<Tr> <Td> 7.0 </Td> <Td>> 0.1 hours </Td> <Td> limitation by dyspnea and mental confusion </Td> </Tr> <P> Normal respiration in divers results in alveolar hypoventilation resulting in inadequate CO elimination or hypercapnia . Lanphier's work at the US Navy Experimental Diving Unit answered the question, "Why don't divers breathe enough?": </P> <Ul> <Li> Higher inspired oxygen (PiO) at 4 atm (400 kPa) accounted for not more than 25% of the elevation in end tidal CO (etCO) above values found at the same work rate when breathing air just below the surface . </Li> <Li> Increased work of breathing accounted for most of the elevation of PACO (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). </Li> <Li> Inadequate ventilatory response to exertion was indicated by the fact that, despite resting values in the normal range, PetCO rose markedly with exertion even when the divers breathed air at a depth of only a few feet . </Li> </Ul> <Li> Higher inspired oxygen (PiO) at 4 atm (400 kPa) accounted for not more than 25% of the elevation in end tidal CO (etCO) above values found at the same work rate when breathing air just below the surface . </Li>

What would cause an increase in the amount of exhaled carbon dioxide