A patient with chronic stable hypercapnia due to COPD has a PaCO2 of 65 mmHg for several weeks, yet his CSF pH is nearly normal. The mechanism responsible is:

• A Increased intrinsic sensitivity of medullary chemoreceptors to CO2
• B Elevated CSF bicarbonate concentration from choroid plexus and renal adaptation over several days ✓
• C Washout of CO2 from CSF by increased cerebral blood flow
• D Loss of blood-brain barrier permeability to hydrogen ions

Explanation

With sustained hypercapnia, the choroid plexus secretes additional bicarbonate into CSF and the kidneys retain bicarbonate, raising buffered base over two to three days and returning CSF pH toward normal. This resets the central chemoreceptors to operate around the new higher PaCO2, which is why chronically hypercapnic patients depend more on hypoxic drive. Sensitivity to CO2 is reduced rather than increased, eliminating option A.

Reference: Ganong's Review of Medical Physiology, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.