Elevated A-a Gradient
February 7th, 2020
Overview
The alveolar-arterial (A-a) gradient is a comparison of the partial pressure of O₂ in the alveoli and in arterial blood.
Normal Range
- 5 - 25
Calculating the A-a Gradient
- The A-a gradient is calculated as the alveolar partial pressure of oxygen (PAO₂) minus the arterial partial pressure of oxygen (PaO₂):
A-a gradient = PAO₂ - PaO₂
The alveolar value is calculated, based on the patient's inspired FiO₂ and the PCO₂ from their blood gas result, while the arterial value is the PaO₂ from the patient's blood gas result.
Calculating the Alveolar Pressure of Oxygen
- To calculate the A-a gradient, first calculate the alveolar partial pressure of oxygen (PAO₂), using the alveolar gas equation:
PAO₂ = ( FiO₂ × ( Patmos - PH₂O ) ) -
PaCO₂R
- FiO₂: the fraction of inspired oxygen that the patient is inspiring - 0.21 on room air
- Patmos: atmospheric pressure - 760mmHg at sea level
- PH₂O: the water vapour pressure - 47mmHg at 37°
- PCO₂: the partial pressure of carbon dioxide, from the patient's ABG result
- R: the respiratory quotient - 0.8
This can be simplified further, assuming that the patient is normothermic and at sea level:
PAO₂ = ( FiO₂ × ( 760 - 47 ) ) -
PaCO₂0.8
To produce the following formula:
PAO₂ = ( FiO₂ × 713 ) -
PaCO₂0.8
Overview
An elevated A-a gradient indicates that the partial pressure of O₂ is higher in the alveoli than in arterial blood, indicating a V/Q mismatch.
Causes of Elevated A-a Gradient
- Dead space ventilation - pneumonia, asthma, COPD, pulmonary embolismVentilation without perfusion
- Left to right shunt - pulmonary oedema, ARDS, pneumoniaPerfusion without ventilation
- Alveolar hypoventilation - pulmonary fibrosis, interstitial lung disease
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