The Impact of Repetitive Long-duration Water Immersion on Vascular Function

Abstract

While physiological responses to water immersion (WI) are well-studied, the vascular responses after WI are less understood. PURPOSE: The objective of this study was to quantify the changes in endothelial function and vascular stiffness following repeated six-hour WIs with surface-supplied air. METHODS: Sixteen healthy subjects (15 male) performed six-hour resting thermoneutral water immersions (WI) at 1.35 atmospheres absolute (ATA) for four consecutive days, with follow-up on the fifth day. Measurements included endothelial function and arterial stiffness (peripheral arterial tonometry), beat-to-beat blood pressure (photoplethysmography), heart rate (HR), and plasma volume (PV) calculated from changes in hemoglobin and hematocrit. RESULTS: The reactive hyperemia index (RHI), a marker of endothelial function, increased with repeated immersions (p=0.008) as did lnRHI (p=0.025). By WI 3, RHI and lnRHI increased 16% and 17%, respectively, compared to WI 1 values, but no significant differences were detected between WI 4 and WI 1 for either measure. Absolute arterial stiffness (augmentation index, AI) increased by an average of 33% (p<0.001) and AI normalized for HR ([email protected]) by 11% (p=0.12) following each WI. PV decreased significantly by 13.2% (p<0.001) following WI and remained 6.8% lower at follow-up compared to pre-WI. Systolic blood pressure significantly decreased by an average of 2.5% following each WI (p=0.012). HR decreased 4.3% after each WI (p<0.001) but increased overall by 6.6% over the course of repeated WI (p<0.001). Total peripheral resistance increased by an average of 13.1% following WI (p=0.003). CONCLUSION: Four consecutive days of six-hour WIs while breathing air at 1.35 ATA results in a transient increase in arterial stiffness following each WI as well as an increase in endothelial function on the third day. Additionally, in the context of acute exposure to WI, blood pressure and endothelial function diverge from their usual direct associations with arterial stiffness