Carotid Body Chemosensitivity is Not Attenuated During Hyperbaric Hypoxia

Water immersion causes CO2 retention, thus increasing the risk of CO2 toxicity. Hyperoxia reduces carotid body (CB) tonic activity, which reduces the ventilatory response to hypercapnia. However, it is not known if CB chemosensitivity is altered during the high partial pressure of oxygen associated with hyperbaria. PURPOSE: We tested the hypothesis that oxygen breathing would lower CB chemosensitivity more than breathing air at 6.1 msw depth. METHODS: Five subjects (age: 23±2 y; BMI: 28±5 kg/m2) completed two, four-hour dry dives at 6.1 msw (1.6 ATA) breathing either 100% O2 or air. CB chemosensitivity was assessed using hypoxic ventilatory response (CBO2) and brief hypercapnic ventilatory response (CBCO2) tests pre-dive, 75 and 155 min into the dives, immediately post-dive, and 60 min post-dive. CBO2 consisted of inhaling 100% N2 for 2-6 breaths, repeated four times, with 2 min between hypoxic exposures. CBCO2 consisted of inhaling 13% CO2, 66% N2, 21% O2 for one breath, repeated four times, with 2 min between hypercapnic exposures. CB chemosensitivity was calculated as the slope of the linear regression line of the peak minute ventilation (MV) in three consecutive breaths vs. the nadir oxygen saturation (pulse oximetry; SpO2) or peak end tidal CO2 tension (capnography; PETCO2) for CBO2 and CBCO2, respectively. Data are reported as a change from pre-dive (mean±SD). RESULTS: SpO2 was higher than pre-dive at all time points (all p<0.01), but was not different between conditions (p=0.24). The change in MV was not different over time (p=0.11) or between conditions (p=0.42). PETCO2 increased during the dive at 75 (Air: 10±5 vs. O2: 7±4 mmHg) and 155 min (Air: 8±5 vs. O2: 5±3 mmHg; p<0.01), but did not differ between conditions (p=0.14). CBO2 and CBCO2 were not different at any time point (p=0.29 and p=0.48, respectively) and were not different between 100% O2 or air conditions (p=0.64 and p=0.32, respectively). CONCLUSIONS: These data indicate that CB chemosensitivity to hypoxia and hypercapnia is not attenuated during hyperbaric hyperoxia. Therefore, the carotid body chemoreceptors do not appear to contribute to CO2 retention in hyperbaria.