Acute effect of hemodialysis on vascular responsiveness: the role of blood viscosity

Background and aim: Vascular responsiveness in conduit arteries is highly influenced by the shear rate of blood cells against the endothelium, generating local nitric oxide-mediated vasodilation. This phenomenon is likely influenced by blood viscosity, as it alters shear rates and the degrees of vasodilation. However, it remains unclear whether changes in blood viscosity (BV%) impact vascular responsiveness. Therefore, the objective of the present study was to investigate the acute effects of hemodialysis on BV% and vascular responsiveness. We hypothesized that BV% would progressively increase during hemodialysis sessions and that this acute change would be correlated with a gradual increase in vascular responsiveness, as assessed by flow-mediated dilation (FMD%). Methods: The degree of intradialytic and interdialytic variation of BV% was measured via hematocrit, while vascular responsiveness was determined by the FMD% of the brachial artery in 17 patients receiving maintenance dialysis for at least three months. Both BV% and FMD% were measured at baseline (T0), at 2 hours (T1) and at 4 hours (T2) during the 4-hour dialysis session (HD) and a 4-hour interdialytic period (Sham) Results: At baseline (T0), FMD% was similar for both Sham and HD (2.5 ± 0.1%; 2.9 ± 0.9%). After 2 and 4 hours, FMD% did not change for the Sham (3.0 ± 1.4% and 3.2 ± 1.3%, respectively). However, a progressive and significant increase in FMD% was observed during HD at T1 (6.7 ± 1.4%; p<0.001) and at T2 (7.8 ± 1.3%; p<0.001). A similar trend was noted for BV%, comparable at T0 in Sham (33.8 ± 3.1%) and HD (33.9 ± 3.1%). No notable difference in BV% was observed during the Sham at T1 (33.9 ± 3.2%) and T2 (33.7 ± 3.1%). In contrast, BV% increased at T1 (35.4 ± 3.2%; p<0.001) and T2 (36.9 ± 2.8%; p<0.001) in HD. The changes between BV% and FMD% were significantly correlated during HD (r = 0.45; p = 0.021). Conclusion: In line with our hypothesis, the progressive increase in BV% measured during dialytic session was parallel to the acute increase in vascular responsiveness. These results highlight that shear rate stimulus-response mechanisms are linearly correlated with blood viscosity. As such, BV% should be considered in the precise analysis of vascular responsiveness in both healthy and dialysis populations. This work was partially funded by the Italian Ministry of Research University (MIUR) This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.