The present invention relates to blood pumps, to methods of using blood pumps, and to control circuits adapted for use with blood pumps.
Implantable blood pumps may be used to provide assistance to patients with late stage heart disease. Blood pumps operate by receiving blood from a patient's vascular system and impelling the blood back into the patient's vascular system. By adding momentum and pressure to the patient's blood flow, blood pumps may augment or replace the pumping action of the heart. For example, a blood pump may be configured as ventricular assist device or “VAD.” Where a VAD is used to assist the pumping action of the left ventricle, the device draws blood from the left ventricle of the heart and discharges the blood into the aorta.
To provide clinically useful assistance to the heart, blood pumps must impel blood at a substantial blood flow rate. For an adult human patient, a ventricular assist device may be arranged to pump blood at about 1-10 liters per minute at a pressure differential across the pump of about 10-110 mmHg, depending on the needs of the patient. The needs of the patient may vary with age, height, and other factors.
It is desirable to monitor the rate at which blood is impelled by a blood pump. For example, if a VAD is operated at a flow rate in excess of the inflow rate of blood to the ventricle, the VAD will create a suction condition within the ventricle, wherein the ventricle is collapsed and essentially devoid of blood. This condition is undesirable. In this condition, the flow rate through the pump will decline rapidly. Likewise, if the intake or outlet of the pump is occluded, the flow rate will decline gradually. If the flow rate through the pump declines, either rapidly (e.g., as a result of suction condition) or gradually (e.g., as a result of an obstruction or occlusion) to the extent that the flow rate is insufficient, the device will not provide sufficient circulatory assistance to the patient. Excessive flow also can create undesirable conditions. Therefore, it would be desirable to provide a blood pump controller which can monitor the blood flow rate produced by the blood pump which it controls.
Furthermore, it is preferable to minimize or reduce any errors in the blood flow monitoring process in order to obtain a more accurate estimation of blood flow in the pump. The relationships between certain properties of the pump (e.g., electrical current supplied to the pump, the speed of rotation of a rotor of the pump, etc.) may be generally known and used to predict blood flow rate in the blood pump, such as in the manner described in co-owned U.S. Published Patent Application No. 2012/0245681, entitled “Flow Estimation in a Blood Pump,” the disclosure of which is incorporated herein in its entirety. However, these relationships are generally estimated, approximated, or otherwise derived from analogous blood pump devices, and in reality may vary from pump to pump. Inaccuracies in the modeling of a pump's characteristic can in turn lead to inaccuracies in the modeling of flow estimation behavior for the pump, which in turn may lead to a poor blood flow estimation. Therefore, it is further desirable to provide a blood pump monitoring system that is capable of optimizing the parameters that are used to determine blood flow in the pump, thereby minimizing or reducing, errors in the blood flow monitoring process.
One aspect of the disclosure provides for a method for monitoring operation of an implantable blood pump, involving, determining an amount of current supplied to the pump, determining a first flow rate value and a second flow rate value based on the amount of current supplied to the pump, and determining a flow rate of blood based on a combination of the first and second flow rate values and a weighting parameter such that the combination of the first and second flow rate values varies depending on the weighting parameter. The weighting parameter may increase or decrease monotonically over the range of operable flow rates for the pump, or over a subset of the range of operable flow rates for the pump. A weight may be determined and assigned to each of the first and second flow rate values based at least in part on a relationship between the weighting parameter and a predetermined threshold value. The assigned weight for the first flow rate value may be inversely proportionate to the relationship between the weighting parameter and the predetermined threshold value, and the assigned weight for the second flow rate value may be proportionate to the relationship between the weighting parameter and the predetermined threshold value.
Determining the relationship between the weighting parameter and the predetermined threshold value may involve comparing the difference between the weighting parameter and the threshold value to a preselected spread value. For example, an absolute difference between the weighting parameter and a predetermined threshold value may be calculated, and the absolute difference may be compared to a predetermined spread value. In such an example, determination of the flow rate of blood based on a combination of the first and second flow rate may be limited to when the absolute difference between the third parameter and a predetermined threshold value is less than the predetermined spread value. For instance, if the absolute difference is greater than the spread value, and the third parameter is less than the predetermined threshold value, the flow rate of blood may be determined based on the first flow rate value and not the second flow rate value. Additionally, or alternatively, if the absolute difference is greater than the spread value, and the third parameter is greater than the predetermined threshold value, the flow rate of blood may be determined based on the second flow rate value and not the first flow rate value. Similarly, if the absolute difference is greater than the spread value, the third parameter is less than the predetermined threshold value, and a determined speed of rotation of the rotor is greater than a threshold speed value to which the determined speed is compared, the flow rate of blood may be determined determined based on the first flow rate value; but if the determined speed is less than the threshold speed value, the flow rate of blood may be determined based on the third parameter and neither of the first and second flow rate values.
In some examples, the implantable blood pump may include a housing having an axis and a rotor disposed within the housing and rotatable around the axis. The weighting parameter may be related to thrust on the rotor along the axis.
Another aspect of the present disclosure provides for a control circuit for monitoring the operation of an implantable blood pump. The control circuit may include a current determination circuit operative to determine an amount of current supplied to the pump, a flow rate value determination circuit operative to determine a first flow rate value and a second flow rate value based on the amount of current supplied to the pump, a transformation circuit operative to determine a weighting parameter, and a flow rate determination circuit operative to determine a flow rate of blood through the pump based on a combination of the first and second flow rate values and the weighting parameter.
Yet another aspect of the present disclosure provides for an implantable blood pump system including a pump with a housing having an axis, and a rotor disposed within the housing, the rotor being rotatable around the axis, and further including the above described control circuit operatively coupled to the pump. The pump may further include a stator operatively coupled to the control circuit. The stator may incorporate a plurality of coils for applying a rotating magnetic field to the rotor, and the weighting parameter may be based on back electromotive force (BEMF) in one or more of the plurality of coils.
Yet a further aspect of the present disclosure provides for a method of optimizing an estimate of flow rate of blood through an implantable blood pump. The method may involve measuring the flow rate of blood through the implantable blood pump, estimating the flow rate of blood through the implantable blood pump based at least in part on a first parameter related to thrust on the rotor along the axis and a second parameter selected from one of a threshold value and a spread value, determining a difference between the measured flow rate and the estimated flow rate, and adjusting the second parameter based on the determined difference such that estimating the flow rate of blood based on the adjusted second parameter results in a reduced difference between the measured flow rate and the estimated flow rate. The estimating, determining, adjusting, and optionally measuring, steps may continue repeatedly only if the determined difference between the measured flow rate and the estimated flow rate is not less than a threshold error value.
The pump also includes a stator 130. The stator includes coils 132a-e (
The control circuit 140 comprises driver circuit 310, current determination module 320, and speed determination module 330. BEMF measuring module 340, transformation module 350, flow determination module 360, and pump control module 370. The modules are depicted and discussed with reference to their individual functions. One or more of the modules 310-270 may be implemented using software operating in a computer system including a general-purpose or special purpose processor, in digital circuitry, or in using analog circuitry.
The driver circuit 310 is an electrical circuit for powering the pump 101 with a 3-phase current. Each phase of the three-phase current preferably is in the form of a generally rectangular wave including alternating off or “open-phase” periods in which power is not applied by the drive circuit and on or “closed-phase” periods during which power is applied. The periods of the various phases are arranged so that at any moment, two pairs of coils are on or closed-phase and one pair is off or open-phase. The open-phase and closed-phase periods of the various phases are arranged so that the various pairs of coils go to an open-phase state in sequence, thus creating the rotating magnetic field that actuates the rotor. Driver circuit 310 applies pulse width modulation during each on or closed-phase period. Thus, during each on or closed-phased period, the voltage applied to the pair of coils varies repeatedly between zero and a selected maximum value at a pulse modulation or chopping frequency much higher than the frequency of the rectangular waveform of the repeating closed-phase and open-phase period.
For example,
Returning to
The control circuit further includes a voltage sampling circuit 315. The voltage-sampling circuit may include an analog-to-digital converter connected across coil pair 132a and 132b and arranged to capture successive samples of the voltage appearing across the coil pair. The voltage-sampling circuit may also include a digital filter for suppressing variations in the sampled voltage at frequencies at or above the pulse-width modulation or chopping frequency used by the drive circuit, so as to provide a filtered series of values. Alternatively, the sampling circuit may include an analog low-pass filter connected between the analog-to-digital converter and the coil pair.
A speed determination module 330 is operatively connected to the sampling circuit 315 to receive the filtered values from sampling circuit. The speed determination module is arranged to deduce the speed of rotation of the magnetic field, and hence the speed of rotation of rotor 120, from these values. For example, the speed determination module may be arranged to record the time when the voltage on coil pair 132a and 132b drops below the threshold value associated with the open-phase periods as the beginning of an open-phase period, and to calculate the interval between the beginnings of successive open-phase periods. The speed of rotation is inversely proportional to this time.
The BEMF measuring module 340 is also connected to receive the stream of sampled voltage values from sampling circuit 315, and to record the filtered voltage values during the open-phase periods. These filtered values represent the BEMF generated by the pump. Transformation module 350 is connected to BEMF measuring module 340. The transformation module processes the data collected by the BEMF measuring module 340 to determine a value of a function of the BEMF. The function is referred to herein as F(BEMF). F(BEMF) may be rate of change of the BEMF with respect to time during each open-phase period, i.e., the absolute value of the slope of the BEMF versus time. Like the BEMF measuring module 340, the transformation module 350 may also be implemented using hardware and/or software.
The flow determination module 360 may include hardware and/or software for determining the rate at which blood is impelled by the pump 101. The flow determination module is operatively connected to current determination module 320, speed determination module 330 and transformation module 350 so that the flow determination module 360 receives values representing current, speed and F(BEMF). The flow determination module is arranged to determine the flow rate from the pump based on this information as further discussed below. Pump control module 370 is operatively linked to flow determination module 360 so that the pump control module 370 receives values representing the flow rate from the flow determination module. The pump control module is also linked to driver circuit 310. The pump control module is arranged to determine a desired pump speed based, at least in part, on the flow rate and to command driver circuit 310 accordingly. Thus, the pump control module can control the pump 100 based on the blood flow rate determined by the flow determination module 370 as further discussed below.
In operation, the control circuit 140 powers the pump 101, via the driver circuit 310, thereby causing the rotor 120 to spin. As the rotor 120 spins, blood enters the pump 101 through the inflow end 380 after which the blood is impelled by the rotor 120 from the outflow end 390. As the blood passes through the pump 101, it imparts a thrust on the rotor 120. The magnitude of this thrust is related to the flow rate of blood through the pump.
As discussed above, the rotor 120 is held in position by magnetic and hydrodynamic forces. However, these forces do not hold the rotor with infinite rigidity. Therefore, thrust imparted to the rotor 120 causes the rotor 120 to move by a displacement distance D towards the inflow end 380. For at least some range of thrust values, distance D is related to the magnitude of the thrust and, thus, related to the blood flow rate. Distance D is greatly exaggerated in
Because F(BEMF) (the rate of change in BEMF in the open phase period) is proportional to BEMF, the same curve 620 depicts the relationship between F(BEMF) and the blood flow rate. Stated another way. F(BEMF) is a parameter related to the thrust on the rotor. The flow determination module 360 determines the flow rate of blood through the pump based in part on this parameter as further explained below. As also shown in
The various modules discussed above with reference to
Data 530 may be retrieved, stored or modified by processor 510 in accordance with the instructions 540. The data may also be formatted in any computer-readable format such as, but not limited to, binary values, ASCII or Unicode. Moreover, the data may comprise any information sufficient to identify the relevant information, such as numbers, descriptive text, proprietary codes, pointers, references to data stored in other memories (including other network locations) or information that is used by a function to calculate the relevant data.
A current-to-flow table 532 is a tabular representation of the function 610 depicted in
Thus, as depicted in Table 1, the current-to-flow map stores plural values of flow rate for each value of current, one associated with the left region and one associated with the right region. At a value of current corresponding to the fiducial flow rate (1.0 amps in the example of Table I), the two values are the same; the current-to-flow table 532 indicates that when the pump 101 is powered with 1.0 amps of current, it pumps blood at the rate of 2 L/min. At a current of 1.2 amps, the blood flow rate is either 1.5 L/min or 3.0 L/min. The current-to-flow relationship varies with the speed of operation of the pump, i.e., the rotation rate of the rotor. The current-to-flow relationship also varies with viscosity of the blood. The viscosity of the blood is directly related to the hematocrit, i.e., the proportion of the blood volume occupied by red blood cells. Therefore, the current-to-flow table stores different sets of values, each associated with a range a particular pump operating speed and blood viscosity. Sets of values for other pump operating speeds and viscosities are calculated from the stored sets by interpolation. The flow calculation module selects the appropriate set of values based on the speed of operation of the pump and on a value of hematocrit or blood viscosity for the patient which has been supplied to the system from an external source through interface 550. The current-to-flow table 532 may be implemented as a file, a data structure, as part of a database, or in any other suitable form.
F(BEMF)-to-flow table 534 may be a tabular representation of the function 620 depicted in
An example of the F(BEMF)-to flow table 534 is provided as Table 2. According to this example, the BEMF-to-flow table 534 indicates that when the BEMF in the coil 132a changes at the rate of 5.5 V/s, the pump 101 impels blood at the rate of 2.5 L/min. The BEMF-to-flow table 534 may be implemented as a file, a data structure, as part of a database, or in any other suitable form.
Fiducial BEMF values 536 for a given pump are also included with the data 530. The fiducial BEMF values 536 provide an indication as to whether the flow determination module should use the relationship between current and flow rate indicated in the left region or right region of curve 610 (
The data may be determined experimentally using the actual pump or a sample pump of similar configuration. In addition, each of the tables may be pre-loaded in the memory 520 before the pump 101 is deployed. With respect to the fiducial BEMF values, these values may also be pre-loaded in the memory 520. The pre-loaded values may be subsequently optimized using the optimization protocol described in greater detail below.
The instructions 540 may be instructions to be executed directly (such as machine code) or indirectly (such as scripts) by the processor. In that regard, the terms “instructions,” “steps” and “programs” may be used interchangeably herein. The instructions may be stored in object code format for direct processing by the processor, or in any other computer language including scripts or collections of independent source code modules that are interpreted on demand or compiled in advance. Functions, methods and routines of the instructions are explained in more detail below. Flow estimation module 542 may include instructions for determining the blood flow rate produced by the pump 101 as further explained below, whereas pump control module 544 may include instructions for controlling the operation of the drive circuit 310 (
The control circuit 140 may optionally include an interface 550 which connects the control circuit 140 to an output device 560. The interface 550 may be an analog interface (e.g., audio interface) or a digital interface, such as Bluetooth. TCP/IP, 3G, and others. Where the control circuit is implemented in an implantable structure adapted to be disposed within the body of the patient, the interface 550 may include known elements for communicating signals through the skin of the patient. The output device 560, may be a speaker, a communications terminal (e.g., computer, cell phone) or any other type of device.
Although
At task 720, the control circuit determines a parameter related to thrust imparted on the rotor 120 by the flow of blood exiting the pump 101. In this embodiment, the determined parameter is the function F(BEMF), the rate of change of BEMF during the open phase periods of coil pair 132a and 132b as discussed above.
At task 730, (
At task 740, the control circuit 140 determines the rate at which blood is impelled by the pump 101 based on the parameter related to thrust determined at task 720.
The tasks included in task 740 are shown in greater detail in
where F(BEMF) is the value of F(BEMF) determined in task 720, and T and S are the respective fiducial BEMF values 536. Depending on the relationship between the BEMF value, T and S, W % may equal more than 1, less than 0, or between 0 and 1. For example, for values of F(BEMF) that exceed than the range of values within the fiducial band, W % equals more than 1. For values of BEMF that are less than the range of values within the fiducial band, W % is less than 0. For values of F(BEMF) within the fiducial band, W % equals between 0 and 1, and increases linearly proportionately with F(BEMF).
At task 940, the control circuit 140 retrieves the function 610 that maps an amount of current supplied to the pump 101 to blood flow rate that is generated by the pump 101, i.e., the current-to-flow table 532 (
At task 950, the control circuit 140 determines which of two different tasks to execute based on the value of W %. The first task 960 determines a first blood flow rate value based on the left portion of the function 610. The second task 970 determines a second blood flow rate value based on the right portion of the function 610. These tasks 960 and 970 are described in greater detail below. A value of W % that is less than 0 indicates with relative certainty that the correct blood flow rate value is on the left portion of the function 610; therefore, only task 960 is executed by the control circuit. A value of W % that is greater than 1 indicates with relative certainty that the correct blood flow rate value is on the right portion of the function 610; therefore, only task 970 is executed by the control circuit. A value of W % that is between 0 and 1 indicates that the correct blood flow rate value may be either on the left or on the right portion of the function 610; therefore, both tasks 960 and 970 are executed.
At task 960, in order to determine a first blood flow rate value based on the left portion of the function 610, the control circuit 140 may use the value of current as an index and retrieve the corresponding value of flow from the entries in the current-to-flow table 532 (and Table 1, above) that pertains to the left portion. Alternatively, the control circuit 140 may obtain two or more values that correspond to the same amount of current and then select the smallest one. In either process, standard interpolation techniques can be used when the value of current falls between stored values.
At task 970, in order to determine a second blood flow rate value based on the right portion of the function 610, the control circuit 140 may use the value of current as an index and retrieve the corresponding value of flow from the entries in the current-to-flow table 532 (and Table 1, above) that pertain to the right portion. Alternatively, the control circuit 140 may obtain two or more values that correspond to the same amount of current and then select the largest one. In either process, standard interpolation techniques can be used when the value of current falls between stored values.
At task 980, the control circuit determines the rate at which blood is impelled by the pump 101 based on the determined first and second blood flow rate values. Under those conditions in which the control circuit determines only the first blood flow rate value (e.g., W % is greater than 1), the determined flow rate (task 980) is the first blood flow rate value. Similarly, under those conditions in which the control circuit determines only the second blood flow rate value (e.g., W % is less than 0), the determined flow rate is the second blood flow rate value. If both the first and second blood flow rate values have been determined, the determined flow rate may be a weighted average of the first and second blood flow rate values. For example, the control circuit may use the following formula:
QT=[(1−W %)*Q1]+[W%*Q2] Formula 2
where Q1 is the first blood flow rate value (task 960) and Q2 is the second blood flow rate value (task 970), to determine QT, which is the determined flow rate (task 980). In the example of Formula 2, the control circuit may use fuzzy logic to weigh the first and second blood flow rate values against one another. The weights attributed to both Q1 and Q2 are adjusted proportionate to the value of W % (inversely linearly for Q1, linearly for Q2). Thus, Q1 is attributed a greater weight than Q2 for values of F(BEMF) that are less than T (but greater than T−S), and Q2 is attributed a greater weight than Q1 for values of F(BEMF) that are greater than T (but less than T+S).
The above calculations for determining flow rate are meant as an example and may performed in any particular order or dividing into any number of steps. For instance, the control circuit 140 may first compare F(BEMF) to an absolute difference between T and S (|T−S|). When |T−S| is greater than F(BEMF), W % percent is either greater than 1 (for values of S greater than T) or less than 0 (for values of T greater than S). Under such circumstances, the control circuit may proceed with either task 760 (for T>S) or task 770 (for S>T) without calculating W % or QT. Otherwise, the control circuit performs both task 760 and 770, and calculates W % and QT.
At task 750 (
The determined flow rate may also, or alternatively, be used to optimize the pre-loaded fiducial F(BEMF) values for the fiducial midpoint and spread. Often, the pre-loaded fiducial F(BEMF) values are estimates determined during development of the pump. These estimates may be further improved or optimized by comparing a flow rate derived using the pre-loaded values (e.g., the flow rate determined using task 980 of
A blood pump system 1000 in accordance with yet another embodiment of the invention incorporates an active control system comprising an active control module which exerts an axial force on the rotor to counteract the effects of thrust on the rotor and maintain the rotor in a substantially constant axial position. Further examples of active control systems are provided in U.S. Published Patent Application No. 20110237863, entitled “Magnetically Levitated Blood Pump With Optimization Method Enabling Miniaturization.”
System 1000 comprises a pump 1001 and control circuit 1070. The pump 1001 comprises a rotor 1020 disposed within housing 1010 and actuated by a stator 1030. The rotor 1020 comprises coils 1030. Unlike the pump 101 (
Control circuit 1070 may include an active control module 1072 and flow determination module 1070. The active control module 1072 may receive input signal(s) 1050 and outputs control signal(s) 1060. In this example, the control signal 1060 controls the magnitude of the magnetic field produced by at least one of the electromagnets 1040a-b. The control signal 1060 may be a digital directed to a controller that operates the electromagnets 1040a-b, an analog current used to power the electromagnets 1040a-b, or any other signal. Because the control signal 1060 sets the magnitude of the magnetic field of the electromagnets 1040a-b, which is used to offset the thrust imparted on the rotor 100 by the flow of blood output by the pump 101, the control signal bears a direct relationship to the thrust.
The signal 1060 constitutes another example of a parameter related to thrust and it may be used to determine blood flow rate. The flow determination module 1074 may determine the blood flow rate produced by the pump 1001 by receiving the control signal 1060 and matching it to a corresponding blood flow rate. For example, a table may be stored in a memory of the control circuit 140 that relates different values for the control signals 1040 to blood flow rate. Here again, the table may include different sets of data for different pump operating speeds and blood viscosities. The flow determination module 1074 may use the table to match the value of the control signal 1060 to a corresponding blood flow rate.
In still other arrangements, thrust can be measured directly. For example, if the pump includes a bearing which retains the rotor against axial movement, the bearing may incorporate a piezoelectric element or other force transducer. The signal from the force transducer, or a function of the signal, may be used as the parameter related to thrust.
In the embodiment discussed above with reference to
As a further alternative, the system can limit determinations of flow directly from F(BEMF) to circumstances in which the value of BEMF is significantly less than the fiducial midpoint T (i.e., less than T−S). As another alternative, the system can further limit determinations of flow directly from F(BEMF) to circumstances in which: (i) the value of BEMF is significantly less than the fiducial midpoint T; and (ii) the speed of rotation of rotor 120 (e.g., determined by the speed determination module 330) is less than 18 kRPM.
In the particular system discussed above with reference to
The control circuit 140 need not store relationships between a parameter such as F(BEMF) and flow or between current and flow in the form of lookup tables as discussed above. The control circuit may retrieve and evaluate a formula that models the rate at which blood is impelled by the pump as a function of the parameter related to thrust, e.g., a formula for the function 620 (
In the embodiments discussed above, the flow rate determined by the control circuit is used to control the operation of the pump. In other embodiments, the control circuit may simply determine the flow rate and send a signal representing the flow rate to an external device, and may not control operation of the pump.
In the embodiment discussed above in connection with
Other parameters related to thrust on the rotor may be employed instead of a function of BEMF. For example, where the pump is equipped with a transducer other than the coils which can directly measure the axial position of the rotor, control circuit 140 may determine the flow rate based in whole or in part on a signal from the transducer which represents displacement. Stated another way, the displacement is a parameter related to thrust on the rotor. Any other parameter related to thrust on the rotor can be used.
More generally, while the above embodiments use a weighting parameter related to thrust on the rotor to determine a blood flow rate of a pump, any parameter that increases or decreases monotonically over the range of operable flow rates for the pump may be used. In other words, as long as a parameter is a monotonic function of operable flow rates for the pump, that parameter may be used to make a determination as to whether the pump is operating on the left or right portion of curve 610 and/or a relative certainty of that determination. Furthermore, any parameter that increases or decreases monotonically over even a subset of the range of operable flow rates for the pump may be used. For example, if the parameter is a monotonic function of flow rates for which there is uncertainty whether the pump is operating on the left or right portion of curve 610, that parameter may be used at least to determine the relative certainty of whether the pump is operating on the left or right portion.
Further aspects of this disclosure are described in the disclosure titled “Fiducial Point Optimization,” which is attached.
As these and other variations and combinations of the features discussed above can be utilized without departing from the subject matter as defined by the claims, the foregoing description of exemplary aspects should be taken by way of illustration rather than by way of limitation of the subject matter as defined by the claims. It will also be understood that the provision of the examples described herein (as well as clauses phrased as “such as,” “e.g.,” “including” and the like) should not be interpreted as limiting the claimed subject matter to the specific examples; rather, the examples are intended to illustrate only some of many possible aspects.
The present application claims the benefit of the filing date of U.S. Provisional Patent Application No. 62/084,742 filed Nov. 26, 2014, the disclosure of which is hereby incorporated herein by reference.
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