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Chapter 7: Functional description
Derived measures for
adjusting the acid-base
status
Delayed effect in the case
of changed blood to
dialysate flow ratio
7-26
If the patient shows signs of metabolic acidosis during the treatment, the
administered citrate volume must be increased or the effective removal
of buffer bases from the blood in the filter must be reduced. In Ci-Ca
CVVHD and Ci-Ca postCVVHDF, this can be achieved by increasing
the set blood flow (which automatically increases the citrate infusion)
and / or by reducing the dialysate flow (which reduces the net removal
of buffer bases from the blood). The latter approach will, however, also
reduce the efficacy of the treatment. Alternatively, separate infusion of
bicarbonate permits an additional administration of buffer bases without
reducing the efficacy of the treatment or causing an additional citrate
load on the metabolism.
With Ci-Ca CVVHD, by changing one of the two named flows by 20%,
you can theoretically expect an effect of approx. 4 mmol/l on the
systemic bicarbonate concentration or base excess. Depending on the
extent of the effect intended, smaller or larger stepwise adjustments
may be necessary. The proper adjusting method should be selected
such that the necessary efficacy of the treatment is ensured and that the
blood flow range which is achievable in practice is taken into
consideration.
For Ci-Ca postCVVHDF, almost the same applies as for Ci-Ca CVVHD.
However, the additional infusion of the bicarbonate-buffered
substitution fluid has a stabilising effect on the resulting acid-base
status. For an effect on the acid-base status, the ratio of blood to
dialysate flow in Ci-Ca postCVVHDF must therefore be changed to a
larger extent than in Ci-Ca CVVHD. By changing one of the two flows
by 30%, you can theoretically expect an effect here of approx. 4 mmol/l
on the systemic bicarbonate concentration or base excess.
Note
With Ci-Ca postCVVHDF, adjusting the blood flow – even to alter the
acid-base status – also requires adjustment of the substituate flow.
Note
In the same way as when changing the calcium dose, the effect of a
changed blood to dialysate flow ratio can only be assessed some time
after the change.
This is caused by the fact that the systemic distribution volume must first
develop a new balance. Depending on the efficacy of the CRRT
treatment and the size of the patient (or his/her distribution volume for
buffer bases, or the essential systemic bicarbonate buffer base), the
first effects can already be seen after a few hours. The full effect can,
however, only be assessed after approximately one day.
This is particularly important to bear in mind if several equivalent
changes are made within short intervals as these may cause an
excessive response.
Fresenius Medical Care multiFiltrate IFU-EN-UK 15A-2015
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