Cerebrovascular Responses to Head Out Water Immersion and a Simulated Rebreather Carbon Dioxide Scrubber Failure

Military divers often utilize rebreathers fitted with CO 2 scrubbers to remove exhaled CO 2 . If the CO 2 scrubber fails, elevations in F I CO 2 will likely increase arterial CO 2 and, therefore, might impact cerebral blood flow velocities as well increase the risk of oxygen toxicity and ‘black out’. It is unclear whether progressive hypercapnia, that simulates a rebreather failure, during cool head out water immersion (HOWI) modifies cerebral blood flow velocity. Purpose: To test the hypothesis that breathing progressive increases in hypercapnic gases increases cerebral blood flow velocity compared to breathing room air during cool HOWI. Methods: Six healthy men (age: 26 ± 7 y) completed two visits in a randomized crossover design: one visit breathing graded hypercapnia (CO2) and another visit breathing room air during cool HOWI (AIR). For the CO2 visit, subjects sat in a dry immersion tank and breathed room air for 10 min followed by 10 minutes of breathing F I CO 2 of 0.005. The immersion tank was then rapidly filled with 25˚ C water up to the sternum. Subjects then completed 15 min of breathing F I CO 2 of 0.005, 30 min of breathing F I CO 2 of 0.010, and 30 minutes of breathing F I CO 2 0.015. AIR visits consisted of the same timeline, but subjects breathed room air for the entire visit. Mean data from the final 5 min of each period were used to compare conditions. Cerebral blood flow velocities (transcranial Doppler ultrasound) of the middle and posterior cerebral arteries (MCA V and PCA V , respectively), MAP (MAP; photoplethysmography), ventilation (VE; pneumotach), and the partial pressure of end tidal carbon dioxide (PETCO 2 ; pneumotach) were continuously recorded. Conductance values were calculated as MCA V /MAP (MCA C ) and PCA V /MAP (PCA C ). Data are presented as mean ± SD within each condition. Results: There were no interaction (P>0.1015) or time (P>0.1210) effects for all variables, except for MAP (Time: P=0.0490) and VE (Time: P=0.0386). There were no differences between conditions for MAP (CO2: 99±19 vs AIR: 98±19 mmHg, P=0.9134), PETCO 2 (CO2: 46±4 vs AIR: 41±8 mmHg, P=0.1211), VE (CO2: 11.1±3.7 vs AIR: 14.2±6.2 L/min, P=0.2409), MCA V (CO2: 60±17 vs AIR: 50±17 cm/s, P=0.1926), PCA V (CO2: 28±5 vs AIR: 27±6 cm/s, P=0.8166), MCA C (CO2: 0.63±0.28 vs AIR: 0.56±0.24 cm/s/mmHg, P=0.5800), or PCA C (CO2: 0.29±0.12 vs AIR: 0.33±0.13 cm/s/mmHg, P=0.5844). Conclusion: Elevations in F I CO 2 did not alter cerebral blood flow velocities during cool HOWI versus room air breathing likely because PETCO 2 was not statistically different between conditions. Future studies should examine whether larger elevations in F I CO 2 that augment PETCO 2 influence cerebral blood flow velocities during cool HOWI. Office of Naval Research N00014-17-1-2878 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.