Method for automatically adjusting the sensitivity of cardiac sense amplifiers

• US 6,112,119 A
• Filed: 10/27/1997
• Issued: 08/29/2000
• Est. Priority Date: 10/27/1997
• Status: Expired due to Term

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Hoffman, David W., Zhou, Ren, Bright, Kevin L., Schuelke, Robert J., Anderson, Russell E., Reinke, James D., Busacker, James W., De La Riva, Virginia
• Primary Examiner(s): Kamm, William E.
• Assistant Examiner(s): EVANISKO, GEORGE ROBERT
• Application Number:
  US08/957,908
• Time in Patent Office: 1,037 Days
• Field of Search: 607/9
• US Class Current: 607/9
• CPC Class Codes: A61N 1/3704 Circuits specially adapted therefor, e.g. for sensitivity control

Abstract

Two mechanisms work together to adjust two variables independently so that the gain of an input amplifier used to find physiologic signals in an implantable medical device can be automatically adjusted to enhance the signal to noise ratio of the electrical input to said amplifier. The first determines whether there has been too long a time between senses found in the body'"'"'s electrical input to the amplifier and if true, and no other conditions override that consideration, it adjusts the value of a long term average parameter which is used as a referent parameter to adjust the actual parameter that is used as the referent for making either a threshold level adjustment or gain change, depending on the structure of the particular design'"'"'s circuitry and/or software. One set of adjustments to the gain referent parameter depends on the relative size of the long term average parameter and the gain referent parameter. The mechanisms preferably employ counting registers, the one which adjusts the gain referent having a process to weight input signals such that those of different types get different weights, and the other mechanism only accepting certain of the input signals. Input signals to the two mechanisms can be either physiologically based or from stimulation pulses issued from an implanted device, such as, for example a pacemaker.

Claims

1. A method for automatically adjusting a present gain/threshold parameter value of an input sense amplifier for use in an implantable medical device which operates said input sense amplifier to receive and, responsive to electrical activity of sufficient intensity that can be called sufficient senses, determine senses of event specific pulses of electrical activity native to a body into which said device is designed for implantation, and from said sufficient senses said amplifier produces a sense amplifier output, said method comprising:

determining adjustments to be made to said input sense amplifier present gain/threshold parameter value based on current values of at least two indicator means for monitoring sense amplifier output, determining a condition and producing a signal output,

keeping track of said current values for said at least two indicator means, a first of said at least two being a Long Time Few Sense (LTFS) indicator means for monitoring sense amplifier output, determining a condition relating to LTFS, and producing a signal output from said LTFS condition determination and a second of said at least two being a Long Term Average (LTA) indicator means for monitoring sense amplifier output, determining a condition relating to LTA, and producing a signal output from said LTA condition determination, said keeping track steps comprising;

tracking said at least two indicator means from each input sense amplifier output by,

operating said LTFS indicator means to (a) monitor said input sense amplifier output to determine whether said LTFS indicator means has received an insufficient number of senses in a tracked period immediately preceding any given sense amplifier output and (b) if said LTFS indicator means has determined a condition of having received such insufficient number of senses, generating an LTFS output signal representative of said condition,

operating said LTA indicator means, to monitor said input sense amplifier for a valid sense output signal, and to count and maintain an LTA count of said valid sense output signals, wherein when a sufficient number of said valid senses has been counted said LTA count is deemed sufficient and, then resetting the LTA count and adjusting an LTA signal value based on comparing a last LTA signal value to said present sense amplifier gain/threshold parameter value, but only adjusting said LTA signal value if there is an LTFS output signal present when said LTA count is deemed sufficient.

2. The method of claim 1 wherein the implantable medical device monitors reversion status and generates a signal on determination of reversion status, the step of operating said LTFS indicator means additionally to perform the step of determining whether there is a reversion status indicated by checking for said signal of determination of reversion status before generating said LTFS output, but only performing the step of generating said LTFS output signal on a determination that such a condition of reversion status exists.

3. The method of claim 2 wherein the implantable medical device monitors paced and sensed events and wherein in operating said LTFS indicator means before said LTFS indicator means generates an LTFS signal, manipulating a value stored in a Paced Event Accumulator (a PEA value) by adding to said PEA value a first incremental value for each paced event, and subtracting a second incremental value larger than said first incremental value for each sensed event to said stored PEA value, wherein said stored value moves further on a sensed than a paced event, but only when said stored PEA value reaches a preset value, does said method generate an LTFS signal.

4. The method of claim 3 wherein the implantable medical device monitors noise reversion status and generates a noise reversion status indicated signal on determination of reversion status, and wherein before said LTFS indicator means generates an LTFS signal, checking for said noise reversion status signal, but only after performing this step of checking, issuing an LTFS signal if there is said noise reversion status indicated signal.

5. The method of claim 4 wherein if there is a condition of noise reversion said LTA indicator means will adjust the present gain/threshold parameter value to increase sensitivity, but only if the LTA value is higher than the present gain/threshold parameter value, and if there is no condition of noise reversion said LTA indicator means will adjust the value of the present gain/threshold parameter value to decrease sensitivity but only if the LTA value is lower than the present gain/threshold parameter value.

6. The method of claim 1 wherein the implantable medical device comprises an Implantable Pulse Generator operating in VDD pacing status and wherein said VDD status comprises sensing of ventricular and atrial events, wherein the step of operating said LTFS indicator means additionally performs the step of determining whether there have been two ventricular events sensed with no intervening atrial sense, but said LTFS indicator means only generates said LTFS output signal on the determination that there has been two ventricular events sensed with no intervening atrial sense.

7. The method of any of claims 1-5 wherein a third one of said at least two indicator means, said third one being a Sense Margin Indicator Accumulator (SMIACC) indicator means for holding an adjustable SMIACC value in a SMIACC memory register means for holding said adjustable SMIACC value which performs steps of monitoring and adjusting said input amplifier output and

if one of said sense amplifier outputs is above a predetermined but adjustable margin value, said third one adjusts said SMIACC value stored in said SMIACC register memory means in a first direction, and wherein these steps of monitoring and adjusting comprise,

if said one of said outputs is below a lower second predetermined level, then adjusting said SMIACC value stored in said register memory means in a second and opposite direction, and

operating said SMIACC indicator means to monitor said SMIACC value wherein;

when said SMIACC value is below a predetermined low value producing an output signal representative of underflow from said third indicator means and,

when said SMIACC value is above a predetermined high value above said register level value, then producing an output signal indicative of overflow, and

at such time as an output value representing underflow or overflow is produced by said SMIACC indicator means, adjusting the input sense amplifier gain/threshold parameter value, upward or downward on the basis of whether there was an underflow or overflow value produced by said third indicator.

8. The method of any of claims 1-5 wherein a third one of said at least two indicator means, said third one being a Sense Margin Indicator Accumulator (SMIACC) indicator means for holding an adjustable SMIACC value in a SMIACC memory register means for holding said adjustable SMIACC value which performs steps of monitoring and adjusting said input amplifier output and

if one of said sense amplifier outputs is above a predetermined but adjustable margin value, said third one adjusts said SMIACC value stored in said SMIACC register memory means in a first direction, and wherein the steps of monitoring comprise,

if said one of said outputs is below a lower second predetermined level, then adjusting said SMIACC value stored in said register means in a second and opposite direction, and

operating said third one to monitor said SMIACC value wherein;

when said SMIACC value is below a predetermined low value producing an output signal representative of underflow from said third one and,

when said SMIACC value is above a predetermined high value above said register level value, then producing an output signal indicative of overflow, and

at such time as an output value representing underflow or overflow is produced by said third one, adjusting the input sense amplifier gain/threshold parameter value, upward or downward on the basis of whether there was an underflow or overflow value produced by said third indicator, and wherein in operating said third one to monitor said SMIACC value, performing a step of determining a range of amplitude for each sense and assigning a weight value for each input amplifier output based on said range, then adding a first predetermined change increment value to said SMIACC value for each input amplifier output above a midrange of values, and subtracting a second predetermined change increment value from said SMIACC value for each input amplifier output in the midrange of values but above a center of said midrange, and subtracting a third predetermined change increment value from said SMIACC value for each input amplifier output that is below said midrange of values, and adding a fourth predetermined change increment value to said SMIACC value for each input amplifier output that is within said midrange but below a center of said midrange.

9. The method of any of claims 1-5 wherein a third one of said at least two indicator means, said third one being a Sense Margin Indicator Accumulator (SMIACC) indicator means for holding an adjustable SMIACC value in a SMIACC memory register means for holding said adjustable SMIACC value which performs steps of monitoring and adjusting said input amplifier output and

if one of said sense amplifier outputs is above a predetermined but adjustable margin value, said third one adjusts said SMIACC value stored in said SMIACC register memory means in a first direction, and wherein the steps of monitoring comprise,

if said one of said outputs is below a lower second predetermined level, then adjusting said SMIACC value stored in said register means in a second and opposite direction, and

operating said third one to monitor said SMIACC value wherein;

when said SMIACC value is below a predetermined low value producing an output signal representative of underflow from said third one and,

when said SMIACC value is above a predetermined high value above said register level value, then producing an output signal indicative of overflow, and

at such time as an output value representing underflow or overflow is produced by said third one, adjusting the input sense amplifier gain/threshold parameter value, upward or downward on the basis of whether there was an underflow or overflow value produced by said third indicator, and wherein in operating said third one to monitor said SMIACC value, performing a step of determining a range of amplitude for each sense and assigning a weight value for each input amplifier output based on said range, then adding a first predetermined change increment value to said SMIACC value for each input amplifier output above a midrange of values, and subtracting a second predetermined change increment value from said SMIACC value for each input amplifier output in the midrange of values but above a center of said midrange, and subtracting a third predetermined change increment value from said SMIACC value for each input amplifier output that is below said midrange of values, and adding a fourth predetermined change increment value to said SMIACC value for each input amplifier output that is within said midrange but below a center of said midrange, and also

wherein a step of determining whether a each input amplifier output is of too long a duration before adding or subtracting any of said increment values to said SMIACC value, and if said sense is of too long duration aborting the step of determining a range of amplitude for each sense.

10. The method of any of claims 1-5 wherein a third one of said at least two indicator means, said third one being a Sense Margin Indicator Accumulator (SMIACC) indicator means for holding an adjustable SMIACC value in a SMIACC memory register means for holding said adjustable SMIACC value which performs steps of monitoring and adjusting said input amplifier output and

if one of said sense amplifier outputs is above a predetermined but adjustable margin value, said third one adjusts said SMIACC value stored in said SMIACC register memory means in a first direction, and wherein the steps of monitoring comprise,

if said one of said outputs is below a lower second predetermined level, then adjusting said SMIACC value stored in said register means in a second and opposite direction, and

operating said third one to monitor said SMIACC value wherein;

when said SMIACC value is below a predetermined low value producing an output signal representative of underflow from said third one and,

when said SMIACC value is above a predetermined high value above said register level value, then producing an output signal indicative of overflow, and

at such time as an output value representing underflow or overflow is produced by said third one, adjusting the input sense amplifier gain/threshold parameter value, upward or downward on the basis of whether there was an underflow or overflow value produced by said third indicator, and wherein in operating said third one to monitor said SMIACC value, performing a step of determining a range of amplitude for each sense and assigning a weight value for each input amplifier output based on said range, then adding a first predetermined change increment value to said SMIACC value for each input amplifier output above a midrange of values, and subtracting a second predetermined change increment value from said SMIACC value for each input amplifier output in the midrange of values but above a center of said midrange, and subtracting a third predetermined change increment value from said SMIACC value for each input amplifier output that is below said midrange of values, and adding a fourth predetermined change increment value to said SMIACC value for each input amplifier output that is within said midrange but below a center of said midrange, and

wherein a step of determining valid input amplifier output for use by said LTA indicator means precedes any addition to said LTA count, wherein said step of determining valid input amplifier output for use by said LTA indicator means comprises determining that an input amplifier output is not a Premature Ventricular Contraction (PVC).

11. The method of any of claims 1-5 wherein a third one of said at least two indicator means, said third one being a Sense Margin Indicator Accumulator (SMIACC) indicator means for holding an adjustable SMIACC value in a SMIACC memory register means for holding said adjustable SMIACC value which performs steps of monitoring and adjusting said input amplifier output and

if one of said sense amplifier outputs is above a predetermined but adjustable margin value, said third one adjusts said SMIACC value stored in said SMIACC register memory means in a first direction, and wherein the steps of monitoring comprise,

if said one of said outputs is below a lower second predetermined level, then adjusting said SMIACC value stored in said register means in a second and opposite direction, and

operating said third one to monitor said SMIACC value wherein;

when said SMIACC value is below a predetermined low value producing an output signal representative of underflow from said third one and,

when said SMIACC value is above a predetermined high value above said register level value, then producing an output signal indicative of overflow, and

at such time as an output value representing underflow or overflow is produced by said third one, adjusting the input sense amplifier gain/threshold parameter value, upward or downward on the basis of whether there was an underflow or overflow value produced by said third indicator, and wherein in operating said third one to monitor said SMIACC value, performing a step of determining a range of amplitude for each sense and assigning a weight value for each input amplifier output based on said range, then adding a first predetermined change increment value to said SMIACC value for each input amplifier output above a midrange of values, and subtracting a second predetermined change increment value from said SMIACC value for each input amplifier output in the midrange of values but above a center of said midrange, and subtracting a third predetermined change increment value from said SMIACC value for each input amplifier output that is below said midrange of values, and adding a fourth predetermined change increment value to said SMIACC value for each input amplifier output that is within said midrange but below a center of said midrange, and

wherein a step of determining valid input amplifier output for use by said LTA indicator means precedes any addition to said LTA count, wherein said step of determining valid input amplifier output for use by said LTA indicator means comprises determining that an input amplifier output is not outside of a hardware overrange.

12. An implantable medical device comprising an input sense amplifier circuit with an automatic adjustment circuit for automatically adjusting a present gain/threshold parameter value of said input sense amplifier wherein said medical device operates said input sense amplifier to receive and evaluate senses of event specific pulses of electrical activity native to a body into which said device is adapted for implantation and to provide an output sense signal representative of said pulses of electrical activity as output subject to said present gain/threshold parameter value if there is sufficient electrical activity, said automatic adjustment circuit comprising:

a recordkeeping circuit for keeping track of at least two indicator values, a first one of said two indicator values being a Long Time Few Sense (LTFS) indicator, and the other of said two indicator values being a Long Term Average (LTA) indicator, and wherein said indicator values are required for adjustments to be made to said gain/threshold parameter value, said recordkeeping circuit comprising;

a plurality of indicator tracking circuits, comprising at least a LTFS indicator tracking circuit and a LTA tracking circuit, each tracking circuit for tracking one of said at least two indicator values based on each input sense amplifier output, wherein,

said LTFS indicator tracking circuit is connected to receive output signals from said input sense amplifier circuit and comprises monitor circuit means for monitoring said input sense amplifier output to determine whether said LTFS indicator tracking circuit has received a number predetermined to be an insufficient number of output signals from said input sense amplifier circuit within a predetermined tracked period immediately preceding any given sense output signal from said input sense amplifier, said LTFS indicator tracking circuit further comprising first indicator output signal means for generating an LTFS output signal when said LTFS indicator tracking circuit has monitored a condition of having received said predetermined to be insufficient number of output sense signals from said input sense amplifier circuit within said predetermined tracking period,

said LTA tracking circuit for monitoring said input sense amplifier output for valid sense output signals, comprising LTA signal value memory means for maintaining an LTA signal value, counter means for maintaining a count of said valid sense output signals, count sufficiency determining means for determining when said count becomes sufficient, and an LTA control circuit means for resetting the count and adjusting said LTA signal value and comprising a comparison circuit, wherein said adjusting of said LTA signal value employs said comparison circuit to compare a last LTA signal value to a current sense amplifier present gain/threshold parameter value if there is said LTFS signal present when said count becomes sufficient.

13. The implantable medical device of claim 12 wherein said LTFS tracking indicator circuit additionally comprises reversion status determination circuit means for determining whether there is a reversion status indicated before said LTFS tracking indicator circuit can generate said LTFS output, and a blocking circuit that only allows said LTFS output to be generated on a determination that a reversion status condition exists.

14. The implantable medical device of claim 12 comprising mode status circuitry for configuring said implantable medical device to be operable in a set of pacing status modes, and when said implantable medical device is operating in one such mode called VDD pacing status, and further comprising device circuitry for interpreting said input sense amplifier output signals for distinguishing ventricular from atrial senses in said output signals, wherein said amplifier output is connected to device circuitry and wherein said device circuitry has an output signal indicating such distinction and wherein said distinction indicating output is provided to said LTFS indicator tracking circuit which LTFS indicator tracking circuit additionally comprises timing and interpretive means for receiving said distinction indicating output, and for determining whether there have been two ventricular senses with no intervening atrial sense, and wherein said LTFS indicator tracking circuit can only generate said LTFS output signal on the determination that there have been two ventricular events without an intervening atrial event.

15. The implantable medical device of claim 13 further comprising monitoring circuitry for monitoring electrical events and pace/sense circuitry for generating a pace/sense signal indication of whether an electrical event monitored by said monitoring circuitry was paced or sensed and for providing signals indicative thereof to said LTFS indicator tracking circuit, and wherein said LTFS indicator tracking circuit further comprises a counting and storage circuit means for maintaining and storing a Paced Event Accumulator value by adding a pace value for each paced event, and subtracting a sense value for each sensed event to said stored PEA value stored in said counting and storage circuit means, wherein said stored value is changed more on a sensed than on a paced event, and wherein said LTFS indicator tracking circuit cannot produce said LTFS output signal until said stored PEA value reaches a preset value.

16. The implantable medical device of claim 15 further having a reversion circuit for determining a condition of noise reversion and producing a signal representative of such noise reversion condition and said LTFS tracking indicator circuit further comprising checking circuit means for checking for said noise reversion signal before generating said LTFS signal.

17. The implantable medical device of claim 16 wherein said LTA tracking circuit further comprises adjustment circuit means for

a) adjusting the value of the sense amplifier present gain/threshold parameter value to increase sensitivity, but only if said LTA signal value is higher than said gain/threshold parameter value, and

b) adjusting the value of the sense amplifier present gain/threshold parameter value to decrease sensitivity if there is no condition of noise reversion but only if the LTA value is lower than the present gain/threshold parameter value.

18. The implantable medical device of any of claims 12-17 further comprising a third tracking indicator circuit means of said plurality of indicator tracking circuits being a Sense Margin Indicator Accumulator (SMIACC) circuit, connected to receive said input sense amplifier output sense signals and to monitor them, and to adjust said input amplifier gain/threshold parameter value, said SMIACC circuit comprising;

within range determining circuit means for determining into which category, above, below and within an output sense signal should fit, relative to a predetermined adjustable amplitude range, and said range determining circuit means for producing a margin indicator signal representing a value indicative of this relation of said amplifier output signal to said range, and

a SMIACC register circuit means for storing a cumulative SMIACC value of all said margin indicator values wherein said SMIACC register circuit means has a predetermined upper value and a predetermined lower value and adjusts said cumulative SMIACC value in a first direction if said one of said sense amplifier outputs is below said range, then adjusting said cumulative SMIACC value in a second and opposite direction if said output signal is above said range, and

an indicator circuit means for interpreting said cumulative SMIACC value and if said cumulative SMIACC value achieves a predetermined upper limit value, producing a SMIACC overflow signal, and if said SMIACC value falls to a predetermined lower limit value, then producing a SMIACC underflow signal.

19. The implantable medical device of any of claims 12-17 further comprising a third tracking indicator circuit means of said plurality of indicator tracking circuits being a Sense Margin Indicator Accumulator (SMIACC) circuit, connected to receive said input sense amplifier output sense signals and to monitor them, and to adjust said input amplifier gain/threshold parameter value, said SMIACC circuit comprising;

within range determining circuit means for determining into which category, above, below and within an output sense signal should fit, relative to a predetermined adjustable amplitude range, and said range determining circuit means for producing a margin indicator signal representing a value indicative of this relation of said amplifier output signal to said range, and

a SMIACC register circuit means for storing a cumulative SMIACC value of all said margin indicator values wherein said SMIACC register circuit means has a predetermined upper value and a predetermined lower value and adjusts said cumulative SMIACC value in a first direction if said one of said sense amplifier outputs is below said range, then adjusting said cumulative SMIACC value in a second and opposite direction if said output signal is above said range, and

an indicator circuit means for interpreting said cumulative SMIACC value and if said cumulative SMIACC value achieves a predetermined upper limit value, producing a SMIACC overflow signal, and if said SMIACC value falls to a predetermined lower limit value, then producing a SMIACC underflow signal and further comprising

a weighting circuit means for providing weighting values wherein when said output from said sense amplifier is within said range, but above a selected point in said range, said weighting circuit means provides a first weighting value and adds said first weighting value to said cumulative SMIACC value and provides and adds to said cumulative SMIACC value a second weighting value when said output from said sense amplifier is above and outside said range, and provides and adds to said cumulative SMIACC value a third weighting value when said output from said sense amplifier is within said range, and below said selected point in said range and provides and adds to said cumulative SMIACC value a fourth weighting value is outside and below said range.

20. The implantable medical device of any of claims 12-17 further comprising a third tracking indicator circuit means of said plurality of indicator tracking circuits being a Sense Margin Indicator Accumulator (SMIACC) circuit, connected to receive said input sense amplifier output sense signals and to monitor them, and to adjust said input amplifier gain/threshold parameter value, said SMIACC circuit comprising;

within range determining circuit means for determining into which category, above, below and within an output sense signal should fit, relative to a predetermined adjustable amplitude range, and said range determining circuit means for producing a margin indicator signal representing a value indicative of this relation of said amplifier output signal to said range, and

a SMIACC register circuit means for storing a cumulative SMIACC value of all said margin indicator values wherein said SMIACC register circuit means has a predetermined upper value and a predetermined lower value and adjusts said cumulative SMIACC value in a first direction if said one of said sense amplifier outputs is below said range, then adjusting said cumulative SMIACC value in a second and opposite direction if said output signal is above said range, and

an indicator circuit means for interpreting said cumulative SMIACC value and if said cumulative SMIACC value achieves a predetermined upper limit value, producing a SMIACC overflow signal, and if said SMIACC value falls to a predetermined lower limit value, then producing a SMIACC underflow signal and further comprising

a weighting circuit means for providing weighting values wherein when said output from said sense amplifier is within said range, but above a selected point in said range, said weighting circuit means provides a first weighting value and adds said first weighting value to said cumulative SMIACC value and provides and adds to said cumulative SMIACC value a second weighting value when said output from said sense amplifier is above and outside said range, and provides and adds to said cumulative SMIACC value a third weighting value when said output from said sense amplifier is within said range, and below said selected point in said range and provides and adds to said cumulative SMIACC value a fourth weighting value is outside and below said range and still further comprising

means for determining a too long sense for determining whether a sense is of too long a duration before accepting any sense output for said manipulating said cumulative SMIACC value.

21. The implantable medical device of any of claims 12-17 further comprising a third tracking indicator circuit means of said plurality of indicator tracking circuits being a Sense Margin Indicator Accumulator (SMIACC) circuit, connected to receive said input sense amplifier output sense signals and to monitor them, and to adjust said input amplifier gain/threshold parameter value, said SMIACC circuit comprising;

within range determining circuit means for determining into which category, above, below and within an output sense signal should fit, relative to a predetermined adjustable amplitude range, and said range determining circuit means for producing a margin indicator signal representing a value indicative of this relation of said amplifier output signal to said range, and

a SMIACC register circuit means for storing a cumulative SMIACC value of all said margin indicator values wherein said SMIACC register circuit means has a predetermined upper value and a predetermined lower value and adjusts said cumulative SMIACC value in a first direction if said one of said sense amplifier outputs is below said range, then adjusting said cumulative SMIACC value in a second and opposite direction if said output signal is above said range, and

an indicator circuit means for interpreting said cumulative SMIACC value and if said cumulative SMIACC value achieves a predetermined upper limit value, producing a SMIACC overflow signal, and if said SMIACC value falls to a predetermined lower limit value, then producing a SMIACC underflow signal and further comprising

a weighting circuit means for providing weighting values wherein when said output from said sense amplifier is within said range, but above a selected point in said range, said weighting circuit means provides a first weighting value and adds said first weighting value to said cumulative SMIACC value and provides and adds to said cumulative SMIACC value a second weighting value when said output from said sense amplifier is above and outside said range, and provides and adds to said cumulative SMIACC value a third weighting value when said output from said sense amplifier is within said range, and below said selected point in said range and provides and adds to said cumulative SMIACC value a fourth weighting value is outside and below said range and still further comprising

means for determining a too long sense for determining whether a sense is of too long a duration before accepting any sense output for said manipulating said cumulative SMIACC value

valid sense determining means for determining valid senses and to exclude invalid senses from use by LTA tracking circuit for addition to said LTA count wherein a step of determining valid senses for use by said LTA precedes any addition to said LTA count, wherein said step comprises determining that a sense is not outside of a hardware overrange.