Krivitski NM
Theory and validation of access flow measurement by dilution
technique during hemodialysis
Kidney Int
(Jul) 48:244-250 1995
A sensor which measures the ultrasonic transit time across a
dialyzer bloodline is normally used to measure blood flow rate. In
this application, the injection of saline upstream to the sensor is
used to estimate access recirculation and, with reversal of the lines,
access blood flow rate. Saline dilution alters the ultrasonic transit
time across a column of flowing blood. If saline is injected upstream
to the sensor, the sensor will register an indicator dilution curve.
If the sensor is placed on the arterial bloodline, injection of blood
into the venous line should not affect the arterial bloodline sensor
reading for the initial 10-15 sec after injection (until the saline
traverses the heart and some of it comes back via the arterial
circulation). If, however, there is access recirculation, then
injection of a 5 ml saline bolus in the venous line will show up at the
arterial bloodline sensor, and the amount of recirculation can be
quantified as a percentage of the dilution curve obtained by injecting
the same amount of saline in the ARTERIAL line just upstream to the
sensor. Access blood flow can be measured by deliberately reversing
the bloodlines for a few minutes before making the injections. With
the lines reversed, the dialyzer is fed from the downstream needle.
The venous line empties into the upstream needle. Saline injected into
the venous line will now go via the upstream needle through the access
segment and downstream needle into the arterial line and will be
detected by the sensor on the arterial line. The percent of injected
saline that will be detected depends on the ratio of blood pump flow to
blood pump flow + access flow. In this way, access blood flow rate can
also be quantified.
The present paper presents details of the
mathematics and in vitro validation using various degrees of separation
of access needles, and also needle orientations. Access recirculation
was accurately quantified with only minor degradation of accuracy at a
3 cm separation of the needles. Access blood flow (in vitro) could be
accurately measured at line flows as low as 100-150 ml/min, and with
blood pump flows down to the same range. When the needles were placed
in the same direction into the "access", the accuracy of access blood
flow measurements was reduced slightly due to imperfect mixing.
This is very exciting new technology. It was validated in patients for
both recirculation and access blood flow at the ASAIO meetings in
Chicago in May of 1995 by Tom Depner. We have used this device to
examine recirculation as well, and find that most patients (about
90-95%) have zero recirculation. We also picked up several patients
with inadvertently reversed needles. The advantage of measuring access
flow is that low flows may predict access failure better than any other
measure.
(Daugirdas)
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