alveolar/arterial gradient


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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(PB-PH2O)-PaCO2/0.8,
where FIO2 is the inspired oxygen concentration; PB, barometric pressure; PH2O, 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.
See also: gradient
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