Patent Description:
<CIT> describes a dialysis fluid heating system including a dialysis fluid heater; a dialysis fluid carrying element in operable communication with the dialysis fluid heater to heat a dialysis fluid; and a logic implementer configured to control the heater using (i) a heater input determined from a dialysis fluid temperature delta and a dialysis fluid flowrate, and (ii) a potential adjustment of the temperature delta using a measured dialysis fluid outlet temperature.

Most HD, HF, and HDF treatments occur in centers. A trend towards home hemodialysis ("HHD") exists today in part because HHD can be performed daily, offering therapeutic benefits over in-center hemodialysis treatments, which occur typically bi- or triweekly. Studies have shown that more frequent treatments remove more toxins and waste products and render less interdialytic fluid overload than a patient receiving less frequent but perhaps longer treatments. A patient receiving more frequent treatments does not experience as much of a down cycle (swings in fluids and toxins) as does an in-center patient, who has built-up two or three days' worth of toxins prior to a treatment. In certain areas, the closest dialysis center can be many miles from the patient's home, causing door-to-door treatment time to consume a large portion of the day. Treatments in centers close to the patient's home may also consume a large portion of the patient's day. HHD can take place overnight or during the day while the patient relaxes, works or is otherwise productive.

Another type of kidney failure therapy is peritoneal dialysis ("PD"), which infuses a dialysis solution, also called dialysis fluid or PD fluid, into a patient's peritoneal chamber via a catheter. The PD fluid comes into contact with the peritoneal membrane in the patient's peritoneal chamber. Waste, toxins and excess water pass from the patient's bloodstream, through the capillaries in the peritoneal membrane, and into the PD fluid due to diffusion and osmosis, i.e., an osmotic gradient occurs across the membrane. An osmotic agent in the PD fluid provides the osmotic gradient. Used PD fluid is drained from the patient, removing waste, toxins and excess water from the patient. This cycle is repeated, e.g., multiple times.

There are various types of peritoneal dialysis therapies, including continuous ambulatory peritoneal dialysis ("CAPD"), automated peritoneal dialysis ("APD"), tidal flow dialysis and continuous flow peritoneal dialysis ("CFPD"). CAPD is a manual dialysis treatment. Here, the patient manually connects an implanted catheter to a drain to allow used PD fluid to drain from the patient's peritoneal cavity. The patient then switches fluid communication so that the patient catheter communicates with a bag of fresh PD fluid to infuse the fresh PD fluid through the catheter and into the patient. The patient disconnects the catheter from the fresh PD fluid bag and allows the PD fluid to dwell within the patient's peritoneal cavity, wherein the transfer of waste, toxins and excess water takes place. After a dwell period, the patient repeats the manual dialysis procedure, for example, four times per day. Manual peritoneal dialysis requires a significant amount of time and effort from the patient, leaving ample room for improvement.

APD is similar to CAPD in that the dialysis treatment includes drain, fill and dwell cycles. APD machines, however, perform the cycles automatically, typically while the patient sleeps. APD machines free patients from having to manually perform the treatment cycles and from having to transport supplies during the day. APD machines connect fluidly to an implanted catheter, to a source or bag of fresh PD fluid and to a fluid drain. APD machines pump fresh PD fluid from a dialysis fluid source, through the catheter and into the patient's peritoneal chamber. APD machines also allow for the PD fluid to dwell within the chamber and for the transfer of waste, toxins and excess water to take place. The source may include multiple liters of dialysis fluid, including several solution bags.

APD machines pump used PD fluid from the patient's peritoneal cavity, though the catheter, to drain. As with the manual process, several drain, fill and dwell cycles occur during dialysis. A "last fill" may occur at the end of the APD treatment. The last fill fluid may remain in the peritoneal chamber of the patient until the start of the next treatment, or may be manually emptied at some point during the day.

Dialysis fluid or treatment for HD, HF, HDF and PD is typically heated prior to being delivered to a dialyzer (HD, HDF), blood line (HF, HDF) or the patient (PD). The dialysis fluid is typically heated to body temperature or <NUM> so that the patient does not experience a thermal shock when the dialysis fluid comingles with the patient's blood or is delivered to the patient's peritoneal cavity. One type of dialysis fluid heater is an inline dialysis fluid heater, which heats the dialysis fluid as it passes through the inline heater. Inline heaters are advantageous because they operate online as treatment is taking place and do not require a separate amount of time offline from the treatment. One drawback to online heating however is that if there is no dialysis fluid flowing when the inline heater is powered, the inline heater may overheat.

There is accordingly a need for an effective, low cost way of preventing or mitigating overheating in an inline heater due to a no or low flow condition.

According to the present invention there is provided an inline heating system according to claim <NUM>.

The present disclosure involves the use of an inline heater in a dialysis machine, which may be any type of dialysis machine, such as a peritoneal dialysis ("PD") machine, hemodialysis ("HD") machine, hemofiltration ("HF") machine, hemodiafiltration ("HDF") machine or continuous renal replacement therapy ("CRRT") machine. The inline heater heats dialysis fluid as it flows through the heater towards the patient (PD), dialyzer (HD, HDF), or blood line (HF, HDF, CRRT) for treatment. The inline heating method is opposed to a batch heater commonly used with PD for heating a bag of dialysis fluid prior to being delivered for treatment. The inline heater of the present disclosure is advantageous because it does not require the footprint involved with maintaining a bag for batch heating. The inline heater also heats the dialysis fluid as it is needed, eliminating the need for a heating period prior to beginning treatment.

The inline heater of the present disclosure is disadvantageous from one standpoint in that if it is attempted to heat dialysis fluid while no dialysis fluid is flowing, the inline heater can overheat. A flow switch may be placed ahead of the inline heater to make sure that flow is present as a condition for energizing the heater. Flow switches add cost however and can become stuck or otherwise not function properly.

The inline fluid heating system of the present solution involves determining the resistance of the heater element of the inline heater. The resistance of the heater element increases in a typical manner with an increase in temperature. The resistance of the heater element may be calculated by knowing an amount of voltage or current applied to the heater element and measuring the other of the current or voltage at the heater element while the inline heater is being powered. Heater element resistance Re is then equal to Ve/Ie. The calculated resistance is applied to an algorithm stored in a control unit of the dialysis machine to determine if a low flow or no flow condition within the inline heater exists.

One possible algorithm sets a maximum heater element resistance increase or resistance derivate (change in heater element resistance over time). By looking at heater element resistance rate of change or increase, a no or low flow condition through the inline heater may be detected early before the temperature increases too much or to a point that could damage the inline heater. When there is no fluid flow in the energized inline heater, the temperature increases rapidly and hence the heater resistance increases rapidly. If the change in heater element resistance over a period or of time (rate of change), e.g., a sample period meets or exceeds a set maximum rate of change of resistance per the period of time, the algorithm triggers the control unit to automatically deenergize the inline heater, stop treatment, and alarm the patient, caregiver or nurse.

One possible implementation of the overheating algorithm is to run a characterizing program that characterizes the inline heater at the time of manufacture and/or servicing of the dialysis machine. The characterizing program characterises how the inline heater resistance relates to temperature. Here, the inline heater is energized while receiving a known flowrate of fluid. Input power is varied while the flowrate is held constant. A temperature sensor may be used to measure the test fluid temperature downstream from the inline heater to confirm that the heater is heating the test fluid properly. The characterising program enables measured heater element resistances (correlated with heater element temperature) to be known for a given fluid flowrate and over a common or use range of input power settings. A resulting calibration data table is then stored in the memory of the control unit of the dialysis machine. The control unit uses the calibration data during treatment to detect when the inline heater is overheating, that is, to detect for a given or commanded flowrate and a commanded input power setting, that the measured heater element resistance Re is greater than it should be for the commanded dialysis fluid temperature.

The algorithm stored in the control unit of the dialysis machine may accordingly analyze two factors (i) rate of change in heater element resistance (derivative) indicating that a no or low flow condition is beginning and (ii) the determine resistance compared to the calibration resistance. That is, the rate of change in heater element resistance may be high, indicating that a no or low flow condition may be beginning, but if the determined heater element resistance (and corresponding temperature) is lower than what the calibration data says the heater element resistance should be for a given flowrate and power input setting, then the algorithm allows the control unit of the dialysis machine to continue treatment.

The cycle rate for the current or voltage measurement may be very high and noisy, and thus the algorithm may be configured to take many measurements, e.g., over the period of a second or more, before making a no or low flow determination, deenergizing the inline heater, stopping treatment, and alarming the patient, caregiver or nurse. Taking many measurements over a safe time period that is too short for the inline heater to overheat significantly helps to prevent false trips. Taking many measurements also provides an adequate number of samples to efficiently supress measurement signal noise.

An analog heater protection circuit may be provided as part of the control unit of the present system to detect the rate of change or derivative, discussed herein, in either one or both of current and/or voltage. The analog heater protection circuit may include one or more of measurement circuitry, divider circuitry, a differential circuit, a comparator, and/or a lowpass/timer circuit.

The control unit may include the at least one of the current or voltage meter.

The control unit may be configured to determine multiple heater element resistances using the applied voltage or current and the measured at least one current or voltage at different times over a sample period to determine if the no flow or low flow condition is detected.

The heater element may be a first heater element, and which includes at least one additional heater element, wherein the control unit is configured to determine a heater element resistance for each heater element as part of the no flow or low flow condition detection algorithm.

The at least one current or voltage meter may be a first current or voltage meter, and which includes at least one additional current or voltage meter arranged to measure the at least one current or voltage at the at least one additional heater element.

The control unit may be configured to stop the voltage or current applied to power the inline heater if the no flow or low flow condition is detected at any of the heater elements.

The control unit may be configured to compare a rate of change in determined heater element resistance to a set maximum rate of change in determined heater element resistance as part of the low flow condition detection algorithm implemented by the control unit.

The rate of change in determined heater element resistance may be a positive rate of change.

As described, a control unit may be configured to compare the determined heater element resistance to a calibrated heater element resistance as part of the low flow condition detection algorithm implemented by the control unit.

The calibrated heater element resistance may be determined remote from the inline heating system and then stored in the control unit.

As described, calibrated heater element resistance may be specific to at least one of (i) fluid flowrate through the inline heater or (ii) input power to the inline heater.

The control unit may be configured to measure the at least one of the current or voltage at a sample rate that is of at least one of (i) <NUM> or greater or (ii) at least two times greater than a frequency of a pulsed voltage source powering the inline heater.

The control unit may include an analog heater protection circuit configured to filter the measured at least one of the current or voltage.

As described an inline heating system may include an inline heater including a heater element; a voltage source or a current source positioned to apply a voltage or current to power the inline heater; and an analog heater protection circuit configured to (i) measure at least one of the current or voltage at the heater element due to the applied voltage or current, (ii) determine a heater element resistance using the applied voltage or current and the measured at least one of the current or voltage, and (iii) provide a stop power signal if the heater element resistance indicates a no flow or low flow condition.

As described, an analog heater protection circuit may include at least one of: measurement circuitry, divider circuitry, a differential circuit, a comparator, or a lowpass/timer circuit.

As described, an inline heating system may include an inline heater including a heater element; and a control unit configured to compare a rate of change in heater element resistance to a set maximum rate of change in heater element resistance as part of a no flow or low flow condition detection algorithm implemented by the control unit, wherein power applied to the inline heater is stopped if the no flow or low flow condition is detected.

As described a control unit is further configured to compare at least one determined heater element resistance to at least one calibrated heater element resistance as part of the low flow condition detection algorithm implemented by the control unit.

As described an at least one calibrated heater element resistance is determined remote from the inline heating system and then stored in the control unit.

As described an at least one calibrated heater element resistance is specific to at least one of (i) fluid flowrate through the inline heater or (ii) input power to the inline heater.

The inline heating system may be provided as part of a peritoneal dialysis machine, hemodialysis machine, hemofiltration machine, hemodiafiltration machine, continuous renal replacement therapy machine, water purification unit, dialysis fluid preparation unit, or blood warmer.

In light of the above aspects and the present disclosure, it is accordingly an advantage of the present disclosure to provide a dialysis machine having an inline heater that is deenergized upon a no or low flow condition.

It is another advantage of the present disclosure to provide a dialysis machine having cost effective inline heater no or low flow protection.

It is a further advantage of the present disclosure to provide a dialysis machine having inline heater no or low flow protection that does not require significant additional hardware.

Referring now to the drawings and in particular to <FIG>, an inline heating system <NUM> is illustrated. Inline heating system <NUM> may be used in many medical fluid applications including but not limited to peritoneal dialysis ("PD"), hemodialysis ("HD") machine, hemofiltration ("HF"), hemodiafiltration ("HDF"), continuous renal replacement therapy ("CRRT") and blood warming. While dialysis is a primary focus for inline heating system <NUM>, the system is not limited to same. For example inline heating system <NUM> may be employed in a water purification unit that prepares purified water for the creation of dialysis fluid, and which heats the water either for downstream use or for disinfection. Inline heating system <NUM> may be employed alternatively in a dialysis fluid preparation unit that prepares dialysis fluid online, and which heats the dialysis fluid for downstream use or for disinfection.

Inline heating system <NUM> includes a fluid source <NUM>, which may be a PD fluid source, HD fluid source, replacement fluid source (HF, HDF, CRRT), or water source for example. Inline heating system <NUM> includes a fluid destination 12d, which may be the patient's peritoneal cavity (PD), a dialyzer (HD, HDF), a blood line (HF, HDF, CRRT), a dialysis fluid preparation unit if inline heating system <NUM> is employed in or provided by a water purification unit, or a dialysis machine (e.g., PD cycler) if dialysis inline heating system <NUM> is employed in or provided by a dialysis fluid preparation unit. Fluid source <NUM> and fluid destination 12d may alternatively both be the patient when inline heating system <NUM> is provided as part of a blood warmer.

Fluid from source <NUM> (which will hereafter be termed dialysis fluid even though the fluid is not limited to same as discussed above) is pumped along a fluid line <NUM> via a pump <NUM>. Pump <NUM> may be any type of fluid pump, e.g., a durable (reusable) pump that contacts the dialysis fluid directly, such as a piston, gear, centrifugal or rotary vane pump. Pump <NUM> may alternatively have a disposable component, such as a pneumatic pump operating with a disposable cassette, an electromechanical pump operating with a disposable cassette, or a peristaltic pump operating with a peristaltic tubing segment.

It is contemplated that there be many different components located along fluid line <NUM> between fluid source <NUM> and fluid destination 12d, such as one or more valve, air trap and various sensors. For ease of illustration inline heating system <NUM> is shown having a pressure sensor 18p and a temperature sensor 18t. The output of pressure sensor 18p may be used as feedback for controlling pump <NUM> so as not to exceed a positive or negative pressure limit. The output of the temperature sensor 18t may be used as feedback for controlling the input power to inline heater <NUM>. Temperature sensor 18t is located downstream from inline heater <NUM> so as to sense the temperature of the dialysis fluid exiting the heater. If desired, an additional temperature sensor may be located upstream of inline heater <NUM> to provide feedforward information concerning the temperature of dialysis fluid entering inline heater <NUM>.

<FIG> further illustrates that inline heating system <NUM> of the present disclosure includes a control unit <NUM>, which may be the control unit of a dialysis machine (e.g., PD cycler), water purification unit or a dialysis fluid preparation unit. Control unit <NUM> in the illustrated embodiment includes one or more processor <NUM>, one or more memory <NUM> and a video controller <NUM>. Control unit <NUM> receives, stores and processes signals or outputs from pressure sensor 18p, temperature sensor 18t and other sensors provided by the machine or unit, such as a conductivity sensor (not illustrated). Control unit <NUM> uses pressure feedback from pressure sensor 18p to control pump <NUM> to pump dialysis fluid at a desired pressure or within a safe pressure limit (e.g., within <NUM> bar (three psig) of positive pressure to a patient's peritoneal cavity). Control unit <NUM> uses temperature feedback from temperature sensor 18t to control inline heater <NUM> to heat the dialysis fluid to a desired temperature, e.g., body temperature or <NUM>.

Video controller <NUM> of control unit <NUM> interfaces with a user interface <NUM> of the machine or unit, which may include a display screen operating with a touchscreen and/or one or more electromechanical button, such as a membrane switch. User interface <NUM> may also include one or more speaker for outputting alarms, alerts and/or voice guidance commands. User interface <NUM> may be provided with the machine or unit as illustrated in <FIG> and/or be a remote user interface operating with control unit <NUM>. Control unit <NUM> may also include a transceiver (not illustrated) and a wired or wireless connection to a network, e.g., the internet, for sending treatment data to and receiving prescription instructions from a doctor's or clinician's server interfacing with a doctor's or clinician's computer.

Referring now to <FIG>, inline heater <NUM> is illustrated in more detail. In the illustrated embodiment, inline heater <NUM> includes a housing <NUM> that surrounds a fluid heating pathway <NUM>. Pathway <NUM> is illustrated as being straight for ease of illustration but may have alternative shapes, such as a coiled or serpentine shape. Also, while inline heater <NUM> is illustrated as a durable component that does not operate with a disposable unit, inline heater <NUM> may alternatively operate with a fluid carrying disposable unit, such as a disposable unit defining a coiled or serpentine heating pathway.

Inline heater <NUM> in the illustrated embodiment includes a plurality of heater elements 26a, 26b, such as two heater elements. Inline heater <NUM> may alternatively include only a single heater element or more than two heater elements 26a, 26b. Heater elements 26a, 26b in the illustrated embodiment are located between housing <NUM> and fluid heating pathway <NUM>, on the inside of housing <NUM> and on the outside of fluid heating pathway <NUM>. Inline heater <NUM> may be configured differently however. For example, heater elements 26a, 26b may instead be located along the center of housing <NUM>. Fluid heating pathway <NUM> here extends around the outside of the centrally located heater elements 26a, 26b, between the heater elements and housing <NUM>, which in any heater embodiment may be thermally insulated. Control unit <NUM>, for any configuration of inline heater <NUM>, controls the power provided to heater elements 26a, 26b. Control unit <NUM> may do so by controlling either the voltage or current to heater elements 26a, 26b. The more voltage or current supplied, the more power is provided to inline heater <NUM>, and thus more heating of dialysis fluid flowing through inline heater <NUM> takes place, resulting in a higher dialysis fluid temperature.

In the illustrated embodiment, control unit <NUM> controls the voltage from a voltage source <NUM> to each heater element 26a, 26b. Voltage source <NUM> may be a <NUM> to <NUM> VAC, a <NUM> to <NUM> VAC voltage source, or a voltage source supplying direct current voltage to heater element 26a, 26b. A positive voltage source line 64a extends to one end of each heater element 26a, 26b, while a negative voltage source line 64b extends to the other end of each heater element 26a, 26b.

Control unit <NUM> in the illustrated embodiment also includes current meters 66a, 66b for heater elements 26a, 26b, respectively. Current meters 66a, 66b are connected in series with heater elements 26a, 26b to be able to measure the current applied to each heater element. In the illustrated embodiment, current meters 66a, 66b are located along positive voltage source lines 64a extending to one end of each heater element 26a, 26b. In an alternative embodiment shown in dashed line, a single current meter <NUM> may be used to read the current flowing through multiple heater elements 26a and 26b, wherein control unit <NUM> may index or switch through readings from each heater element in a cyclical manner. Control unit <NUM> may alternatively operate with single current meter <NUM> so as to read both currents (through lines 64a and 64b) along the common line having single sensor <NUM>. Here, it is not required to cycle sensor <NUM> to read the different lines 64a, 64b individually.

It is contemplated for control unit <NUM> to instead measure both voltage and current simultaneously to obtain good accuracy for the resistance value Re of each heater element 26a, 26b. As discussed below in connection with <FIG>, an analog heater protection circuit may be provided with control unit <NUM> to detect the rate of change or derivative, discussed herein, in either one or both of current and/or voltage.

Inline heater <NUM> of inline heating system <NUM> is disadvantageous from one standpoint in that if it is attempted to heat dialysis fluid while no dialysis fluid is flowing, inline heater <NUM> can overheat. A flow switch may be placed ahead of inline heater <NUM> to make sure that flow is present as a condition for control unit <NUM> to energize the heater. Flow switches add cost however and can become stuck or otherwise not function properly.

Inline heating system <NUM> solves the overheating problem by configuring control unit <NUM> to determine the resistance Re of each heater element 26a, 26b of inline heater <NUM>. The resistance Re of each heater element 26a, 26b increases in a typical manner upon an increase in temperature. The resistance Re of each heater element 26a, 26b is in one embodiment calculated by knowing an amount of voltage or current applied to each heater element 26a, 26b (in <FIG> voltage Ve to the heater elements is controlled by control unit <NUM>) and measuring the other of the current or voltage in each heater element 26a, 26b (in <FIG> current Ie along heater elements 26a, 26b is measured by control unit <NUM>) while inline heater <NUM> is being powered. Heater element resistance Re is then equal to Ve/Ie. The calculated resistance is applied to an algorithm stored in memory <NUM> of control unit <NUM>, which is interrogated by processor <NUM> to determine if a low flow or no flow condition within inline heater <NUM> exists.

Referring now to <FIG>, in an alternative embodiment, system <NUM> of the present disclosure may employ an analog heater protection circuit <NUM>, which performs some or all of the functionality described above as being performed via software using one or more processor <NUM> and one or more memory <NUM>. It should be appreciated that control unit <NUM> as discussed herein also includes analog heater protection circuit <NUM> and any other heater protection circuitry that may be employed by system <NUM>. Control unit <NUM> includes all supervisory control side and protective side hardware and software and all lower level hardware (including heater protection circuit <NUM>) and software.

As illustrated in <FIG>, analog heater protection circuit <NUM> may include measurement circuitry <NUM>, which takes measurements Ve and Ie. Circuitry <NUM> may include a current probe, e.g., an ACS723 current probe, which measures the current Ie. Measurement circuitry <NUM> may include a suitably sized resistor grid to measure the voltage Ve. Measurement circuitry <NUM> may also include a low pass filter that filters two voltage signals (one representing current Ie and the other representing voltage Ve). A standard RC lowpass filter, for example, may be used to suppress the most prevalent noise created by the pulsed controller. The two voltage signals (possibly filtered) are fed from measurement circuitry <NUM> to divider circuitry <NUM>, which includes a divider, e.g., an AD538 divider, creating a new voltage corresponding to heater element resistance Re.

The voltage corresponding to heater element resistance Re is fed from divider circuitry <NUM> to a differential circuit <NUM> that creates a second new voltage representing a rate of change of heater element resistance Re. Differential circuit <NUM> may be (i) a passive capacitive differential circuit that includes a capacitor and resistor, (ii) a passive inductive differential circuit that includes an inductor and a resistor, or (iii) an active differential circuit, which may also include a capacitor and a resistor as well as an operation amplifier. The second new voltage representing the rate of change of heater element resistance Re is then delivered from differential circuit <NUM> to a comparator <NUM>, the output of which may be used to disable or to turn off inline heater <NUM> if the rate of change of heater element resistance Re meets or exceeds a threshold rate of change of heater element resistance Re set at comparator <NUM>. A lowpass/timer circuit <NUM> may be provided after comparator <NUM> to remove potential glitches/disturbances. Lowpass/timer circuit <NUM> ensures that the no/low flow signal does not go high based on noise that may have gotten through to the lowpass filter/timer circuit. Lowpass/timer circuit <NUM> instead ensures that the signal is stable/triggered for a certain amount of time and that inline heater <NUM> is not depowered or shut off for small signal spikes or noises that may occur.

Referring now to <FIG>, a collection of graphs over a common time period correlating inline heater power input applied to heater elements 26a, 26b, dialysis fluid flowrate through inline heater <NUM> and heater element resistance Re is illustrated. The graphs highlight for example that as power to heater elements 26a, 26b increases, e.g., from <NUM> Watts, to <NUM> Watts, to <NUM> Watts, heater element resistance Re increases in turn from about <NUM> Ohms, to about <NUM> Ohms, to about <NUM> Ohms, respectively, which coincides with an increase in heater element temperature. The graphs also highlight that whenever dialysis fluid or other fluid flowrate is stopped, while input power continues to be applied, heater element resistance Re increases sharply, which also coincides with a sharp increase in heater element temperature.

One possible algorithm stored in memory <NUM> and interrogated by processor <NUM> sets a maximum heater element resistance increase or resistance derivate (change in heater element resistance over time). With control unit analysing heater element resistance Re rate change or increase over time, a no or low flow condition through the inline heater <NUM> is detectable before the temperature of heater elements 26a, 26b increases to a point that could damage inline heater <NUM>. When there is no fluid flow in an energized inline heater <NUM>, the temperature of heater elements 26a, <NUM> increases rapidly and hence the heater element resistance Re increases rapidly. Control unit <NUM> is accordingly programmed to look to determine if a change in heater element resistance Re over a period or of time (rate of change), e.g., to determine if a sample period rate of change of resistance Re meets or exceeds a set maximum rate of change of resistance Re over the period of time, the algorithm triggers control unit <NUM> to automatically deenergize inline heater <NUM>, stop treatment at the corresponding machine or cycler (or stop preparing purified water or stop preparing dialysis fluid at the corresponding unit), and alarm the patient, caregiver or nurse, e.g., at user interface <NUM>.

One possible implementation of the overheating algorithm of inline heating system <NUM> is to run a characterizing program that characterizes inline heater <NUM> at the time of manufacture and/or servicing of the dialysis machine (or other unit). The characterizing program characterises how heater element resistance Re relates to input power and flowrate. Here, the inline heater is energized while receiving a known flowrate of fluid. Input power is varied while the flowrate is held constant. Temperature sensor 18t may be used to measure the test fluid temperature downstream form inline heater <NUM> to confirm that the inline heater is heating the test fluid properly to the commanded temperature. The output of pump <NUM> is monitored to confirm that the commanded flowrate is met (a weight scale may be used in the test setting). The characterising program enables measured heater element resistances Re (correlated with heater element temperature) for the particular inline heater <NUM> tested to be known for a commanded flowrate and fluid temperature, and over a common or use range of input power settings. A resulting calibration data table is then stored in memory <NUM> of control unit <NUM> of the dialysis machine (or other unit). Control unit <NUM> uses the calibration data during treatment to detect when inline heater <NUM> is overheating, that is, to detect that for a given or commanded flowrate and input power setting to achieve a commanded temperature, whether a measured and determined heater element resistance Re is greater than it should be for the commanded dialysis fluid temperature.

The algorithm stored in memory <NUM> of control unit <NUM> of the dialysis machine (or other unit) may accordingly analyze two factors (i) rate of change in heater element resistance (derivative) indicating that a no or low flow condition may be beginning and (ii) a comparison between a determined heater element resistance Re to the calibration resistance for the same input power and commanded flowrate. That is, the rate of change in heater element resistance Re may be high, indicating that a no or low flow condition may be underway, but if the determined heater element resistance (and corresponding temperature) is lower than what the calibration data says the heater element resistance should be for a given flowrate and power input setting, then the algorithm allows control unit <NUM> of dialysis machine (or other unit) to continue treatment.

The cycle rate for the current or voltage measurement to determine heater element resistance Re may be very high, and thus the algorithm may be configured to take many measurements, e.g., over the period of a second or more, before making a no or low flow determination, deenergizing inline heater <NUM>, stopping treatment, and alarming the patient, caregiver or nurse. Taking many measurements over a safe time period that is too short for inline heater <NUM> to overheat significantly helps to prevent false trips.

Taking many measurements also provides an adequate number of samples to efficiently supress measurement signal noise. The measurement signal for heater element resistance Re tends to be very noisy due to its environment including a pulsed power source. In one implementation, the high sampling rate was <NUM>, and a multi- or four step decimation procedure was applied. Each decimation step was performed by applying an 8th order Chebyshev lowpass filter type I and by removing samples to a 10th of a fraction of the original or prior step, e.g., to <NUM> samples, then <NUM> samples, etc. There are numerous ways of performing the filtering leading to the heater element resistance Re graph at the top of <FIG>. It is likely, however, that successful filtering requires a large sample rate, such as <NUM>. The sample rate will depend on the frequency of the pulsed voltage source, e.g., if a typical <NUM>/<NUM> AC voltage source, a sample rate of at least two times as fast, e.g., <NUM> to <NUM> is likely required. However, the sample rate may desirably be at least ten times higher, e.g., <NUM> to <NUM>.

Claim 1:
An inline heating system (<NUM>) comprising:
an inline heater (<NUM>) including a heater element (26a);
a control unit (<NUM>) configured to cause one of voltage or current to be applied to power the inline heater (<NUM>);
a memory (<NUM>) communicatively coupled to the control unit (<NUM>), the memory configured to store a calibration data table that relates calibrated heater element resistances of the inline heater (<NUM>) to at least one of fluid flowrates through the inline heater (<NUM>) or input voltages or currents to the inline heater (<NUM>); and
at least one of a current or voltage meter (<NUM>, 66a) outputting to the control unit (<NUM>) and arranged to measure at least one of the current or voltage at the heater element (26a) due to the applied voltage or current,
wherein the control unit (<NUM>) is further configured to
determine a heater element resistance (Re) using the applied voltage or current and the measured at least one of the current or voltage as part of a no flow or low flow condition detection algorithm implemented by the control unit (<NUM>),
compare the determined heater element resistance (Re) and at least one of a commanded fluid flowrate or the applied voltage or current to the calibration data table to determine whether the no flow or low flow condition is detected, and
stop the voltage or current that is applied to power the inline heater (<NUM>) when the no flow or low flow condition is detected.