Lindsay RM, Burbank J, Brugger J, Bradfield E, Kram R, Malek
P, Blake PG
A device and a method for rapid and accurate measurement of access
recirculation during hemodialysis
(Apr) 49:1152-1160 1996
This paper describes the hemodynamic recirculation monitor (HDM), a
practical, accurate and relatively non-invasive method for measurement
of hemodialysis access recirculation. Data from over 1000 measurements
were used to validate this method.
The HDM measures the relative conductivities of blood entering and
leaving the fistula via the venous and arterial lines. The
conductivity sensors require the blood lines to be modified by the
insertion of paired toroids into the arterial and venous lines.
Differential conductivity is measured by comparing the electromagnetic
inductance of the arterial and venous toroids.
For recirculation measurements, one ml of hypertonic (4000 mM) saline
is injected rapidly upstream of the venous toroid. The saline 'tracer'
is detected at the venous toroid as a transient increase in
conductivity. If recirculation is present, some of the saline is
immediately drawn into the arterial line and detected at the arterial
toroid a short time later. By comparing the size of the arterial and
venous signals, the recirculation may be quantified.
The data presented in this paper suggest that the HDM has a
precision of 2% absolute. The CV at 10% recirculation was 20%.
Recirculation measured by the HDM agreed with the traditional
three-sample BUN method (r=0.92), however, Bland-Altman analysis was
not done. The paper confirmed previous observations that the
three-sample BUN method is imprecise and will 'detect' recirculation
at around 5% even when no recirculation exists. The paper also uses
the large number of recirculation measurements to illustrate some
important clinical points regarding access recirculation;- True access
recirculation is rare (2/27 patients with fistulae, 0/22 patients with
PTFE grafts). Massive recirculation may occur even in a fistula which
appears normal. Timely intervention by angioplasty may restore access
function and prevent loss of the access by thrombosis. Access
recirculation is inevitable in central venous catheters with flow
reversed. Access recirculation, if present, is critically dependent on
blood flow rate.
Comment: In principle, the HDM may be re-programmed to measure
cardio-pulmonary recirculation and, therefore, cardiac output. This
possibility was discussed in the paper. The principle of measuring
hemodialysis recirculation by tracer dilution is not new. At least
three non-invasive recirculation monitors are already commercially
available. All three methods are less invasive and arguably more
practical than the HDM; The Gambro Fistula Assessment Monitor (FAM)
has been marketed in Europe since 1985. Injected isotonic saline is
used as the tracer and is detected optically. The equipment was
designed as a module for the AK10 dialysis machine which is now
obsolete. Transonics Inc. market a recirculation monitor which detects
isotonic saline tracer using ultrasound. This equipment has not been
subjected to as rigorous validation as described in Lindsay's paper
but is probably at least as precise. Fresenius dialysis machines now
contain an optional integral recirculation monitor which uses a bolus
of cool blood as the tracer. This method is the simplest to operate as
all functions are carried out automatically by the dialysis machine.
No injection is required as the cool bolus is achieved by allowing the
dialysis fluid to cool. The Fresenius method is probably not as
precise as the HDM and cannot distinguish between access and
cardio-pulmonary recirculation. None of these three methods need
modification of the blood lines or hypertonic saline injection as does
Lindsay does discuss the potential drawbacks of the
HDM method, namely the potential for vascular endothelial damage
caused by the hypertonic saline and the need for blood line
modification. I am also uneasy about the additional turbulence and
potential for clotting induced by the toroids.
this paper uses a large number of observations to validate the
saline-tracer method of recirculation measurements. Many valuable
lessons may be learned from Lindsay's observations. This paper will
increase awareness and interest in recirculation measurement using one
of the commercially available monitors.
(James Tattersall, M.D.)