For many years hemodialysis has been used for treating patients and has proved successful all over the world. Although the hemodialysis technology has reached an advanced stage, there are some important aspects by reason of which hemodialysis methods are in need of development. In particular the financial costs for the long-term treatment of patients suffering from chronic diseases plays an important role. Due to the rising costs in the health systems of the industrialized countries and the financial problems in the developing countries, it is necessary to further reduce the hemodialysis costs to be able to treat a still increasing number of patients. The cost factor of hemodialysis treatments largely depends on whether the method can be carried out in an optimum way. On the one hand the treatment time can be shortened when the method is optimally performed and, on the other hand, the additional nursing costs incurred by a patient who is well tended thanks to an optimum hemodialysis treatment are reduced due to the fact that the patient is less morbid and therefore needs less care.
Within the NCCLS (National Committee for Clinical and Laboratory Standards) the morbidity of a large group of patients was examined in the United States in response to the dialysis dose. It turned out that the morbidity falls to a low constant value when the value for K t/V rises from 0.8 to 1 or more.
K is here the clearance, t the treatment time and V the whole body water of a patient. It is therefore desirable to control each treatment in such a way that K t/V becomes 1 for each treatment.
For an optimum hemodialysis, and for avoiding an increase in the morbidity, it is thus imperative to either exactly determine the above-mentioned ratio K t/V or to increase the dialysis time for reasons of safety. Apart from possible health risks, the latter entails considerable costs. To optimize hemodialysis, the patient's whole body water V may be determined, which can be done in a relatively easy way as the same is known over a long period. The whole body water can be deduced from the patient's body weight, his age and sex and the estimated fat portion. It can also be calculated on the basis of urea measurements and with the aid of the urea model. In addition, a whole body impedance measurement can be carried out for the purpose of determination. Furthermore, it is relatively easy to determine the treatment time t for the respective treatment of a patient.
By contrast, the prior art does not disclose any methods which could be employed in vivo for exactly determining the clearance of a hemodialyzer. As a rule, the details given by the manufacturer are taken as a basis. These details, however, include considerable errors. For instance, the manufacturer does not check each single dialyzer as to its clearance. Rather, the manufacturer guarantees a specific clearance range, which however does not exclude the possibility that some dialzyers do not comply with this range.
Another reason for changes in the clearance is that dialyzers are reconditioned and reused, above all in the United States, for reasons of costs and biocompatibility.
Another reason why the information given by the manufacturer about the clearance cannot be fully relied on is that the manufacturer's information is based on measurements in aqueous solutions, while the relevant clearance during the treatment of a patient relates to the aqueous portion of blood. The flow of this aqueous amount is calculated from the entire blood flow and the hematocrit of a patient. Although the hematocrit is normally known, it may vary during hemodialysis due to ultrafiltration.
In addition, it must be taken into account that the blood flow is only approximately known, as it normally depends on the delivery rate, i.e. the rotational speed of a conventionally used peristaltic blood pump and the flexible tube diameter of the system. Since the tube diameter is only known at an accuracy of +/- 5%, and since, moreover, the cross-section of the pump may be reduced due to the suction vacuum, considerable tolerances result therefrom.
It follows from the above that it has not been possible in the prior art to measure the clearance in vivo and that the formerly known calculation methods have been subject to considerable inaccuracies.
To overcome these problems, in-vivo urea measurements are carried out in the prior art, as the relevant clearance of the dialyzer is the urea clearance and the above-mentioned calculation formula is based on the urea model. For urea measurements samples must be taken and examined in the laboratory. As a result, there are considerable time delays between the taking of the samples and the availability of the measurement results. The dialysis treatment can thus not be controlled. Furthermore, these examinations are very expensive. As a rule, the patient is therefore treated on the basis of estimated values and a margin of safety, a quality control being only carried out at greater time intervals, e.g. once a month, through urea measurements.
Furthermore, it is known from the prior art that the clearance for sodium chloride ions is equal to the urea clearance. Since the ion concentration and thus the conductivity of the dialysis fluid and of blood substantially depend on Na and Cl ions, it is possible to determine the clearance through a conductivity measurement. In a known method the concentration is given at the blood inlet side of the hemodialyzer and set to zero at the dialysis fluid inlet side, or vice versa, for determining the clearance. However, this method has the disadvantage that during the dialysis process it is not possible to set the inlet concentration at the dialysis fluid inlet side to zero. Furthermore, the inlet concentration of blood is not known. Hence, this method is not suited for an in-vivo control of the hemodialysis process.