Posted tagged ‘V/Q Mismatch’

Why do COPD patients retain CO2 when given too much oxygen?

August 12, 2009

Most people think that this is due to loss of hypoxic drive, and subsequent hypoventilation.  Although this may be true for a small minority of patients, the majority will retain carbon dioxide due to V/Q mismatching.

Localised low PaO2 in the lung leads to localised vasoconstriction, to areas of poor ventilation have appropriately poor perfusion, redirecting blood to areas of better ventilation.  This protects the COPD patient from V/Q mismatch.

If a high concentration of oxygen is given to a patient, loco-regional low oxygen concentration will be abolished, and with local hypoxic vasoconstriction.  Thus, perfusion returns to normal, but ventilation is poor.  This is termed a shunt, and results in worsening hypoxia, and most importantly, hypercarbia.