Alquist M, Thysell H, Ungerstedt U, Hegbrant J
Urea concentration gradient between muscle interstitium and
plasma develops during dialysis
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Sep) 7:1505 1996
The rate at which urea can be removed from a patient during dialysis is
partly dependent on the rate at which urea transfers into the dialysis
access from peripheral parts of the body. During dialysis, this finite rate
of inter-compartment urea transfer results in the urea concentration at the
dialyzer falling below the level found in other parts of the body. This
reduces the concentration gradient at the dialyzer membrane and the rate at
which urea is removed by around 10-15% depending on the duration of
treatment and Kt/V delivered. After the end of dialysis, the continuing
inter-compartment transfer causes the post-dialysis rebound.
The mechanism of this inter-compartment transfer may be diffusion (eg
across cell membranes) or blood flow (eg in poorly perfused areas of the
body). Since it is hard to measure these components directly there is
currently some debate around the mechanism of the inter-compartment
transfer. The effect of resting muscle is particularly of interest.
Resting muscle comprises a significant proportion of the urea distribution
volume and has a relatively low blood blood flow rate. If the main delay in
inter-compartment diffusion results from the poor perfusion of the muscles
then dialysis efficiency could be improved by exercise during dialysis.
Diffusion across cell membranes, on the other hand, is not so easy to
modify.
This paper reports a pilot study of three patients during hemodialysis. The
urea concentration in the in the quadriceps muscle was continuously sampled
using a micro-dialysis technique. The muscle urea concentration was
compared with the blood access concentration.
The results show that the urea concentrations are higher in the muscle
interstitial fluid than in the blood during dialysis. This difference
increases progressively from zero at the start of dialysis to around 30% by
the end of dialysis.
Normally, post-dialysis BUN is around 30% of the starting level and this
increases to around 40% by 30 minutes after dialysis. This means that by
the end of a typical dialysis, the equilibrated or mean body urea
concentration is about 30% higher than the blood concentration. Therefore,
the micro-dialysis samples in this study represent the expected
equilibrated concentration. This suggests that the muscle interstitial
fluid represents a transitional compartment, possibly between intracellular
and main vascular compartments. If inter-compartment transfer is mainly
dependent on low blood flow in the muscles, interstitial concentrations
would have had to be higher than observed in this study to account for a
30% urea rebound.
Comment:
Since only a single muscle site in three patients was studied, it is not
possible to draw any general conclusions from this paper. If confirmed by
larger studies, the pilot data tend to suggest that low blood flow in the
muscles was not a main determinant of inter-compartment transfer.
(James Tattersall, M.D., United Kingdom)
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Am Soc Nephrol
Basic hemodialysis :
Physiology
Basic hemodialysis :
Adequacy, prescription, urea kinetics