Diagnosis
Blood gas tests may be performed, typically by radial artery puncture, in the setting of acute breathing problems or other acute medical illness. Hypercapnia is generally defined as an arterial blood carbon dioxide level over 45 mmHg (6 kPa). Since carbon dioxide is in equilibrium with carbonic acid in the blood, hypercapnia drives serum pH down, resulting in respiratory acidosis.
EEG changes denoting minor narcotic effects can be detected for expired gas end tidal partial pressure of carbon dioxide (
  
    
      
        
          
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    {\displaystyle {P_{ET_{CO_{2}}}}}
  ) increase from 40 torrs (0.053 atm) to approximately 50 torrs (0.066 atm).