Kapoian T, Miller AJ, Sherman RA
Validation of a revised slow/stop flow recirculation method
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Sep) 7:1409 1996
Although there is much discussion as to how to draw the postdialysis
BUN sample in terms of rebound, all agree that a method must be used
which eliminates the possibility that recirculated blood is being
sampled. Otherwise, in patients with severe access recirculation
(about 3-5% of the population, usually), the postdialysis BUN will be
contamined with dialyzer outlet blood, and will underestimate the true
postdialysis BUN. This can result in a severe overestimation of
delivered Kt/V or URR.
In the past, to eliminate access recirculation (AR), the suggested
strategy was to slow the pump to 50-100 ml/min for about 15-30 sec.
Recirculation occurs usually when access flow (Qa) falls below the
extracorporeal blood flow (Qb). Under such conditions, the blood pump
begins to pull in blood from the downstream limb of the access. As a
corollary, to stop recirculation, one needs to simply reduce Qb so it
is less than Qa. A pump flow of 50-100 ml/min is adequate for this
purpose. Blood samples are usually drawn from the sampling port,
which almost always (in US blood lines) is about 8-10 ml downstream
from the tip of the vascular access needle. To clear this deadspace,
one should ideally pass double the volume of blood through the line
(20 ml) before stopping the pump completely and obtaining the sample.
Sherman concluded that, at a flow of 120 ml/min, or 2 ml/sec, 10 sec
would be sufficient to clear the "deadspace" in the line of
recirculated blood. Waiting longer than necessary is problematic,
because the A/V urea gradient established during dialysis begins to
close rapidly as soon as the blood pump is slowed. As such, an early
increase in access urea level is seen within about 10-15 sec of
slowing the blood pump.
Sherman et al used this 10 sec slow flow (120 ml/min) method in 50
patients, and also measured AR by ultrasound dilution (UD). Access flow
was also measured. In 7/50 patients, AR by ultrasound dilution (UD)
was 33%, Qa by UD averaged 254 ml/min, and AR by this new urea method
was 27%. Since Qb was 500 ml/min, these patients by rights should
have true AR. In the remaining 43 patients, AR by UD was 2%, Qa by UD
averaged 870 ml/min, and recirculation by slow flow averaged 0%. It
appeared that even with this method, due to variability in urea
measurements, an AR of up to 10% by the urea method should be considered
as a negative test.
Comment: This is a nice piece of work, especially as Qa was
also measured. It suggests that even a 20 sec slow flow period may be
too long in measuring recirculation, although the waiting period
depends on the slow flow Qb and also on the deadspace volume. The
waiting period would theoretically be less than 10 sec if one draws
the sample from the line attached to the needle segment (deadspace
only a few mls). It is very important to STOP THE PUMP after the slow
flow period prior to sampling. This freezes the desired sample in the
dialysis tubing, and prevents further changes in the composition of
blood to be sampled during the delay associated with obtaining the
sample. See the
FAQ on HDCN regarding this topic for more information.
(John T. Daugirdas, M.D., University of IL at Chicago)
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Am Soc Nephrol
Basic hemodialysis :
Vascular Access: graft/fistula