System and method for estimating cardiac pressure using parameters derived from impedance signals detected by an implantable medical device

• US 7,794,404 B1
• Filed: 11/13/2006
• Issued: 09/14/2010
• Est. Priority Date: 03/31/2006
• Status: Active Grant

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Original Assignee: Pacesetter Incorporated (Abbott Laboratories Incorporated)
• Inventors: Eigler, Neal L., Panescu, Dorin, Whiting, James S., Gutfinger, Dan E.
• Primary Examiner(s): Layno; Carl H
• Assistant Examiner(s): Porter, Jr.; Gary A
• Application Number:
  US11/559,235
• Time in Patent Office: 1,401 Days
• Field of Search: 600/508, 600/517, 600/485, 600/486, 600/506, 600/513, 600/526, 600/547, 607/17, 607/23
• US Class Current: 600/486
• CPC Class Codes: A61B 5/021 Measuring pressure in heart or blood vessels A61B5/0205 takes precedence
• 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

Abstract

Techniques are provided for estimating left atrial pressure (LAP) or other cardiac pressure parameters based on various parameters derived from impedance signals. In particular, effective LAP is estimated based on one or more of: electrical conductance values, cardiogenic pulse amplitudes, circadian rhythm pulse amplitudes, or signal morphology fractionation values, each derived from the impedance signals detected by the implantable device. Predetermined conversion factors stored within the device are used to convert the various parameters derived from the electrical impedance signal into LAP values or other appropriate cardiac pressure values. The conversion factors may be, for example, slope and baseline values derived during an initial calibration procedure performed by an external system, such as an external programmer. In some examples, the slope and baseline values may be periodically re-calibrated by the implantable device itself.

Claims

1. A method for estimating cardiac pressure within a patient using an implantable medical device, the method comprising:

applying an electrical impedance detection pulse from which electrical impedance can be measured to tissues of the patient including cardiac tissues;

measuring a parameter influenced by the electrical impedance detection pulse, the parameter being affected by cardiac pressure, the measuring comprising;

detecting an electrical impedance (Z) signal within the patient along a sensing vector wherein impedance is affected by cardiac pressure; and

deriving the parameter from the electrical impedance (Z) signal; and

estimating, by a processor of the implantable medical device, cardiac pressure within the patient by applying predetermined conversion factors to the measured parameter by recalling predetermined conversion factors from a memory of the implantable medical device, wherein the predetermined conversion factors are configured to convert the parameter to cardiac pressure by inputting slope and baseline values representative of a linear relationship between cardiac pressure and the parameter.

2. The method of claim 1 wherein the predetermined parameter is a cardiogenic pulse amplitude value and wherein the conversion factors are cardiogenic pulse amplitude-based conversion factors for converting cardiogenic pulse amplitude values to cardiac pressure values.

3. The method of claim 2 wherein estimating cardiac pressure includes applying the cardiogenic pulse amplitude-based conversion factors to cardiogenic pulse amplitudes derived from an electrical impedance (Z) signal.

4. The method of claim 1 wherein the predetermined parameter is a circadian rhythm pulse amplitude value and wherein the conversion factors are circadian rhythm-based conversion factors for converting circadian rhythm pulse amplitude values to cardiac pressure values.

5. The method of claim 4 wherein estimating cardiac pressure includes applying the cardiogenic pulse amplitude-based conversion factors to cardiogenic pulse amplitudes derived from the electrical impedance (Z) signal.

6. The method of claim 1 wherein the predetermined parameter is a cardiogenic signal fractionation parameter and wherein the conversion factors are fractionation-based conversion factors for converting fractionation parameters to cardiac pressure values.

7. The method of claim 6 wherein estimating cardiac pressure includes applying the fractionation-based conversion factors to cardiogenic fractionation parameters derived from an electrical impedance (Z) signal.

8. The method of claim 6 wherein the fractionation parameter represents the fractionation of a cardiogenic component of the electrical impedance (Z) signal.

9. The method of claim 6 wherein the fractionation parameter represents the fractionation of a circadian component of the electrical impedance (Z) signal.

10. The method of claim 1 wherein the parameter includes one or more of:

an electrical conductance (G) value, a cardiogenic pulse amplitude value, a circadian rhythm pulse amplitude, and a cardiogenic signal morphology fractionation parameter.

11. The method of claim 1 wherein the predetermined parameter is an electrical conductance (G) value and wherein the predetermined conversion factors are conductance-based conversion factors for converting electrical cardiac conductance (G) values to cardiac pressure values.

12. The method of claim 11 wherein electrical conductance (G) value is derived from the detected impedance (Z) by calculating the reciprocal of the detected impedance (Z).

13. The method of claim 11 wherein the predetermined conversion factors includes slope and baseline values representative of a linear relationship between conductance (G) and cardiac pressure;

and

wherein estimating cardiac pressure includes applying the slope and baseline values to convert electrical conductance (G) values to cardiac pressure values.

14. The method of claim 13 wherein estimating cardiac pressure includes calculating:

Cardiac Pressure=G*Slope+Baseline.

15. The method of claim 13 wherein electrical conductance (G) value is derived from impedance values (Z) measured along a sensing vector through the left atrium of the heart of the patient and wherein the resulting cardiac pressure value is an estimated left atrial pressure (eLAP) value obtained by calculating:

eLAP=G*Slope+bLAP;

wherein Slope is a slope value specifically for use with left atrial pressure (LAP) and bLAP is a baseline value specifically for use with LAP.

16. The method of claim 13 further including an initial calibration step for determining the slope and baseline values that are representative of the linear relationship between conductance (G) and cardiac pressure for the particular patient.

17. The method of claim 16 wherein determining the slope and baseline values includes:

transmitting, to an external calibration system, a first conductance calibration value (G₁) detected by an implantable medical device and a corresponding first cardiac pressure calibration value (Pressure₁) measured by a device other than the implantable medical device at a first time within the patient; and

inputting, to the external calibration system, a second conductance calibration value (G₂) detected by the implantable medical device and a corresponding second cardiac pressure calibration value (Pressure₂) measured by a device other than the implantable medical device at a second time within the patient, wherein the first and second cardiac pressure values (Pressure₁, Pressure₂) differ substantially.

18. The method of claim 17 wherein determining the slope is performed by calculating:

Slope=(Pressure₂−

Pressure₁)/(G₂−

G₁).

19. The method of claim 18 wherein determining the baseline is performed by calculating:

Baseline=Pressure₂−

Slope*G₁.

20. The method of claim 17 wherein the first and second conductance calibration values (G₁, G₂) are derived from first and second impedance calibration values (Z₁, Z₂), respectively, measured within the patient at times when the first and second cardiac pressure values (Pressure₁, Pressure₂), respectively, are measured.

21. The method of claim 20 further including additional calibration steps of:

detecting the first cardiac pressure value (Pressure₁) and the first conductance calibration value (G₁) while the patient is at rest; and

detecting the second cardiac pressure value (Pressure₂) and the second conductance calibration value (G₂) while the patient is subject to a condition significantly affecting cardiac pressure within the patient, wherein detecting the first and second conductance calibration values (G₁, G₂) are measured within the patient using the implantable medical device and detecting the first and second cardiac pressure values (Pressure₁, Pressure₂) are measured within the patient using a Swan-Ganz catheter equipped to measure pulmonary capillary wedge pressure (PCWP).

22. The method of claim 21 wherein the condition significantly affecting cardiac pressure within the patient includes one or more of:

isometric muscle contraction, vasodilatation, vasoconstriction, rapid pacing and performance of the Valsalva maneuver.

23. The method of claim 20 wherein the cardiac pressure value to be estimated is an effective pressure value and wherein the method further includes additional calibration steps of:

detecting the first cardiac pressure value (Pressure₁) using a Swan-Ganz catheter equipped to measure pulmonary capillary wedge pressure (PCWP) and the first impedance calibration value (Z₁) within the patient at a time of device implant; and

detecting an additional impedance calibration value (Z_N) during performance of a Valsalva maneuver by the patient at a time subsequent to implant, wherein detecting the first and second impedance calibration values (Z₁, Z_N) are measured within the patient using the implantable medical device.

24. The method of claim 23 wherein following device implant, only the baseline value is recalibrated based on an additional conductance value (G_N) derived from the additional impedance value (Z_N):

Baseline=−

Slope*G_N.

25. The method of claim 23 wherein following device implant the slope and baseline values are re-calibrated based on an additional conductance value (G_N) derived from the additional impedance value (Z_N) and on a first effective cardiac pressure value (Pressure₁):

Slope_G=−

LAP₁/(G_N−

G₁) and

Baseline_G=−

New_Slope_G*G_N,

where Slope_G is a articular sloe value LAP₁ is a first cardiac pressure calibration value, Baseline_G is a articular baseline value, and New Slope_G is a new sloe value.

26. The method of claim 23 wherein the effective cardiac pressure value might not drop to zero within the patient during Valsalva and wherein an additional correction factor is calculated at device implant.

27. The method of claim 13 further including an initial calibration step of determining the slope and baseline values that are representative of the linear relationship between conductance (G) and cardiac pressure for any of a plurality of patients.

28. The method of claim 26 wherein determining the slope and baseline values includes:

transmitting, to an external calibration system, a plurality of conductance values (G) detected by the internal medical device and corresponding cardiac pressure values (Pressure) measured by a device other than the implantable medical device within a plurality of test subjects; and

determining, by the external calibration system, slope and baseline values by applying linear regression to the plurality of conductance values (G) and cardiac pressure values (Pressure) obtained from the test subjects.

29. The method of claim 27 wherein the conductance values (G) are derived by the external calibration system from impedance values (Z) measured by implantable medical devices along sensing vectors passing through the left atria of the hearts of each of the test subjects and wherein the cardiac pressure values obtained by temporary pressure sensors inserted within each of the test subjects.

30. The method of claim 1 further including controlling therapy based on estimated cardiac pressure.

31. The method of claim 30 wherein the implantable device is equipped with an implantable drug pump and wherein drug therapy is controlled based on estimated cardiac pressure.

32. The method of claim 1 further including detecting heart failure based on estimated cardiac pressure.

33. The method of claim 32 wherein the implantable device is equipped with an implantable warning device and wherein a warning is generated upon detection of heart failure.

34. The method of claim 32 wherein the implantable device is equipped to transmit signals to an external display device and wherein the warning is transmitted to the external device.

35. The method of claim 1 wherein the implantable device is equipped to transmit signals to an external display device and wherein estimated cardiac pressure values are transmitted to the external device.

36. A system for estimating cardiac pressure within a patient using an implantable medical device, the system comprising:

means for applying an electrical field to tissues of the patient including cardiac tissues;

means for measuring an electrical conductance (G) value, the electrical conductance (G) value being affected by cardiac pressure; and

means for estimating cardiac pressure within the patient by applying slope and baseline values representative of a linear relationship between the electrical conductance (G) value and cardiac pressure to convert electrical conductance (G) values into cardiac pressure values according to the relationship;

Cardiac Pressure=G*Slope+Baseline.

37. The system of claim 36 wherein the electrical conductance (G) value is derived from impedance values (Z) measured along a sensing vector through the left atrium of the heart of the patient and wherein the resulting cardiac pressure value is an estimated left atrial pressure (eLAP) value obtained by calculating:

eLAP=G*Slope+bLAP;

wherein Slope is a slope value specifically for use with left atrial pressure (LAP) and bLAP is a baseline value specifically for use with LAP.

38. The system of claim 36 further including an initial calibration means for determining the slope and baseline values that are representative of the linear relationship between electrical conductance (G) value and cardiac pressure for the particular patient.

39. The system of claim 37 wherein the initial calibration means comprises:

means for transmitting, to an external calibration system, a first conductance calibration value (G₁) detected by an implantable medical device and a corresponding first cardiac pressure calibration value (Pressure₁) measured by a device other than the implantable medical device at a first time within the patient; and

means for inputting, to the external calibration system, a second conductance calibration value (G₂) detected by the implantable medical device and a corresponding second cardiac pressure calibration value (Pressure₂) measured by a device other than the implantable medical device at a second time within the patient, wherein the first and second cardiac pressure values (Pressure₁, Pressure₂) differ substantially.

40. The system of claim 39 wherein determining the slope is performed by calculating:

Slope=(Pressure₂−

Pressure₁)/(G₂−

G₁); and

determining the baseline is performed by calculating;

Baseline=Pressure₂−

Slope*G₁.

41. The system of claim 39 wherein the first and second conductance calibration values (G₁, G₂) are derived from first and second impedance calibration values (Z₁, Z₂), respectively, measured within the patient at times when the first and second cardiac pressure values (Pressure₁, Pressure₂), respectively, are measured.

42. The system of claim 36 further comprising an initial calibration means for determining the slope and baseline values that are representative of the linear relationship between electrical conductance (G) values and cardiac pressures for any of a plurality of patients.

43. A method for estimating cardiac pressure within a patient using an implantable medical device, the method comprising:

applying an electrical field to tissues of the patient including cardiac tissues;

measuring an electrical conductance (G) value, the electrical conductance (G) value being affected by cardiac pressure; and

estimating, by a processor of the implantable medical device, cardiac pressure within the patient by applying slope and baseline values representative of a linear relationship between the electrical conductance (G) value and cardiac pressure to convert electrical conductance (G) values into cardiac pressure values according to the relationship;

Cardiac Pressure=G*Slope+Baseline.

44. The method of claim 43 further including controlling therapy based on estimated cardiac pressure.

45. The method of claim 44 wherein the implantable device is equipped with an implantable drug pump and wherein drug therapy is controlled based on estimated cardiac pressure.

46. The method of claim 43 further including detecting heart failure based on estimated cardiac pressure.

47. The method of claim 46 wherein the implantable device is equipped with an implantable warning device and wherein a warning is generated upon detection of heart failure.

48. The method of claim 46 wherein the implantable device is equipped to transmit signals to an external display device and wherein a warning is transmitted to the external device.

49. The method of claim 43 wherein the implantable device is equipped to transmit signals to an external display device and wherein estimated cardiac pressure values are transmitted to the external display device.

50. A method for estimating cardiac pressure within a patient using an implantable medical device, the method comprising:

applying an electrical field to tissues of the patient including cardiac tissues;

measuring a parameter influenced by the electrical field, the parameter being affected by cardiac pressure, wherein the parameter is an electrical conductance (G) value; and

estimating, by a processor of the implantable medical device, cardiac pressure within the patient by applying predetermined conversion factors to the measured parameter, wherein the predetermined conversion factors are conductance-based conversion factors for converting electrical cardiac conductance (G) values to cardiac pressure values, the predetermined conversion factors include slope and baseline values representative of a linear relationship between conductance (G) and cardiac pressure, wherein the estimating further comprises;

applying the slope and baseline values to convert electrical conductance (G) values to cardiac pressure values; and

calculating;

Cardiac Pressure=G*Slope+Baseline.

51. The method of claim 50 further including an initial calibration step for determining the slope and baseline values that are representative of the linear relationship between electrical cardiac conductance (G) values and cardiac pressure for the particular patient.

52. The method of claim 51 wherein determining the slope and baseline values includes:

transmitting, to an external calibration system, a first conductance calibration value (G₁) detected by an implantable medical device and a corresponding first cardiac pressure calibration value (Pressure₁) measured by a device other than the implantable medical device at a first time within the patient; and

inputting, to the external calibration system, a second conductance calibration value (G₂) detected by the implantable medical device and a corresponding second cardiac pressure calibration value (Pressure₂) measured by a device other than the implantable medical device at a second time within the patient, wherein the first and second cardiac pressure values (Pressure₁, Pressure₂) differ substantially.

53. The method of claim 52 wherein the first and second conductance calibration values (G₁, G₂) are derived from first and second impedance calibration values (Z₁, Z₂), respectively, measured within the patient at times when the first and second cardiac pressure values (Pressure₁, Pressure₂), respectively, are measured.

54. The method of claim 53 further including additional calibration steps of:

detecting the first cardiac pressure value (Pressure₁) and the first conductance calibration value (G₁) while the patient is at rest; and

detecting the second cardiac pressure value (Pressure₂) and the second conductance calibration value (G₂) while the patient is subject to a condition significantly affecting cardiac pressure within the patient, wherein detecting the first and second conductance calibration values (G₁, G₂) are measured within the patient using the implantable medical device and detecting the first and second cardiac pressure values (Pressure₁, Pressure₂) are measured within the patient using a Swan-Ganz catheter equipped to measure pulmonary capillary wedge pressure (PCWP).

55. The method of claim 53 wherein the cardiac pressure value to be estimated is an effective pressure value and wherein the method further includes additional calibration steps of:

detecting the first cardiac pressure value (Pressure₁) using a Swan-Ganz catheter equipped to measure pulmonary capillary wedge pressure (PCWP) and the first impedance calibration value (Z₁) within the patient at a time of device implant; and

detecting an additional impedance calibration value (Z_N) during performance of a Valsalva maneuver by the patient at a time subsequent to implant, wherein detecting the first and second impedance calibration values (Z₁, Z_N) are measured within the patient using the implantable medical device.

56. The method of claim 55 wherein the effective cardiac pressure value might not drop to zero within the patient during Valsalva and wherein an additional correction factor is calculated at device implant.

57. The method of claim 56 wherein determining the slope and she baseline values includes:

transmitting, to an external calibration system, a plurality of conductance values (G) detected by the internal medical device and corresponding cardiac pressure values (Pressure) measured by a device other than the implantable medical device within a plurality of test subjects; and

determining, by the external calibration system, slope and baseline values by applying linear regression to the plurality of conductance values (G) and cardiac pressure values (Pressure) obtained from the test subjects.

58. The method of claim 50 further including an initial calibration step of determining the slope and baseline values that are representative of the linear relationship between electrical cardiac conductance (G) values and cardiac pressure for any of a plurality of patients.

59. The method of claim 58 wherein the conductance values (G) are derived by an external calibration system from impedance values (Z) measured by the implantable medical device along sensing vectors passing through a left atria of the hearts of each of the plurality of patients and wherein the cardiac pressure values obtained by temporary pressure sensors inserted within each of the plurality of patients subjects.

60. The method of claim 50 wherein the electrical cardiac conductance (G) value is derived from a detected impedance (Z) value by calculating the reciprocal of the detected impedance (Z) value.