DETECTING WORSENING HEART FAILURE BASED ON IMPEDANCE MEASUREMENTS

• US 20100030292A1
• Filed: 07/31/2008
• Published: 02/04/2010
• Est. Priority Date: 07/31/2008
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Sarkar, Shantanu, Hettrick, Douglas A., Stadler, Robert W.
• Granted Patent: US 8,255,046 B2
• Time in Patent Office: Days
• US Class Current: 607/6
• CPC Class Codes: A61B 5/0031 Implanted circuitry
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body
• CPC Class Codes: A61B 5/6846 specially adapted to be brought in contact with an internal body part, i.e. invasive
• CPC Class Codes: A61N 1/3627 for treating a mechanical deficiency of the heart, e.g. congestive heart failure or cardiomyopathy
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance
• CPC Class Codes: A61N 1/37235 Aspects of the external programmer
• CPC Class Codes: A61N 1/37258 Alerting the patient
• CPC Class Codes: A61N 1/39622 Pacing therapy

Abstract

Techniques for processing impedance data to provide an early warning for heart failure decompensation are described. An example device may be configured to measure intrathoracic impedance values, and increment an index when a determined impedance is less than a reference impedance. The incrementing may be based on the difference between the reference impedances and the determined impedance. In some examples, the amount of incrementing is reduced based on a variability of the impedances, or increased over time so long as the index remains above a threshold, e.g., zero. In some examples, the manner is which the reference impedances are determined changes over time to, for example, address rapid changes in impedance after device or system implantation. In some examples, the index is compared to a threshold to determine whether to provide an alert. In some examples, two thresholds are used to provide hysteresis.

Claims

1. A method comprising:

periodically determining an impedance of a patient based on measured impedances;

comparing each of the determined impedances to a respective reference impedance, wherein the respective reference impedance for each of the determined impedances is determined based on a plurality of previously determined impedances;

modifying an index over time based on the comparisons between the determined impedances and the reference impedances;

comparing the index to at least one threshold;

determining whether to provide an alert based on the comparison of the index to the at least one threshold; and

varying at least one other parameter over time that, in addition to the determined and reference impedances, affects a slope of the index.

2. The method of claim 1, wherein varying at least one other parameter comprises determining a variability value based on the plurality of previously determined impedance values, and modifying the index based on the comparisons comprises modifying the index based on the comparisons and the variability value.

3. The method of claim 2, wherein modifying the index based on the comparisons and the variability comprises:

determining a modification based on one of the comparisons; and

reducing the modification based on the variability value.

4. The method of claim 3, wherein the variability value is scaled by a time dependent value that reduces an affect of the variability value on the modification over time.

5. The method of claim 4, wherein modifying an index over time based on the comparisons between the determined impedances and the reference impedances comprises:

increasing the index by differences between the reference impedances and the determined impedances less the scaled variability value so long is the determined impedances are less than the reference impedances.

6. The method of claim 2, wherein determining the variability value comprises:

determining a plurality of absolute differences between successive one of the previously determined impedances; and

determining a median of the differences as the variability value.

7. The method of claim 1, wherein varying at least one other parameter comprises:

determining a value that increases as a function of time over which the index is greater than one of the thresholds; and

adding the value to the index when the index is greater than the threshold.

8. The method of claim 7, wherein adding the value to the index when the index is greater than the threshold comprises adding value to the index when the index has been greater than the threshold for a threshold duration.

9. The method of claim 1, further comprising determining a subsequent one of the reference impedances based on a comparison of a current one of the determined impedances to a current one of the reference impedances, wherein determining the subsequent one of the reference impedances comprises:

increasing the current one of the reference impedances by a predetermined increment value if the current one of the determined impedances is greater than the current one of the reference impedances; and

decreasing the current one of the reference impedances by a predetermined decrement value if the current one of the determined impedances is less than the current one of the reference impedances,

wherein varying at least one other parameter comprises varying at least one of the increment value or the decrement value over time.

10. The method of claim 9, wherein varying at least one of the increment value or the decrement value over time comprises decreasing the at least one of the increment value or the decrement value over time.

11. The method of claim 9, wherein varying at least one of the increment value or the decrement value over time comprises varying the at least one of the increment value or the decrement value over a predetermined period of time from implantation of electrodes used to measure the impedance of the patient within the patient or a modification of a system comprising the electrodes.

12. The method of claim 1,

wherein comparing each of the determined impedances to a respective reference impedance comprises determining differences between the reference impedances and the determined impedances, and

wherein modifying an index over time based on the comparisons between the measured impedances and the reference impedances comprises determining the index as a sum of the previous X or fewer differences so long as the measured impedance is less then the reference impedance,

wherein X is a predetermined constant value.

13. The method of claim 1,

wherein comparing the index to at least one threshold comprises comparing the index to a first threshold and a second threshold,

wherein determining whether to provide an alert based on the comparison of the index to the at least one threshold comprises:

providing the alert when the index is at least one of greater than or equal to the first threshold, and

stopping the alert when the index is subsequently at least one of less than or equal to the second threshold,

wherein the first threshold is greater than the second threshold.

14. The method of claim 1, wherein the alert indicates a possible heart failure decompensation event.

15. The method of claim 1, wherein the impedance comprises a intrathoracic impedance.

16. The method of claim 1, wherein periodically measuring, comparing, modifying comparing, determining, and varying comprises periodically measuring, comparing, modifying comparing, determining, and varying by an implantable medical device.

17. A method comprising:

periodically determining an impedance of a patient based on a plurality of measured impedances;

comparing each of the determined impedances to a respective reference impedance, wherein the respective reference impedance for each of the determined impedances is determined based on a plurality of previously determined impedances;

determining an index based on the previous X or fewer comparisons, wherein X is a predetermined constant value;

comparing the index to at least one threshold; and

determining whether to provide an alert based on the comparison of the index to the at least one threshold.

18. The method of claim 17,

wherein comparing each of the determined impedances to a respective reference impedance comprises determining differences between the reference impedances and the determined impedances, and

wherein determining the index based on the previous X or fewer comparisons comprises determining the index as a sum of the previous X or fewer differences so long as the determined impedance is less then the reference impedance.

19. The method of claim 18, wherein determining the index as a sum of the previous X or fewer differences comprises:

storing the differences in a buffer with a limit of X differences; and

summing the index values stored in the buffer.

20. The method of claim 19, further comprising resetting the buffer when a current determined impedance is greater than a respective reference impedance value.

21. A method comprising:

periodically determining an impedance of a patient based on a plurality of measured impedances;

comparing each of the determined impedances to a respective reference impedance, wherein the respective reference impedance for each of the determined impedances is determined based on a plurality of previously determined impedances;

modifying an index over time based on the comparisons between the determined impedances and the reference impedances;

comparing the index to a plurality of thresholds; and

determining whether to provide an alert based on the comparison of the index to the plurality of thresholds.

22. The method of claim 21,

wherein comparing the index to a plurality of thresholds comprises comparing the index to a first threshold and a second threshold,

wherein determining whether to provide an alert based on the comparison of the index to the at least one threshold comprises:

providing the alert when the index is at least one of greater than or equal to the first threshold, and

stopping the alert when the index is subsequently at least one of less than or equal to the second threshold,

wherein the first threshold is greater than the second threshold.

23. A system comprising:

a plurality of electrodes;

a medical device coupled to the electrodes that periodically measures an impedance of the patient; and

a processor that;

periodically determines an impedance value based on a plurality of the measured impedances,

compares each of the determined impedance values to a respective reference impedance, wherein the respective reference impedance for each of the determined impedance values is determined based on a plurality of previously determined impedance values,

modifies an index over time based on the comparisons between the determined impedance values and the reference impedances,

compares the index to at least one threshold,

determines whether to provide an alert based on the comparison of the index to the at least one threshold, and

varies at least one other parameter over time that, in addition to the determined impedance values and reference impedances, affects a slope of the index.

24. The system of claim 23, wherein the processor determines a variability value based on the plurality of previously determined impedance values, and modifies the index based on the comparisons and the variability value.

25. The system of claim 24, wherein the processor determines an index modification based on one of the comparisons, and reduces the modification based on the variability value.

26. The system of claim 25, wherein the processor scales variability value by a time dependent value that reduces an affect of the variability value on the modification over time.

27. The system of claim 26, wherein the processor increases the index by differences between the reference impedances and the determined impedance values less the scaled variability value so long is the determined impedance values are less than the reference impedances.

28. The system of claim 23, wherein the processor determines a value that increases as a function of time over which the index is greater than one of the thresholds, and adds the value to the index when the index is greater than the threshold.

29. The system of claim 28, wherein the processor adds the value to the index when the index has been greater than the threshold for a threshold duration.

30. The system of claim 23,

wherein the processor determines a subsequent one of the reference impedances based on a comparison of a current one of the determined impedance values to a current one of the reference impedances, and

wherein the processor increases the current one of the reference impedances by a predetermined increment value if the current one of the determined impedance values is greater than the current one of the reference impedances, and decreases the current one of the reference impedances by a predetermined decrement value if the current one of the determined impedance values is less than the current one of the reference impedances, and varies at least one of the increment value or the decrement value over time.

31. The system of claim 30, wherein the processor decreases the at least one of the increment value or the decrement value over time.

32. The system of claim 30, wherein the processor varies the at least one of the increment value or the decrement value over a predetermined period of time from implantation of electrodes used to measure the impedance of the patient within the patient.

33. The system of claim 23, wherein the electrodes comprise a plurality of implantable electrodes, the medical device comprises an implantable medical device, and the processor comprises a processor of the implantable medical device.

34. The system of claim 33, wherein the implantable medical device comprises at least one of a pacemaker, a cardioverter, or a defibrillator.

35. A system comprising:

a plurality of electrodes;

a medical device coupled to the electrodes that periodically measures an impedance of a patient; and

a processor that;

periodically determines an impedance value based on a plurality of the measured impedances,

compares each of the determined impedance values to a respective reference impedance, wherein the respective reference impedance for each of the determined impedance values is determined based on a plurality of previously determined impedance values;

determines an index based on the previous X or fewer comparisons, wherein X is a predetermined constant value;

compares the index to at least one threshold; and

determines whether to provide an alert based on the comparison of the index to the at least one threshold.

36. The system of claim 35, wherein the processor determines differences between the reference impedances and the determined impedance values, and determines the index as a sum of the previous X or fewer differences so long as the measured impedance is less then the reference impedance.

37. The system of claim 36, further comprising a memory, wherein the processor stores the differences in a buffer with a limit of X differences in the memory and sums the index values stored in the buffer.

38. The system of claim 37, wherein the processor resets the buffer when a current measured impedance is greater than a respective reference impedance value.

39. A system comprising:

a plurality of electrodes;

a medical device coupled to the electrodes that periodically measures an impedance of a patient; and

a processor that;

periodically determines an impedance value based on a plurality of the measured impedances,

compares each of the determined impedance values to a respective reference impedance, wherein the respective reference impedance for each of the determined impedance values is determined based on a plurality of previous measured impedances;

modifies an index over time based on the comparisons between the determined impedance values and the reference impedances;

compares the index to a plurality of thresholds; and

determines whether to provide an alert based on the comparison of the index to the plurality of thresholds.

40. The system of claim 39,

wherein the processor compares the index to a first threshold and a second threshold, provides the alert when the index is at least one of greater than or equal to the first threshold, and stops the alert when the index is subsequently at least one of less than or equal to the second threshold, and

wherein the first threshold is greater than the second threshold.

41. A system comprising:

means for periodically determining an impedance of a patient based on a plurality of measured impedances;

means for comparing each of the determined impedances to a respective reference impedance, wherein the respective reference impedance for each of the determined impedances is determined based on a plurality of previous measured impedances;

means for modifying an index over time based on the comparisons between the determined impedances and the reference impedances;

means for comparing the index to at least one threshold;

means for determining whether to provide an alert based on the comparison of the index to the at least one threshold; and

means for varying at least one other parameter over time that, in addition to the determined and reference impedances, affects a slope of the index.

42. A system comprising:

means for periodically determining an impedance of a patient based on a plurality of measured impedances;

means for comparing each of the determined impedances to a respective reference impedance, wherein the respective reference impedance for each of the determined impedances is determined based on a plurality of previously determined impedances;

means for determining an index based on the previous X or fewer comparisons, wherein X is a predetermined constant value;

means for comparing the index to at least one threshold; and

means for determining whether to provide an alert based on the comparison of the index to the at least one threshold.

43. A system comprising:

means for periodically determining an impedance of a patient based on a plurality of measured impedances;

means for comparing each of the determined impedances to a respective reference impedance, wherein the respective reference impedance for each of the determined impedances is determined based on a plurality of previously determined impedances;

means for modifying an index over time based on the comparisons between the determined impedances and the reference impedances;

means for comparing the index to a plurality of thresholds; and

means for determining whether to provide an alert based on the comparison of the index to the at least one threshold.