Acid-base disorders and the anion gap

What it is

An arterial blood gas is read in a fixed order, and the order is what stops a second disturbance from being missed. Ask whether there is acidemia or alkalemia; whether the primary problem is respiratory or metabolic; if respiratory, whether it is acute or chronic; if metabolic acidosis, what the anion gap is; for both, whether the compensation is what it should be; and last, whether the imbalance is mixed.

The arterial pH opens the algorithm. A pH below 7.35 is acidosis and above 7.45 is alkalosis; one slide sets the line at 7.4, with a lower value acidotic and a higher one alkalotic. In acidosis, a PaCO2 above 45 mm Hg makes it respiratory, and a PaCO2 at or below 45 mm Hg, or a low HCO3, makes it metabolic. In alkalosis, a PaCO2 below 32 mm Hg makes it respiratory, and a PaCO2 of 32 mm Hg or more, or a high HCO3, makes it metabolic. A second slide uses PCO2 above 44 mm Hg or HCO3 below 20 mEq/L on the acidemic side, and PCO2 below 36 mm Hg or HCO3 above 28 mEq/L on the alkalemic side.