SYSTEM AND METHOD FOR CALIBRATING CARDIAC PRESSURE MEASUREMENTS DERIVED FROM SIGNALS DETECTED BY AN IMPLANTABLE MEDICAL DEVICE

• US 20080262361A1
• Filed: 04/24/2008
• Published: 10/23/2008
• Est. Priority Date: 11/13/2006
• Status: Active Grant

• Current Assignee: Pacesetter Incorporated (Abbott Laboratories)
• Original Assignee: Pacesetter Incorporated (Abbott Laboratories)
• Inventors: Eigler, Neal L., Panescu, Dorin, Whiting, James S., Gutfinger, Dan E.
• Granted Patent: US 8,202,224 B2
• Time in Patent Office: Days
• US Class Current: 600/486
• CPC Class Codes: A61B 5/0215 by means inserted into the body catheters A61M25/00
• 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

Various techniques are provided for calibrating and estimating left atrial pressure (LAP) using an implantable medical device, based on impedance, admittance or conductance parameters measured within a patient. In one example, default conversion factors are exploited for converting the measured parameters to estimates of LAP. The default conversion factors are derived from populations of patients. In another example, a correlation between individual conversion factors is exploited to allow for more efficient calibration. In yet another example, differences in thoracic fluid states are exploited during calibration. In still yet another example, a multiple stage calibration procedure is described, wherein both invasive and noninvasive calibration techniques are exploited. In a still further example, a therapy control procedure is provided, which exploits day time and night time impedance/admittance measurements.

Claims

1. 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;

inputting default conversion factors for use in converting the measured value to an estimated cardiac pressure value, wherein the default conversion factors are values representative of a correlation between the measured parameter and the cardiac pressure derived from a population of patients; and

estimating cardiac pressure within the patient by applying the default conversion factors to the measured parameter.

2. The method of claim 1 wherein applying an electrical field to tissues of the patient includes applying a detection pulse from which one or more of impedance (Z), admittance (Y) and conductance (G) can be measured.

3. The method of claim 1 wherein measuring the parameter includes:

measuring one or more of impedance (Z), admittance (Y) and conductance (G) signal within the patient along a sensing vector passing through cardiac tissue; and

deriving the parameter from the detected signal.

4. The method of claim 1 wherein the default values include slope and baseline values representative of the correlation.

5. The method of claim 4 wherein estimating cardiac pressure includes calculating:

Cardiac Pressure=Parameter*Slope+Baseline

wherein Parameter represents the measured parameter.

6. The method of claim 1 further including controlling at least one device function based on estimated cardiac pressure.

7. The method of claim 6 wherein the device function to be controlled includes one or more of generating cardiac pressure diagnostics, generating heart failure diagnostics, controlling pacing therapy and controlling delivery of medications and controlling the prescription of medications.

8. The method of claim 1 wherein different sets of default conversion factors are employed based on one or more of patient age, gender, weight, height, cardiac function, cardiac compliance, and cardiovascular capacitance.

9. A method for calibrating a cardiac pressure estimation system of an implantable medical device for implant within a patient, wherein the estimation procedure exploits conversion factors representative of a correlation between a measured electrical parameter within the patient and the cardiac pressure of the patient, the method comprising:

calibrating the cardiac pressure estimation system by determining an initial set of conversion factors for the patient for use in converting measured electrical parameters within the patient to cardiac pressure estimates;

determining whether there is a linear relationship between the conversion factors for the patient;

re-calibrating cardiac pressure estimation system by selectively adjusting the conversion factors based, at least in part, on whether there is a linear relationship between the conversion factors for the patient.

10. The method of claim 9 wherein the conversion factors include slope and baseline values representative of a linear relationship between measured parameter and cardiac pressure, wherein there is also a linear relationship between the slope and baseline values for the patient, and wherein re-calibrating cardiac pressure estimation system includes

determining one of the slope and baseline values and then calculating the other of the slope and baseline values based on the linear relationship between the slope and baseline values.

11. The method of claim 9 wherein the conversion factors include slope and baseline values representative of a linear relationship between measured parameter and cardiac pressure, wherein there is no linear relationship between the slope and baseline values for the patient, and wherein re-calibrating cardiac pressure estimation system further includes

determining whether one of the slope and baseline values remains substantially unchanged within the patient while the other varies and, if so, then re-calibrating only the value that varies.

12. A method for estimating cardiac pressure within a patient using an implantable medical device wherein the estimation exploits conversion factors representative of a correlation between a measured electrical parameter within the patient and the cardiac pressure of the patient, the method comprising:

determining a set of conversion factors for converting measured electrical parameters to estimates of cardiac pressure, wherein the conversion factors are determined, at least in part, based on a physiological state of the patient; and

estimating cardiac pressure within the patient by applying the set of conversion factors to measured electrical parameters.

13. The method of claim 12 wherein the physiological state is either an acute state wherein a relatively short period of time has resulted in an acute change of fluid volume within the intra-vascular fluid compartment without affecting the fluid volume within the interstitial fluid compartment or a chronic state wherein a relatively longer period of time has resulted in the equilibration of fluid volume between the intra-vascular and interstitial fluid compartments.

14. The method of claim 13 wherein determining the set of conversion factors includes:

inputting a first set of conversion factors appropriate for converting electrical parameters measured while the patient is in the acute state;

inputting an adjustment factor for use in adjusting the first set of conversion factors to yield a second set of conversion factors appropriate for use with electrical parameters measured while the patient is in the chronic state; and

applying the adjustment factor to the first set of conversion factors to generate the second set of conversion factors.

15. The method of claim 14 wherein the first and second sets of conversion factors include slope and baseline values representative of a linear relationship between the measured parameter and cardiac pressure and wherein the adjustment factor is applied only to the slope value.

16. The method of claim 15 wherein the slope of the first set of conversion factors is adjusted by dividing the slope by adjustment factor in the range of 3 to 5 to yield the slope of the second set of conversion factors.

17. The method of claim 14 wherein adjustment factor is specific to the particular patient in which the device is implanted.

18. The method of claim 13 wherein determining the set of conversion factors includes:

determining the current patient physiological state of the patient; and

retrieving and applying the appropriate set of conversion factors for the current physiological state.

19. The method of claim 18 wherein the implantable device stores a first set of conversion factors appropriate for use in the acute state and a second set of conversion factors appropriate for use in the chronic state and wherein retrieving and applying the appropriate set of conversion factors for the current physiological state includes retrieving the first set of conversion factors if the patient is within the acute state and retrieving the second set of conversion factors if the patient is within the chronic state.

20. The method of claim 12 wherein determining the set of conversion factors based on the physiological state of the patient includes:

determining a first set of acute conversion factors based, at least in part, on calibration parameters measured while the patient is undergoing an acute physiological maneuver affecting only the fluid volume within the intra-vascular fluid compartment; and

determining a second set of chronic conversion factors based, at least in part, on calibration parameters measured while the patient is in a chronic steady state in which the fluid volume within the intra-vascular and interstitial fluid compartments have equilibrated.

21. A method for calibrating a cardiac pressure estimation system of an implantable medical device for implant within a patient, wherein the estimation procedure exploits conversion factors representative of a correlation between a measured electrical parameter within the patient and the cardiac pressure of the patient, the method comprising:

determining an initial set of conversion factors using an invasive calibration procedure wherein cardiac pressure calibration values are obtained using a pressure sensor implanted within the patient;

adjusting the set of conversion factors using a non-invasive calibration procedure wherein cardiac pressure calibration values are estimated from electrical parameters detected within the patient; and

estimating cardiac pressure values within the patient based on electrical parameters detected within the patient using the latest set of conversion factors.

22. The method of claim 21 wherein determining an initial set of conversion factors using an invasive calibration procedure exploits pressure values obtained using a Swan-Ganz catheter.

23. The method of claim 21 wherein adjusting the set of conversion factors using a non-invasive calibration procedure exploits cardiac pressure values estimated based on one or more of:

circadian immittance signals;

the relative sizes of V-waves to A-waves within cardiogenic immittance signals;

echocardiography signals; and

cardiac immittance signals obtaining during performance of one or more of isometric muscle contraction, vasodilatation, vasoconstriction, rapid pacing, posture changes, raising the lower extremities, and the Valsalva maneuver.

24. A method for controlling therapy delivered by an implantable medical device for implant within a patient, the method comprising:

measuring a first set of immittance values during a first interval of time within the patient while the patient is in a chronic physiological state corresponding to an equilibration of fluid volume between the intra-vascular and interstitial fluid compartments within the thorax of the patient;

estimating cardiac pressure within the patient following the first interval of time based on the first set of immittance values and controlling therapy based on the estimated cardiac pressure;

measuring a second set of immittance values during a second interval of time within the patient while the patient is also in a chronic physiological state; and

estimating cardiac pressure within the patient following the second interval of time based on the second set of immittance values and controlling therapy based on the estimated cardiac pressure.

25. The method of claim 24 wherein the first interval of time is during the night such that therapy may be adjusted in the morning based on the immittance values measured during the night and wherein the second interval of time is during the day such that therapy may be adjusted in the evening based on the immittance values measured during the day.