A-a Gradient (Alveolar-Arterial Oxygen Gradient) & Age-Expected Threshold Calculator

Compute PAO₂, A-a O₂ gradient, and age-adjusted hypoxemia cutoff.

Alveolar-Arterial (A-a) O₂ Gradient
21.73 mmHg
Calculated Alveolar PAO₂ 99.73 mmHg
Age-Adjusted Upper Normal Limit 16 mmHg
Excess Gradient Above Age Limit 5.73 mmHg
Inputs
⚡ Scenarios:
%
Fraction of Inspired Oxygen FiO₂ % (21% on room air).
mmHg
Measured arterial oxygen partial pressure (PaO₂) in mmHg.
mmHg
Measured arterial carbon dioxide (PaCO₂) in mmHg.
years
Patient age in years for expected A-a gradient ceiling.
mmHg
Ambient barometric pressure (760 mmHg at sea level).
Results & Graph
Alveolar-Arterial (A-a) O₂ Gradient
21.73 mmHg
Calculated Alveolar PAO₂
99.73 mmHg
Age-Adjusted Upper Normal Limit
16 mmHg
Excess Gradient Above Age Limit
5.73 mmHg

Alveolar-Arterial (A-a) O₂ Gradient

Graph 1 • A-a Oxygen Gradient • Domain Architecture & Live Telemetry Maps Inspired Fio2 Pct, Arterial Pao2 Mm Hg, Arterial Paco2 Mm Hg through A-a Oxygen Gradient Formula.
A-a Oxygen Gradient • Clinical & Physiological Reference Scale Low / Baseline Target / Reference Elevated / Watch High / Action Alveolar-Arterial (A-a) O₂ Gradient: 21.73 mmHg Calculated Alveolar PAO₂ 99.73 mmHg Age-Adjusted Upper Normal 16 mmHg Excess Gradient Above Age 5.73 mmHg
Graph 2 • Alveolar-Arterial (A-a) O₂ Gradient Across Inspired Fio2 Pct Sweep (40% → 160%) Evaluates PAO₂ = (FiO₂/100) × (P_baro − 47) − (PaCO₂ / 0.8); A-a Gradient = PAO₂ across 7 operating points (current input highlighted).
0 mmHg 61.36 mmHg 122.7 mmHg 21.73 8.40% 21.73 12.60% 21.73 16.80% 21.73 21% 51.68 25.20% 81.62 29.40% 111.6 33.60%
Graph 3 • Joint Sensitivity: Inspired Fio2 Pct × Arterial Pao2 Mm Hg Compares Alveolar-Arterial (A-a) O₂ Gradient when Arterial Pao2 Mm Hg is shifted +25% (solid) vs. −25% (dashed).
0 mmHg 47.38 mmHg 94.77 mmHg --- Arterial Pao2 Mm Hg @ 75% 2.23 -30% 2.23 -15% 2.23 +0% 24.69 +15% 47.15 +30%
Summary: Alveolar PAO₂ is 99.7 mmHg and A-a gradient is 21.7 mmHg (age-adjusted normal ceiling ≤16 mmHg for age 48).
Summary: Alveolar PAO₂ is 99.7 mmHg and A-a gradient is 21.7 mmHg (age-adjusted normal ceiling ≤16 mmHg for age 48).
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Formula & Mathematical Methodology: A-a Oxygen Gradient Formula

PAO₂ = (FiO₂/100) × (Pbaro − 47) − (PaCO₂ / 0.8)
A-a Gradient = PAO₂ − PaO₂
ExpectedMax = Age/4 + 4
PAO₂ = (FiO₂/100) × (P_baro − 47) − (PaCO₂ / 0.8); A-a Gradient = PAO₂ − PaO₂; ExpectedMax = Age/4 + 4
  • inspiredFio2Pct: Inspired Fio2 Pct (%)
  • arterialPao2MmHg: Arterial Pao2 Mm Hg (mmHg)
  • arterialPaco2MmHg: Arterial Paco2 Mm Hg (mmHg)
  • patientAgeYears: Patient Age Years (years)
  • barometricPressureMmHg: Barometric Pressure Mm Hg (mmHg)

Rounding: Clinical scores round to statutory integer or decimal precision; physiological metrics round to 2 decimal places.

Worked Example: Clinical / Physiology Scenario: A-a Oxygen Gradient

Compute PAO₂, A-a O₂ gradient, and age-adjusted hypoxemia cutoff.

  1. Enter lab values, anthropometric measurements, or performance test metrics.
  2. Evaluate the reference equation: PAO₂ = (FiO₂/100) × (P_baro − 47) − (PaCO₂ / 0.8); A-a Gradient = PAO₂ − PaO₂; ExpectedMax = Age/4 + 4.
  3. Inspect the primary classification, supporting indices, and 3 SVG charts.
Result: Computes A-a Oxygen Gradient via PAO₂ = (FiO₂/100) × (P_baro − 47) − (PaCO₂ / 0.8); A-a Gradient = PAO₂ − PaO₂; ExpectedMax = Age/4 + 4.

Frequently Asked Questions

How does the A-a Gradient (Alveolar-Arterial Oxygen Gradient) & Age-Expected Threshold Calculator compute its result?

It evaluates PAO₂ = (FiO₂/100) × (P_baro − 47) − (PaCO₂ / 0.8); A-a Gradient = PAO₂ − PaO₂; ExpectedMax = Age/4 + 4 directly in your browser and updates three custom SVG visualizations in real time.

Can I use Goal Seek to solve backwards for a target lab value or pace?

Yes. Click "🎯 Goal Seek" in the results toolbar to solve backwards for any numeric input parameter.

Are my health and biometric inputs kept private?

Yes. All calculations run 100% locally on your device with zero external transmission.

Health & Fitness · Pulmonary, Critical Care & Acid-Base

A-a Gradient (Alveolar-Arterial Oxygen Gradient) & Age-Expected Threshold Calculator: inputs, method, and how to read the result

Compute PAO₂, A-a O₂ gradient, and age-adjusted hypoxemia cutoff.

Choose inputs for the case you are checking

The defaults are illustrative. Replace them with values for your case and keep the units shown beside each field.

  • Inspired Fio2 Pct: Fraction of Inspired Oxygen FiO₂ % (21% on room air).
  • Arterial Pao2 Mm Hg: Measured arterial oxygen partial pressure (PaO₂) in mmHg.
  • Arterial Paco2 Mm Hg: Measured arterial carbon dioxide (PaCO₂) in mmHg.
  • Patient Age Years: Patient age in years for expected A-a gradient ceiling.

What the calculation includes

With the default values shown, Alveolar-Arterial (A-a) O₂ Gradient is 21.73 mmHg. The calculation follows A-a Oxygen Gradient Formula; the formula and variable definitions above show how the inputs combine.

One-input check: With the other defaults held constant, changing Inspired Fio2 Pct from 21 % to 23.1 % changes Alveolar-Arterial (A-a) O₂ Gradient from 21.73 mmHg to 36.7 mmHg. This is a comparison inside the calculator's stated model, not a prediction.

Where to use caution

  • Uses peer-reviewed clinical and sports physiology formulas.
  • For educational and informational reference only; clinical scores do not replace physician judgment or laboratory reference ranges.

Scope: Global Clinical & Sports Science Consensus Check the assumptions and limitations against your situation before relying on the estimate.

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