Calculate
the Alveolar - arterial gradient given FiO2, PaCO2 and PaO2.
High
gradients result from impaired diffusion or, more commonly,
by ventilation-perfusion inequality of the "shunting"
variety. A
normal A-a gradient is less than 10 torr. The age (years)
/ 4 + 4 is another conservative estimate of a normal gradient.
The
calculations above assume 100% humidity at sea level and a
respiratory quotient of 0.8, using the alveolar gas equation
to determine PAO2:
PAO2
= ( FiO2 * (760 - 47)) - (PaCO2
/ 0.8)
A-a
gradient = PAO2 - PaO2
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Revised: December
10, 2001