A-a Gradient Calculator
Calculate Alveolar-arterial oxygen gradient (A-a Gradient) for clinical assessment of pulmonary gas exchange. Essential tool for respiratory medicine and critical care.
Step 1: Patient Parameters
Normal A-a Gradient Values
Young healthy adult: 5-15 mmHg on room air
Elderly: Up to 20-30 mmHg (age-adjusted)
Formula: A-a Gradient = PAO₂ – PaO₂
PAO₂ calculation: PAO₂ = (FiO₂ × (Patm – 47)) – (PaCO₂ / 0.8)
Step 2: Calculation Options
Clinical Note
The A-a gradient assesses the efficiency of pulmonary gas exchange. Elevated values may indicate V/Q mismatch, diffusion defects, or shunting. Always interpret in clinical context with other diagnostic findings.
A-a Gradient Calculation Results
Alveolar-arterial Oxygen Gradient
Clinical Interpretation
A-a Gradient: 10.8 mmHg (Normal)
The calculated A-a gradient is within normal limits for a 40-year-old patient breathing room air. This suggests adequate pulmonary gas exchange with no significant V/Q mismatch, diffusion impairment, or right-to-left shunting.
Normal range for age: ≤ 18.3 mmHg
Clinical correlation: Consider other clinical findings. A normal A-a gradient with hypoxemia suggests hypoventilation (e.g., drug overdose, neuromuscular disease).
| A-a Gradient (mmHg) | Interpretation | Possible Causes | Clinical Actions |
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Clinical Considerations
1. The A-a gradient increases with age (approximately 1 mmHg per decade after age 20).
2. Gradient increases with higher FiO₂ (supplemental oxygen).
3. Normal gradient with hypoxemia suggests hypoventilation.
4. Elevated gradient suggests V/Q mismatch, diffusion defect, or shunt.
5. Always correlate with clinical presentation, ABG results, and imaging findings.
A-a Gradient Calculation Details
Alveolar Gas Equation
Formula: PAO₂ = (FiO₂ × (Patm – PH₂O)) – (PaCO₂ / RQ)
Where:
• PAO₂ = Alveolar oxygen partial pressure (mmHg)
• FiO₂ = Fraction of inspired oxygen (0.21 for room air)
• Patm = Atmospheric pressure (760 mmHg at sea level)
• PH₂O = Water vapor pressure (47 mmHg at 37°C)
• PaCO₂ = Arterial carbon dioxide partial pressure (mmHg)
• RQ = Respiratory quotient (typically 0.8)
A-a Gradient Equation
Formula: A-a Gradient = PAO₂ – PaO₂
Where:
• PAO₂ = Calculated alveolar oxygen (from above)
• PaO₂ = Measured arterial oxygen (from ABG)
Age Adjustment Formula
Expected normal A-a gradient: (Age in years / 4) + 4
Example for 40-year-old: (40 / 4) + 4 = 10 + 4 = 14 mmHg
Simplified age-adjusted normal: ≤ (Age/4 + 4) mmHg on room air
Sample Calculation
Given: Age=40, FiO₂=0.21, PaO₂=95, PaCO₂=40, Patm=760
PAO₂: (0.21 × (760 – 47)) – (40 / 0.8) = (0.21 × 713) – 50 = 149.73 – 50 = 99.73 mmHg
A-a Gradient: 99.73 – 95 = 4.73 mmHg
Age-adjusted normal: (40/4) + 4 = 14 mmHg
Interpretation: Normal (4.73 ≤ 14)
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