Clinically, the effect of hypercapnia on pH is estimated using the ratio of the arterial pressure of carbon dioxide to the concentration of bicarbonate ion, 
  
    
      
        
          
            P
            
              
                a
                
                  C
                  
                    O
                    
                      2
                    
                  
                  
                    /
                  
                  H
                  C
                  
                    O
                    
                      3
                    
                    
                      −
                    
                  
                
              
            
          
        
      
    
    {\displaystyle {P_{a_{CO_{2}/HCO_{3}^{-}}}}}
  .
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.