Thursday, June 25, 2026
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Respiratory Physiology Calculator

A-a Gradient Calculator: PAO2 − PaO2

Calculate alveolar oxygen tension using the alveolar gas equation, then compute the alveolar-arterial oxygen gradient with optional age-adjusted expected gradient for hypoxemia mechanism assessment.

Quick Answer

The A-a gradient (alveolar-arterial oxygen gradient) is PAO2 minus measured PaO2, where PAO2 comes from the alveolar gas equation: PAO2 = FiO2 × (Patm − PH2O) − PaCO2/RQ. An elevated gradient suggests impaired oxygen transfer from alveoli to arterial blood — from V/Q mismatch, diffusion limitation, or shunt — but does not identify the cause alone. Age-adjusted expected gradient on room air is commonly age/4 + 4 mmHg.

Alveolar Gas Equation and A-a Gradient
PAO2 = FiO2 x (Patm - PH2O) - PaCO2 / RQ
A-a gradient = PAO2 - PaO2
Pressures in mmHg. FiO2 is a fraction. Common defaults: Patm 760, PH2O 47, RQ 0.8. Expected gradient ≈ age/4 + 4 on room air.

Calculate A-a Gradient

Compute alveolar oxygen tension from the alveolar gas equation, then subtract measured PaO2.

Inspired oxygen and pressure

Room air is 0.21; 50% oxygen is 0.50.

Blood gas values

Common default is 0.8 unless a different physiology assumption is justified.

Optional. Displays expected gradient as age / 4 + 4 mmHg.

A-a Gradient
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mmHg
Calculated PAO2
-
mmHg
Expected Gradient
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mmHg, if age entered

Hypoxemia Mechanism Reference

Normal A-a gradient

Hypoxemia likely from hypoventilation (high PaCO2) or low inspired PO2 (altitude, low FiO2). Correct ventilation or oxygen source.

Elevated A-a gradient

Suggests V/Q mismatch, diffusion limitation, or shunt. Does not specify which — correlate with clinical context and imaging.

Age adjustment

Expected gradient ≈ age/4 + 4 mmHg on room air. Gradient increases with normal aging even without lung disease.

How to Use This Calculator

1
Enter FiO2 as a fraction. Room air is 0.21; 50% oxygen is 0.50.
2
Use local atmospheric pressure for altitude when available; sea-level default is 760 mmHg.
3
Enter PaCO2 and PaO2 from the arterial blood gas, and leave RQ at 0.8 unless a different assumption is justified.
4
Enter age to display the common expected room-air gradient estimate: age / 4 + 4.
Worked Example

FiO2 0.21, Patm 760, PH2O 47, PaCO2 40, RQ 0.8, PaO2 80, age 60.

PAO2 = 0.21 × (760 − 47) − 40 / 0.8 = 99.7 mmHg. A-a gradient = 99.7 − 80 = 19.7 mmHg. Expected gradient at age 60 = 60/4 + 4 = 19 mmHg.

Interpretation Caveats

The A-a gradient is easiest to interpret on room air. On supplemental oxygen, especially high FiO2 or positive-pressure ventilation, the expected gradient changes and the result should be interpreted with oxygen delivery method, shunt physiology, timing, and ventilator settings.

At altitude, lower barometric pressure reduces inspired oxygen pressure and PAO2. Use local Patm rather than sea-level default when interpreting hypoxemia away from sea level.

Pharma & clinical trial context

A-a gradient and derived PAO2 appear in respiratory physiology teaching for clinical trial medical monitors, pulmonary safety assessments, and critical care pharmacology studies where drug effects on gas exchange must be distinguished from hypoventilation or low FiO2. Sponsors document FiO2 source, barometric pressure assumptions, and RQ in protocol appendices when A-a gradient is used as an exploratory endpoint.

Use the Alveolar Gas Equation Calculator for standalone PAO2 estimation, the Oxygenation Index Calculator for ventilated patient severity metrics, and the Ventilation Index Calculator for CO2 clearance burden in ICU trial documentation.

Inhaled pulmonary vasodilator trials, ARDS intervention studies, and altitude physiology research should pre-specify whether P/F ratio, A-a gradient, or shunt fraction is the primary oxygenation endpoint — each captures different physiology and responds differently to FiO2 changes.

Evidence & sources

Frequently Asked Questions

The alveolar-arterial oxygen gradient is the difference between calculated alveolar oxygen tension (PAO2) and measured arterial oxygen tension (PaO2). It helps separate hypoxemia caused by low inspired oxygen or hypoventilation from impairment due to V/Q mismatch, diffusion limitation, or right-to-left shunt physiology.
First calculate PAO2 using the alveolar gas equation: FiO2 × (atmospheric pressure − water vapor pressure) − PaCO2 / respiratory quotient. Then subtract measured PaO2 from PAO2. All pressures are in mmHg; FiO2 is a fraction. This calculator performs both steps automatically.
A common age-adjusted estimate on room air is age divided by 4 plus 4 mmHg — for example, approximately 19 mmHg at age 60. Normal ranges vary with FiO2, altitude, posture, age, pregnancy, and measurement conditions. Expected gradient increases with age even in healthy lungs.
It can be calculated at any FiO2, but the expected gradient is less straightforward on high supplemental oxygen because shunt physiology may not respond fully to increased FiO2. Oxygen delivery method, ventilator settings, sampling timing, and FiO2 accuracy all affect interpretation.
An elevated A-a gradient suggests impaired oxygen transfer from alveoli to arterial blood, commonly from V/Q mismatch, diffusion limitation, or right-to-left shunt. It does not identify the exact cause — correlate with history, exam, imaging, echocardiography, and response to supplemental oxygen.
Hypoxemia with a normal A-a gradient typically reflects hypoventilation (elevated PaCO2 reduces PAO2) or low inspired oxygen pressure (high altitude, low FiO2). The alveolar gas equation shows PaCO2 and FiO2 directly affect PAO2 — correcting ventilation or inspired oxygen may normalize PaO2 without intrinsic lung pathology.
At altitude, lower barometric pressure reduces inspired oxygen pressure and calculated PAO2. Use local atmospheric pressure rather than the sea-level 760 mmHg default. Hypoxemia at altitude may reflect low inspired PO2 with a relatively normal gradient, or additional lung pathology with an elevated gradient.
RQ 0.8 is the standard default, reflecting mixed carbohydrate and fat metabolism. RQ approaches 1.0 with high carbohydrate load and may be lower with ketogenic metabolism. RQ affects the PaCO2 term in the alveolar gas equation — use 0.8 unless a different physiology assumption is clinically justified.
P/F ratio is a simple oxygenation index (PaO2 divided by FiO2) used in ARDS classification. A-a gradient quantifies the gap between calculated alveolar and arterial oxygen, separating hypoventilation and low FiO2 effects from intrinsic gas exchange impairment. Both are complementary in blood gas interpretation.
The age/4 + 4 mmHg rule is most applicable to room-air interpretation in stable patients without significant lung disease. It provides a rough expected upper limit for comparison. On supplemental oxygen or in acute respiratory failure, clinical context matters more than the age-adjusted estimate alone.
A-a gradient elevation suggests shunt or V/Q mismatch but does not quantify shunt fraction directly. Shunt estimation requires additional calculations (e.g., mixed venous oxygen content, cardiac output) or 100% FiO2 testing. Use the gradient as a screening metric, not a shunt calculator.
No. A-a gradient interpretation requires clinical correlation with history, physical examination, imaging, echocardiography, ventilator assessment, and disease-specific workup. This tool supports educational calculation checking — not diagnosis, treatment selection, or disposition decisions.

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PAO2Alveolar Gas EquationOIOxygenation IndexVIVentilation IndexFENaFENa CalculatorSpO2SpO2/FiO2 Ratio

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