MAP & A–a Gradient

The A–a gradient separates causes of hypoxaemia. Pure hypoventilation raises PaCO₂, which lowers alveolar oxygen and so leaves the gradient normal despite a low PaO₂. A raised gradient points instead to shunt, V/Q mismatch or a diffusion problem.

The expected gradient widens on oxygen. The (age ÷ 4) + 4 rule assumes room air, so a gradient that looks acceptable on a high FiO₂ should not reassure.

The alveolar gas equation here uses a barometric pressure of 760 mmHg and water vapour pressure of 47 mmHg, both sea level values, with a respiratory quotient of 0.8. At altitude the alveolar PO₂ falls and the arithmetic shifts.

MAP degrades in tachycardia. Weighting diastole twice assumes it occupies about two thirds of the cardiac cycle, which stops being true when the rate climbs and diastole shortens disproportionately. An arterial line measures MAP directly and should be preferred when one is in.