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====Alveolo-arterial oxygen gradient A-aDO<sub>2</sub>====
The alveolar-arterial oxygen gradient A-aDO<sub>2</sub>, sometimes referred to as the '''alveolar-arterial difference''', is a parameter used '''to assess the degree of impaired oxygenation of the lungs'''. It primarily '''indicates the quality of alveocapillary diffusion.'''

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A–aDO<sub>2</sub> = pAO<sub>2</sub> &minus; paO<sub>2</sub>
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A–aDO<sub>2</sub> = (760 x FiO<sub>2</sub>) &minus; {(paO<sub>2</sub> + paCO<sub>2</sub>) + 47}
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*''FiO<sub>2</sub> is given as a decimal number''
*''paO<sub>2</sub> and paCO<sub>2</sub> are given in torr''
*''760 = barometric pressure at sea level''
*''47 = partial pressure of water vapour in the inhaled air''

This formula can be used if data on inhaled O<sub>2</sub> concentration and arterial gas values are available. Values >350 are indicative of '''respiratory insufficiency''', values >'''550''' are one of the criteria for '''ECMO''' (extracorporeal membrane oxygenation).