Cardiovascular and autonomic responses to physiological stressors before and after six hours of water immersion

The physiological responses to water immersion (WI) are known; however, the responses to stress following WI are poorly characterized. Ten healthy men were exposed to three physiological stressors before and after a 6-h resting WI (32-33°C): 1) a 2-min cold pressor test, 2) a static handgrip test to fatigue at 40% of maximum strength followed by postexercise muscle ischemia in the exercising forearm, and 3) a 15-min 70° head-up-tilt (HUT) test. Heart rate (HR), systolic and diastolic blood pressure (SBP and DBP), cardiac output (Q), limb blood flow (BF), stroke volume (SV), systemic and calf or forearm vascular resistance (SVR and CVR or FVR), baroreflex sensitivity (BRS), and HR variability (HRV) frequency-domain variables [lowfrequency (LF), high-frequency (HF), and normalized (n)] were measured. Cold pressor test showed lower HR, SBP, SV, Q , calf BF, LFnHRV, and LF/HFHRVand higher CVR and HFnHRVafter than before WI (P < 0.05). Handgrip test showed no effect of WI on maximum strength and endurance and lower HR, SBP, SV, Q , and calf BF and higher SVR and CVR after than before WI (P < 0.05). During postexercise muscle ischemia, HFnHRVincreased from baseline after WI only, and LFnHRVwas lower after than before WI (P < 0.05). HUT test showed lower SBP, DBP, SV, forearm BF, and BRS and higher HR, FVR, LF/HFHRV, and LFnHRVafter than before WI (P < 0.05). The changes suggest differential activation/depression during cold pressor and handgrip (reduced sympathetic/elevated parasympathetic) and HUT (elevated sympathetic/reduced parasympathetic) following 6 h of WI. Copyright © 2013 the American Physiological Society.