Patent Description:
Fluid pumps, for example blood pumps for the extracorporeal circulation of blood are used in a number of medical applications, for example in hemodialysis, and haemodiafiltration.

In hemodialysis machines, it is known to use a disposable cartridge comprising a rigid frame defining fluid pathways and chambers and a flexible membrane covering a surface of the cartridge. The cartridge is loaded into a hemodialysis machine, where pressure, typically pneumatic pressure, exerted on the outside surface of the flexible membrane, causes the membrane to move back and forth. This back and forth action in the region of a chamber acts as a fluid pump, which is thus often referred to as a membrane pump. Such a machine is disclosed in <CIT>.

The movement may also be used to mix two or more fluids in a chamber, such as bicarbonate and acid to create dialysate.

The movement of the flexible membrane may also be used to open and close valves defined in the rigid frame of the disposable cartridge. Such a system is disclosed in <CIT>.

Because of its use in hemodialysis, in pumping and mixing fluids, the cartridge may be referred to as a haemodialysis pulsatile pumping cartridge. Such a disposable cartridge is typically made of a rigid frame of polyethylene and a flexible membrane of polyvinyl chloride (PVC).

An instrument for measuring blood volume is disclosed in <CIT>. However, the system of this document does not comprise a signal generator arranged to provide an output signal when an electrical characteristic is indicative of a deterioration of the deformable membrane.

<CIT> shows a system for balancing flows in a dialysis machine.

In use, the disposable cartridge is loaded in a hemodialysis machine and undergoes repeated deformations in the localised regions of the pump chambers, mixing chambers and valves. In a typical cycle, the machine will perform a priming stage, a treatment stage, including flow balance and ultrafiltration stages following by a purge stage.

Such a dialysis machine relies on volumetric control. Dosing and mixing of fluids is controlled by the volume of the pump chambers which in turn is affected by the flexibility of the membrane. The specific flexibility of the membrane is therefore essential to the accurate running of the dialysis machine. Similarly, flow of fluids into and out of the pump chambers is controlled by inlet and outlet valves which rely on the deformation of the flexible membrane.

Repeated deformations gradually cause plastic deformation of the flexible membrane. Thus the disposable cartridges may be engineered to withstand a specific cycle loading, along with a reserve factor. Once used, the disposable cartridge is disposed of and should not be used again.

Re-use of a cartridge designed for a single cycle, or re-use of a cartridge designed for a specific number of cycles, beyond that number of cycles, may reduce the efficacy of treatment and has the potential to cause damage to the dialysis machine.

Given the typical make-up of the disposable cartridge, and that a typical cycle ends with a purge stage that washes and empties the cartridge to remove any residual fluids, a used cartridge does not visibly show any distinct features, as compared to an unused cartridge. Therefore there is a risk of cartridge re-use, either deliberate or accidental.

Valve leak systems are installed in the dialysis machine to detect if any of the valves on the dialysis cartridge are not closing properly, such that there is a leak of fluid through the valve. The valve leak system comprises first and second conductivity electrodes arranged upstream and downstream of the valve respectively. A conductivity measurement is taken across the electrodes when the valve is closed. For a normally functioning valve, a low conductivity, or indeed no conductivity should be detected when the valve is closed. If a valve is leaking however, a high conductivity is detected, as the leaking fluid carries the charge through the valve, between the first and second conductivity electrodes.

The present invention aims to provide a membrane pump usage condition detection system and a method of determining a membrane pump usage condition and that mitigates one or more of the above problems.

According to a first aspect of the present invention, there is provided a membrane pump usage condition detection system according to claim <NUM>.

Thus the system provides an output signal characterising the membrane pump usage condition using data from a valve leak system. This gives an operator of a cartridge incorporating the membrane pump, information concerning the overall condition of the cartridge. As data from the valve leak system is used, the membrane pump usage condition detection system requires no additional hardware.

The electrical characteristic may be one of conductance, impedance or capacitance or any other electrical characteristic measurable across the valve.

The detection system further comprises a processor arranged to receive the output signal. The output signal may be stored in the processor. Thus further analysis may be performed on the identified characteristic. Patterns of deliberate or accidental cartridge re-use may then be monitored.

The membrane pump is provided on a cartridge.

The processor may calculate the specific number of cartridge uses from the electrical characteristic indicative of the membrane pump usage condition.

The output signal is an error message preventing further cartridge use. This prevents deliberate or accidental cartridge re-use, or re-use of a cartridge beyond a specified number of uses. This may be notified to the user/operator of the dialysis machine in the form of an audible or visual alarm.

The comparator may compare the electrical characteristic with a pre-determined threshold value. Alternatively, the comparator may compare the electrical characteristic with a dynamic threshold value.

The output signal may be provided when the electrical characteristic is above the threshold value. Alternatively, the output signal is provided when the electrical characteristic falls below the threshold value.

The output signal may be an alarm indicating a net fluid removal error. A valve leak could cause net fluid removal error by not fully filing up or emptying the pump with dialysate. Hence, when a valve leak is detected by measuring the frequency an alarm is generated to stop treatment.

The processor may be programmed to permit a set number of cartridge re-uses.

The measuring device may be a pair of electrodes. Alternatively the measuring device may be a pair of capacitance probes.

According to a second aspect of the present invention, there is provided a method of determining a membrane pump usage condition as set out in claim <NUM>.

Thus the method of the present invention provides a measure of cartridge deterioration (and re-use) using the data output of the existing valve leak detection system.

The electrical characteristic may be one of conductance, impedance or capacitance. The cycle may include at least one of a priming stage, a treatment stage, and a purge stage.

The step of determining the membrane pump usage condition includes recording a characteristic value above a limit value. Alternatively, the step of determining the membrane pump usage condition may include recording a series of characteristic values above a limit value.

The characteristic value may be a decay rate of the valve leak frequency following a spike in the valve leak frequency. The characteristic values are observed in the data of the valve leak detection system. The so-called background levels change with cartridge re-use. The background level of the valve leak detection system is observed to spike to higher levels during cartridge re-use. The background level of the valve leak detection system is observed to spike to higher levels and then more rapidly decay during cartridge re-use.

The decay rate of the valve leak frequency following a spike in the valve leak frequency may be indicative of a specific number of cartridge uses. Thus the specific number of cartridge uses may be determined.

The characteristic value may be the mean, median or modal average valve leak frequency during the cycle. This value may be compared to a signature value for a specific cartridge, providing an indication of the level of cartridge re-use.

The cartridge usage condition may be determined using a processor provided on the dialysis machine.

The valve leak frequency may be recorded at a sample rate of <NUM> sample a second during the cycle. Alternatively, the valve leak frequency may be recorded at an intermittent sample rate during the cycle.

Embodiments of the present invention will now be described, by way of example only, and with references to the accompanying drawings, in which:.

A cross section of a dialysis machine <NUM> is shown schematically in <FIG>. The dialysis machine <NUM> has machine body <NUM>. The machine body <NUM> houses pneumatic actuators <NUM> and a controller <NUM>. The dialysis machine <NUM> includes a first platen <NUM> and a second platen <NUM>. The first and second platens <NUM>, <NUM> together define a cavity into which a cartridge <NUM> is received in a known manner.

The cartridge <NUM> (see <FIG> and <FIG>) has a rigid body <NUM> covered by a flexible membrane <NUM>, providing a machine facing surface <NUM>. The cartridge <NUM> in part embodies pump chambers and valves. In this case, the chambers are flow balance pump chamber "A" <NUM>, flow balance pump chamber "B" <NUM>, dialysate mixing pump chamber <NUM>, acid mixing pump chamber <NUM> and ultrafiltration pump chamber <NUM>. The flow balance pump chamber "A" <NUM> and flow balance pump chamber "B" <NUM> each have two inlet valves <NUM> and two outlet valves <NUM>.

The pneumatic operation of each of the chambers <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are substantially similar, such that only the flow balance pump chamber "A" <NUM> shall be described in detail.

Furthermore, the two inlet valves <NUM> and the two outlet valves <NUM> are substantially similar, such that only one inlet valve <NUM> and one outlet valve <NUM> shall be described in detail.

Referring back to <FIG>, the flow balance pump chamber "A" <NUM> and inlet and outlet valves <NUM>, <NUM> are defined between respective concave cavities <NUM> formed in the rigid body <NUM> of the cartridge <NUM> and the flexible membrane of the cartridge <NUM>. The cartridge <NUM> defines fluid pathways <NUM> between the flow balance pump chamber "A" <NUM> inlet and outlet valves <NUM>, <NUM>.

In use, the cartridge <NUM> is retained between the first platen <NUM> on a first side of the cartridge <NUM> and the second platen <NUM> on a second side of the cartridge <NUM>. The second platen <NUM> has a cartridge engaging surface <NUM> and a non-cartridge engaging surface <NUM>. Cavities <NUM> are defined within the cartridge engaging surface <NUM>, which correspond to the concave cavities <NUM> on the cartridge <NUM>. A fluid port <NUM> is defined in each of the concave cavities <NUM>, fluidly connecting the cartridge engaging surface <NUM> and the non-cartridge engaging surface <NUM>, of the second platen <NUM>.

The pneumatic actuators <NUM> are arranged in fluid communication with the first side of the cartridge <NUM>, through the second platen <NUM> via the fluid ports <NUM>, and hence the machine facing surface <NUM> of the flexible membrane <NUM>. The pump chambers and valves are operated pneumatically by actuating the flexible membrane <NUM> using the pneumatic actuators <NUM> provided in the machine body <NUM>. In an alternative embodiment the pump chambers and valves are operated hydraulically.

The pump chambers and valves are provided with sensing arrangements <NUM>, each of which include two sensing electrodes, generally termed <NUM> (see <FIG>). The sensing electrodes <NUM> are rotationally symmetrical and are made of a conductive material. The sensing electrodes <NUM> include a pointed tip <NUM>. The sensing electrodes <NUM> are mounted in the rigid body <NUM> of the cartridge <NUM>. The pointed tip <NUM> is arranged to face the second side of the cartridge <NUM> and the flexible membrane <NUM>.

The sensing arrangements <NUM> monitor the flow of fluids through the pump chambers and valves along the various fluid pathways. Referring to <FIG>, one such fluid pathway is the fluid pathway <NUM> associated with the flow balance pump chamber "B" <NUM>. As the sensing arrangements <NUM> are substantially similar, only the sensing arrangement <NUM> associated with the flow balance pump chamber "B" <NUM> shall be described in detail.

The flow balance pump chamber "B" sensing arrangement <NUM> is arranged with an inlet valve sensing electrode <NUM> and outlet valve sensing electrode <NUM>. The inlet valve sensing electrode <NUM> is fixed to the rigid body <NUM> of the cartridge <NUM> with the pointed sensing tip <NUM> exposed to the fluid flowpath <NUM> at the entrance to the inlet valve <NUM>. The outlet valve sensing electrode <NUM> is fixed to the rigid body <NUM> of the cartridge <NUM> with the pointed sensing tip <NUM> exposed to the fluid flowpath <NUM> at the exit of the outlet valve <NUM>.

Thus the inlet valve sensing electrode <NUM> is provided upstream of the flow balance pump chamber "B" <NUM>, and outlet valve sensing electrode <NUM> is provided downstream of the flow balance pump chamber "B" <NUM>.

When the cartridge <NUM> is loaded into the dialysis machine <NUM>, the inlet valve sensing electrode <NUM> and outlet valve sensing electrode <NUM> line up with sprung contacts <NUM>, <NUM> provided in the second platen <NUM>, sandwiching the flexible membrane <NUM> therebetween.

The sprung contacts <NUM>, <NUM> are electrically connected to a processor <NUM>, incorporating a sensor circuit, a comparator and a power source, provided in the machine body <NUM> via electrical connectors <NUM>, <NUM> respectively.

Thus inlet valve sensing electrode <NUM> and outlet valve sensing electrode <NUM> are electrically connected to the processor <NUM> through the flexible membrane <NUM>. The inlet valve sensing electrode <NUM> and outlet valve sensing electrode <NUM>, together with the sprung contacts <NUM>, <NUM>, processor <NUM> and respective connectors <NUM>, <NUM> form the sensing arrangement <NUM>.

In use at least one of the inlet valve <NUM> and the outlet valve <NUM> will always be closed. That is, there are three modes of operation. In an idle mode, both the inlet valve <NUM> and the outlet valve <NUM> are closed. Thus there should be no continuous flowpath between the inlet valve sensing electrode <NUM> and the outlet valve sensing electrode <NUM>. In a fill mode, the inlet valve <NUM> is open, and the outlet valve <NUM> is closed. This allows flow balance pump chamber "B" to be filled. However, there should still be no continuous flowpath between the inlet valve sensing electrode <NUM> and the outlet valve sensing electrode <NUM>, as the outlet valve is closed. In an empty mode, the inlet valve <NUM> is closed, and the outlet valve <NUM> is open. This allows flow balance pump chamber "B" to be emptied. However, there should still be no continuous flowpath between the inlet valve sensing electrode <NUM> and the outlet valve sensing electrode <NUM>, as the inlet valve <NUM> is closed. Thus the valve leak system may detect when either of the inlet or outlet valves <NUM>, <NUM> are leaking using the sensing arrangement <NUM> shown in <FIG>.

During operation of the dialysis machine <NUM>, the sensing arrangements <NUM> are used to detect leakage across the pump chambers and valves of the Dialysis Machine.

The sensor circuit of the processor <NUM>, includes an operational amplifier based relaxation oscillator whose frequency is determined by electrical conductance of the fluid path.

In use, an alternating potential difference from the power source is applied across the fluid flowpath <NUM> by the inlet valve sensing electrode <NUM>. The conductance of the fluid flowpath <NUM> between the inlet valve <NUM> and outlet valve <NUM> of the flow balance pump chamber "B" <NUM> is measured at the outlet valve sensing electrode <NUM> by measuring the potential difference detected at the outlet valve sensing electrode <NUM>, as will be described in more detail below. The potential differential provides an indication the conductivity of the fluid flowpath <NUM>. The relaxation oscillator ensures that the sensing arrangement <NUM> operates with an alternating current with minimal direct current offset. This reduces the galvanic effects on the inlet valve sensing electrode <NUM> and the outlet valve sensing electrode <NUM>.

The sensor circuit of the processor <NUM> generates the pulse train from the relaxation oscillator which is sent through the sensing arrangement <NUM>, to output at the processor <NUM>. The output at the processor is a series of pulses. From this series of pulses, a frequency is determined by measuring the time between the pulses, and hence fluid conductivity. This frequency value is known as the valve leak frequency.

The sensing arrangement <NUM> detects a valve leak in the inlet and outlet valves <NUM>, <NUM> of the flow balance chamber "B" <NUM> by performing conductivity checks during operation of the flow balance system. The conductivity along a flow path should not exceed a defined limit if the flow path is interrupted by valves <NUM>, <NUM>. The test is performed once every pump operation. If the inlet or outlet valves <NUM>, <NUM> fail to close, then the respective pump may draw or expel the fluid associated with that pump the wrong way, which is undesirable. The protective system for this error uses conductivity of the fluid flowpath <NUM> as a means to determine this failure. Thus in normal operation of the Dialysis Machine, there should never be a conductive path across the whole of the pump, from before the inlet valve <NUM> to after the outlet valve <NUM>, that has a conductivity of a value equal to or greater than a limit value set by the particular geometry of the cartridge in question. If a conductive path is seen, this may be indicative of one of the valves <NUM>, <NUM> having failed to close.

Hence the valve leak detection system measures a valve leak frequency value. The valve leak signal is generated by an oscillator and the frequency of the signal is determined by the feedback resistor. The sensing electrodes are connected in parallel to the feedback resistor so that a lower impedance across the valves would cause the total feedback resistor value to decrease, increasing the oscillating frequency.

During normal operation of an exemplary Dialysis Machine having an exemplary cartridge, the relaxation oscillator is tuned to generate a signal of <NUM> for a resistance of <NUM> kOhms across the sensing electrodes. The expected detected valve leak frequency value is between <NUM> and <NUM>. Should a valve leak frequency value in excess of <NUM> be detected, a valve leak has occurred.

The valve leak detection system described above may be used to determine membrane pump usage and hence cartridge usage.

A partial valve leak (e.g. due to re-used cartridges) is detected when the variation of detected valve leak frequency value within a pumping cycle increases.

A variation in the detected valve leak frequency value is detected by the comparator within the processor <NUM> measuring the difference between the minimum and maximum valve leak frequency values measured within one pump cycle.

For the normal operation of an exemplary Dialysis Machine having an exemplary cartridge referred to above, variation of valve leak frequency value is between <NUM> and <NUM>. Variations above <NUM> are considered to be partial valve leak. Thus for this exemplary Dialysis Machine having an exemplary cartridge, the pre-determined threshold value is a valve leak frequency value difference of <NUM>.

The value used for the partial valve leak is not the absolute frequency but the variation of the frequency within a pumping cycle. When the valve is partially leaking (e.g. due to re-use of cartridges) the frequency signal is not as stable as it normally is: maximum relative difference of the valve leak frequency value within one pump cycle is more than <NUM>. Normal expected values of absolute frequency are <NUM> to <NUM> with a variation of less than <NUM> within a pumping cycle.

As the valve leak signal is generated by an oscillator and the frequency of the signal is determined by the feedback resistor, a dynamic threshold value for the valve leak frequency value difference may be used instead of the pre-determined threshold value.

The effects on a cartridge <NUM> during a dialysis treatment cycle can be represented by a typical test cycle. A typical test cycle includes three main stages, flow balance, ultrafiltration and purge. In the first <NUM> minutes, the dialysis machine <NUM> is taken through a flow balance stage of the test cycle. The flow balance stage tests the flow balance valves. During the next <NUM> minutes, the dialysis machine <NUM> is taken through an ultrafiltration stage of the test cycle. The ultrafiltration stage tests the ultrafiltration valves. The test cycle is then ended with a purge stage. The purge stage empties the cartridge <NUM> of all dialysate fluids, and cleans the fluid flow paths with reverse osmosis water.

In order to determine the deterioration rates, the same cartridge <NUM> is forced through repeated test cycles.

With reference to <FIG>, a valve leak frequency profile between inlet valve <NUM> and outlet valve <NUM>, i.e. the fluid flowpath <NUM> across the flow balance chamber "B" <NUM>, on a cartridge <NUM> undergoing a typical cycle is shown. The valve leak frequency values are measured every second, a comparison is made between the minimum and maximum valve leak frequency values and plotted as point readings <NUM> with respect to the Y-axis. A general distribution over the typical cycle time as shown on the X-axis. The first stage of the typical cycle represents the ultrafiltration stage <NUM> of the cycle, whereas the second stage of the typical cycle represents the flow balance stage <NUM> of the cycle. During ultrafiltration the blood pump is stopped at <NUM> and re-started at <NUM>. Similarly, during flow balance, the blood pump is stopped at <NUM> and re-started at <NUM>.

As can be seen in <FIG>, whilst the measured valve leak frequency difference varies during a normal treatment session, the magnitude of any single difference value <NUM> does not exceed <NUM>. The typical value throughout the cycle for valve leak frequency difference is between <NUM> and <NUM>. A spike <NUM> in the measured valve leak frequency difference value is seen during the ultrafiltration stage <NUM> of the cycle, the spike <NUM> reaching a maximum value of approximately <NUM>.

With reference to <FIG>, a cartridge <NUM> is taken through a series of five typical cycles, numbered as first cycle <NUM>, second cycle <NUM>, third cycle <NUM>, fourth cycle <NUM> and fifth cycle <NUM>. For each cycle, the valve leak frequency difference values are again plotted as point readings with respect to the Y-axis giving a general distribution over the typical cycle time as shown on the X-axis. Each cycle <NUM>, <NUM>, <NUM>, <NUM>, <NUM> includes an ultrafiltration stage <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, a flow balance stage <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and a purge stage. The purge stage follows the flow balance stage <NUM>, <NUM>, <NUM>, <NUM>, <NUM> for each of the cycles <NUM>, <NUM>, <NUM>, <NUM>, <NUM> respectively.

The valve leak frequency difference limit <NUM> is shown as a dashed line at <NUM>. The valve leak frequency difference limit <NUM> may be manipulated depending on the deterioration rates displayed by the cartridge <NUM>.

An increase in the valve leak frequency difference values can be seen from the first cycle <NUM> to the second cycle <NUM>. This increase in the valve leak frequency difference value represents a deterioration in the flexible membrane of the cartridge. A yet greater increase in the valve leak frequency difference values is seen from the second cycle <NUM> to the third cycle <NUM>. An alarm is raised during the third cycle <NUM>, as the point readings regularly breach the <NUM> valve leak frequency difference limit <NUM>.

Thus <FIG> shows that the valve leak frequency difference values increase over several cycles with the method described above, with the rate of deterioration being worse after each re-use, until the flexible membrane is effectively plastically deformed. Further re-use of the same cartridge will thus prevent meaningful treatment sessions, as evidenced in the fourth and fifth cycles <NUM>, <NUM>.

With reference to <FIG>, a cartridge <NUM> is taken through a series of four typical cycles, numbered as first cycle <NUM>, second cycle <NUM>, third cycle <NUM> and fourth cycle <NUM>. For each cycle, the valve leak frequency difference values are again plotted as point readings with respect to the Y-axis giving a general distribution over the typical cycle time as shown on the X-axis. Each cycle <NUM>, <NUM>, <NUM>, <NUM> includes an ultrafiltration stage <NUM>, <NUM>, <NUM>, <NUM>, and a flow balance stage <NUM>, <NUM>, <NUM>, <NUM>, respectively, however, unlike in <FIG>, no purge stage. Instead, the dialysis machine <NUM> is switched off just before the purge stage.

The valve leak frequency difference limit <NUM> is shown as a dashed line at <NUM>.

<FIG> shows that the degree of cartridge <NUM> deterioration is minimal if the purge stage is avoided, thereby allowing an alternative method to re-use the cartridge if necessary. This method of ageing the cartridge <NUM> does not appear to cause any noticeable deterioration.

Thus the method of determining a cartridge usage condition records the valve leak frequency during the cycle to obtain a characteristic value, and determines the cartridge usage condition based on the characteristic value. The characteristic value may be a single breach of the <NUM> valve leak frequency difference limit <NUM>. The characteristic value may be a discreet number of breaches of the <NUM> valve leak frequency difference limit <NUM>. The characteristic value may be a decay rate of the valve leak frequency difference following a spike in the valve leak frequency difference. The characteristic value may be the mean, median or modal average valve leak frequency difference during the cycle. The processor <NUM> may be programmed to monitor any of the preceding characteristic values. On receipt of a characteristic value, the process may send a signal to a graphical user interface, or to an audible or visual alarm to indicate the cartridge usage condition or a signal to prevent activation of the dialysis machine cycle.

Thus the detection system is sensitive enough so that it detects a deterioration of the cartridge membrane before a leak across the valve is established. This allows an operator to prevent use of a cartridge not fit for purpose.

Claim 1:
A membrane pump usage condition detection system (<NUM>) comprising:
a cartridge (<NUM>) having a deformable membrane,
a membrane pump provided on the cartridge (<NUM>) and defining a flow path (<NUM>) arranged to be opened and closed by at least one valve (<NUM>, <NUM>),
a measuring device (<NUM>, <NUM>);
a comparator;
a processor; and
a signal generator, wherein
the measuring device (<NUM>, <NUM>) is configured to determine an electrical characteristic between two points on the flow path of the membrane pump, one point arranged upstream of the at least one valve (<NUM>, <NUM>) and the other point arranged downstream of the at least one valve (<NUM>, <NUM>),
whereby the measuring device (<NUM>, <NUM>) measures the electrical characteristic when the at least one valve (<NUM>, <NUM>) is closed, and
wherein the comparator is configured to monitor the electrical characteristic, and
the signal generator is arranged to provide an output signal when the electrical characteristic is indicative of a deterioration of the deformable membrane, and the processor is arranged to receive the output signal.