Effect of repeated water immersions on orthostatic tolerance and cardiovascular regulation during 70° head‐up tilt testing

Abstract

Water immersion (WI) reduces orthostatic tolerance (OT) and alters physiological responses to orthostatic stress, but the effect of repeated WI is unknown. Eight healthy men (35±2 yrs; mean±SEM) underwent 70° head‐up tilt (HUT) testing before and after 6‐hr O2‐breathing WI on 5 consecutive days (e.g. WI/1..WI/5). Heart rate (HR), blood pressure (BP: SBP, DBP, MAP), cardiac output (Q), limb blood flow (BF), stroke volume (SV), plasma volume (PV) and systemic vascular resistance (SVR) were measured during supine baseline (BL) and HUT. All data are compared to pre‐WI/1 BL values. Supine: HR and MAP were unchanged after repeated WI. Q, SV, and PV were reduced, and SVR increased after WI/1 (p<0.001). The drop in SV (~15 ml) persisted despite a return of PV to BL before WI/4 and/5. HUT: After WI/1, HR increased more than pre‐WI/1 (+32±3 vs +23±3 bpm) to compensate for lower SV, but SBP was reduced (−10±4 mmHg) (p<0.001). The tachycardic response to HUT was augmented over the week (+42±3 bpm, p<0.001), and SBP was further reduced after WI/5 (p=0.03), even though SV, SVR, and limb BF at post‐WI/5 were comparable to post‐WI/1. OT (max ΔHR during HUT) declined after WI/1 (p<0.001) and continued to diminish over repeated WI (p=0.02). We conclude that repeated WI exposures exacerbate immersion‐related reductions in OT. The responses suggest a shift in systemic and regional mechanisms to maintain BP and BF.Support: ONR N0001409WX20220