Besarab A, Frinak S, Dumler F, Goldman J, De Vita MV, Sherman
R, Kapoian T
Dialyzer venous drip chamber accurately reflects intra-
access pressure
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Sep) 7:1403 1996
Although measuring access flow is now acknowledged to be the best
method of monitoring access patency, practical methods for doing this
are not yet available. An alternative is measurement of venous access
pressures during dialysis as advocated by Schwab, but these depend on
blood flow,
viscosity (and thereby hematocrit), needle size, and possibly needle
placement. It has been shown that measurement of intraaccess
pressures under conditions of no extracorporeal flow is more
predictive of future access failure than measurement with blood
flowing, as the former method eliminates the problems associated with
needle resistance (e.g. needle size, blood flow rate, and viscosity).
When access pressure is measured with a transducer placed at heart level,
when this pressure at zero extracorporeal flow is greater than 40% (or
certainly
50%) of the MAP, access failure is likely.
Measuring access pressures with a transducer at heart level is also
cumbersome. What is needed is a method to use the venous pressure
monitor of the dialysis machine to measure intra-access pressure.
There are three potential problems: (1) machine-to machine and
manufacturer-to manufacturer variations in transducer height, (2) the
need for proper transducer calibration, and (3) the need to clamp the
venous line between the dialyzer and venous pressure monitor (to
obviate transmission of upstream pressures to the venous monitor).
In this abstract, Besarab et al, who originated the concept of
measuring intra-access pressures
(see KI abstract 1995), compared
intra-access pressures measured directly by transducer (PT) with those
measured by the venous pressure monitor (VPM) at the venous drip
chamber with the venous line clamped. They looked at 3 different
types of machines at 3 different centers. Because the VPM is usually
above heart level, VPM pressures will always be higher than intra-
access pressures measured by a heart-level transducer. With Fresenius
machines, this pressure difference averaged 17 mm Hg; with Cobe Centry
3 machines the differences were 16 mm Hg at one unit and 12 at another,
whereas
with Althin machines the difference averaged 10 mm Hg. If the
transducer is properly calibrated, one could simply measure the
difference in height between the patient's heart level and the VPM in
cm, convert to mm Hg, and have the pressure difference.
Besarab then computed a normalized access pressure (access pressure
divided by mean BP) using the VPM corrected for the machine-specific
offset to estimate access pressure. When normalized access pressure
was < 0.5, Doppler access flow averaged 555 ml/min, whereas when it
was > 0.5, Doppler access flow averaged 1229 ml/min, showing that
this method could be used to predict flows and therefore access
failure.
Comment: This represents the first practical method which can
be applied unit-wide to measure access flow, which can be done prior
to every dialysis with minimal staff imposition. Remember to clamp
the venous line upstream to the monitor when taking the measurement,
and use different offsets for patients who are seated vs. those being
dialyzed on gurneys or beds.
(John T. Daugirdas, M.D., University of IL at Chicago)
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Am Soc Nephrol
Basic hemodialysis :
Vascular Access: graft/fistula