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Respiratory Physiology Calculator

Alveolar Gas Equation Calculator: PAO2 Estimate

Estimate PAO2 from inspired oxygen, atmospheric pressure, water vapor pressure, PaCO2, and respiratory quotient — for blood gas interpretation, A-a gradient calculation, and altitude physiology.

Quick Answer

The alveolar gas equation estimates alveolar oxygen tension (PAO2): PAO2 = FiO2 × (Patm − PH2O) − PaCO2/RQ. Sea-level defaults are FiO2 0.21, Patm 760 mmHg, PH2O 47 mmHg, and RQ 0.8. PAO2 is the calculated alveolar target; subtract measured PaO2 to obtain the A-a gradient. Adjust Patm for altitude and FiO2 for supplemental oxygen delivery method.

Alveolar Gas Equation
PAO2 = FiO2 x (Patm - PH2O) - PaCO2 / RQ
FiO2 is a fraction; pressures are in mmHg; RQ is often approximated as 0.8. Sea-level room air with PaCO2 40: PAO2 ≈ 99.7 mmHg.

Calculate PAO2

Estimate alveolar oxygen tension from FiO2, atmospheric pressure, water vapor, PaCO2, and respiratory quotient.

Inspired oxygen and pressure

Room air is 0.21. Delivered FiO2 may be approximate on low-flow devices.

Use local barometric pressure at altitude when relevant.

Blood gas values

Common default is 0.8 unless a different value is clinically justified.

Calculated Alveolar Oxygen Tension

- mmHg

-

Variable Effects on PAO2

FiO2 increase

Raising FiO2 directly increases the inspired oxygen term. Effect is linear with FiO2 × (Patm − PH2O).

PaCO2 increase

Higher PaCO2 reduces PAO2 via the PaCO2/RQ term — hypoventilation lowers alveolar oxygen even at constant FiO2.

Altitude (lower Patm)

Reduced barometric pressure lowers (Patm − PH2O), decreasing PAO2 at the same FiO2 and PaCO2.

How to Use This Calculator

1
Enter FiO2 as a fraction. For nasal cannula and mask oxygen, delivered FiO2 may be only an approximation.
2
Use local atmospheric pressure when altitude is clinically relevant; lower Patm lowers PAO2.
3
Enter PaCO2 from the arterial blood gas and use RQ 0.8 unless a different physiology assumption is needed.
4
Compare calculated PAO2 with measured PaO2 using the A-a Gradient Calculator when assessing hypoxemia mechanisms.
Worked Example

FiO2 0.21, Patm 760, PH2O 47, PaCO2 40, RQ 0.8.

PAO2 = 0.21 × (760 − 47) − 40 / 0.8 = 149.7 − 50 = 99.7 mmHg.

If measured PaO2 = 80 mmHg, A-a gradient = 99.7 − 80 = 19.7 mmHg.

Oxygen Therapy and Altitude Caveats

The equation assumes a known FiO2. On low-flow oxygen devices, actual FiO2 changes with inspiratory flow, respiratory pattern, mouth breathing, mask fit, and entrainment. Ventilator FiO2 is usually more controlled but still needs timing matched to the blood gas sample.

At altitude, the default sea-level atmospheric pressure can substantially overestimate PAO2. Adjust Patm to local barometric pressure when interpreting oxygenation in high-altitude care, transport, aviation, or research settings.

Pharma & clinical trial context

The alveolar gas equation underpins respiratory physiology training for clinical trial medical monitors, pulmonary safety reviewers, and critical care pharmacology teams assessing whether hypoxemia reflects hypoventilation, low inspired oxygen, or intrinsic gas exchange impairment. PAO2 calculation method should be documented when used in exploratory trial endpoints.

Compute the A-a gradient with the A-a Gradient Calculator, assess ventilated patient severity via the Oxygenation Index, and quantify CO2 clearance burden with the Ventilation Index when respiratory endpoints appear in ICU or pulmonary drug development protocols.

Altitude physiology studies, inhaled drug trials, and ARDS intervention research should pre-specify Patm source (measured versus standard atmosphere), PH2O assumption, and RQ default in statistical analysis plans to ensure PAO2 reproducibility across sites.

Evidence & sources

Frequently Asked Questions

The alveolar gas equation estimates alveolar oxygen tension, PAO2, from inspired oxygen fraction, atmospheric pressure, water vapor pressure, arterial carbon dioxide tension, and respiratory quotient. It represents the oxygen pressure in alveoli if CO2 and RQ assumptions hold.
The simplified form is PAO2 = FiO2 × (Patm − PH2O) − PaCO2/RQ. FiO2 is a fraction; pressures are in mmHg. The PaCO2/RQ term reflects CO2 displacement of alveolar oxygen — higher PaCO2 or lower RQ reduces calculated PAO2.
Common defaults are FiO2 0.21 on room air, atmospheric pressure 760 mmHg, water vapor pressure 47 mmHg at body temperature (37°C), and respiratory quotient 0.8. With PaCO2 40 mmHg, sea-level room-air PAO2 ≈ 99.7 mmHg.
Altitude lowers atmospheric pressure, which lowers inspired oxygen pressure and calculated PAO2. At 5,000 feet (~632 mmHg Patm), room-air PAO2 is substantially lower than at sea level. Use local barometric pressure instead of the 760 mmHg default when altitude matters.
Supplemental oxygen increases FiO2 and therefore calculated PAO2, but delivered FiO2 can be variable with nasal cannula, Venturi masks, and non-rebreather masks depending on flow, fit, and respiratory pattern. Ventilator FiO2 is usually more controlled but requires timing matched to the blood gas sample.
PAO2 is calculated alveolar oxygen tension from the alveolar gas equation. PaO2 is measured arterial oxygen tension from an arterial blood gas. The difference (PAO2 − PaO2) is the A-a gradient, which quantifies gas exchange impairment.
Inspired gas is humidified to body temperature in the airways, so water vapor occupies part of the total barometric pressure. At 37°C, PH2O is approximately 47 mmHg. Only the remaining pressure (Patm − PH2O) is available for oxygen and CO2 partial pressures.
RQ 0.8 is standard for mixed metabolism. High carbohydrate intake pushes RQ toward 1.0; fasting or ketogenic states may lower RQ. RQ affects the PaCO2/RQ term — a higher RQ reduces the CO2 correction and increases calculated PAO2 slightly.
Yes. With normal PaCO2 (40 mmHg) and RQ 0.8 at sea level, room-air PAO2 is approximately 100 mmHg — higher than the commonly cited "100% oxygen saturation" reference because alveolar PO2 exceeds arterial PO2 when gas exchange is efficient.
Yes, when ventilator FiO2 is known and PaCO2 is measured from an arterial blood gas sampled near documented ventilator settings. PEEP and mean airway pressure do not appear directly in the simplified equation — they affect gas exchange efficiency reflected in the A-a gradient, not in PAO2 calculation itself.
The equation assumes uniform alveolar gas composition, steady-state CO2, known FiO2, and a single RQ value. It does not account for diffusion limitation, shunt, or V/Q heterogeneity — those appear when comparing calculated PAO2 to measured PaO2 via the A-a gradient.
No. PAO2 is an estimate for educational and calculation-checking purposes. Clinical decisions require measured PaO2, pH, PaCO2, hemoglobin, oxygen saturation, clinical context, and qualified interpretation — not calculated PAO2 alone.

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