MO651: The Blood to Extracellular Volume Relationship is Stable and in the Physiologic Range in Chronic Haemodialysis Patients
Authors: Kron Susanne , Schneditz Daniel , Kron Joachim 
Published: May 2022

Abstract

BACKGROUND AND AIMS
Blood volume (BV) is tightly controlled by the kidneys and the interstitial space plays an important role in this control. The interstitial space serves as a volume buffer in case of volume excess or volume depletion. It is the purpose of this buffer to maintain adequate circulatory and diastolic filling pressures, and thereby to maintain a functional circulation of the blood. BV is approximately one-third of the extracellular volume (ECV) under physiologic conditions. In haemodialysis patients without renal function, ECV is expanded because fluid is accumulated in the interdialytic period. This volume excess has to be removed during dialysis by ultrafiltration (UF) of blood.
The question therefore arises, i) how is this volume excess distributed between intra-and extravascular compartments in dialysis patients and ii) how is this distribution affected by UF?

METHOD
Blood and extracellular volumes were measured in 79 stable chronic haemodialysis patients with moderate volume overload (1.85 ± 1.22 L).
Immediately before treatment, ECV and volume overload were evaluated by bioimpedance spectroscopy using the body composition monitor [BCM, Fresenius Medical Care (FMC), Bad Homburg, Germany]. ECV at the end of treatment was calculated by subtracting the intra-dialytic weight loss from pre-dialysis ECV.
The actual BV at the beginning of dialysis was determined by indicator dilution, using an online infusate bolus of 240 mL which was administered immediately after the beginning of the dialysis session and subsequent calculation using the data of the relative BV monitor integrated into the haemodiafiltration machine 5008 [Fresenius Medical Care (FMC), Bad Homburg, Germany]. BV at the end of the dialysis session was calculated from the measurement of BV at the beginning and the relative BV at the end of dialysis.

RESULTS
The relationship of BV (5.76 ± 1.54 L) to ECV (17.91 ± 3.90 L) was 0.321 ± 0.039 at dialysis start and 0.319 ± 0.040 at the end (BV 5.20 ± 1.41 L, ECV 16.22 ± 3.57 L). The mean deviation between pre- and post-dialysis was −0.002 ± 0.018. There were strong correlations (P &lt; .001) between BV and ECV both at the beginning (r = 0.88) and at the end (r = 0.88) of dialysis.

CONCLUSION
These data show that i) the blood to extracellular volume ratio in haemodialysis patients is close to one-third assumed under physiologic conditions and that ii) this ratio remains unchanged in spite of UF.
The constant relationship between the two compartments is maintained under conditions of haemodialysis with the rapid volume removal by UF. This confirms previous findings that the vascular refilling volume is only about 70% of UF volume during dialysis.
Despite the lack of kidney function, the mechanisms to adequately distribute volume excess between intra- and extravascular appear to be maintained. The volume shift between plasma and interstitial volume prevents both an inter-dialytic cardio-vascular burden and intra-dialytic hypotension. Therefore, haemodialysis patients with moderate volume overload are likely to be adequately protected from volume-related complications.
To the best of our knowledge, this is the first time that these relationships are reported in dialysis patients.