Alveolar Gas Equation: Calculating PAO2 | Pulmonics 17/100

The alveolar gas equation derived from Dalton's law, nitrogen balance and the respiratory exchange ratio, its assumptions, and how to use it to predict alveolar oxygen at any FiO₂, PaCO₂ and barometric pressure, including why a few percent of oxygen corrects the hypoxemia of pure hypoventilation while the CO₂ and the pH stay where they were. Taught by Kasim Hassen, MD, RRT. Part of Pulmonics — respiratory care from first principles. Chapters 0:00 Hook 0:57 Objectives 1:38 The alveolus as a mixing chamber 2:45 Deriving it: R and the nitrogen balance 6:31 What the equation assumes 8:20 PAO₂ at any FiO₂, PaCO₂ and altitude 9:57 Hypoventilation alone: room air versus oxygen 11:51 Why a little oxygen fixes the oxygen and not the CO₂ 14:54 Board traps 16:16 Case resolved and recap 17:21 Recall questions 18:03 Next References 1. Fenn WO, Rahn H, Otis AB. A theoretical study of the composition of the alveolar air at altitude. Am J Physiol. 1946;146(5):637–53. doi:10.1152/ajplegacy.1946.146.5.637 2. Riley RL, Cournand A. 'Ideal' alveolar air and the analysis of ventilation-perfusion relationships in the lungs. J Appl Physiol. 1949;1(12):825–47. doi:10.1152/jappl.1949.1.12.825 3. Curran-Everett D. A classic learning opportunity from Fenn, Rahn, and Otis (1946): the alveolar gas equation. Adv Physiol Educ. 2006;30(2):58–62. doi:10.1152/advan.00076.2005 4. Hedov J, Elzén Junker J, Hallbäck M, Lönnqvist PA, Karlsson J. Novel approach to estimate respiratory quotient from the alveolar gas equation: a proof-of-concept study in pigs. Intensive Care Med Exp. 2026;14(1):118. doi:10.1186/s40635-026-00969-3 5. Dickstein ML. Extracorporeal CO2 removal and the alveolar gas equation. Am J Respir Crit Care Med. 2020;202(7):1057–8. doi:10.1164/rccm.202005-1609LE 6. Sarkar M, Niranjan N, Banyal PK. Mechanisms of hypoxemia. Lung India. 2017;34(1):47–60. doi:10.4103/0970-2113.197116 7. Fu ES, Downs JB, Schweiger JW, Miguel RV, Smith RA. Supplemental oxygen impairs detection of hypoventilation by pulse oximetry. Chest. 2004;126(5):1552–8. doi:10.1378/chest.126.5.1552 8. Severinghaus JW. Simple, accurate equations for human blood O2 dissociation computations. J Appl Physiol Respir Environ Exerc Physiol. 1979;46(3):599–602. doi:10.1152/jappl.1979.46.3.599 9. National Institute of Standards and Technology. Thermophysical properties of fluid systems (water, saturation properties). In: NIST Chemistry WebBook, NIST Standard Reference Database Number 69 [Internet]. Gaithersburg (MD): NIST; [cited 2026 Oct 6]. Available from: https://webbook.nist.gov/chemistry/fl... 10. National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration, United States Air Force. U.S. Standard Atmosphere, 1976. NOAA-S/T 76-1562; NASA-TM-X-74335. Washington (DC): U.S. Government Printing Office; 1976. Available from: https://ntrs.nasa.gov/citations/19770... 11. Giosa L, Busana M, Bonifazi M, Romitti F, Vassalli F, Pasticci I, et al. Mobilizing carbon dioxide stores. An experimental study. Am J Respir Crit Care Med. 2021;203(3):318–27. doi:10.1164/rccm.202005-1687OC 12. Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371(15):1434–45. doi:10.1056/NEJMra1003327 Educational content only — not medical advice for any individual patient.