Hypoxia And Apnea Training During Water Immersion, But Not Air, Increases Spleen Volume

Indigenous breath-hold divers have comparatively large spleen volumes (SV), likely due to natural selection. SV is positively associated with apnea duration in breath-hold divers. Thus, increasing SV may be an adaptation to diving related stressors, such as apneas, relative hypoxemia, water immersion, and thermal stress. Twelve days of apnea training increases apnea duration and SV. Moreover, SV is positively associated with thyroid hormone bioavailability, which is increased with cold exposure, suggesting that cold water may augment training-induced splenic growth. However, it is unknown whether repeated exposure to diving related stressors modifies SV in people without breath hold diving experience or ancestry. PURPOSE: Test the hypothesis that 14 d of moderate hypoxia for 3 h and 10 maximal apneas increases SV, and that increases in SV will be greater with cold water immersion. METHODS: 27 healthy adults completed 14 days of training within a 16-day period. Training consisted of 5 apneas prior to and following 3 h of exposure to normobaric hypoxia (FiO2 = 0.14). Subjects underwent one of three conditions for the entirety of the training: thermoneutral air (Air, n = 8, 5 women), thermoneutral (TN) head out water immersion (HOWI) (~35.5 °C, n = 9, 5 women), or cool (CW) HOWI (~28 °C, n = 10, 5 women). On Days 1 and 14, SV was measured (ultrasound) while seated in air and calculated using the Pilström equation. RESULTS: Average apnea duration increased from Day 1 (79 ± 33 s) to Day 14 (98 ± 36 s, p < 0.01), but no condition or interaction (p ≥ 0.41) effects were observed. SV increased from Day 1 (163 ± 37 ml) to Day 14 (180 ± 39 ml, main effect of time: p < 0.01). Post hoc analysis indicated that SV increased with HOWI (TN HOWI: mean diff. 15 ml, p = 0.02, CW HOWI: mean diff. 26 ml, p < 0.01), but not Air (mean diff: 9 ml, p = 0.39). The change in SV from Day 1 to Day 14 was not different between Air and TN HOWI (p = 0.68) or TN and CW HOWI (p = 0.26) but was higher in CW HOWI compared to Air (p = 0.05). Changes in SV were not correlated with changes in apnea duration (r = -0.29, p = 0.15). CONCLUSIONS: SV increased when 14 days of exposure to moderate hypoxia and apneas took place in HOWI. Further research is needed to better understand the diving relevant functional benefits underlying the observed splenic growth. Support: Office of Naval Research (N00014-20-1-2593) Office of Naval Research (N00014-20-1-2593)