Chapter 5: Acid–Base Disorders
A 22-year-old woman is referred for evaluation of progressive weakness of 5 months duration. The patient denies vomiting or ingestion of diuretics, but notes “salt craving” as manifest by eating highly salty food snacks several times daily. On physical examination the BP is 100/60 and declines to 80/42 with standing. There is no jugular vein distention (JVD) or peripheral edema and the examination of the heart reveals normal heart sounds and no murmurs, rubs or gallops.
Metabolic profile: Na 140, K 2.8, Cl 90, tCO2 38, BUN 28, Cr 1.1, Serum Osm 297
Urine: Na 46, K 42, Cl 56, pH 5.0, Osm 450
Arterial blood gas: pH 7.48, PCO2 50, HCO3– 36
Which ONE response from the list below best describes the acid–base disorder in this patient?
A. Mixed metabolic acidosis—metabolic alkalosis
B. Metabolic alkalosis—respiratory acidosis
The answer is C. Clearly response D is incorrect because the arterial pH is high, not low. Taken together, the laboratory values clearly suggest a metabolic alkalosis (low [tCO2], elevated arterial pH, and an increased PCO2). How does one determine if the increase in the PCO2 to a value of 50 mm Hg, is the expected compensatory response or, in other words, is there a concomitant respiratory acidosis (response B)? The easiest approximation of the expected PCO2 can be obtained by simply adding the number 15, to the measured tCO2 concentration; or, as in this case 38 + 15 = 53 (mm Hg). This approach can be used in the range of serum [tCO2] values from 10 to 40 mEq/L and is reasonably accurate within a range of ±3. Also, it’s important to note, that values of PCO2 above 50 mm Hg in compensation for metabolic alkalosis are unusual as the hypercapnic response becomes limited by the associated hypoxia, and the age of the patient, as well as whether there is lung disease present. Therefore, response B, is not correct and by elimination, the most accurate response is response C (pure metabolic alkalosis). Now, how does one place the obvious hypokalemia into perspective; what is the cause in this case? Examination of the urine electrolytes reveals that the urine [K+] of 42 mEq/L is inappropriately high (>10 mEq/L) for the prevailing hypokalemia. If desired, the TTKG can be calculated from the data available. The value for TTKG is 11, indicating therefore, that there is inappropriate secretion of K+ by the DCT2 and principal cells of the CCD, in this patient ...