Alveolar Oxygen Tension Calculator

Calculates PAO₂ from FiO₂, barometric pressure, water vapor pressure, PaCO₂, and RQ.

Inputs

FiO₂ %

Barometric pressure, P atm mmHg

Water vapor pressure, PH₂O mmHg

PaCO₂ mmHg

Respiratory quotient, RQ

Equation:

Solving steps:

P atm − PH₂O

—

mmHg

CO₂ term

—

mmHg

PAO₂

—

mmHg

Reference: Sea-level defaults are P atm 760 mmHg and PH₂O 47 mmHg. The reference notes that when FiO₂ is over 60%, RQ should not be used.

Alveolar oxygen tension estimates oxygen pressure in the alveolus using the alveolar gas equation.

PAO₂ is used for A–a gradient, a/A ratio, shunt interpretation, and oxygenation assessment.

The equation is sensitive to FiO₂ entry, barometric pressure, and whether the high-FiO₂ simplification is appropriate.

Attached equations reference supplied by the user for calculator equation confirmation.