P1091DYNAMICS OF VASCULAR REFILLING IN EXTENDED NOCTURNAL HAEMODIALYSIS
Authors: Kron Susanne , Schneditz Daniel , Leimbach Til , Kron Joachim 
Published: June 2020

Abstract

Background and Aims
Refilling volume had not been a measurable parameter in clinical practice so far, because the knowledge of absolute blood volume (BV) is necessary. Recently, we developed a simple method to determine absolute BV, thus enabling quantification of the refilling volume. The overnight dialysis is particularly suitable for studying the refilling process due to negligible external influences and the extended treatment time.

Method
At the beginning of the dialysis (t=0), before starting the ultrafiltration (UF), a defined online bolus of 240 ml dialysate was infused by pressing the emergency button on the haemodiafiltration (HDF) machine (5008, FMC). The subsequent increase in relative BV (RBVpost-RBVpre) was measured with the integrated BV monitor, and absolute BV at t=0 was calculated:
absolute BV in ml = bolus volume 240 ml x 100% / increase in relative BV in%.
Absolute BV at any other time t was calculated as: Vt = V0 x RBVt (in %) / 100
At constant UF rate refilling volume (Vref) is given for every hour (h) as:
Vref h = UFV h – drop in absolute BVh (beginning – end)
The refilling fraction is expressed as refilling/UF ratio:
Ref% = Vref/UFV x 100, in %
Before treatment, volume overload was evaluated by bioimpedance spectroscopy (BIS) using the body composition monitor (FMC).

Results
Refilling volume and refilling fraction were studied in 10 stable chronic haemodialysis (HD) patients during an extended (7 hour) nocturnal HD session. Specific BV at treatment start was 74.9 ± 11.8 mL/kg and dropped to 68.6 mL/kg at dialysis end. The constant UF rate was set to 406 ± 120 mL/h (specific UF rate 4.81 ± 1.83 mL/kg/h). In the first hour refilling volume amounted only to 23% of UF volume. Refilling fraction reached its maximum in the 2nd, 3 rd and 4th hour at about mean 90% (91.5%, 88.7%, and 91.1% respectively) of UF volume. From the 5th hour on, refilling volume and refilling fraction decreased (5th hour 81.3%, 6th hour 72.5%, and 7th hour 70.0% of UF volume). In the 5th hour, cumulative UF volume exceeded volume overload (1.1 ± 1.0 L) measured by BIS in 9 of 10 patients. Refilling fraction cumulatively increased up to the 4th hour. Thereafter, cumulative ratio remained constant at 70 – 75% due to the decrease in refilling volume. This did not change during the further course of dialysis. Cumulative refilling volume showed a strong correlation (r2 = 0.88; p&lt;0.001) with UF volume.

Conclusion
After the first hour, when sufficient refilling takes place, refilling volume is in a stable ratio of approximately 90 % of UF volume. Refilling is low in the first hour, and, therefore, refilling volume amounted cumulatively only about 70 to 75% of UF volume during usual dialysis duration. Thereby, absolute BV will be reduced by approximately 250 to 300 ml per liter UF. This must be taken into account in volume management in order to avoid intradialytic complications.
As expected, refilling decreases when volume overload in the interstitial space is removed. Due to the reduced refilling, there will be an additional gap in BV.