A a gradient
The term A-a gradient, or alveolar-arterial gradient, refers to the difference between the oxygen concentration in the alveoli of the lungs and in the arteries. The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia.
It is calculated by the following formula
A-a (O2) = (FiO2%/100) * (Patm - 47 mmHg) - (PaCO2/0.8) - PaO2
FiO2 Room Air = 21 %
Atmospheric Pressure = 760 mm Hg at sea level
Water vapor pressure pH2O (mmHg) = 47 mm Hg at 37 degrees Celsius
Respiratory quotient RQ (VCO2/VO2) = 0.8 (usual)
Another way is: PiO2 – PaCO2/0.8
- PiO2 = FiO2 x (barometric pressure – saturated vapour pressure of H20)
- PiO2 = 0.21 x (760 – 47) – sea level.
- PiO2 = 150mmHg.
The A–a gradient is helpful in determining the source of hypoxemia such as intrapulmonary (within the lungs) or extrapulmonary (elsewhere in the body).
A normal A–a gradient for a young adult non-smoker breathing air, is between 5–10 mmHg.
Age and A-a gradient
Normally, the A–a gradient increases with age. For every decade a person has lived, their A–a gradient is expected to increase by 1 mmHg. A conservative estimate of normal A–a gradient is less than [age in years/4] + 4. Thus, a 40-year-old should have an A–a gradient less than 14.