Answer: Statement D is NOT True
Respiratory compensation in metabolic alkalosis very rarely leads to a PCO2 >55 mmHg, making option D the incorrect statement.
Why Statement D is False
In metabolic alkalosis, the body's compensatory mechanism involves hypoventilation to retain CO2 and normalize pH. However, this respiratory compensation is inherently limited by the body's need to maintain adequate oxygenation. The respiratory drive to prevent hypoxemia acts as a "brake" on compensation.
The PCO2 typically increases by approximately 0.5-1.0 mmHg for every 1 mEq/L increase in bicarbonate, but compensation rarely exceeds a PCO2 of 50-55 mmHg 1. This is because further hypoventilation would risk significant hypoxemia, which the body will not tolerate. The respiratory center prioritizes oxygen delivery over acid-base correction.
Why the Other Statements ARE True
Statement A: Elevated Bicarbonate and Alkalemia ✓
Metabolic alkalosis is definitionally characterized by:
Statement B: Volume Assessment is Important ✓
Physical examination to assess circulating volume is critical because the pathophysiology and treatment differ based on volume status 3:
- Volume-depleted (chloride-responsive): Requires saline administration
- Volume-expanded (chloride-resistant): Requires different management (treating mineralocorticoid excess, potassium repletion)
The ECF volume-centered diagnostic approach remains the most practical framework for evaluation and treatment 3.
Statement C: Urinary Chloride is Useful ✓
Urinary chloride concentration is the key diagnostic tool for distinguishing metabolic alkalosis subtypes 1, 3:
- Urine Cl- <20 mEq/L: Chloride-responsive (saline-responsive) - typically from vomiting, diuretics after discontinuation, or volume depletion
- Urine Cl- >20 mEq/L: Chloride-resistant (saline-resistant) - typically from mineralocorticoid excess, ongoing diuretic use, or severe hypokalemia
This distinction directly guides whether isotonic saline administration will be effective.
Statement E: Diuretics are a Common Cause ✓
Loop and thiazide diuretics are among the most common causes of metabolic alkalosis in hospitalized patients 4, 1. They cause:
- Volume contraction (maintains alkalosis)
- Chloride depletion
- Hypokalemia (perpetuates alkalosis)
- Enhanced distal sodium delivery with increased H+ secretion
Clinical Pitfall
A common error is expecting robust respiratory compensation in metabolic alkalosis similar to what occurs in metabolic acidosis. Unlike metabolic acidosis where respiratory compensation can be quite vigorous (PCO2 can drop to 10-15 mmHg), metabolic alkalosis compensation is self-limited by hypoxemic drive. Seeing a PCO2 >55 mmHg in the setting of alkalemia should prompt consideration of a mixed disorder (concurrent primary respiratory acidosis) rather than simple compensation 1.
The practical implication: If you see metabolic alkalosis with PCO2 >55 mmHg, you're likely dealing with a mixed acid-base disorder, not pure compensated metabolic alkalosis.