No Effect Of Water Immersion On Training-induced Improvements In Apnea Duration

Apnea training increases tolerance to hypercapnia and hypoxemia. Apnea training often occurs out of water or when submerged in water, the latter of which increases thoracic pressure and causes mild CO2 retention. We have previously demonstrated that apnea time is acutely elevated during head-out water immersion (HOWI). However, it is unknown whether training improvements in apnea duration are modified when completed during HOWI compared to air. Purpose: Test the hypothesis that training-induced improvements in apnea duration are heightened during HOWI. Methods: Nineteen healthy adults completed 14 days of training within a 16-day period. Training consisted of five maximal apneas prior to 3 h of moderate normobaric hypoxia (FiO2: 0.14) and five maximal apneas post-hypoxia. Apneas were separated by 2.5 min of rest. The HOWI group (n = 9, 5 women) were immersed (mid-sternum) while seated in the upright position in thermoneutral water (~35.5 °C). The air (AIR) group (n = 10, 5 women) sat in the upright seated position in moderate temperature air (~23 °C). During training, apnea duration, heart rate nadir and arterial oxyhemoglobin saturation (SpO2) nadir were recorded. In this analysis, pre-hypoxia apneas on Days 1, 7 and 14 are included and data were averaged over apneas 3-5 (to control for splenic contraction). Data are presented as mean ± SD. Results: On Day 1, apnea duration (HOWI: 80 ± 32 s, AIR: 71 ± 19 s, p = 0.430), heart rate nadir (HOWI: 56 ± 7 bpm, AIR: 54 ± 17 bpm, p = 0.728) and SpO2 nadir (HOWI: 88 ± 8%, AIR: 93 ± 2%, p = 0.114) did not differ between groups. Compared to Day 1, apnea duration increased by 21 ± 5 s on Day 7 and 23 ± 12 s on Day 14 (main effect of time: p = 0.002) but did not change differently between groups (group x time: p = 0.324). Neither heart rate nadir nor SpO2 nadir changed with training (main effect of time: p ≥ 0.264) and did not change differently between groups (group x time: p ≥ 0.541). Conclusions: Training-induced improvements in apnea duration were not modified by water immersion. Furthermore, indices of the activation of the dive reflex (e.g., heart rate and SpO2 nadirs) during apneas were not impacted by 14 days of apnea training. Support: Office of Naval Research (N00014-20-1-2593) Office of Naval Research (N00014-20-1-2593)