alveolargassligningen

The alveolargassligningen, known in English as the alveolar gas equation, is a fundamental relation in respiratory physiology. It describes how the partial pressure of oxygen in the alveoli (PAO2) is determined from the inspired oxygen fraction and the alveolar level of carbon dioxide, and it forms the basis for interpreting oxygenation in arterial blood gases.

Formally, PAO2 = FiO2 × (Pb − PH2O) − (PaCO2 / R). Here FiO2 is the inspired oxygen fraction, Pb is barometric pressure, PH2O is the water vapor pressure in humidified inspired air (about 47 mmHg at body temperature), PaCO2 is the arterial partial pressure of carbon dioxide, and R is the respiratory exchange ratio (the ratio of CO2 production to O2 consumption, typically ~0.8). Some texts use PACO2 (the alveolar CO2 tension) instead of PaCO2; in healthy individuals these values are close.

At sea level (Pb ≈ 760 mmHg) with FiO2 = 0.21, PH2O ≈ 47 mmHg, and PaCO2 ≈ 40 mmHg, PAO2 ≈ 0.21 × (760 − 47) − 40/0.8 ≈ 150 − 50 ≈ 100 mmHg. The alveolar-arterial gradient (A-aO2 = PAO2 − PaO2) is used to assess oxygenation status; a normal A-a gradient is roughly 5–15 mmHg in young adults and increases with age.

Clinically, the equation is used to estimate expected PAO2, interpret hypoxemia, and distinguish causes such as hypoventilation, diffusion limitation, V/Q mismatch, or right-to-left shunts. It also helps understand the effects of altitude and supplemental oxygen on alveolar oxygen tension.

Limitations include assumptions of steady-state conditions and exclusion of complex ventilation-perfusion abnormalities; the value of R and PH2O can vary with physiology and environment.