Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Reexamination Certificate
2000-12-11
2003-02-04
Kim, John (Department: 1723)
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
C073S861000, C073S861050, C073S861070, C073S861080, C073S861180, C210S745000, C210S746000, C210S805000
Reexamination Certificate
active
06514419
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of kidney dialysis processes and more particularly to such processes for measuring arterio-venous shunt blood flow and undesirable recirculation during hemodialysis.
Dialysis is a process by which an artificial kidney replaces the function of a patient's kidney. Blood is removed from the patient's vascular system via an arterial line, is passed through a dialyzer and is returned to the patient via a venous line for normal circulation through the patient's vascular system. A majority of dialysis patients have an arterio-venous shunt implanted in a location having a high blood flow that simplifies the withdrawal of blood from the part that is closer to the arterial side of the shunt and the return of purified blood downstream of the withdrawal site, closer to venous side of the shunt. In some cases the shunt clots or stenoses and the resulting reduction in blood flow necessitates surgery that is costly and invasive for the patient. In the situation of low blood flow in the shunt or, if there is any other problem with the venous outflow, some part of the freshly dialyzed blood from the venous return line flows directly to the arterial withdrawal line where it is again filtered. If this undesired direct recirculation level is high enough, some amount of blood will be repeatedly refiltered and the rest of the patient's blood will not be sufficiently filtered to provide the patient with adequate dialysis.
One method of measuring shunt blood flow currently uses color coded duplex sonography. This is very expensive and involves operation by highly-qualified professionals. Measurements are therefore made only rarely and the onset of reduced flow, when treatment could be made without surgery can be missed.
The standard test for undesired direct recirculation requires three blood samples while the patient is on dialysis. This method requires blood samples from the patient, time from the nurses, and high laboratory costs. Dialysis patients generally have lower hematocrit than the normal population and are at greater risk from losing blood, so this is not very satisfactory.
Another technique involves injection of a saline solution intravenously and recording changes of blood optical properties for detecting recirculation qualitatively. This technique leaves open the question of whether recirculation is quantitatively reduced sufficiently to warrant intervention.
SUMMARY OF THE INVENTION
The present invention avoids the problems encountered with previous methods and techniques by providing an accurate determination of shunt blood flow and undesired recirculation at lower cost.
Blood flow, Q, measured by the dilution method (A. C. Guyton Textbook of Medical Physiology, Sixth Edition, p. 287, 1981) is given by: Q=V/S (Eq. 1) where V is the amount of injected indicator and S is the area under a dilution curve and is equal to the average concentration of indicator in the blood for the duration of the curve, multiplied by the duration of the curve.
A dilution curve is obtained by measuring changes in a physical parameter of the blood over a period of time, and plotting the resulting variations. For example, if the blood parameter being measured is sound velocity, the injection of an indicator such as a saline solution, having a different sound velocity than blood, will produce a change in the measured parameter as the indicator passes the sensor location. The indicator dilutes the blood, and produces a sound velocity curve which is a measure of that dilution. Although injection of a saline solution is convenient for producing a measurable change in a blood parameter such as sound velocity, other changes of parameters may also be suitable. Thus, changes in temperature, electrical impedance, optical characteristics, and the like may also be used as indicators to produce dilution curves. For purposes of this disclosure, however, reference will primarily be made to the use of saline solution as the indicator, with resulting changes in sound velocity in the blood being measured to provide a dilution curve.
To facilitate the measurement of shunt blood flow in accordance with the present invention, the blood line connection is reversed from normal; that is, the arterial inlet which removes the blood from the patient for dialysis is located downstream (not upstream as normal) of the venous outlet in the shunt. A volume of indicator, such as a saline solution, is injected into the venous line (V
ven
), where it is mixed with the dialyzer blood flow Q
dial
and the mixture is delivered to the shunt where it is combined with the blood flow in the shunt (Q
shunt
). The blood shunt flow (Q
shunt
) can be calculated from Equation 1 by measuring the dilution area in the arterial line S
art
:
Q
shunt
=+Q
dial
=V
ven
/S
art
(Eq. 2)
or
Q
shunt
=V
ven
/S
art
−Q
dial
(Eq. 3)
Equation 3 shows that if the blood flow through the dialyzer Q
dial
is measured and the absolute concentration of indicator in the arterial blood line S
art
is recorded, then the blood flow through the shunt Q
shunt
can be calculated.
In some methods applicable to hemodialysis, sensors are clamped onto the exterior of the arterial or venous line, or tube. However, it is difficult to measure the absolute concentration of indicator in the blood through the hemodialysis tube. For example, if a sound velocity sensor is used to record protein concentration changes in blood due to a saline indicator injection, the sound beam will have to pass through both the tube and the blood. Recorded measurements of absolute sound velocity will be influenced not only by the blood, but also by the unknown sound properties of the tube. The same problem occurs if an optical sensor is clamped onto tube; i.e., the recorded amplitude of a light beam is not only the function of hemoglobin concentration but of tube properties.
This problem may be solved by an additional calibration injection of the same indicator, which is injected in the arterial line, but upstream of the place where the measurements are made. The equation for this case will be:
Q
dial
=V
cal
/S
cal
(Eq. 4)
where V
cal
is the known quantity of indicator in the calibration injection and S
cal
is the area under the resulting dilution curve. This area is the average concentration of indicator in the blood for the duration of the curve, times the duration of the curve.
From Equations 2 and 4 the formula for shunt blood flow will be:
Q
shunt
=Q
dial
(
V
ven
/V
cal
*S
cal
/S
art
−1) (Eq. 5) or
Q
shunt
=(
V
ven
/S
art
−V
cal
/S
cal
) (Eq. 6)
Equation 5 is suitable if blood flow in the tube can be measured accurately. The ratio S
cal
/S
art
shows that the recorded dilution areas only need to be proportional to relative changes in concentrations in this case. Assuming that tube properties are constant during the measurements, the value of this ratio can be calculated with high accuracy for most type of sensors, including sound velocity, optical, etc.
Equation 6 can be used where tube blood flow is unknown but absolute concentrations are measured, for instance by withdrawing the blood from the arterial blood line and using an optical densitometer for optical dye dilution measurements.
To avoid the need for a calibration injection, an additional sensor that is matched to the arterial line sensor is located on the venous line downstream of the location of the intravenous indicator injection. For this case, the injected indicator will be mixed with the venous line tube flow, so by analogy with the calibration injection of Equation 4:
Q
dial
=V
ven
/S
ven
(Eq. 7)
where S
ven
is the area under the dilution curve and is calculated as the average concentration of indicator in the blood for the duration of curve, times the duration of the curve. From the same injection, the area S
art
is generated. The formula for blood flow by substituting in Equation 5 is:
Q
shunt
=Q
dial
(
S
Harter Secrest & Emery LLP
Kim John
Salai Esq. Stephen B.
Shaw Esq. Brian B.
Transonic Systems Inc.
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