CO2 retention during hyperbaric exercise while breathing 40/60 nitrox.

We evaluated CO2 retention in 24 Navy construction divers breathing air at 1 atm abs (101.3 kPa) and 40% O2 (40/60) nitrox at 4 atm abs (Po2 of 162.1 kPa) inside a pressure chamber. The divers sat immersed to the sternal notch and exercised against pneumatically loaded pedals at a Vo2 of approximately 1.3 liter/min. The mean end-tidal CO2 tension (PET(CO2)2) at 1 atm abs (45.7 +/- 5.0 SD torr) was significantly higher than that of non-divers and diving trainees (40 +/- 5.0) but did not increase significantly at depth (47.1 +/- 6.3). The ranking of CO2 retention was not maintained at depth. Unpredictable upward and downward shifts of up to 10 torr occurred in some divers. The PET(CO2) of six of the divers at pressure was greater than 50 torr, which based on animal studies markedly increases the risk of central nervous system oxygen toxicity. We translated their values into individual depth limits with 40/60 nitrox: three with 50 < PET(CO2) < 55 torr were forbidden to dive beyond 25 m and three with values > 55 torr were restricted to 20 m. We propose that whenever possible, PET(CO2) during exercise at pressure be measured in potential nitrox users and that the above PO2 limits be enforced on moderate and extreme CO2 retainers, respectively.