A 44-year-old patient hyperventilated to a PaCO2 of 24 mm Hg for 48 hours. What [HCO3-] would you expect (normal [HCO3-] is 24 mEq/L)?

A 44-year-old patient hyperventilated to a PaCO2 of 24 mm Hg for 48 hours. What [HCO3-] would you expect (normal [HCO3-] is 24 mEq/L)?

• A. 10 mEq/L
• B. 12 mEq/L
• C. 14 mEq/L
• D. 16 mEq/L

Answer: (D)

This item tests renal compensation in chronic respiratory alkalosis. When someone hyperventilates for a while, PaCO2 falls and the body tries to keep pH from rising too high by reducing bicarbonate via the kidneys. For a chronic decrease in PaCO2, the bicarbonate falls by roughly 4–5 mEq/L for every 10 mm Hg drop in PaCO2. Here, PaCO2 drops from 40 to 24, a 16 mm Hg decrease. Applying the chronic compensation rule gives a bicarbonate decrease of about 6–8 mEq/L (roughly 4–5 per 10 mm Hg times 1.6). Starting from a normal bicarbonate of 24 mEq/L, the expected value is around 16–18 mEq/L. The most fitting choice is 16 mEq/L. Other values (10, 12, or 14) would reflect a greater-than-expected drop in bicarbonate for this degree of PaCO2 reduction, or an additional metabolic disturbance, which isn’t consistent with simple chronic respiratory alkalosis after about 48 hours.

This item tests renal compensation in chronic respiratory alkalosis. When someone hyperventilates for a while, PaCO2 falls and the body tries to keep pH from rising too high by reducing bicarbonate via the kidneys. For a chronic decrease in PaCO2, the bicarbonate falls by roughly 4–5 mEq/L for every 10 mm Hg drop in PaCO2.

Here, PaCO2 drops from 40 to 24, a 16 mm Hg decrease. Applying the chronic compensation rule gives a bicarbonate decrease of about 6–8 mEq/L (roughly 4–5 per 10 mm Hg times 1.6). Starting from a normal bicarbonate of 24 mEq/L, the expected value is around 16–18 mEq/L. The most fitting choice is 16 mEq/L.

Other values (10, 12, or 14) would reflect a greater-than-expected drop in bicarbonate for this degree of PaCO2 reduction, or an additional metabolic disturbance, which isn’t consistent with simple chronic respiratory alkalosis after about 48 hours.