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Hypercapnia is an excess of carbon dioxide in the blood.
The arterial partial pressure of carbon dioxide (PaCO2) reflects alveolar ventilation. Thus hypoventilation results in an increased PaCO2. In contrast PaO2 is a poor indicator of alveolar ventilation - it is most influenced by alveolar diffusion and ventilation-perfusion balance.
An increase in PaCO2 is associated with a reduction in blood and urinary pH. There is a consequent increased urinary reabsorption of bicarbonate (to increase the blood buffering capacity). There is renal sodium retention and increased excretion of ammonium salts.
If the PaCO2 is greater than 9.3kPa (70mmHg) then there is a direct depressive effect on the respiratory centre (in the reticular formation in the medulla). When this occurs the hypoxic drive becomes important.