Does A Simulated Rebreather Failure Influence The Exercise Pressor Reflex During Cool Head-out Water Immersion?

Underwater diving operations can require personnel to use a rebreather during physical tasks. Periodically, the rebreather CO2 scrubber can fail; exposing the diver to the inhalation of hypercapnic gas that increases the risk of syncope and oxygen toxicity. It is unknown whether breathing hypercapnic gas during head-out water immersion (HOWI) influences the exercise pressor reflex. PURPOSE: Test the hypothesis that cardiovascular responses during the exercise pressor reflex would be greater when breathing hypercapnic gas vs breathing room air during cool HOWI. METHODS: 6 men (age: 26 ± 7 y) completed 2 visits in a randomized crossover design. Subjects sat in a water immersion tank and completed four 2-min submaximal static handgrip exercises (HG) immediately followed by 3 min of circulatory occlusion (PECO). Subjects breathed either graded hypercapnia (CO2), with FICO2 increasing by 0.5 every 30 min to simulate a rebreather failure, or room air (Air). HG and PECO were performed during baseline breathing room air in a dry tank, after 10 min of breathing room air or a FICO2 of 0.5 in a dry tank, after 10 min of HOWI (25 °C water at sternum level) breathing room air or FICO2 of 0.5, and after 60 minutes of HOWI breathing room air or FICO2 of 1.5. Heart rate (HR; ECG), mean arterial pressure (MAP; photoplethysmography), systolic (SBP), and diastolic blood pressure (DBP) were continuously recorded. 30 sec averages during HG and PECO periods were used to determine maximum changes (Δ) compared to baselines. Data are presented as mean ± SD. RESULTS: There were no significant time (all P > 0.301) or interaction (all P > 0.097) effects. There were no differences between conditions during HG for ΔHR (CO2: 14 ± 6; Air: 15 ± 8 bpm; P = 0.802), ΔMAP (CO2: 21 ± 13; Air: 30 ± 20 mmHg; P = 0.156), ΔSBP (CO2: 22 ± 14; Air: 36 ± 25 mmHg; P = 0.153), or ΔDBP (CO2: 18 ± 12; Air: 26 ± 16 mmHg; P = 0.152). There were no differences between conditions during PECO for ΔHR (CO2: 6 ± 3; Air: 6 ± 3 bpm; P = 0.922), ΔMAP (CO2: 18 ± 13; Air: 28 ± 16 mmHg; P = 0.134), ΔSBP (CO2: 23 ± 16; Air: 39 ± 25 mmHg; P = 0.134) or ΔDBP (CO2: 15 ± 9; Air: 22 ± 12 mmHg; P = 0.109). CONCLUSIONS: While breathing hypercapnic gas during underwater diving can be hazardous, our data indicate that it likely does not influence cardiovascular responses to the exercise pressor reflex during immersion. Supported by: N00014-17-1-2878