Regulation of blood flow during intradialytic exercise: the role of blood viscosity

Introduction: Intradialytic exercise (IE) is an effective non-pharmacological intervention for patients undergoing hemodialysis (HD). During exercise training, increased blood flow (BF) within the exercising muscle is driven by elevated endothelial shear stress (SS) that is proportional, at least in part, to blood viscosity (BV). Although ultrafiltration during HD session reduces plasma volume, thereby increasing BV, it remains unclear whether this change in BV influences peripheral circulation during IE. Therefore, the aim of this cross-sectional study was to examine the effects of IE on BV and peripheral/central hemodynamics. Methods: Patients receiving maintenance 4-hour haemodialysis sessions for at least 3 months were included. BF and vascular conductance (VC) at the brachial artery were measured using Doppler ultrasound. Stroke volume (SV), cardiac output (CO), mean arterial pressure (MAP) and heart rate (HR) were continuously monitored via finger plethysmography. BV was determined from hematocrit. All measurements were obtained at baseline (T0) and at 4 hours (T1) during a 4-hour dialysis session and a 4-hour interdialytic period (Sham). At each time point, patients performed IE through intermittent handgrip exercise at workloads of 3, 6, and 9 kg. Demographic and laboratory data were also collected. Results: 12 out of 27 patients were enrolled. BF increased progressively with workload in sham and HD periods. Compared to Sham period, BF in HD was statically significant higher at 6kg (199.1±48.2 vs. 156.1±39.2 ml/min; p< 0.05) and 9kg (289.1±54.9 vs. 223.8±49.5 ml/min; p< 0.05) but not at 3kg (139.1±32.1 vs. 135.6±26.3 ml/min). Similarly, VC increased with workload in HD (3% at 3kg; 34% at 6kg, p< 0.05; 43% at 9kg, p< 0.05). Conversely, SV, CO and MAP, indices of central hemodynamic, were significantly lower in HD (-22% at 3kg, -15% at 6kg, -9% at 9kg, p< 0.05) than Sham period. Conversely, HR increased in HD at T1 (+16%), suggesting a compensatory chronotropic response to reduced plasma volume. BV significantly higher in HD period at T1 compared to Sham (4.2±0.5 vs. 3.3±0.6 cP; p < 0.05). Conclusion: IE increased BV and peripheral vasodilation, resulting in greater BF and VC. This augmented peripheral response occurred despite reductions in central hemodynamics, suggesting that viscosity-mediated SS plays a key role in regulating peripheral hemodynamics during IE. This abstract was presented at the American Physiology Summit 2026 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.