Surgery – Blood drawn and replaced or treated and returned to body
Reexamination Certificate
1997-11-26
2001-04-17
Sykes, Angela D. (Department: 3762)
Surgery
Blood drawn and replaced or treated and returned to body
C604S005040, C210S739000, C600S366000, C600S454000
Reexamination Certificate
active
06217539
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of in-vivo determination of hemodialysis parameters, and to a device for carrying out the method.
BACKGROUND OF THE INVENTION
Hemodialysis has been used successfully for many years for treatment of patients with renal failure and has proven successful throughout the world.
Human kidneys have several functions, e.g., eliminating water, removing metabolic waste products (urea, creatinine) and helping to adjust the concentrations of various substances such as blood electrolytes (sodium, bicarbonate, etc.) at certain levels.
Hemodialysis is a treatment method for compensating for renal dysfunction by removing metabolic waste products and helping to adjust the blood electrolyte concentrations.
This treatment method is carried out with a dialyzer, which is essentially an exchanger with two chambers separated from each other by a semi-permeable membrane, a blood chamber for connection to an extracorporeal blood circulation and a chamber for a dialysis fluid which is connected to a container for dialysis fluid in a dialysate circuit. A classical dialysate fluid contains the main blood electrolytes in a concentration close to the concentrations in the blood of a healthy person.
During a treatment, the patient's blood and the dialysis fluid are passed by both sides of the membrane, usually in countercurrent flow, at a predetermined flow rate. The metabolic elimination products diffuse through the membrane from the blood chamber to the chamber for dialysis fluid, while the electrolytes present in the blood and in the dialysis fluid at the same time diffuse from the chamber with a higher concentration to the chamber with a lower concentration. The metabolism can also be influenced by applying a transmembrane pressure (ultrafiltration).
To be able to optimize the treatment method, hemodialysis parameters must be determined in vivo, i.e., while the procedure is being carried out. One such parameter in particular is the value for the exchange efficiency of the iS dialyzer, represented by the “clearance” or “dialysance D.” The following definitions are conventional:
According to DIN 58,352, part 1, the clearance for a certain substance K denotes the virtual (calculated) blood volume from which this substance is removed completely by the dialyzer per minute.
Dialysance is another term for determining the efficiency of a dialyzer, which also takes into account the concentration of the substance in the dialysis fluid which is involved in the mass exchange in the dialyzer.
In addition to these dialyzer performance characteristics, other parameters are also important, in particular the values of the aqueous portion of the blood (the effective blood flow), the hematocrit and the blood concentration at the inlet.
It is relatively complex to quantify mathematically the blood purification methods on the basis of measurement technology and, in conjunction with this, to determine the above-mentioned parameters of dialysis. Reference is made in this regard to the book by H. E. Franz, Blutreinigungsverfahren [Blood Purification Methods], published by Georg Thieme Verlag, Stuttgart, New York, 1990, specifically pages 479-492.
Accordingly, the following is obtained in particular for determining the dialysance or the clearance for a given electrolyte, e.g., sodium as the substance, if there is no ultrafiltration. The dialysance D is equal to the ratio of the mass transport for this electrolyte on the blood side Qb×(cbi−cbo) and the difference in concentration of this electrolyte between the blood and the dialysis fluid at the inlet of the dialyzer (cbi−cdi).
D
=
Qb
(
cbi
-
cbo
)
cbi
-
cdi
(
1
)
For reasons of mass balance (the quantity of substance removed from the blood is equal to the quantity of substance carried away in the dialysate in the same period of time), the following holds:
QB
·(
cbi−cbo
)=−
Qd·
(
cdi−cdo
) (2)
It follows from (1) and (2) for the dialysance on the dialysate side that:
D
=
-
Qd
(
cdi
-
cdo
)
cbi
-
cdi
(
3
)
where in (1) to (3):
Qb=effective blood flow
Qd=dialysis fluid flow
cb=concentration of the substance in the solution volume of the blood
cd=concentration of the substance in the dialysis fluid
i=inlet of the dialyzer
o=outlet of the dialyzer
The effective blood flow is the flow of the blood component in the whole blood flow in which the substances to be removed are dissolved, i.e., it is based on the complete (aqueous) solution volume for this substance. Depending on the substance, this may be the plasma water flow or the blood water flow, i.e., the total amount of water in the whole blood.
For the case of a specific metabolic elimination product (such as urea), cdi=0, and then we speak of the clearance K for this metabolic product instead of the dialysance.
K
=
Qb
(
cbi
-
cbo
)
cbi
=
Qd
cdo
cbi
All the known methods of in-vivo determination of hemodialysis parameters begin with these considerations, most of which have in common the attempt to avoid a direct measurement procedure on the blood side because this could represent a not insignificant source of risks. Therefore, there has been an effort to derive the quantities of measured values that are to be determined merely from measurements on the dialysate side, even with regard to the quantities on the blood side. A customary basic method is to measure the concentration of the substance in the dialysis fluid upstream and downstream of the dialyzer and then calculate from this the mass transport on the dialysate side Qd×(cdi−cdo), and derive from these values by means of the above equations other quantities, in particular the value for the blood concentration at the inlet cbi, which enters into the equations as a mathematical unknown. The two values cdi and cdo need not necessarily be measured. The inlet value cdi can also be adjusted in a defined manner in the fresh dialysis fluid.
If the value cbi of an electrolyte is to be determined in particular, cdi and cdo can be determined by conductivity measurements. In the case of NaCl, a nonspecific measurement is sufficient, because NaCl is responsible for most of the conductivity of the fluids involved. This basic method is known from European Patent EP 97,366.
Starting from the preceding basic method, the individual known methods differ in the methods of measurement and analysis. They will be explained in greater detail below.
European Patent EP 291,421 B1 discloses a method of determining the blood concentration at the inlet, where the dialysate inlet concentration is varied according to a ramp function to determine the point at which there is no further transfer of electrolyte across the membrane. Therefore, the known method works according to the principle of varying the inlet conductivity of the dialysis fluid to the extent that it no longer differs from the outlet conductivity. Then it must have assumed the blood input conductivity (cbi=cdi). Other parameters of hemodialysis can then be derived on the basis of equations (1) through (3). A disadvantage of this method is the relatively long measurement time due to the period of time until reaching the stable equilibrium state in adjusting the dialysis fluid at the new input concentration value, which is not immediately effective at each point in the dialyzer. Due to the system, a certain amount of time is required until a jump in conductivity at the dialysate inlet leads to stable conditions at the dialysate outlet. The period of time required to reach the stable equilibrium state is determined essentially by the extent of the change in conductivity per unit of time. Within this long period of time, however, parameters of dialysis can change and can thus falsify the value to be determined. It should be noted in particular that the known method mentioned above (like all other methods) can change the blood concentration at the inlet cbi through the induced electrolyte transfer. In the kno
Bianco Patricia
Fresenius Medical Care Deutschland GmbH
Kenyon & Kenyon
Sykes Angela D.
LandOfFree
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