The A-a Gradient Calculator is a tool used in respiratory medicine to assess the efficiency of gas exchange in the lungs. The A-a Gradient represents the difference between the partial pressure of oxygen in the alveoli (A) and the partial pressure of oxygen in arterial blood (a).
A-a Gradient Calculator
The A-a Gradient Calculator is divided into two versions: Basic and Advanced.
Basic Version:
- FiO₂: Fraction of inspired oxygen
- PaO₂ (mmHg): Alveolar Partial Pressure of Oxygen
- PaCO₂ (mmHg): Arterial partial pressure of carbon dioxide
Advanced Version:
- FiO₂: Fraction of inspired oxygen
- PaO₂ (mmHg): Alveolar Partial Pressure of Oxygen
- PaCO₂ (mmHg): Arterial partial pressure of carbon dioxide
- Patm: Atmospheric Pressure (mmHg):
- PH₂O: water vapor pressure
A-a Gradient Formula
A−aGradient = (FiO2×(Patm−Ph2O)) – (PaCO2 / R) – PaO2
Standard Value:
- Patm = 760 mmHg (standard atmospheric pressure)
- Ph₂O = 47 mmHg (partial pressure of water vapor at body temperature)
- R = 0.8 (Respiratory quotient)
For example:
- FiO₂ (Fraction of Inspired Oxygen): 0.10
- PaO₂ (Partial Pressure of Oxygen in Arterial Blood, mmHg): 10
- PaCO₂ (Partial Pressure of Carbon Dioxide in Arterial Blood, mmHg): 10
First, we need to calculate the Alveolar Oxygen (A):
A = FiO2 × (Patm−Ph2O)
A = 0.10×(760−47)=0.10×713 = 71.3mmHg
Second, we need to calculate the Correction for CO₂ (B):
B = R/PaCO2 = 0.8/10 = 12.5mmHg
In the third step, you can calculate the A-a Gradient:
A−a Gradient = A − B − PaO2
A−a Gradient = 71.3−12.5−10 = 48.8mmHg