Medicine:Alveolar air equation
The alveolar air equation is the method for calculating partial pressure of alveolar gas (PAO2). 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 PaO2 one has to also know the patient's PaCO2, FIO2 (fraction of inspired oxygen) and the PB (barometric pressure), all components of the equation for PAO2.
Equations
Several possible equations exist to calculate the alveolar air.[1][2][3][4][5][6][7]
- [math]\displaystyle{ \begin{align} P_A \ce{O2} & = F_I \ce{O2} \left(PB-P \ce{H2O}\right) - P_A C\ce{O2} \left(F_I \ce{O2} + \frac{1-F_I \ce{O2}}{R}\right) \\ & = P_I \ce{O2} - P_A C\ce{O2} \left(F_I \ce{O2} + \frac{1-F_I \ce{O2}}{R}\right) \\ & = P_I \ce{O2} - \frac{V_T}{V_T-V_D}\left(P_I \ce{O2} - P_E \ce{O2}\right) \\ & = \frac{P_E \ce{O2} - P_I \ce{O2} \left(\frac{V_D}{V_T}\right)}{1- \frac{V_D}{V_T}} \end{align} }[/math]
Abbreviated alveolar air equation
- [math]\displaystyle{ P_A \ce{O2} = \frac{P_E \ce{O2} - P_i \ce{O2} \frac{V_D}{V_T}}{1- \frac{V_D}{V_T}} }[/math]
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.[8]
Respiratory quotient (R)
- [math]\displaystyle{ R = \frac{P_E \ce{CO2} (1-F_I \ce{O2})}{P_i \ce{O2} - P_E \ce{O2} - (P_E \ce{CO2} * F_i \ce{O2})} }[/math]
Physiologic dead space over tidal volume (VD/VT)
- [math]\displaystyle{ \frac{V_D}{V_T} = \frac{P_a \ce{CO2} - P_E \ce{CO2} }{P_a \ce{CO2} } }[/math]
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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