Repetitive Cold-Water Immersion Augments Norepinephrine Concentrations but Not Ventilatory Responses During Rest at High Altitude

Study Objective: To compare the effects of pre-ascent cold-water (CW) and thermoneutral (TN) water immersions on resting ventilation (RV) and catecholamine concentrations during hypobaric hypoxia (HH). Hypothesis: We hypothesized that six CW (12°C) immersions would reduce norepinephrine (NE) and epinephrine (Epi) as well as RV responses during an acute HH exposure (468 mmHg) compared to a TN (35°C) immersion control. Methods: Ten males (age: 26 ± 8 years, body mass: 82.9 ± 7.0 kg, height: 176.1 ± 6.9 cm, body fat: 23.2 ± 6.1%) completed six, 10-minute head-out water immersions in CW and TN water in a randomized cross-over manner. Immersions were performed twice per day across 3 consecutive days, separated by at least 4 hours. The morning following their last water immersion, participants, after a baseline venous blood sample at sea level [SL (~760 mmHg)], ascended in a hypobaric altitude chamber where RV was recorded for 10-15 minutes at three acute time points [30 minutes, 90 minutes, 150 minutes (RV1, RV2, RV3)]. Minute ventilation (V E ), tidal volume (V T ), respiration rate (RR), oxygen saturation (SpO2), and partial pressure of end-tidal carbon dioxide (PetCO2) were averaged across the final 10 minutes of each RV. Following RV3, a venous blood sample was collected to assess Epi/NE concentrations. A linear mixed model with random intercept for volunteer was used to compare RV 1-3 between CW and TN conditions. A paired samples t-test was used to compare the Epi/NE responses between CW and TN conditions separately at SL and post RV3. Results: Circulating NE was significantly greater following CW compared to TN when sampled at SL (415.9 ± 95.3 vs 315.7 ± 67.7 pg/mL, p = 0.008) and post RV3 (626.4 ± 189.2 vs 531.5 ± 207.6 pg/mL, p = 0.024), while Epi remained unchanged ( p >0.05). V E , RR, and V T showed no significant main effects for time or condition and no significant interaction ( p >0.05). Not unexpectedly, both SpO2 ( p = 0.014) and PetCO2 ( p <0.001) demonstrated a significant main effect of time with RV3 reflecting higher SpO2 and lower PetCO2 (84.2 ± 3.4%, 36 ± 3 mmHg) compared to RV1 (82.3 ± 4.4%, p = 0.035; 39 ± 4 mmHg, p < 0.001) and RV2 (82.3 ± 3.4%, p = 0.030; 39 ± 3 mmHg, p = 0.001). Conclusion: Circulating NE, but not Epi, was elevated proportionally at both SL and after 150 minutes of high-altitude exposure when preceded by repeated CW immersions compared to TN immersions. Ventilatory responses to altitude were not different between CW and TN despite the NE elevation (a known ventilatory stimulant). Repeated CW immersion may have lasting influence on baseline SL circulating NE that is retained at rest once altitude exposed. Funded by DHP JPC-5; author views not representative of the Army/ DoD. 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.