Specific symptoms attributable to early hypercapnia are dyspnea (breathlessness), headache, confusion and lethargy. Clinical signs include flushed skin, full pulse (bounding pulse), rapid breathing, premature heart beats, muscle twitches, and hand flaps (asterixis). The risk of dangerous irregularities of the heart beat is increased.
Increased work of breathing accounted for most of the elevation of 
  
    
      
        
          
            P
            
              
                a
                
                  C
                  
                    O
                    
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    {\displaystyle {P_{a_{CO_{2}}}}}
   (alveolar gas equation) in exposures above 1 atm (100 kPa), as indicated by the results when helium was substituted for nitrogen at 4 atm (400 kPa).