A-a Gradient Calculator
Calculate the alveolar-arterial oxygen gradient for assessment of oxygenation disorders
Patient Information & Blood Gas Values
Age is used to calculate expected A-a gradient
Choose preferred pressure units
21% for room air, higher values for supplemental oxygen
Normal range: 80-100 mmHg
Normal range: 35-45 mmHg
Atmospheric pressure affects oxygen calculations
Input Validation Warnings
A-a Gradient Results
Calculation Example
Case: 57-year-old patient with pneumonia
Given values:
• Age: 57 years
• FiO2: 21% (room air)
• PaCO2: 45 mmHg
• PaO2: 70 mmHg
• Atmospheric pressure: 760 mmHg (sea level)
Calculation:
• PAO2 = 0.21 × (760 - 47) - 45/0.8 = 149.7 - 56.3 = 93.4 mmHg
• A-a gradient = 93.4 - 70 = 23.4 mmHg
• Expected for age = 57/4 + 4 = 18.3 mmHg
Interpretation: Elevated A-a gradient suggests intrapulmonary pathology (V/Q mismatch consistent with pneumonia)
Normal Reference Ranges
Arterial Blood Gas (ABG)
PaO2: 80-100 mmHg
PaCO2: 35-45 mmHg
pH: 7.35-7.45
HCO3: 22-26 mEq/L
A-a Gradient
Normal: 5-15 mmHg
Formula: (Age ÷ 4) + 4 mmHg
Room air: <20 mmHg (young adults)
100% O2: <100 mmHg
Quick Reference
Normal A-a Gradient
Extrapulmonary hypoxemia
Hypoventilation, low FiO2
Elevated A-a Gradient
Intrapulmonary hypoxemia
V/Q mismatch, shunt
Formula
A-a = PAO2 - PaO2
PAO2 = FiO2×(Patm-47) - PaCO2/0.8
Clinical Pearls
A-a gradient increases with age: expect ~2.5 mmHg increase per decade
Normal A-a gradient rules out significant intrapulmonary disease
Elevated gradient on 100% O2 suggests shunt physiology
Use with P/F ratio for complete oxygenation assessment
Understanding the A-a Gradient
What is the A-a Gradient?
The alveolar-arterial (A-a) oxygen gradient represents the difference between the calculated alveolar oxygen pressure (PAO2) and the measured arterial oxygen pressure (PaO2). This gradient helps differentiate between extrapulmonary and intrapulmonary causes of hypoxemia.
Clinical Significance
- •Normal gradient: Suggests extrapulmonary hypoxemia (hypoventilation, low FiO2)
- •Elevated gradient: Indicates intrapulmonary disease (V/Q mismatch, shunt)
- •Age-related: Normal gradient increases with age due to physiologic changes
Pathophysiology
Normal Gas Exchange
- • Optimal ventilation-perfusion matching
- • Minimal A-a gradient (<15 mmHg in young adults)
- • Efficient oxygen transfer
V/Q Mismatch
- • Uneven ventilation-perfusion distribution
- • Moderately elevated A-a gradient
- • Responds to supplemental oxygen
Intrapulmonary Shunt
- • Blood bypasses ventilated alveoli
- • Severely elevated A-a gradient
- • Poor response to supplemental oxygen
Extrapulmonary Causes
- • CNS depression (opioids, sedatives)
- • Neuromuscular disease
- • Chest wall abnormalities
- • High altitude
- • Low inspired oxygen concentration
V/Q Mismatch
- • Pneumonia
- • Asthma
- • COPD exacerbation
- • Pulmonary embolism
- • Interstitial lung disease
True Shunt
- • ARDS
- • Severe pneumonia
- • Pulmonary edema
- • Arteriovenous malformation
- • Intracardiac shunt
Related Respiratory Calculators
Medical Disclaimer
This A-a gradient calculator is provided for educational and informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. The A-a gradient is a clinical tool that must be interpreted in the context of the patient's complete clinical presentation, including physical examination, medical history, and other diagnostic tests. Always consult with a qualified healthcare provider or pulmonologist for proper interpretation of arterial blood gas results and respiratory assessment. This tool cannot replace professional medical judgment and should only be used by qualified healthcare professionals as a reference aid in clinical decision-making.