Learning Center/Formulas/PAO₂

oxygenation

Alveolar Oxygen Tension

PAO₂ = FiO₂ × (Patm − 47) − PaCO₂ / RQ

Normal: ~100 mmHg on room air at sea level (FiO₂ 0.21, PaCO₂ 40)

Variables

PAO₂

Alveolar partial pressure of oxygen(mmHg)

Normal: ~100 mmHg on room air

FiO₂

Fraction of inspired oxygen(decimal)

Normal: 0.21 (room air)

Patm

Atmospheric pressure(mmHg)

Normal: 760 mmHg at sea level

Water vapor pressure at 37°C(mmHg)

Normal: 47 mmHg

PaCO₂

Arterial CO₂ tension(mmHg)

Normal: 35–45 mmHg

Respiratory quotient(unitless)

Normal: 0.8 (assumed)

What It Measures

PAO₂ represents the theoretical oxygen tension in the alveolar gas. It is the driving pressure available to push oxygen across the alveolar-capillary membrane into pulmonary blood. The alveolar gas equation accounts for the dilution of oxygen by water vapor and CO₂. It is not measured directly but calculated from known variables.

When to Use

Used as a prerequisite to calculate the A-a gradient. Required whenever assessing whether hypoxemia is due to a problem in gas exchange (elevated A-a gradient) or simply low inspired oxygen/hypoventilation (normal A-a gradient).

Interactive Calculator

FiO₂(decimal)

Atmospheric Pressure(mmHg)

PaCO₂(mmHg)

Worked Examples

Room air at sea level

→ ~100 mmHg

Normal alveolar oxygen tension on room air

Patient on 60% O₂

→ ~379 mmHg

Markedly elevated PAO₂; any measured PaO₂ < 360 indicates A-a gradient elevation

COPD with high CO₂

→ ~75 mmHg

Elevated PaCO₂ displaces alveolar O₂, contributing to hypoxemia

Clinical Significance

A calculated PAO₂ that is much higher than the measured PaO₂ indicates impaired gas exchange (elevated A-a gradient) from V/Q mismatch, shunt, or diffusion impairment. If PAO₂ is low but the A-a gradient is normal, the problem is hypoventilation or low FiO₂.

Memory Tips

💡PAO₂ uses the ALVEOLAR gas equation — think of alveoli as a bucket: you fill it with FiO₂ × dry pressure, but CO₂ takes up some space
💡The 47 mmHg for water vapor is always subtracted first from atmospheric pressure
💡RQ of 0.8 means for every 10 mmHg of CO₂ produced, about 12.5 mmHg of O₂ is consumed (1/0.8)

NBRC Pearls

Normal PAO₂ on room air ≈ 100 mmHg; increases by ~5 mmHg for every 1% increase in FiO₂ above 21%
PAO₂ falls with elevated PaCO₂ — COPD patients with CO₂ retention will have lower PAO₂ even without lung disease
At high altitude, Patm falls, reducing PAO₂ — explains why supplemental O₂ is needed on aircraft

← All Formulas

Variables

PAO₂

Alveolar partial pressure of oxygen(mmHg)

Normal: ~100 mmHg on room air

FiO₂

Fraction of inspired oxygen(decimal)

Normal: 0.21 (room air)

Patm

Atmospheric pressure(mmHg)

Normal: 760 mmHg at sea level

Water vapor pressure at 37°C(mmHg)

Normal: 47 mmHg

PaCO₂

Arterial CO₂ tension(mmHg)

Normal: 35–45 mmHg

Respiratory quotient(unitless)

Normal: 0.8 (assumed)

What It Measures

Memory Tips

💡PAO₂ uses the ALVEOLAR gas equation — think of alveoli as a bucket: you fill it with FiO₂ × dry pressure, but CO₂ takes up some space

💡The 47 mmHg for water vapor is always subtracted first from atmospheric pressure

💡RQ of 0.8 means for every 10 mmHg of CO₂ produced, about 12.5 mmHg of O₂ is consumed (1/0.8)

NBRC Pearls

Normal PAO₂ on room air ≈ 100 mmHg; increases by ~5 mmHg for every 1% increase in FiO₂ above 21%

PAO₂ falls with elevated PaCO₂ — COPD patients with CO₂ retention will have lower PAO₂ even without lung disease

At high altitude, Patm falls, reducing PAO₂ — explains why supplemental O₂ is needed on aircraft