CA1316987C - Method and apparatus for measuring the lead current in a pacemaker - Google Patents

Method and apparatus for measuring the lead current in a pacemaker

Info

Publication number
CA1316987C
CA1316987C CA000611722A CA611722A CA1316987C CA 1316987 C CA1316987 C CA 1316987C CA 000611722 A CA000611722 A CA 000611722A CA 611722 A CA611722 A CA 611722A CA 1316987 C CA1316987 C CA 1316987C
Authority
CA
Canada
Prior art keywords
output
current
voltage
transistors
pacemaker
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Fee Related
Application number
CA000611722A
Other languages
French (fr)
Inventor
David L. Thompson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Medtronic Inc
Original Assignee
Medtronic Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Medtronic Inc filed Critical Medtronic Inc
Application granted granted Critical
Publication of CA1316987C publication Critical patent/CA1316987C/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

Links

Classifications

    • AHUMAN NECESSITIES
    • A61MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61NELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
    • A61N1/00Electrotherapy; Circuits therefor
    • A61N1/18Applying electric currents by contact electrodes
    • A61N1/32Applying electric currents by contact electrodes alternating or intermittent currents
    • A61N1/36Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
    • A61N1/362Heart stimulators
    • A61N1/37Monitoring; Protecting
    • A61N1/3706Pacemaker parameters

Abstract

ABSTRACT
Disclosed is an improved pacemaker arrangement which permits the measurement of output current to the heart so that, if necessary, a physician can adjust the output of the pace-maker. Prior art pacemakers include a series resistor in the output current path and the voltage drop across it can be measured and telemetered out. This requires a high precision resistor to reduce measurement errors and, if it fails, a "no output" condition can result. Also, the resistor wastes output energy. The present invention uses a large number, e.g. 200, of field effect transistors (FETS) operated in parallel to discharge a capacitor through the heart tissue. Pacer lead current is monitored by measuring current through a small number, e.g. 2, of the FETS. The current monitoring function is performed by a current-to-voltage converter coupled to an A/D converter. The voltage from the current-to-voltage con-verter may be applied through additional circuitry to the gates of the FETS to provide for a constant current output pulse.

Description

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~ETHOD AND APPARAT~S FOR MEAS~RING THE ~EAD
CURRENT IN A PACEMAKER

FIELD OF THE INVENTION
. . _ _ The present invention relates generally to implantable pacemakers and more particularly to an output circuit topology which permits the measurement of output current to the heart.
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BACKGROUND OF THE INVENTION
Implantable pacemakers are coupled to the heart through a lead system. In operation, the pacemaker periodically provides an electrical output stimulus which is delivered to the complex impedance oE the heart. The amount of energy delivered to the heart depends on the resistance of the lead and the heart tissue. It has been routine to measure this impedance or threshold at the time of implantation to permi.t optimizing the location oE the pacing lead and to maximize longevity of the pacer. These acute measurements are made with an oscilloscope or, more frequently, with a special instrument called a pacing system analyzer or PSA~
Later, after~the lead has~"healed" into the heart tissue, the margin of capture and delivered energy are estimated by the physician. With the advent of output pulse~programmable~pacers, physicians have been able to adapt the pacer's output to the threshold requirements of the patient and thus~prolong the longevity of the implanted device.
To assi~st the~physician with followup care, modern pacers use internal circuitry to monitor the output pulse parameters~and~to telemeter this in~ormation to the ; phys~ician~via~a programmer. Thls inEormation is used in ass~essing the performance of the~pacemaker and the associated lead.~
A ~problem which is presented by~this technology is a discrepancy between the measured values of lead current and delivered~energy pr~esented by the programmer, PSA and .. :
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13~ ~987 oscilloscope. These differences result primarily from differ-ent measurement methodologies.
Typically, prior art pacers place a series resistor in the output path of the lead current. During the delivery of a pacing stimulus, the voltage drop across the resistor is measured and telemetered out. This technique requires the use of a high preclsion resistor to reduce measurement errors. It also introduces a component whose failure can lead to an un-desirable "no output" condition. Also, such a system is waste-ful of output energy because of the inclusion of the measuring resistor.
SUMMARY OF T~E INVENTION
In contrast to this prior ~rt, the present invention includes a large number (typically 200) of FET transistors operated in parallel to discharge a capacitor through the heart tissue. Pacer lead current is monitored by measuring the current through a small number (typically 2) of these ~, ~' transistors.
The current monitoring function is performed by a current-to-voltage converter coupled to an analog-to-digital converter whlch may make one or more voltage measurements during the output pulse~. The ability to time the measurement with respect to the leading and trailing edges of the output pulse provides flexibility to match the telemetered pacing ;~ current data with operating room measurements thus reducing the confusion that dis~crepancies can cause.

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~a 66742-311 Additionally, the voltage developed by the current-to-voltage converter may be applied through additional circuitry to the gates of the FETS to provide for a constant current output pulse.
In accordance with a broad aspect of the invention ~here is provided, in a pacemaker having output circuit means for generation of stimulation pulses, timing means for triggering said output circuit means to provide said stimulation pulses, and terminal means coupled to said output circuit means for providing said stimulation pulses to said heart~ the improvement wherein:
said output circuit means comprises an output capacitor and swltching means for providing current flow between said output capacitor and said terminal means in response to said timing ~:~ means, said switching means comprising a plurality of parallel connected metal oxide semiconductor output transistors; and :~
wherein said pacemaker further comprises measuring means ,~ coupled to at least one of said output transistors for monitoring the drain to source current of said at least one of said output translstors, when current flows between said output capacitor and said terminal means.
BRIEF D~SCRIPTIO~ OF THE DRAWI~GS
Fig. 1 is a schematic dlag:ram of an illustrative embodiment of the present invention.

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~31~7 Fig. 2 is a schematic diagram oE an alternate embodiment of the present invention.
Fig. 3 is a waveform diagram of a ~ypical pacer output pulse.

DESCRIPTION OF THE PREFEEtRED EMBODIMENT
The circuitry depicted in Figs. 1 and 2 show simplified pacer output circuits wherein like numerals are used to identify like components.
A logic level pace signal generated by pulse generator logic (not shown) is coupled to terminal 12 to initiate an output pulse. The output pulse is coupled to the heart 10 via a conventional lead system which may include a conventional tip electrode and a ca~ or ring electrode.
At the conclusion of the output pulse hown in Fig.
3, the recharging resistor 22 will rec~large the output capacitor 24 through the heart 10.
The control voltage present at terminal 12 is distributed to the gates of the number of monolithic FET
tran istors. These voltage controlled devices divide the output current and share it. Within the constraints oE
die topology and geometry, each FET is identical and therefore share~ the output current equally. One or more o~ t~e~transistors from the array are selected for connection to current monitoring circuitry. ~he current flow in these sampling transistors ref lect the total current delivered by the array. Illustrative circuitry for monitoring current may include an operational amplifier 18 coupled to an analog to digital converter 2~.
In operation, the drain source current is converted to a voltage by the~operational amplifier 18 and resistor 25 (where V out = -i X R). This voltage may be converted to a ~I~ary number by the analog-to-digital converter. The value of the curren~ measurement may be transmitted to a remote or ex~ernal receiver. The external instrument - :., ~ .....

3L 3 ~ 7 converts the telemetered information to a measurement of output current, lead impedance or output pulse energy.
The waveform generated between the output terminal 14 and circuit ground 16 connected to the heart load 10 is depicted in Fig. 3.
The output pulse shown in Fig. 3 will occur as the output capacitor 24 is discharged through the current limiting impedance of the heart 10 and the array of drain source resistance of output transistors. The duration of the output pulse corresponds to the "on" time of the transistors.
The voltage on the capacitor 24 declines expotentially Erom an initial voltage Vl to a final voltage V2 during the "on" time of the transistors. The difference in the voltages i9 proportional to the load present at the output terminal 14.
It is contemplated that the time at which the output current is monitored (tm) will be selected to conform the current mesurement displayed to the physician ~to the value mesured by other equipment. This calibration technique assumes that the~lead impedance is constant, and therefore the concomitant instantaneous output current is linearly proportional to the instantaneous output voltage. In this instance,~the current measurement made when the output voltage is V1 will be higher than the current value when the measurement is made when the output voltage is V2.
It should also be apparent that one could make a current~measurement at both the V1 and V2 voltage~and compute :An arithmetic average output current. Likewise a measurement at Vl would give peak values. A measurement at a fixed t:ime ater Vl ~tm =~90 microseconds) would indicate values similar to commonly available pacing system analyzers.
Additionally, as shown in Fig. 2, the output vol~age oE the operational~amplifier derived from the sampling transistor current flow can also be used as a control voltage to drive the gates of output transistors to provide a constant current output through gate control circuitry 26. In operation, the operational amplifier 18 . : .

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output is biased to a nominal drive voltage corresponding to the desired drain source current for the output transistors shown in the figure as Q1, Q2, Q3, Q4 and QN.
If the current delivered to -the load by the array is low due to increased lead impedance, the control voltage at the output of the operational amplifier will increase the gate voltage to increase the drain/source current of the array transistors thus regulating the output current to the desired level.
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Claims (4)

1. In a pacemaker having output circuit means for generation of stimulation pulses, timing means for triggering said output circuit means to provide said stimulation pulses, and terminal means coupled to said output circuit means for providing said stimulation pulses to said heart, the improvement wherein:
said output circuit means comprises an output capacitor and switching means for providing current flow between said output capacitor and said terminal means in response to said timing means, said switching means comprising a plurality of parallel connected metal oxide semiconductor output transistors; and wherein said pacemaker further comprises measuring means coupled to at least one of said output transistors for monitoring the drain to source current of said at least one of said output transistors, when current flows between said output capacitor and said terminal means.
2. A pacemaker according to claim 1 wherein said measuring means comprises a current to voltage transducer connected between the source of said at least one of said output transistors and a reference potential.
3. A pacemaker according to claim 2 wherein said measuring means further comprises an analog-to-digital converter coupled to a voltage output of said current to voltage transducer whereby the current monitored through said at least one of said output transistors is converted to a number.
4. A pacemaker according to claim 2 or claim 3 wherein a voltage output of said current to voltage transducer is supplied to the gates of said output transistors to regulate the gate voltage of said output transistors to control the drain/source current of said output transistors.
CA000611722A 1988-09-20 1989-09-18 Method and apparatus for measuring the lead current in a pacemaker Expired - Fee Related CA1316987C (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US07/246,600 1988-09-20
US07/246,600 US4949720A (en) 1988-09-20 1988-09-20 Apparatus for measuring the lead current in a pacemaker

Publications (1)

Publication Number Publication Date
CA1316987C true CA1316987C (en) 1993-04-27

Family

ID=22931349

Family Applications (1)

Application Number Title Priority Date Filing Date
CA000611722A Expired - Fee Related CA1316987C (en) 1988-09-20 1989-09-18 Method and apparatus for measuring the lead current in a pacemaker

Country Status (4)

Country Link
US (1) US4949720A (en)
EP (1) EP0360551A3 (en)
JP (1) JPH02116383A (en)
CA (1) CA1316987C (en)

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US5137021A (en) * 1990-11-29 1992-08-11 Medtronic, Inc. Lead current measurement circuit
US5201865A (en) * 1991-10-28 1993-04-13 Medtronic, Inc. Medical lead impedance measurement system
SE9300283D0 (en) * 1993-01-29 1993-01-29 Siemens Elema Ab PROCEDURE AND DEVICE FOR ELECTRODOOD MONITORING BY ELECTRICAL HEART STIMULATORS
SE9602440D0 (en) * 1996-06-20 1996-06-20 Pacesetter Ab Medical apparatus
EP1011801A1 (en) * 1997-01-29 2000-06-28 Cardiac Pacemakers, Inc. Current and temperature compensated voltage reference
US6292050B1 (en) * 1997-01-29 2001-09-18 Cardiac Pacemakers, Inc. Current and temperature compensated voltage reference having improved power supply rejection
US6141585A (en) 1998-05-08 2000-10-31 Intermedics Inc. Implantable cardiac stimulator with electrode-tissue interface characterization
US6620186B2 (en) 2000-05-25 2003-09-16 International Business Machines Corporation Method and system for medical lead impedance test
US8301248B1 (en) 2002-03-06 2012-10-30 Boston Scientific Neuromodulation Corporation Sequenced and simultaneous stimulation for treating congestive heart failure
US20030199938A1 (en) * 2002-04-22 2003-10-23 Karel Smits Precise cardiac lead placement based on impedance measurements
US7089057B2 (en) * 2002-04-26 2006-08-08 Medtronic, Inc. Detection of possible failure of capacitive elements in an implantable medical device
US7539538B2 (en) 2004-05-28 2009-05-26 Boston Science Neuromodulation Corporation Low power loss current digital-to-analog converter used in an implantable pulse generator
US7930028B2 (en) * 2006-10-20 2011-04-19 Biotronik Crm Patent Ag Implantable cardiac device with a shock lead
US8972005B2 (en) 2013-03-12 2015-03-03 Medtronic, Inc. Subthreshold lead impedance measurement for subcutaneous device
US9364673B2 (en) 2013-10-16 2016-06-14 Boston Scientific Neuromodulation Corporation Power supply disconnect current measurement for an implantable medical device
US9956422B2 (en) 2014-04-24 2018-05-01 Medtronic, Inc. Therapy delivery methods and circuits for an implantable medical device
US10625087B2 (en) 2014-04-24 2020-04-21 Medtronic, Inc. Therapy delivery methods and circuits for an implantable medical device
US10159847B2 (en) 2015-05-20 2018-12-25 Medtronic, Inc. Implantable medical devices with active component monitoring
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Also Published As

Publication number Publication date
JPH02116383A (en) 1990-05-01
EP0360551A3 (en) 1992-06-10
US4949720A (en) 1990-08-21
EP0360551A2 (en) 1990-03-28

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