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Difficulty: EasyNephrology

A 62-year-old woman comes to the clinic for a follow-up visit. Two months ago, she was prescribed hydrochlorothiazide for hypertension. She now complains of generalized weakness and occasional muscle cramps. Her blood pressure is 118/76 mm Hg, and her pulse is 78/min. Physical examination shows dry mucous membranes and no peripheral edema. Laboratory studies show:

TestResult
Sodium138 mEq/L
Potassium2.9 mEq/L
Chloride92 mEq/L
Bicarbonate34 mEq/L
Arterial pH7.52
pCO2pCO_248 mm Hg

Which of the following best describes this patient's acid-base status?

  1. Primary metabolic alkalosis with appropriate respiratory compensationAnswer
  2. B
    Primary respiratory acidosis with appropriate metabolic compensation
  3. C
    Primary metabolic alkalosis with a superimposed respiratory acidosis
  4. D
    Primary respiratory alkalosis with appropriate metabolic compensation
  5. E
    Primary metabolic acidosis with a superimposed respiratory alkalosis

Answer

Primary metabolic alkalosis with appropriate respiratory compensation
This patient has a pH of 7.52, indicating alkalemia. The elevated serum bicarbonate (34 mEq/L34 \text{ mEq/L}) identifies the primary disorder as metabolic alkalosis, likely secondary to hydrochlorothiazide use, which causes volume contraction and hypokalemia. The expected respiratory compensation is calculated using the formula: Expected pCO2=40+0.7×(ΔHCO3−)pCO_2 = 40 + 0.7 \times (\Delta HCO_3^-). Here, 40+0.7×(34−24)=47±2 mm Hg40 + 0.7 \times (34 - 24) = 47 \pm 2 \text{ mm Hg}. Since the patient's measured pCO2pCO_2 of 48 mm Hg48 \text{ mm Hg} falls squarely within this expected range, she has a simple primary metabolic alkalosis with appropriate respiratory compensation.

Step-by-Step Solution

1
Analyze the pH to determine the primary disturbance.
The arterial pH is 7.52.
A pH greater than 7.44 indicates that the primary acid-base disorder is an alkalosis.
2
Identify the primary driver of the alkalemia.
The serum bicarbonate is elevated at 34 mEq/L, and the pCO2 is elevated at 48 mm Hg.
An elevated bicarbonate directly causes alkalemia, identifying the primary disorder as a metabolic alkalosis (often caused by thiazide-induced volume contraction). The elevated pCO2 is a compensatory response.
3
Calculate the expected respiratory compensation.
Expected pCO2=40+[0.7×(34−24)]=40+7=47±2pCO_2 = 40 + [0.7 \times (34 - 24)] = 40 + 7 = 47 \pm 2 mm Hg.
Because the patient's measured pCO2 (48 mm Hg) falls within the expected range (45-49 mm Hg), the respiratory compensation is appropriate and there is no superimposed secondary disorder.

Key Concept

Thiazide diuretics can cause volume contraction and hypokalemia, leading to a primary metabolic alkalosis. The expected respiratory compensation for metabolic alkalosis is an increase in pCO2 by roughly 0.7 mm Hg for every 1 mEq/L rise in bicarbonate.
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Topics

Nephrology

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