alveolar/arterial gradient

Abbreviation: A/a gradient
The difference between the calculated oxygen pressure available in the alveolus and the arterial oxygen tension. It measures the efficiency of gas exchange. Synonym: respiratory index.

The formula for measuring the gradient is known as the partial pressure of oxygen dissolved in arterial blood and is shown below.

PaO₂=FIO₂(P_B-P_H2O)-PaCO₂/0.8,

where FIO₂ is the inspired oxygen concentration; P_B, barometric pressure; P_H2O, partial pressure of water vapor (47 mm Hg); PaCO₂, partial pressure of carbon dioxide dissolved in arterial blood; and PaO₂, partial pressure of oxygen dissolved in arterial blood.