Overview of Plasma Anion Gap VIDEO Respiratory acidosis is suggested by P co 2 > 40 mm Hg; HCO 3 − should compensate by increasing 3 to 4 mEq / L (3 to 4 mmol / L) for each 10-mm Hg rise in P co 2 sustained for 4 to 12 hours (there may be no increase or only an increase of 1 to 2 mEq / L [1 to 2 mmol / L], which slowly increases to 3 to 4 mEq / L [3 to 4 mmol / L] over days).