Supplementary Material for: Transcapillary Refilling Rate and Its Determinants during Haemodialysis with Standard and High Ultrafiltration Rates

Background: Achieving euvolaemia using ultrafiltration (UF) during haemodialysis (HD) without inducing haemodynamic instability presents a major clinical challenge. Transcapillary refill is a key factor in sustaining the circulating blood volume (BV) during UF, which is in turn predicted by the rate of refilling. However, absolute plasma refilling rate (PRR), its determinants and variability with UF rate (UFR), have not been reported in the literature. Method: We studied paired HD sessions (n = 48) in 24 patients over 2 consecutive mid-week HD treatments. Plasma refilling was measured using real-time, minute-by-minute relative BV changes obtained from the integrated BV monitoring device during UF. A fixed bolus dilution approach at the start of HD was used to calculate absolute BV. The first control HD session was undertaken with a standard UFR required to achieve the prescribed target weight, while during the second study session, a fixed (high) UFR (1 L/h) was applied, either in the first (n = 12 patients) or in the final hour (n = 12 patients) of the HD session. Participants’ had their hydration status measured pre- and post-HD using multifrequency bioimpedance (BIS). Blood pressure was measured at 15-min intervals and blood samples were collected at 7 intervals during HD sessions. Results: The mean PRR during a standard 4-hr HD session was 4.3 ± 2.0 mL/kg/h and varied between 2 and 6 mL/kg/h. There was a mean time delay of 22 min (range 13.3–35.0 min) for onset of plasma refilling after the application of UF irrespective of standard or high UFRs. The maximum refilling occurred during the second hour of HD (mean max PRR 6.8 mL/kg/h). UFR (beta = 0.60, p < 0.01) and BIS derived pre-HD overhydration index (beta = 0.44, p = 0.01) were consistent, independent predictors of the mean PRR (R² = 0.49) in all HD sessions. At high UFRs, PRR exceeded 10 mL/kg/h. The total overall plasma refill contribution to UF volume was not significantly different between standard and high UF. During interventions no significant haemodynamic instability was observed in the study. Conclusion:We describe absolute transcapillary refilling rate and its profile during HD with UF. The findings provide the basis for the development of UF strategies to match varying PRRs during HD. An approach to fluid removal, which is tailored to patients’ refilling rates and capacity, provides an opportunity for more precision in the practice of UF.