It is believed that the initial BP depression with the decreased heart rate is due to the direct depressant effect of CO2 upon the heart and that the return of blood pressure to its original level was due to the rapid rise of 
  
    
      
        
          
            P
            
              
                a
                
                  C
                  
                    O
                    
                      2
                    
                  
                
              
            
          
        
      
    
    {\displaystyle {P_{a_{CO_{2}}}}}
  .
A significant percentage of rebreather deaths have been associated with CO2 retention.