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]

{\begin{aligned}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{aligned}}

Abbreviated alveolar air equation

P_{A}{\ce {O2}}={\frac {P_{E}{\ce {O2}}-P_{i}{\ce {O2}}{\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}{\ce {CO2}}(1-F_{I}{\ce {O2}})}{P_{i}{\ce {O2}}-P_{E}{\ce {O2}}-(P_{E}{\ce {CO2}}*F_{i}{\ce {O2}})}}

Physiologic dead space over tidal volume (VD/VT)

{\frac {V_{D}}{V_{T}}}={\frac {P_{a}{\ce {CO2}}-P_{E}{\ce {CO2}}}{P_{a}{\ce {CO2}}}}

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

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[1] Raymond L, Dolan W, Dutton R, et al: Pulmonary function and gas exchange during altitude hypoxia (abstract). Clin Res 19:147, 1971

[2] 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

[3] Rossier P-H, Blickenstorfer E: Espace mort et hyperventilation. Helv Med Acta 13:328-332, 1946

[4] 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

[5] McNicol MW, Campbell EJM: Severity of respiratory failure: arterial blood gases in untreated patients. Lancet 1:336-338, 1965

[6] 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

[7] Suwa K, Geffin B, Pontoppidan H, et al: A nomogram for deadspace requirement during prolonged artificial ventilation. Anesthesiology 29:1206-1210, 1968

[8] Fenn WO, Rahn H, Otis AB: A theoretical study of the composition of alveolar air at altitude. Am J Physiol 146:637-653, 1946

Mechanical ventilation
Fundamentals	Modes of mechanical ventilation Mechanical ventilation in emergencies Nomenclature of mechanical ventilation
Modes	IMV/SIMV CMV ACV CSV PAP BPAP/NIV CPAP APRV MMV PAV ASV HFV
Related illness	ARDS Atelectotrauma Biotrauma Pulmonary barotrauma Pulmonary volutrauma Rheotrauma 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 Mechanical power