alveolar-arterial gas pressure difference, the difference between the partial pressure of a gas, such as CO2, in the alveoli and that in systemic arterial blood. The difference may indicate ventilation-perfusion mismatching. A negative difference indicates that the partial pressure of the gas is higher in systemic arterial blood than it is in alveolar air. It is measured in torr or millimeters of mercury (mm Hg).
