \frac{V_D}{V_T}}$

Abbreviated alveolar air equation

$P_A O_2 = \frac{P_E O_2 - P_i O_2 \frac{V_D}{V_T}}{1- \frac{V_D}{V_T}}$

P_AO₂, P_EO₂, and P_iO₂ 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.

Respiratory quotient (R)

$R = \frac{P_E CO_2 (1-F_I O_2)}{P_i O_2 - P_E O_2 - (P_E CO_2 * F_i O_2)}$

Physiologic dead space over tidal volume (VD/VT)

$\frac{V_D}{V_T} = \frac{P_a CO_2 - P_E CO_2}{P_a CO_2}$