A patient with chronic stable hypercapnia (PaCO2 65 mmHg for months) has a plasma bicarbonate of 34 mEq/L. A second patient with acute respiratory depression has PaCO2 60 mmHg with bicarbonate 26 mEq/L. The difference in bicarbonate response between the two patients is best explained by:

• A Greater buffering capacity of extracellular proteins in chronic disease
• B Renal adaptation: increased ammoniagenesis and hydrogen ion secretion take days to raise bicarbonate generation ✓
• C Chronic patients retain more chloride, which pairs with bicarbonate
• D Acute hypercapnia suppresses renal carbonic anhydrase activity completely

Explanation

Acute respiratory acidosis is buffered almost entirely by intracellular proteins and haemoglobin, adding only 2 to 4 mEq/L to bicarbonate within minutes. Over 3 to 5 days, the kidney responds to elevated PaCO2 by increasing proximal tubular hydrogen secretion, ammoniagenesis, and new bicarbonate generation, allowing a larger compensatory rise. Chloride falls, not rises, in chronic compensation, eliminating option C.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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