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.^[1]^[2]^[3]^[4]^[5]^[6]^[7]

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})

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

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

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.^[8]

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}

Mechanical ventilation

Fundamentals	Modes of mechanical ventilation Mechanical ventilation in emergencies Mechanical ventilation in neonates Nomenclature of mechanical ventilation

Modes	IMV/SIMV CMV ACV CSV PAP BPAP CPAP APRV MMV PAV ASV HFV

Related illness	ARDS Pulmonary barotrauma Pulmonary volutrauma Ventilator-associated pneumonia Oxygen toxicity Ventilator-associated lung injury

Pressure	P_EEP FiO₂ Δ_P P_IP P_S P_AW P_plat

Volumes	V_T V_E V_f

Other	C_dyn C_static P_AO₂ V_D/V_T OI A-a gradient

References

↑ Raymond L, Dolan W, Dutton R, et al: Pulmonary function and gas exchange during altitude hypoxia (abstract). Clin Res 19:147, 1971
↑ Spaur WH, Raymond LW, Knott MM, et al: Dyspnea in divers at 49.5 ATA: Mechanical not chemical in origin. Undersea Biomed Res 4:183-198, 1977
↑ Rossier P-H, Blickenstorfer E: Espace mort et hyperventilation. Helv Med Acta 13:328-332, 1946
↑ 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
↑ McNicol MW, Campbell EJM: Severity of respiratory failure: arterial blood gases in untreated patients. Lancet 1:336-338, 1965
↑ Begin R, Renzetti AD Jr: Alveolar-arterial oxygen pressure gradient: I. Comparison between an assumed and actual respiratory quotient in stable chronic pulmonary disease; Relationship to aging and closing volume in normal subjects. Respir Care 22:491-500, 1977
↑ Suwa K, Geffin B, Pontoppidan H, et al: A nomogram for deadspace requirement during prolonged artificial ventilation. Anesthesiology 29:1206-1210, 1968
↑ Fenn WO, Rahn H, Otis AB: A theoretical study of the composition of alveolar air at altitude. Am J Physiol 146:637-653, 1946

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