CO2 RETENTION DURING HYPERBARIC EXERCISE WHILE BREATHING 40 60-NITROX/

We evaluated CO2 retention in 24 Navy construction divers breathing ai r at 1 atm abs (101.3 kPa) and 40% O-2 (40/60) nitrox at 4 atm abs (PO 2 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)) at 1 arm abs (45.7 +/- 5.0 SD ton) 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 ret ention was not maintained at depth. Unpredictable upward and downward shifts of up to 10 ton occurred in some divers. The PETCO(2) of six of the divers at pressure was greater than 50 ton, 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 ton were forbidden to div e beyond 25 m and three with values > 55 ton were restricted to 20 m. We propose that whenever possible, Per during exercise at pressure be measured in potential nitrox users and that me above PO2 limits be enf orced an moderate and extreme CO2 retainers, respectively.