Aa Gradient Calculator

The Aa (alveolar-arterial) gradient calculator helps medical professionals assess the difference between alveolar and arterial oxygen concentrations. This measurement is crucial for differentiating between various causes of hypoxemia and evaluating pulmonary function.

What is the Aa Gradient?

The Aa gradient, or alveolar-arterial gradient, measures the difference between:

• The oxygen concentration in the alveoli (tiny air sacs in the lungs)
• The oxygen concentration in the arterial blood

A normal Aa gradient varies with age and can be estimated using the formula: Age/4 + 4

Clinical Uses

The Aa gradient helps differentiate between:

Extrapulmonary Causes of Hypoxemia

• Hypoventilation
• Low FiO2 (fraction of inspired oxygen)

Intrapulmonary Causes

• Ventilation-perfusion mismatch (V/Q mismatch)
• Intrapulmonary or cardiac shunt

Understanding Your Results

The calculator provides two key values:

1. Calculated Aa Gradient: Based on your input values
2. Expected Aa Gradient: Based on the patient’s age

Interpretation:

• If the calculated gradient is higher than expected: Suggests intrapulmonary causes
• If the calculated gradient is normal or lower than expected: Suggests extrapulmonary causes

Formula

The Aa gradient is calculated using these formulas:

1. Aa gradient = PAO2 − PaO2

2. Where PAO2 (alveolar oxygen pressure) is calculated as: PAO2 = [FiO2 × (Patm − PH2O) − PaCO2 / 8]

Where:

• PaO2: Arterial oxygen pressure
• 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 body temperature)
• PaCO2: Arterial carbon dioxide pressure

Normal Values

• Young healthy adults: 5-10 mmHg
• Elderly: Higher values expected (calculated using Age/4 + 4)
• Values >20 mmHg may indicate pathology

Important Notes

• All pressure values should be in mmHg
• Results should always be interpreted in clinical context
• This calculator is for informational purposes and should not replace professional medical judgment

This calculator is intended for use by healthcare professionals. Always consult with a qualified medical professional for interpretation of results and medical advice.