Patent Application
20120228226
The invention relates to a method and a device to store and display a history of Kt/V or other adequacy parameters trends in a machine able to perform kidney substitution treatments. The stored trends can be evaluated against the ongoing treatment offering some parameters, which may be used by the medical staff to assess said ongoing treatment.
Dialysis adequacy is the topic that has got and gets more attention when one thinks about patient outcome. In order to estimate dialysis adequacy one needs a parameter establishing a relation between dialysis dosage and patient outcome. The most accepted parameter to estimate the quantity of dialysis delivered or dosage is the Kt/V, where K is the effective clearance for urea, t is the treatment time and V is the urea distribution volume which matches the total body water.
The NCDS (National Cooperative Dialysis Study) and the HEMO study found, after analyzing a large patient group, that morbidity and mortality in end stage renal disease (ESRD) was strongly correlated with the Kt/V value or dialysis dose. Data obtained from these studies resulted in guidelines regarding hemodialysis treatments, which demand a minimum dose of Kt/V=1.2 for non-diabetic patients and 1.4 for diabetics (DOQI guidelines). It is worthy to point out that a morbidity decrease not only improves the patient well-being, but also reduces significantly the medical costs as the patient requires less care.
The need of a reliable and cost effective method to monitor the Kt/V and by extension control dialysis adequacy and morbidity, is therefore easily understood.
In the Kt/V calculation, the main problems are K and V estimation along with the multi-compartment urea kinetics. V can be estimated by bioimpedance, anthropometric measurements or applying the urea kinetic model (UKM), all these methods have a certain degree of error. K can be estimated so far by measuring the urea blood concentration before and after the treatment or by monitoring inlet and outlet conductivity changes in the dialysate side.
Blood samples method is the reference one. After taking the blood samples and applying either UKM or Daugirdas formula a single pool Kt/V (spKt/V) is estimated, further, Daugirdas second generation formulas should be used to get an equilibrated Kt/V (eKt/V) which accounts for the urea rebound caused by the fact that urea kinetic's does not follow a single pool model but a multi-compartment one. This method has two main problems: it is not possible to know whether the treatment is adequate or not before it finishes, therefore it is not possible to perform any action to improve the situation; it is not an easy to apply method: sampling time is very important to get an accurate value, and the medical staff must send the samples to the lab, wait for the results and calculate Kt/V values with the help of a computer. These facts result on a monthly basis Kt/V measurements in best case, which means that in worst case scenario a patient might be under-dialyzed for one whole month.
Conductivity methods are based on the observation that sodium clearance is almost equal to urea clearance and that the relationship between dialysate conductivity and dialysate sodium concentration can be considered linear on the temperature range of interest. Therefore it is possible to get urea clearance by measuring the sodium diffusion transport through the membrane in the dialyzer.
It is important to introduce the concept of Dialysance, as it slightly differs from Clearance:
Clearance is defined as the ratio between transport rate and concentration multiplied by flow, and it is applicable when the diffusing substance is on the blood side but not on the dialysate, that is the case for urea.
Dialysance is defined as the ratio between transport rate and concentration gradient multiplied by flow, and it is applicable when the diffusing substance is in both dialyzer sides. When one applies conductivity methods to measure urea Clearance, one actually measures sodium Dialysance (Depner T, Garred L. Solute transport mechanisms in dialysis. Hörl W, Koch K, Lindsay R, Ronco C, Winchester J F, editors. Replacement of renal function by dialysis, 5th ed. Kluwer academic publishers, 2004:73-91).
During conductivity based clearance measurements, a dialysate inlet conductivity different to the blood one is produced, which results in a net transfer of sodium either from blood to dialysate or from dialysate to blood due to the generated gradient. There are currently several methods which are applied in the industry:
In a first method a one-step conductivity profile is performed; in a second method a two-step conductivity profile is performed; and in a third method an integration of conductivity peaks is used. (Polaschegg H D, Levin N W. Hemodialysis machines and monitoris. Hörl W, Koch K, Lindsay R, Ronco C, Winchester J F, editors. Replacement of renal function by dialysis, 5th ed. Kluwer academic publishers, 2004:414-418). The main advantage of this approach is that it is relatively easy to implement and cost effective as it only needs an extra conductivity/temperature sensor downstream the dialyzer. It offers Kt/V measurements during the treatment allowing the medical staff to react and perform some actions in case the treatment is not going as it should. However, conductivity based methods have also some limitations: they can induce some sodium load in the patient during the measurement; they are not useful to obtain other interesting parameters like nPCR or TRU. The maximum measurement frequency offered so far by the industry is about 20 minutes, which means that in worst case scenario the patient could be under-dialyzed for 20 minutes. And although there are some publications claiming it, so far, conductivity methods haven't been applied with enough reliability to hemofiltration or hemodiafiltration treatments.
Another method to estimate hemodialysis adequacy is by direct measurement of the waste products (urea) concentration in the effluent dialysate, this method assumes that the evolution of urea concentration over the time in the dialysate side is proportional to the one in the blood, therefore the slope of the line obtained after applying the natural logarithm to the registered concentration values over the time will be the same on both sides: dialysate side and blood side. And by definition such slope is K/V, which multiplied by the therapy time results in the Kt/V value.
There are two different methods available to measure online the concentration of waste products in effluent dialysate: Urea sensors and UV spectrophotometry.
The limitations of the urea sensors are well known. Recent works carried out by Fridolin I. et al (I. Fridolin, M. Magnusson, L.-G. Lindberg. On-line monitoring of solutes in dialysate using absorption of ultraviolet radiation: Technique description. The International Journal of Artificial Organs. Vol. 25, no. 8, 2002, pp. 748-761) and Uhlin F. (Uhlin F. Haemodialysis treatment monitored online by ultra violet absorbance. Linkoping University Medical Dissertations no 962. Department of Medicine and Care Division of Nursing Science & Department of Biomedical Engineering. 2006.) have shown that UV spectrophotometry is a reliable and cost affordable method to monitor waste products in effluent dialysate. Additionally, the European Patent EP1083948B1 describes a sensor coupled with the dialysate flow system of a dialysis machine, which is actually an UV spectrophotometer measuring UV absorbance of UV absorbing products in spent dialysate.
By direct measuring waste products in spent dialysate, a curve representing the concentration or absorbance decay of uremic compounds over the whole dialysis period is obtained. These curves tightly reflect the course of the treatment and manifest patient based haemodynamic singularities, which influence the dialysis efficiency. The obtained concentration-curves could be saved either in the dialysis machine or in a patient card. During a dialysis procedure, the saved curves could be displayed by user request. Visual comparisons between the ongoing and the previously recorded therapies could help the physician to identify problems. Moreover, a comparison algorithm could generate some parameters to quantify these discrepancies. A signal could be triggered when the differences get over a certain threshold to draw the attention of the medical staff, and help them on deciding if an action is necessary.
The problem of this invention is to provide a reliable method and device to record and display concentration or absorbance curves obtained during different dialysis therapies; and to offer objective parameters evaluating the differences between the ongoing and previous therapies. This problem is solved by a method with the features described in claim 1. Furthermore this problem is solved by a device with the features described in claim 11. Preferred embodiments of the invention are described in the claims 2 to 10.
Further goals, advantages, features and possibilities of use of this invention arise out of the subsequent description of the embodiments of the invention. Therefore every described or depict feature of its own or in arbitrary meaningful combination forms the subject matter of the invention even independent of its summary in the claims or its reference to other claims.
It shows:
The UV-sensor 37 can be substituted by an Urea-sensor, in this case will the urea concentration in spent dialysate measured instead of the light absorbance. The disclosed dialysis machine is provided with several other means as is conventional. These other means are not disclosed, since they are not relevant for the operation of the present invention.
The obtained absorbance curves are stored in a patient card attached to the computer 14 or in a database hosted in the computer 14. The number of curves which may be saved is variable and depends on the capacity of the storage medium. In our preferred embodiment the last 15 treatments are stored in a suitable patient card. The stored treatments are overwritten following a FIFO (first in first out) procedure. Additionally, a given therapy can be set as not over-writable, in such a case the therapy will be kept until it is set back to over-writable. It is also possible to store Kt/V or URR curves.
The different stored curves can be displayed by selecting them on a menu in the display of the dialysis machine (
Specific curves can be identified by special names. In our preferred embodiment the treatment curve which delivered the best Kt/V is saved as “Best treatment”; and the treatment which delivered the worst Kt/V is saved as “Worst treatment”. These specific treatments can be kept out of the FIFO overwriting functionality, being only respectively overwritten if a better or worse treatment is recorded. The tracking of specific treatments may be reset by user action or after a predefined amount of time.
In our preferred embodiment an average curve out of all the curves stored in the patient card is calculated and identified as “average”. This average curve is more valuable when comparing it against the ongoing curve because is less affected by extreme values and therefore more representative of the patient status.
The main factors affecting the shape of the absorbance curves are the blood flow and the dialysate flow. The stronger the counter current effect is, the more efficient the dialysis treatment is, and by extension the more steeper the UV-absorbance curve is. Ideally the recorded curves should be normalized to the ongoing treatment parameters to make them comparable.
Kety (Kety S S: Physiological and physical factors governing the initial states of drug distribution, in Teorell T, Dedrick R L, Condliffe P G (eds): Pharmacology and Pharmacokinetics. New York, N.Y., Plenum Press, 1974, pp 233-240) developed a mathematical model describing the contribution of both blood flow and solute diffusion to solute transport across capillary membranes. This model relates dialyzer clearance with extracorporeal blood flow when applied to a dialyzer:
K=Q
b(1−e−PS/Q
where Qb is the extracorporeal blood flow and PS is the product between dialyzer membrane permeability and membrane surface.
The mass balance of any compound during a dialysis therapy can be described as follows:
K·C
b
=Q
d
·C
d (2)
where K is the dialyzer clearance; Cb is the blood concentration; Qd is the dialysate flow; and Cd is the dialysate concentration.
Because our goal is to normalize recorded treatments to the current treatment parameters regardless of the blood concentration, we can consider Cb as a constant. Thus, it is possible to normalize each point of a recorded concentration curve to the ongoing curve as follows:
Where Cd-current is the normalized dialysate concentration; or in other words, the concentration that we would get in the recorded treatment if we had use the blood flow and dialysate flow of the current treatment.
By knowing the PS of the dialyzer it is possible to use equation 1 to calculate the K's, which are required by equation 4. It is also possible to use UV-absorbance values instead of concentrations in all the equations disclosed above.
In the preferred embodiment the PS factors for each dialyzer type are available in the dialysis machine, new PS factors for other dialyzer types can be added at any time either by the technician servicing the machine or by the medical staff. The recorded therapy data includes the dialysate absorbance, blood flow and dialysate flow at each time point, and the PS factor. Before the start of the treatment the user selects the dialyzer. With the dialyzer selection and the current Qd, the recorded curve is normalized to the current curve on real-time. If for example during the first half of a four hours therapy, the patient is treated with a K of 240 ml/min (300 ml/min blood flow) and during the second half with a K of 180 ml/min (200 ml/min blood flow), the recorded curve will be normalized to 240 ml/min for 2 hours and to 180 ml/min for 2 hours.
The normalization of the recorded curves to the current one is not mandatory. The dialysis machine offers the user the possibility to switch between two modes: normalized and not normalized.
During a dialysis procedure the dialysate flow prescription may change, such a change has important influence on the plotted concentration due to dilution effects. An increase of dialysate flow from 400 ml/min to 800 ml/min will cause a 50% drop of the measured concentration, which can be misleading. It is important to account for dilution effects during a treatment. In our preferred embodiment the plotted concentration is on real time normalized to a standard dialysate flow of 500 ml/min. The normalization is achieved with the following equation:
Where Cn is normalized concentration; C is concentration; Qd-std is the standard dialysate flow to which the concentration is normalized; and Qd-current is the current dialysate flow. The concentrations may be substituted by UV-absorbance values.
The dilution effects can be illustrated by the following example. Let's suppose that the treatment is started with a dialysate flow of 500 ml/min and the first absorbance value is 3; after one hour the flow is set to 400 and the obtained absorbance is 3.1; furthermore, after two hours the flow is set to 800 ml/min and the obtained absorbance is 1.3. The normalized absorbance at each of the time points will be:
This approach avoids misleading the medical staff.
A visual comparison between recorded and ongoing treatments supposes a valuable help for the experienced physician, however an objective parameter that evaluates the difference between the compared therapies would help on taking objective decisions regarding the current treatment or future prescriptions. In the preferred embodiment the difference between the area under the curve between the recorded and the current therapy is calculated. The integration of the concentration curve may be or not multiplied by the dialysate flow, in any of the cases it gives an idea of the amount of substances that were cleared from the patient. The difference between the two therapies quantifies how much more (or less) substances were removed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09011353.1 | Sep 2009 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/005454 | 9/6/2010 | WO | 00 | 5/22/2012 |
