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. Fifteen male subjects performed six-hour resting thermoneutral water immersions (WI) at 1.35 atmospheres absolute for four consecutive days, with follow-up on the fifth day. Measurements included peripheral endothelial function and augmentation index (PAT, peripheral arterial tonometry), beat-to-beat blood pressure (BP, photoplethysmography), heart rate (HR), and plasma volume (PV) calculated from changes in hemoglobin and hematocrit. The reactive hyperemia index (RHI), a marker of peripheral endothelial function, increased with repeated immersions (p = 0.008). By WI2 and WI3, RHI increased 12% and 16%, respectively, compared to WI1 values, but no significant differences were detected between WI4 and WI1 for either measure. Absolute augmentation index (AI) increased by an average of 33% (p<0.001) and AI normalized for HR (AI@75) by 11% (p = 0.12) following each WI. PV decreased significantly by 13.2% 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). Compared to pre-WI HR, average post-WI HR decreased 4.3% lower (p<0.001), but increased overall by 8.2% over the course of repeated WI (p<0.001). Total peripheral resistance increased by an average of 13.1% following WI (p = 0.003). Thus, peripheral endothelial function increases after two days of WI, and PAT-derived measures of arterial stiffness increase transiently post-WI. Additionally, BP and PAT-derived endothelial function diverge from their usual associations with arterial stiffness (i.e. augmentation index) in the context of WI.

The diving response and cardiac vagal activity: A systematic review and meta‐analysis

“…Due to unclear RMSSD data (Berry et al, 2017; Louis et al, 2015; Schirato et al, 2018), three articles were removed from the meta‐analyses and retained only for the systematic review. Ten articles that reported heart rate (Ferrigno et al, 1991; Hayashi et al, 1997; Hurwitz & Furedy, 1986; Khurana & Wu, 2006; Marabotti et al, 2013; Simmons et al, 2017) or HRV parameters other than RMSSD (Costalat et al, 2021; Kinoshita et al, 2006; Kiviniemi et al, 2012; Wieske et al, 2013) had to be excluded because no RMSSD data were provided. One article did not provide pre‐ exposure RMSSD values (Lemaître et al, 2008).…”