Fault-tolerant elective replacement indication for implantable medical device

• US 5,402,070 A
• Filed: 07/15/1994
• Issued: 03/28/1995
• Est. Priority Date: 06/22/1993
• Status: Expired due to Term

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Starkson, Ross O., Schmidt, Craig L., Markowitz, H. Toby, Shelton, Michael B.
• Primary Examiner(s): MANUEL, GEORGE C
• Application Number:
  US08/275,863
• Time in Patent Office: 256 Days
• Field of Search: 324/426, 324/428, 324/429, 324/430, 324/432, 324/433, 607/29, 607/27
• US Class Current: 324/433
• CPC Class Codes: A61N 1/3708 for power depletion
• CPC Class Codes: A61N 1/378 Electrical supply
• CPC Class Codes: G01R 31/389 Measuring internal impedance, internal conductance or related variables

Abstract

A pacemaker having a fault-tolerant elective replacement indicator (ERI) triggering scheme in which transient excursions of parameters used as criteria for triggering ERI are rejected as triggering events. Periodic assessments of certain indicia of battery depletion are made, and subjected to a long-term low-pass filtering operation in order to reduce the effects of transient excursions of the indicia which result from non-ERI conditions. Over a long period of time (e.g., a day) predetermined threshold values of the indicia of interest must be exceeded a predetermined number of times in order for the device to issue an ERI. In one disclosed embodiment of the invention, the battery'"'"'s loaded terminal voltage and internal impedance are used as indicators of the battery'"'"'s depletion level. Periodically, these values are measured and converted to digital values. The digital values are subjected to a low-pass filtering operation to prevent temporary or transient excursions of the impedance and voltage curves from causing ERI to be triggered. In another disclosed embodiment of the invention, a long term fading average of periodically measured values is maintained. When the measured values are found to fulfill the ERI criteria, assessment of the parameters of interest is performed at an increased rate. While assessments are performed at the increased rate, the continuously updated fading average value must fulfill the ERI triggering criteria at least a predetermined number of times before ERI is triggered. If the fading average fails to fulfill the ERI triggering criteria the required number of times, assessments are resumed at the first, slower periodic rate.

Claims

1. An apparatus for indicating when a battery'"'"'s depletion level exceeds a predetermined maximum, comprising:

a first monitoring circuit, coupled to said battery and adapted to periodically assess a first condition of said battery known to reflect said battery'"'"'s depletion level, and to periodically generate at least one first condition signal indicative of said first condition;

a first comparison circuit, coupled to said first monitoring circuit and responsive to said at least one first condition signal to assert a first comparison signal when said first condition signal indicates that said first condition reflects a level of battery depletion greater than said predetermined maximum;

a second monitoring circuit, coupled to said battery and to said first comparison circuit, said second monitoring circuit responsive to said assertion of said first comparison signal to assess a second condition of said battery known to reflect said battery'"'"'s depletion level, and to generate at least one second condition signal indicative of said second condition;

a second comparison circuit, coupled to said second monitoring circuit and responsive to said at least one second condition signal to assert a second comparison signal when said second condition signal indicates that said second condition reflects a level of battery depletion greather than said predetermined maximum;

an accumulator circuit, coupled to said second comparison circuit and responsive to assertion of said second comparison signal to increment an accumulator value;

a depletion indicator circuit, coupled to said accumulator circuit and adapted to assert a depletion indicator signal when said accumulator value exceeds a predetermined maximum, said depletion indicator circuit further adapted to periodically reset said accumulator value.

2. An apparatus in accordance with claim 1, wherein said first condition is said battery'"'"'s loaded terminal voltage and wherein said second condition is said battery'"'"'s internal impedance.

3. A method of indicating when a battery has reached a predetermined level of depletion, comprising the steps of:

(a) assessing at least one condition of said battery known to reflect said battery'"'"'s depletion level;

(b) incrementing an accumulator value when said at least one condition assessed in step (a) reflects a depletion level greater than said predetermined level of depletion;

(c) repeating steps (a) and (b) once each time a first predetermined time interval elapses;

(d) resetting said accumulator value to zero once each time a second predetermined time interval, greater than said first predetermined time interval, elapses;

(e) asserting a depletion indicator signal when said accumulator value exceeds a predetermined maximum level.

4. A method in accordance with claim 3, wherein said first predetermined time interval is three hours and wherein said second predetermined time interval is twenty-four hours.

5. A method in accordance with claim 4, wherein said step of incrementing comprises adding sixty-four to said accumulator value and wherein said predetermined maximum accumulator value level is 255.

6. A method in accordance with claim 5, wherein said at least one condition includes said battery'"'"'s loaded terminal voltage and said battery'"'"'s internal impedance level.

7. An apparatus for indicating when a battery'"'"'s depletion level exceeds a predetermined maximum, comprising:

a monitoring circuit, coupled to said battery and periodically obtaining a measured value of a characteristic of said battery, where said characteristic is known to reflect said battery'"'"'s depletion level, and periodically generating signals corresponding to said measured values;

a low-pass filter circuit, coupled to said monitoring circuit, receiving said measured values, and digitally filtering them to yield filtered values;

an indicator circuit, coupled to said low-pass filter circuit, responsive to assert an indicator signal when said filtered value exceeds a predetermined maximum value.

8. An apparatus in accordance with claim 7, wherein said characteristic of said battery is said battery'"'"'s loaded terminal voltage.

9. An apparatus in accordance with claim 7, wherein said characteristic of said battery is said battery'"'"'s internal impedance.

10. An apparatus for indicating when a battery'"'"'s depletion level exceeds a predetermined maximum, comprising:

a depletion monitoring circuit, coupled to said battery and adapted to periodically generate a digital output value reflecting a level of a characteristic of said battery known to reflect said battery'"'"'s depletion level;

a low-pass filtering circuit, coupled to said depletion monitoring circuit and adapted to perform a digital low-pass filtering operation on a plurality of said periodically generated digital output values to produce a filtered value;

an indicator circuit, coupled to said low-pass filter circuit and adapted to assert an indicator signal when said filtered value exceeds a predetermined threshold.

11. An apparatus for indicating when a battery'"'"'s depletion level exceeds a predetermined maximum, comprising:

a voltage measurement circuit, coupled to said battery and adapted to periodically measure said battery'"'"'s loaded terminal voltage and to periodically generate a first digital output value;

a first comparator circuit, coupled to said voltage measurement circuit and adapted to assert a first comparator signal when said periodically generated first digital output value is below a predetermined minimum voltage;

an impedance measurement circuit, coupled to said battery and to said first comparator circuit, said impedance measurement circuit responsive to said assertion of said first comparator signal to measure said battery'"'"'s internal impedance, and to generate a second digital output value corresponding to said measured battery loaded terminal voltage;

a second comparator circuit, coupled to said impedance measurement circuit and adapted to assert a second comparator signal when said second digital output value exceeds a predetermined maximum value;

an accumulator circuit, coupled to said second comparator circuit and responsive to said assertion of said second comparator signal to increment an accumulator value;

an indicator circuit, coupled to said accumulator circuit and adapted to assert an indicator signal when said accumulator value exceeds a predetermined maximum.

12. An apparatus for indicating when a battery reaches a predetermined level of depletion, comprising:

a voltage measurement circuit, coupled to said battery and adapted to generate voltage signals representing a measured value of said battery'"'"'s loaded terminal voltage once each time a first predetermined time interval elapses;

an averaging circuit, coupled to said voltage measurement circuit and responsive to said generation of said voltage signals to update a running average of measured loaded terminal values;

a comparison circuit, coupled to said voltage measurement circuit and adapted to assert a first threshold signal when said voltage signals represent a loaded terminal voltage below a predetermined minimum voltage;

said voltage measurement circuit being responsive to assertion of said first threshold signal to generate voltage signals representing a measured value of said battery'"'"'s loaded terminal voltage once during each of a plurality of second predetermined time intervals, wherein said second predetermined time intervals are shorter than said first predetermined time intervals;

said comparison circuit being further adapted to assert a second threshold signal during one of said plurality second time intervals when said running average of measured loaded terminal values is below said predetermined minimum voltage.

13. A method of indicating when a battery reaches a predetermined depletion level, said method comprising the following steps:

(a) measuring said battery'"'"'s loaded terminal voltage;

(b) after each measurement in step (a), updating a running average of loaded terminal voltage measurements;

(c) comparing each measurement from step (a) to a predetermined minimum loaded terminal voltage;

(d) repeating steps (a) through (c) at a first periodic rate until said comparison step (c) above indicates that said measurement from step (a) is below said predetermined minimum voltage;

(e) repeating steps (a) and (b) after said comparison step (c) above indicates that said measurement from step (a) is below said predetermined minimum voltage;

(f) comparing said updated running average to said predetermined minimum loaded terminal voltage;

(g) asserting an indicator signal when said comparison in step (f) above indicates that said running average is below said predetermined minimum loaded terminal voltage;

(h) repeating steps (e) through (g) above a predetermined number of times at a second periodic rate, faster than said first periodic rate;

(i) after said step (h) of repeating a predetermined number of times, repeating step (d).