Respiratory

Question 1

What is the A-a Gradient? what are normal values and how is this affected by age?

Answer 1

A-a Gradient = (150 - 5/4CO2) - PaO2
This is calculated from 21% of inspired air at sea level - water vapour - CO2 concentration in alveolar.

Generally A-a gradient increases by 1 with every decade.

Normal values are 5-10mmHg with increases with age.

Question 2

What does an increased A-a Gradient indicate?

Answer 2

Generally used in situation of hypoxia, and a NORMAL A-a gradient in setting of hypoxia is typically hypoventilation of the alveolar or low oxygen inspiration (High altitude or decreased O2 inspired.)

Hypoxia with Elevated A-a Gradient, indicated an insufficient oxygenation of the blood. With either increased diffusion capability, V/Q mismatch, increased extraction of the O2, or right-to-left shunt of either cardiac or pulmonary origin.