US20060047367A1 - Method of controlling an implantable medical device with optimized pump drive - Google Patents

Abstract

Method of controlling an implantable medical device having a power source having a voltage, a therapeutic substance pump driven by a motor, electrically coupled to the power source, supplied with a series of drive pulses having a duty cycle. The voltage of the power source is measured. The duty cycle of drive pulses to the motor are varied by increasing the duty cycle as the voltage of the power source decreases.

Description

RELATED APPLICATION
• This application is related to the following co-pending applications entitled “Implantable Therapeutic Substance Infusion Device With Motor Stall Detector” by inventors Seifert et al. (attorney docket number P8905.00) and “Implantable Therapeutic Substance Infusion Device With Active Longevity Projection” by inventors Rogers et al. Ser. No. 09/809,809 Filed Mar. 16, 2001 (attorney docket number P8904.00), which are not admitted as prior art with respect to the present invention by its mention in this cross reference section.
• This application is a divisional of U.S. patent Application Ser. No. 10/125,871, Charles R. Rogers, filed Apr. 22, 2002, published as U.S. patent application Publication No. US 2003/0199855 A1, which is not admitted as prior art with respect to the present invention by its mention in this cross reference section.
FIELD OF THE INVENTION
• This disclosure relates to a medical device and more particularly to an implantable therapeutic substance infusion device, also known as an implantable drug pump, optimized peristaltic pump motor drive circuit.
BACKGROUND OF THE INVENTION
• The medical device industry produces a wide variety of electronic and mechanical devices for treating patient medical conditions. Depending upon medical condition, medical devices can be surgically implanted or connected externally to the patient receiving treatment. Clinicians use medical devices alone or in combination with therapeutic substance therapies and surgery to treat patient medical conditions. For some medical conditions, medical devices provide the best, and sometimes the only, therapy to restore an individual to a more healthful condition and a fuller life. One type of medical device is an implantable therapeutic substance infusion device.
• An implantable therapeutic substance infusion device is implanted by a clinician into a patient at a location appropriate for the therapy. Typically, a therapeutic substance infusion catheter is connected to the device outlet and implanted to infuse the therapeutic substance such as a drug or infusate at a programmed infusion rate and predetermined location to treat a condition such as pain, spasticity, cancer, and other medical conditions. Many therapeutic substance infusion devices are configured so the device can be refilled with therapeutic substance through a septum while the device is implanted. Then the time the device can be implanted may not be limited by therapeutic substance stored capacity of the device. An example of an implantable therapeutic substance infusion is shown in Medtronic, Inc. product brochure entitled “SynchroMed.RTM. Infusion System” (1995).
• Electrically powered implanted therapeutic substance infusion devices consume energy delivered typically by a battery, also called a power source, and can require replacement once implanted due to depletion of the battery. Typically the most significant power-consuming component in an implantable infusion device is the therapeutic substance metering motor such as a stepper motor.
• A stepper motor is an electromechanical device whose rotor rotates a discrete angular amount when an electrical drive pulse is applied to the stator windings. The amplitude and the width of the electrical drive pulse must be tailored to the electromechanical properties of the motor in order to achieve rotation, stability, and optimal energy consumption. Examples of instability include the motor rotating backwards, stepping ahead then “flipping back” to its starting position, and not stepping at all. For a stepper motor to function normally and efficiently over a wide power source voltage range, the motor drive pulse needs to be adjusted proportional to the voltage change of the power source.
• If all motor drive pulse parameters are held constant while the power source voltage decreases, a decrease due to normal consumption of power source energy, excess energy above that needed by the motor is delivered at the beginning of the service life of the device. This occurs because the pulse parameters needed for the end of service life, for example, pulse width, are greater than needed at the beginning.
• Thus, unless the pulse parameters are appropriately varying as the power source voltage is varying, the excess energy drawn from the power source undesirably reduces the service life of the implantable pump. This may cause an early need to replace the pump which is undesirable.
• Since replacement of the implanted device requires an invasive procedure of explanting the existing device and implanting a new device, it is desirable to extend battery life to the greatest extent practicable. Some previous implantable infusion devices have reduced power consumption by varying the pulse width of the motor drive signal, but stepper motors can become unstable or stall under some circumstance when the pulse width is varied from the optimal pulse width that the motor is typically designed to use. An example of a motor drive signal with a varying pulse width is shown in Japanese Patent 11,042,286 “Intracorporealy Embedded Type Liquid Medicine Supplying Apparatus” by Yamazaki (Feb. 16, 1999).
• For the foregoing reasons, there is a need for an implantable therapeutic substance infusion device with optimized pump motor drive to increase the infusion device's effective service life. This increased service life reduces the overall cost of the medical therapy and the inconvenience to the patient and clinician for future device replacement surgeries.
BRIEF SUMMARY OF THE INVENTION
• In an embodiment, the present invention provides a method for reducing energy consumption in an implantable infusion device having a stepper motor driving an infusion pump and powered by a power source having a power source voltage, the stepper motor being driven by drive pulses. An instruction is received to infuse a therapeutic substance. The power source voltage is measured. A duty cycle of the drive pulses for delivery within a substantially fixed drive interval to operate the stepper motor is calculated. The duty cycle of the drive pulses within the substantially fixed drive interval to operate the stepper motor is increased as the power source voltage decreases. The therapeutic substance is infused when the stepper motor operates the infusion pump to meter the therapeutic substance.
• In an embodiment, a fixed drive interval for the drive pulses to be delivered to the stepper motor to operate the infusion pump according to a therapy program is specified and a predetermined number of drive pulses within the drive interval are generated according to the power supply voltage to reduce motor energy consumption.
• In an embodiment, the present invention provides a method of controlling an implantable medical device having a power source having a voltage, a therapeutic substance pump driven by a motor, electrically coupled to the power source, supplied with a series of drive pulses having a duty cycle. The voltage of the power source is measured. The duty cycle of drive pulses to the motor are varied by increasing the duty cycle as the voltage of the power source decreases.
• In an embodiment, the motor comprises a stepper motor.
• In an embodiment, a series of drive pulses is generated in a substantially fixed drive interval.
• In an embodiment, the fixed drive interval is specified according to a therapy program and a predetermined number of drive pulses within the drive interval are generated according to the power supply voltage to reduce motor energy consumption.
• In an embodiment, the predetermined number of drive pulses is in a range from about 10 to about 25 drive pulses within the substantially fixed drive interval.
• In another embodiment, the duty cycle comprises a range from about 75% to about 100% of the drive interval.
• In another embodiment, the predetermined number of drive pulses is selected to operate the stepper motor while substantially maintaining motor stability and substantially avoid motor stall.
• In another embodiment, the power source voltage is forecasted and the duty cycle is adjusted in response thereto.
• In another embodiment, the implantable infusion device has electronics coupled to the motor and to the power source, the electronics having a processor, memory coupled to the processor and an infusion program residing in the memory, the infusion program is modified once the therapeutic infusion device is implanted.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows the environment of an implantable therapeutic substance infusion device embodiment;
• FIG. 2 shows an implantable therapeutic substance infusion device embodiment;
• FIG. 3 shows an implantable therapeutic substance infusion device with catheter embodiment;
• FIG. 4 shows an exploded view of an implantable therapeutic substance infusion device with peristaltic pump embodiment;
• FIG. 5 shows a block diagram of an implantable therapeutic substance infusion device embodiment;
• FIG. 6 shows flow diagram for the motor drive control embodiment;
• FIG. 7 shows a schematic diagram of stepper motor control system embodiment;
• FIG. 8 shows an electrical pulse diagram versus time for the chopper motor control embodiment;
• FIG. 9 shows a diagram of the power source voltage versus depth of discharge of the power source superimposed with drive pulse duty cycle embodiment;
• FIG. 10 shows the drive pulse duty cycle with varying duty cycles embodiment, and
• FIG. 11 shows that the motor drive pulse energy is substantial constant over the useful voltage range of the power source embodiment.
DETAILED DESCRIPTION OF THE INVENTION
• FIG. 1 shows the environment of an implantable medical device known as an implantable therapeuticsubstance delivery device30, also known as a drug pump, having a peristaltic pump with optimized peristaltic pump motor drive embodiment. The therapeuticsubstance delivery device30 operates to infuse atherapeutic substance36 stored intherapeutic substance reservoir44 at a programmed flow rate into apatient38. The therapeuticsubstance delivery device30 can be used for a wide variety of therapies such as pain, spasticity, cancer, and many other medical conditions.
• The implantable therapeuticsubstance delivery device30 is typically implanted by a surgeon in a sterile surgical procedure performed under local, regional, or general anesthesia Before implanting the therapeuticsubstance delivery device30, acatheter32 is typically implanted with the distal end position at the desired therapeuticsubstance delivery site34 and the proximal end tunneled to the location where the therapeuticsubstance delivery device30 is to be implanted. The implantable therapeuticsubstance delivery device30 is generally implanted subcutaneous about 2.5 cm (1.0 inch) beneath the skin where there is sufficient tissue to support the implanted system. Once the therapeuticsubstance delivery device30 is implanted into thepatient38, the incision can be sutured closed and the therapeuticsubstance delivery device30 can begin operation.
• FIG. 2 shows an implantable therapeuticsubstance delivery device30 with optimized pump motor drive embodiment withhousing41 and fillport septum40.FIG. 3 shows implantable therapeuticsubstance delivery device30 connected tocatheter32 prior to implantation into apatient38 by a surgeon.
• Thetherapeutic substance36 inpump reservoir44 inside the pump is a substance intended to have a therapeutic effect such as pharmaceutical compositions, genetic materials, biologics, and other substances. Pharmaceutical compositions are chemical formulations intended to have a therapeutic effect such as intrathecal antispasmodics, pain medications, chemotherapeutic agents, and the like. Pharmaceutical compositions are often configured to function in an implanted environment with characteristics such as stability at body temperature to retain therapeutic qualities, concentration to reduce the frequency of replenishment, and the like. Genetic materials are substances intended to have a direct or indirect genetic therapeutic effect such as genetic vectors, genetic regulator elements, genetic structural elements, DNA, and the like. Biologics are substances that are living matter or derived from living matter intended to have a therapeutic effect such as stem cells, platelets, hormones, biologically produced chemicals, and the like. Other substances are substances intended to have a therapeutic effect yet are not easily classified such as saline solution, fluoroscopy agents, and the like.
• Thetherapeutic substance36 inreservoir44 can be replenished in some embodiments of the implanted therapeuticsubstance delivery device30 by inserting a non-coring needle connected to a syringe filled withtherapeutic substance36 through the patient's skin into afill port septum40 on the therapeuticsubstance delivery device30 to fill the implanted device. The contents of the syringe are then injected into thepump reservoir44.
• If the therapeuticsubstance delivery device30 requires replacement due to conditions such as power source depletion or other condition, an incision is made near the implanted therapeuticsubstance delivery device30, and the old therapeuticsubstance delivery device30 is removed, also known as explanted. After the old therapeuticsubstance delivery device30 has been explanted, typically a new therapeuticsubstance delivery device30 is then implanted.
• FIG. 4 shows an exploded view of an implantable therapeutic substance infusion device with optimized pump motor drive comprised of ahousing41, apower source42, atherapeutic substance reservoir44, atherapeutic substance pump46, andelectronics48.
• Thehousing41 is manufactured from a material that is biocompatible and hermetically sealed such as titanium, tantalum, stainless steel, plastic, ceramic, and the like. Thepower source42 is carried in thehousing41. Thepower source42, selected to operate thetherapeutic substance pump46 andelectronics48, may be a lithium ion (Li+) battery, a capacitor, and the like.
• Thetherapeutic substance reservoir44 is carried in thehousing41 and is configured to containtherapeutic substance36. The therapeuticsubstance pump assembly46 is carried in thehousing41, and is fluidly coupled to thetherapeutic substance reservoir44 and electrically coupled to thepower source42. The therapeuticsubstance pump assembly46 is a pump sufficient for infusingtherapeutic substance36 such as the peristaltic pump with stepper motor drive that can be found in the SynchroMed& Infusion System available from Medtronic, Inc.
• A stepper motor is an electromechanical device whose rotor rotates a discrete angular amount when an electrical drive pulse is applied to the stator windings. The amplitude and the width of the pulse must be tailored to the electromechanical properties of the motor in order to achieve rotation, rotational stability, and optimal energy consumption. An example is a motor that rotates 180 degrees with the application of a 3 volt, 11.2 millisecond, square pulse. A second pulse is then applied at minus 3 volts to rotate an additional 180 degrees making a complete revolution.
• The stepper motor is mechanically coupled by gears to the peristaltic roller pump where the rollers rotate in such a way as to squeeze a compressible tube and drive liquid through the tube lumen in one direction. In effect thetherapeutic substance36 from thereservoir44 flows in the tube and is metered to thepatient38 viacatheter32 toanatomical sight34.
• Examples of instability include the motor rotating backwards, stepping ahead then “flipping back” to its starting position, and not stepping at all. For a stepper motor to function normally and efficiently over a wide power source voltage range, the motor drive pulse parameters need to be adjusted proportional to the voltage change of the power source.
• FIG. 5 shows a block diagram device embodiment. Theelectronics48 are carried in thehousing41 and coupled to thetherapeutic substance pump46 and thepower source42. Theelectronics48 include a processor, memory, an infusion program, and transceiver circuitry.
• At the processor can be a microprocessor, an application specific integrated circuit (ASIC) state machine, a gate array, a controller, and the like. Theelectronics48 are configured to control thetherapeutic substance pump46 infusion rate and can be configured to operate many other features such as patient alarms and the like. The infusion program and other device parameters and patient information reside in memory and are capable of being modified once the therapeutic substance infusion device is implanted. The transceiver circuitry is coupled to the processor for externally receiving and transmitting therapeutic substance infusion device information.
• FIG. 6 shows a flow chart of a method for operating a therapeuticsubstance infusion device30 with optimized pump motor drive. The method embodiment includes receiving an instruction to infuse atherapeutic substance61. Together with the elements of measuring, receiving and storing the power source voltage signal in62, the drive pulse duty cycle is calculated or provided by a look-up table in63. The drive pulse duty cycle is used for delivery ofmotor drive pulses64 to the motor that generates the infusion of therapeutic substance from thepump65.
• FIG. 7 shows a schematic diagram of stepper motor control system to illustrate the components of the preferred embodiment. Thestepper motor106 is connected to thepower source102 via themotor drive circuit105, and mechanically coupled withgears107 to the therapeuticsubstance infusion pump108. Themotor drive circuit105 is configured with a driveinterval timing circuit105aand adrive pulse circuit105b. The driveinterval timing circuit105aspecifies a substantially fixed drive interval for drive energy to be delivered to themotor106 to operate atherapeutic substance pump108 according to a therapy program. Thedrive pulse circuit105bis coupled to the powersource measurement circuit140 to generate a predetermined number of drive pulses within the drive interval according to thepower supply102 voltage to reducemotor106 energy consumption. The duty cycle of short drive pulses varies according to thepower source voltage95 ofFIG. 9 atterminal103 as measured by the powersource measurement circuit140 to reduce motor energy consumption.
• FIG. 8 shows the repetition rate of the long fixed drive period t2 varies dependent on the programmed pump flow rate in accordance with the therapy program. For example, the repetition rate is high for high therapy flow rates, and the repetition rate is low for low therapy flow rates.
• InFIG. 7 thepower source voltage95 atterminal103 is feed to the quad switches111,112,113, and114 as well the powersource measurement circuit140. The powersource measurement circuit140 is composed of a standard analog-to-digital converter as well as other electrical components. The powersource measurement circuit140 generates a signal or digital word proportional to the power source voltage that is fed to theprocessing unit141. Theprocessing unit141 produces a duty cycle control signal or digital word that is fed to thecontroller104 in themotor drive circuit105. Thecontroller104 in the motor drive circuit is composed of standard digital logic, registers and other logic components to configure the motor drive pulses. Registers may include but are not limited to a pulse interval register, a drive pulse width register, a drive pulse duty cycle register, and a drive pulse expiration register.
• Thecontroller104 outputs electrical pulses to drive the four motor drive switches111,112,113, and114.Outputs116 and118 of the quad switch assembly produce a motor drive pulse train that energizes themotor stator magnet122, in turn rotating themotor rotor120. Themotor stator coil124 is wound around themagnetic material stator122. Therotor120 is mechanically coupled by-gears107 to thepump mechanism108. Therotor120 of the stepper motor rotates in such a way as to propel and meter thetherapeutic infusion substance36 from thepump30 to thepatient38.
• This invention achieves motor drive energy optimization by dividing the usual long drive pulse into numerous shorter pulses. For example, the long single drive pulse can be performed by23 shorter pulses applied over the time period t2 inFIG. 8. Due to the large inductance of the motor's stator winding124, the flow of current is not significantly disrupted at each short pulse. The duty cycle, ratio of “on time” to “total time”, of the short pulses is directly proportional to the energy savings compared to a 100% “on time” drive pulse. The duty cycle changes in small discrete increments over the usable power source voltage range such that the energy that drives the motor is substantially constant and of proper amplitude for stable motor function. For example, the duty cycle increases in 1/16 steps as the power source voltage decreases and vice versa.
• FIG. 8 illustrates how the stepper motor pulse duty cycle is achieved. The long pulse t2 is combined or commutated withchopper oscillator pulses80 fromchopper oscillator110 in thecontroller104 of themotor drive circuit105. The long motor drive pulse width t2 is fixed at 11.2 milliseconds. The 2048hertz chopper oscillator110 providessquare wave pulses80 that are used to achieve the short drive pulse t5. The duty cycle of the 2048 hertz pulses t5 in periods t2 and t4 are dependent on the power source voltage as described above. The predetermined number of short drive pulses t5 are about 23 pulses within the substantially fixed drive interval of 11.2 milliseconds of t2 and t4. The drive periods t2 and t4 have a duty cycle in the range from about 75% to about 100%. This combination of varying short pulse parameters achieves substantially constant motor drive energy to reliably operate a therapeutic substance pump according to a therapy program.
• FIG. 9 illustrates the continuously varyingpower source voltage95 versus the Depth of Power Source Discharge during the service life of the infusion device. Superimposed onFIG. 9 is the motor drive shortpulse duty cycle91 expressed in percent. As the power source voltage decreases, the duty cycle increases, ranging from 75% to 100%. The end of service life92 of thepower source42 is where 100% duty cycle occurs. Until the end of service life is reached, the duty cycle of the stepper motor varies to compensate for the decreasing power source voltage.
• FIG. 10 illustrates the duty cycle, defined as t5 divided by t6, as it varies depending onpower source voltage95. At 100% duty cycle, the short drive pulses t5 are on during the entire 11.2 millisecond drive period t2 inFIG. 8. At 75% duty cycle, the long drive pulse is composed of 23 consecutive short drive pulses each at about 0.75/2048 Hertz=0.366 milliseconds long.
• FIG. 11 illustrates the long pulsemotor drive energy1100 is substantially constant versus power source voltage. This near constant delivered energy assures stable andreliable motor120 rotation even when thepower source voltage95 decreases.
• Many other related functions could be derived from this optimized motor drive invention. For example, if a motor stall is detected, the drive pulse duty cycle could be increased or decreased to overcome the motor stall or to do diagnostics of the stall. An intermittent motor stall might occur due to significant environmental magnetic field interactions such as experienced with magnetic resonance imaging technology.
• In addition, if a very large change of power source current is needed, the resultant anticipated power source voltage reduction could be predicted using a power source forecast circuit. The power source forecast circuit identifies that the power source voltage will change abruptly and that the duty cycle must be adjusted. Typically the duty cycle is increased in anticipation of lower power source voltage. Such a dynamic prediction may be needed because of inherent time delays of the power source measuring or processing system. Large power source current changes might occur when the pump infusion rate is abruptly changed from a very low rate to a very high rate or an alarm is activated.
• Thus, the implantable therapeuticsubstance infusion device30 embodiments with optimized pump motor drive achieves the maximum service life of the power source while maintaining assurance that the stepper motor will function normally over the entire range of power source voltages. An energy efficient motor drive is achieved based on measuring power source voltage and continuously controlling the motor drive pulse parameters to minimize the electrical energy delivered to the pump motor. A predetermined number of drive pulses is selected to operate the stepper motor while substantially maintaining motor stability and avoid motor stall.
• Thus, embodiments of the method of controlling an implantable medical device with optimized pump drive are disclosed to increase the infusion devices effective service life. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.

Claims (19)

1. A method for reducing energy consumption in an implantable infusion device having a stepper motor driving an infusion pump and powered by a power source having a power source voltage, said stepper motor being driven by drive pulses, comprising:

receiving an instruction to infuse a therapeutic substance;

measuring said power source voltage;

calculating a duty cycle of said drive pulses for delivery within a substantially fixed drive interval to operate said stepper motor;

increasing said duty cycle of said drive pulses within said substantially fixed drive interval to operate said stepper motor as said power source voltage decreases; and

infusing said therapeutic substance when said stepper motor operates said infusion pump to meter said therapeutic substance.

2. A method as inclaim 1 further comprising the steps of:

specifying a fixed drive interval for said drive pulses to be delivered to said stepper motor to operate said infusion pump according to a therapy program; and

generating a predetermined number of drive pulses within said drive interval according to said power supply voltage to reduce motor energy consumption.

3. A method as inclaim 2 wherein said predetermined number of drive pulses is in a range from about 10 to about 25 drive pulses within said substantially fixed drive interval.

4. A method as inclaim 2 wherein said drive interval has a duty cycle in a range from about 75% to about 100% of the step interval.

5. A method as inclaim 2 wherein said predetermined number of drive pulses is selected to operate the stepper motor while substantially maintaining motor stability and substantially avoid motor stall.

6. A method as inclaim 1 further comprising the step of using logic contained in registers to configure said drive pulses.

7. A method as inclaim 6 wherein said registers include a pulse interval register, a drive pulse width register, a drive pulse duty cycle register, and a drive pulse expiration register.

8. A method as inclaim 1 further comprising the step of forecasting said power source voltage and adjusting said duty cycle in response thereto.

9. A method of controlling an implantable medical device having a power source having a voltage, a therapeutic substance pump driven by a motor, electrically coupled to said power source, supplied with a series of drive pulses having a duty cycle, comprising the steps of:

measuring said voltage of said power source; and

varying said duty cycle of drive pulses to said motor by increasing said duty cycle as said voltage of said power source decreases.

10. A method as inclaim 9 wherein said motor comprises a stepper motor.

11. A method as inclaim 9 further comprising the step of generating said series of drive pulses in a substantially fixed drive interval.

12. A method as inclaim 11 further comprising the steps of:

specifying said fixed drive interval according to a therapy program; and

generating a predetermined number of drive pulses within said drive interval according to said power supply voltage to reduce motor energy consumption.

13. A method as inclaim 12 wherein said predetermined number of drive pulses is in a range from about 10 to about 25 drive pulses within said substantially fixed drive interval.

14. A method as inclaim 12 wherein said duty cycle comprises a range from about 75% to about 100% of said drive interval.

15. A method as inclaim 12 wherein said predetermined number of drive pulses is selected to operate the stepper motor while substantially maintaining motor stability and substantially avoid motor stall.

16. A method as inclaim 9 further comprising the step of forecasting said power source voltage and adjusting said duty cycle in response thereto.

17. A method as inclaim 9 wherein said implantable infusion device has electronics coupled to said motor and to said power source, said electronics having a processor, memory coupled to the processor and an infusion program residing in said memory, further comprising the step of modifying said infusion program once said therapeutic infusion device is implanted.

18. A method as inclaim 17 further comprising the step of externally receiving therapeutic substance infusion device information.

19. A method as inclaim 18 further comprising the step of externally transmitting therapeutic substance infusion device information.

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