Sands J, Glidden D, Jacavage W, Jones B
Difference between delivered and prescribed blood flow in
hemodialysis
42nd Annual Conference of the ASAIO
ASAIO J
(May) 42:(2):74 1996
One of the difficulties in properly doing urea modeling is the fact that the
blood flow on the blood
pump reading is not always equal to the delivered flow. This occurs
particularly at high blood
flows, i.e., greater than 400 ml/min. Lets call the blood flow gap the %
difference in actual flow
from nominal (the reading on the machine) flow.
The BFG is due to the fact that the blood pump tubing segment within the
roller pump becomes oval
instead of round at high negative prepump pressures. For each stroke of the
roller pump, ovaling of
the tubing causes a lower volume to be delivered. With smaller needles, in
particular, the negative
pressure increases markedly at high blood flow rates. When prepump pressure
is below -250 mm Hg,
you get a substantial blood flow gap, and in general, the blood flow rate
should not be increased
beyond this point. For this reason, most centers using high efficiency
dialysis now no longer use
16 gauge needles.
Tom Depner did some work on this published in the ASAIO Transactions, and
found that, at 200 ml/min
nominal flow, the blood flow gap (BFG) was 0%, and 300 ml/min it was about
5%, and at 400 ml/min it
was about 10%. Based on this Tom and I developed a correction formula used
in a
nomogram approach to urea modeling. Recent data suggests that such a
correction may be bit too
severe, especially when 15 gauge needles are used. However, at high blood
flows, the prescribed dialyzer
clearance decreases somewhat for still unexplained reasons, and the above
correction does hold up
well for modeling purposes (see ASN 1996 abstracts from the NIH Hemo Study
Group).
In the present abstract, in 64 patients with fistulas and in 31 with AV
grafts, actual blood flow
was measured using the Transonics flowmeter. 16, 15, and 14 gauge needles
were used. There was no
BFG at 200 ml/min, and BFG was minimal at 300 ml/min (about 2%). BFG
averaged 5% at 400 ml/min, but
was 10% or less in 30% of patients. Most importantly, there was a huge BFG
in 20 patients dialyzed
with permcaths. BFG was 6, 12, and 22% at nominal Qbs of 200, 300, and 400
ml/min. Predictably,
BFG correlated with needle size. Unfortunately, prepump pressure was not
measured; this is where
the best correlation should be found.
Comment: These findings are important. The clinical implications are:
when trying to
dialyze with permcaths, you need to overprescribe dialysis, because estimated
clearance, even if
based in an A/V difference, will often be too low due to a large BFG. Large
BFGs can also occur if
one tries to do high efficiency dialysis with 16 gauge needles. Optimally,
everyone should be
measuring prepump pressure, and keeping it "above" -250 mm Hg. Even then,
one can expect a BFG of
about 10%.
(John T. Daugirdas, M.D., University of Illinois at Chicago)
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