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.

Fig 1. Water immersion and experimental timeline.

Four consecutive water immersions (WI) were performed, immediately preceded and followed by blood draws (WI1, WI3, and Recovery), EndoPAT testing (WI1-4 and Recovery), and Finometer measurement (WI1-4 and Recovery).

Fig 2. Peripheral endothelial function before and after repeated 6-h water immersions.

Values are mean ± SE. *P<0.05 compared to water immersion 1 (WI1). †P<0.05 compared to WI1 pre. Immersion resulted in increased reactive hyperemia index (RHI) by the second and third days of WI. RHI during follow-up (F/U) remained elevated compared to baseline (WI1 pre).

Fig 3. Arterial stiffness parameters before and after repeated 6-h water immersions.

Values are mean ± SE. *P<0.05 compared to water immersion 1 (WI1). Increases in augmentation index (AI—A) and AI corrected for heart rate (AI@75—B) were seen following WI but there was no cumulative effect of repeated WI.

Fig 4. Heart rate before and after repeated 6-h water immersions.

Values are mean ± SE. *P<0.05 compared to water immersion 1 (WI1). †P<0.05 compared to WI1 pre. Heart rate was lower post-WI but increased over the course of repeated immersions. Heart rate during follow-up (F/U) remained elevated compared to baseline (WI1 pre).