Surgery – Diagnostic testing – Measuring or detecting nonradioactive constituent of body...
Reexamination Certificate
1999-11-30
2001-07-10
Sykes, Angela D. (Department: 3736)
Surgery
Diagnostic testing
Measuring or detecting nonradioactive constituent of body...
C600S573000, C210S646000
Reexamination Certificate
active
06258027
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for calculating dialysis efficiency using values obtained from a urea sensor for the calculations. The calculations can also predict certain conditions which may require intervention.
BACKGROUND OF THE INVENTION
In hemodialysis it is common to dialyse a patient three times per week, with time periods of three to four hours per treatment. The object of this treatment is to provide an adequate dose of dialysis to the patient. Such a dose of treatment can be defined in different ways.
One commonly used definition uses the urea molecule as a marker molecule and prescribes that the dialysis clearance (K) divided by the distribution volume (V) of urea times total treatment time (t) should exceed a certain constant, for example Kt/V is greater than one per treatment. The weekly dialysis dose is then Kt/V greater than three.
One common way of measuring Kt/V is by measuring the concentration of urea (c
b
) in the plasma before and after the treatment. The ratio R=c
bpost
/c
bpre
is correlated to Kt/V. A number of different equations have been suggested for the calculation of Kt/V, such as:
Kt/V
=−ln (
R
−0.03)+(4−3.5
·R
)·
UF/W
(1)
where UF=ultrafiltration volume removed in liters and W=post-dialysis weight in kg.
Several clinical studies have been performed evaluating Kt/V in which post-dialysis plasma urea (c
bpost
) has been measured immediately after the dialysis, usually less than two minutes after ending the treatment. However, most patients have a rebound of c
bpost
. If an equilibrated post-treatment (c
bpost
) is measured after about 30 minutes, for example, a more “true” Kt/V can be measured.
The measurement of c
b
is not unproblematic. It is required that a blood sample be taken before and after the dialysis treatment. Such samples are then analysed by the hospital's laboratory. The resulting values are thus provided with a substantial time delay. In this way it is not possible to adjust the actual treatment so that a prescribed dose is obtained.
The post-treatment sample must be taken with care, especially regarding timing, to avoid false values due to cardiopulmonary or access recirculation. Another source of error is the rebound mentioned above.
If an equilibrated post-treatment sample should be taken, such sample should be taken about 30 to 60 minutes after terminating the treatment, which is not practical for the patient. The amount of rebound and the rate of rebound varies considerably from patient to patient.
These problems have been addressed in the prior art in different manners.
International Application No. WO 94/08641 describes the use of a urea monitor for assessing the adequacy of an dialysis treatment. The urea monitor is connected to the dialysis effluent line and measures the concentration of urea in the dialysate leaving the dialyzer.
According to this specification, it is necessary to know or measure the pre-dialysis plasma urea value (c
bpre
). Such measurements can be made by measuring the urea concentration in an equilibrated sample taken before initiation of the treatment. However, such initial measurement takes time and the dialysis machine needs to be specially constructed to obtain such pre-dialysis urea value.
Other indicators of adequate dialysis are URR and SRI:
URR=
1
−R=
1
−c
b post
/c
b pre
(2)
SRI=
(
m
urea pre
−m
urea post
)/
m
urea pre
(3)
where m
urea pre
and m
urea post
are pre and post amounts of urea in the body, respectively.
The object of the present invention is to provide a method and apparatus for determining the efficiency of a dialysis treatment and monitoring delivered dose of treatment on-line.
Another object of the present invention is to provide a method and apparatus for continuously monitoring the dialysis efficiency for adjusting the dialysis treatment on line when required, for example if the dialyzer is clotted.
A further object of the present invention is to provide a method and apparatus for estimating the dose of dialysis delivered without the need for taking blood samples or requiring the dialysis machine to make any special adjustments such as taking an equilibrated pre-dialysis plasma urea concentration.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now been realized by the invention of method for calculating a parameter of the mass exchange of a solute in a fluid comprising:
passing the solute in a predetermined volume of the fluid on one side of a semi-permeable membrane in a mass exchange device,
passing an exchange fluid on the other side of the semi-permeable membrane,
obtaining a solute concentration curve by repeatedly measuring the concentration of the solute in the exchange fluid,
fitting an approximation curve having a logarithm comprising a substantially straight line with at least a portion of the concentration curve,
determining a parameter of the approximation curve, and
calculating the mass of the solute in the predetermined volume of the fluid by the formula m=Q
d
·c
d
/P wherein
m comprises the mass of the solute,
Q
d
comprises the flow rate of the exchange of fluid,
c
d
comprises the concentration of the solute in the exchange fluid, and
P comprises the parameter.
In a preferred embodiment, the method includes determining the flow rate of the exchange fluid, integrating the product of the flow rate of the exchange fluid and the concentration of the solute in the exchange fluid over time in order to calculate the accumulated mass of the solute in the exchange fluid, and calculating a solute reduction index by the formula:
SRI
=(
U−G·t
)/(
m+U−G·t
)
wherein
SRI comprises the solute reduction index,
U comprises the accumulated mass of the solute in the exchange fluid,
G comprises the production of the solute over time, and
t comprises time.
In accordance with one embodiment of the method of the present invention, the mass exchange device comprises a dialyzer, the mass exchange fluid comprises a dialysate fluid, and the solute comprises urea.
In accordance with another embodiment of the method of the present invention, the parameter corresponds to a whole body clearance divided by the predetermined volume of the fluid.
In accordance with another embodiment of the method of the present invention, the passing of the exchange fluid on the other side of the semi-permeable membrane comprises passing a finite initial concentration of the solute and wherein the measuring of the solute concentration comprises repeatedly measuring the difference of the solute concentration across the semi-permeable membrane.
In accordance with another embodiment of the method of the present invention, the fitting of the approximating curve comprises obtaining a compensated concentration by subtracting a predetermined concentration term from the concentration of the solute, obtaining the logarithm of the compensated concentration, and fitting a straight line to the logarithm of the compensated concentration, whereby the predetermined concentration term compensates for the generation of the solute.
In accordance with another embodiment of the method of the present invention, the parameter of the approximation curve comprises a slope of the substantially straight line.
In accordance with another embodiment of the method of the present invention, the fitting of the approximation curve excludes a predetermined initiation period. Preferably, the predetermined initiation period comprises about 60 minutes.
In accordance with another embodiment of the method of the present invention, the method includes periodically interrupting the passing of the exchange fluid on the other side of the semi-permeable membrane for a first time period, and adjusting the time scale by replacing the first time period with a second time period, the second time period being less than the first time period.
In accordance with another embodiment of the method o
Carter Ryan
Gambro Lundia AB
Lerner David Littenberg Krumholz & Mentlik LLP
Sykes Angela D.
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