A 41-year-old woman with glaucoma is treated with acetazolamide. Several weeks later the woman has an aerial pH of 7.34, an aerial PCO2 of 29 mm Hg, and a plasma HCO3- of 15 mEq/L. Which of the following abnormalities has this women most likely developed?
Correct Answer: Metabolic acidosis
Description: The laboratory results indicate that the aerial pH, aerial PCO2, and plasma HCO3- concentrations are all low. These changes clearly demonstrate metabolic acidosis, which occurs commonly when a carbonic anhydrase inhibitor is administered. The carbonic anhydrase enzyme attached to the brush border of the tubular epithelial cells normally catalyzes the dissociation of carbonic acid into water and carbon dioxide. Inhibition of carbonic anhydrase prevents the removal of bicarbonate ions from the tubular fluid, which initially increases urine pH. The result is heavy spillage of bicarbonate in the urine, which is the hallmark of type 2 A (renal tubular acidosis). However, once the plasma levels of bicarbonate have decreased sufficiently, the bicarbonaturia ceases and the plasma HCO3- levels stabilize at a lower than normal level. Consequently, the urine pH falls typically to 4.5-5.0. Ref: Reilly R.F., Jackson E.K. (2011). Chapter 25. Regulation of Renal Function and Vascular Volume. In L.L. Brunton, B.A. Chabner, B.C. Knollmann (Eds), Goodman & Gilman's The Pharmacological Basis of Therapeutics, 12e.
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