Implantable medical devices using heuristic filtering in cardiac event detection

• US 20080045850A1
• Filed: 08/01/2006
• Published: 02/21/2008
• Est. Priority Date: 08/01/2006
• Status: Active Grant

• Current Assignee: Cameron Incorporated (Schlumberger NV)
• Original Assignee: Cameron Incorporated (Schlumberger NV)
• Inventors: Phillips, James William
• Granted Patent: US 7,623,913 B2
• Time in Patent Office: Days
• US Class Current: 600/509
• CPC Class Codes: A61B 5/316 Modalities, i.e. specific diagnostic methods
• CPC Class Codes: A61B 5/346 Analysis of electrocardiograms
• CPC Class Codes: A61N 1/37 Monitoring; Protecting

Abstract

Methods for performing cardiac signal analysis in an implanted medical device, and devices configured to perform illustrative methods of cardiac signal analysis. A cardiac signal is captured by an implanted device using implanted electrodes and, during at least certain conditions, the cardiac signal undergoes heuristic filtering. In some embodiments, heuristic filtering is achieved by modifying a signal or value that is used as an indicator of received signal amplitude. In an illustrative example, the heuristic filtering includes periodically incrementing or decrementing the signal or value toward a desired quiescent point, where the heuristic filter period is significantly longer than the sampling period for the signal itself. In another illustrative example, the heuristic filter frequency can be adjusted dynamically to keep the signal average near the desired quiescent point.

Claims

1. A method of cardiac signal analysis comprising:

capturing a cardiac signal using implanted electrodes; and

applying a heuristic filter to the captured cardiac signal.

2. The method of claim 1, wherein the step of applying a heuristic filter includes:

applying the heuristic filter at a first, higher frequency during a first period of time; and

applying the heuristic filter at a second, lower frequency during a second period of time.

3. The method of claim 1, wherein the heuristic filter is a dynamic heuristic filter having a frequency that changes in response to an observed variable.

4. An implantable cardiac stimulus device comprising implantable electrodes and operational circuitry configured to perform the method of claim 2.

5. The method of claim 1, wherein the heuristic filter is a dynamic heuristic filter having a frequency, the frequency of the heuristic filter being varied as follows:

an average of a variable related to the amplitude of the captured cardiac signal is calculated;

if the average is relatively closer to a predetermined quiescent point for the variable, the frequency is lowered; and

if the average is relatively farther from a predetermined quiescent point for the variable, the frequency is raised.

6. The method of claim 1, wherein the heuristic filter is a dynamic heuristic filter having a frequency, the frequency of the heuristic filter being varied as follows:

an average of a variable related to the amplitude of the captured cardiac signal is calculated; and

the average is compared to a first threshold the variable and, if the first threshold is exceeded, the frequency is increased.

7. The method of claim 1, wherein the heuristic filter is a dynamic heuristic filter having a frequency, the frequency of the heuristic filter being varied as follows:

an average of a variable related to the amplitude of the captured cardiac signal is calculated; and

the average is compared to a first threshold and, if the first threshold is not exceeded, the frequency is reduced.

8. The method of claim 1, wherein the heuristic filter is a dynamic heuristic filter having a frequency, the frequency of the heuristic filter being varied as follows:

an average of a variable related to the amplitude of the captured cardiac signal is calculated;

the average is compared to a first threshold and, if the first threshold is not exceeded, the frequency is reduced; and

the average is compared to a second threshold and, if the second threshold is exceeded, the frequency is increased.

9. The method of claim 1, wherein:

the captured cardiac signal is sampled and converted to digital value stored on an accumulator;

the heuristic filter is applied by modifying a value stored on the accumulator at periodic intervals.

10. The method of claim 9, wherein the value stored on the accumulator is modified by:

observing at least one value or an average of several values stored on the accumulator;

if the at least one value or an average of several values is greater than a desired quiescent point, the value stored on the accumulator is decreased; and

if the at least one value or an average of several values is less than a desired quiescent point, the value stored on the accumulator is increased.

11. The method of claim 1, wherein:

the captured cardiac signal is sampled and converted to digital form;

the heuristic filter is applied by generating a heuristic offset and adding the heuristic offset to the digitally converted cardiac signal to generate an adjusted digital representation of the captured cardiac signal; and

the heuristic offset is generated by observation a value of at least one sample of the adjusted digital representation.

12. The method of claim 1, wherein the heuristic filter is applied in the analog domain by the use of an integrator configured to estimate an average amplitude of a signal related to the amplitude of the captured cardiac signal, coupled to a comparator configured to compare a voltage on the integrator to a reference voltage, the comparator being coupled to charging/discharging circuitry that manipulates a signal that is summed with a signal received from an amplifier that amplifies the captured cardiac signal, the integrator, comparator, and charging/discharging circuitry forming at least part of a feedback loop.

13. An implantable cardiac stimulus device comprising implantable electrodes and operational circuitry configured to perform the method of claim 1.

14. A method of operating an implantable cardiac stimulus device comprising:

delivering a stimulus to the heart of a patient;

for a period of time following stimulus delivery, performing the method of claim 1 until a cardiac event is detected.

15. The method of claim 14, wherein the cardiac event is detected by comparing a received cardiac signal to a detection threshold profile until the detection threshold profile is crossed by the received cardiac signal.

16. A method of cardiac signal analysis comprising:

receiving a signal representative of cardiac electrical activity following delivery of cardiac stimulus; and

converting the signal to a form suitable for analysis;

wherein the converting step includes modifying the signal by the use of a heuristic filter.

17. A method of cardiac signal analysis comprising:

capturing a cardiac signal between implanted electrodes;

conditioning the cardiac signal for use in analysis, including sampling the cardiac signal;

creating an adjusted signal, the adjusted signal representing a series of samples of the conditioned cardiac signal added to an offset; and

periodically;

a) calculating an average of the adjusted signal over a predetermined period of time; and

b) modifying the offset depending upon whether the average of the adjusted signal is greater than or less than a predetermined value.

18. A method of treating a patient using an implanted cardiac stimulus device system, the method comprising:

utilizing a first method of cardiac signal analysis during a first time period;

delivering cardiac stimulus; and

during a second time period following delivery of the cardiac stimulus, performing the method of claim 17.

19. The method of claim 18, further comprising:

during the second time period, comparing the adjusted signal to a predetermined threshold to detect a threshold crossing;

ending the second time period when a threshold crossing is detected.

20. The method of claim 18, further comprising:

selecting a frequency for periodically performing steps a) and b) at the start of the second time period;

adjusting the frequency later in the second time period.

21. A method of cardiac signal analysis comprising:

providing analog-to-digital converter (ADC) functionality in a form that calculates a sample value as the output of an accumulator which is configured to change from sample-to-sample by receiving an up/down signal for a period of time related to a change in an analog signal; and

providing a heuristic filter that is selectively operable with a selectable frequency, the heuristic filter operable to adjust a stored value on the accumulator toward a predetermined quiescent point.

22. The method of claim 21, further comprising, after delivery of a cardiac stimulus:

operating the heuristic filter using a first periodicity having a first fundamental period; and

observing whether the value on the accumulator is approaching the quiescent point and, if so, operating the heuristic filter using a second periodicity having a second fundamental period that is longer than the first fundamental period.

23. The method of claim 21, further comprising, after delivery of a cardiac stimulus:

operating the heuristic filter;

increasing the period of the heuristic filter if the difference between the accumulator output and the quiescent point is less than a first threshold;

decreasing the period of the heuristic filter if the difference between the accumulator output and the quiescent point is greater than a second threshold; and

terminating operation of the heuristic filter if a cardiac event is detected.

24. An implantable cardiac rhythm management device comprising:

at least a pair of sensing electrodes; and

an analog-to-digital converter (ADC) configured to convert a signal received from the sensing electrodes into a digital representation;

wherein the output of the ADC is selectively modified by a heuristic filter.

25. The implantable cardiac rhythm management device of claim 24, wherein the heuristic filter is selectively operable and, when operated, the heuristic filter provides a heuristic offset for summing with the digital representation.

26. The implantable cardiac rhythm management device of claim 24, wherein the heuristic filter has a frequency of operation that can be modified in light of detected conditions.

27. The implantable cardiac rhythm management device of claim 24, wherein the heuristic filter has a frequency of operation that can be modified in light of a calculated average of the output of the ADC.