IMPLANTABLE MEDICAL DEVICES EVALUATING THORAX IMPEDANCE

• US 20080300504A1
• Filed: 05/29/2007
• Published: 12/04/2008
• Est. Priority Date: 05/29/2007
• Status: Active Grant

• Current Assignee: Biotronik SE & Company KG
• Original Assignee: Biotronik CRM Patent AG (Biotronik SE & Company KG)
• Inventors: Muessig, Dirk, Garner, Garth, Hastings, David F., de Voir, Christopher S., Kraetschmer, Hannes, Lefkov, Sharon
• Granted Patent: US 7,970,462 B2
• Time in Patent Office: Days
• US Class Current: 600/547
• CPC Class Codes: A61B 5/053 Measuring electrical impedance or conductance of a portion of the body
• CPC Class Codes: A61B 5/0538 invasively, e.g. using a catheter
• CPC Class Codes: A61B 5/0809 by impedance pneumography
• CPC Class Codes: A61B 5/4878 Evaluating oedema
• CPC Class Codes: A61B 5/6846 specially adapted to be brought in contact with an internal body part, i.e. invasive
• CPC Class Codes: A61B 5/7285 for synchronising or triggering a physiological measurement or image acquisition with a physiological event or waveform, e.g. an ECG signal A61B6/5205, A61B6/541, A61B8/5207, A61B8/543 take precedence
• CPC Class Codes: A61N 1/36521 the parameter being derived from measurement of an electrical impedance
• CPC Class Codes: A61N 1/36585 controlled by two or more physical parameters

Abstract

Implantable medical device with an impedance determination unit with constant current/voltage source having current feed terminals connected to electrodes for intracorporal placement which generates measuring current pulses having constant current/voltage, for causing a current through a body via intracorporally placed electrodes, a measuring unit for measuring voltage/current strength of voltage/current fed through body, an impedance value determination unit connected to the current/voltage source and adapted to determine an impedance value for each measuring current pulse, and an impedance measuring control and evaluation unit connected to the impedance determination unit which controls the unit and evaluates a sequence of consecutive impedance values, the impedance determination unit further adapted to determine at least intrathoracic and intracardiac impedance values for same period of time, the intrathoracic values sampled with a lower sampling rate than the intracardiac values.

Claims

1. An implantable medical device comprising:

an impedance determination unit comprising

a constant current source or a constant voltage source having current feed terminals that are connected or can be connected to electrodes for intracorporal placement and that is adapted to generate measuring current pulses having a constant current strength or a constant voltage, respectively, for causing a current to be fed through a body via intracorporally placed electrodes;

a measuring unit for measuring a voltage corresponding to a current fed through a body by said constant current source or a current strength of a current fed through a body by said constant voltage source, respectively;

an impedance value determination unit that is connected to said constant current source or said constant voltage source and the measuring unit that is adapted to determine an impedance value for each measuring current pulse; and

,

an impedance measuring control and evaluation unit, that is connected to said impedance determination unit and that is adapted to control said impedance determination unit and to evaluate a sequence of consecutive impedance values determined by said impedance determination unit, wherein the impedance determination unit is further adapted to determine at least intrathoracic impedance values and intracardiac impedance values for same period of time, wherein the intrathoracic impedance values are sampled with a lower sampling rate than the intracardiac impedance values.

2. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to determine at least two kinds of intrathoracic impedance values comprising:

a first kind corresponding to an impedance between an IMD case and at least one electrode in one ventricle of a heart; and

,

a second kind corresponding to an impedance between one of the previous electrodes and at least one electrode in one ventricle of a heart.

3. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to determine at least two kinds of intrathoracic impedance values comprising:

a first kind corresponding to an impedance between an IMD case and an electrode in a right ventricle of a heart, and

a second kind corresponding to an impedance between an IMD case and an electrode in a left ventricle of a heart.

4. The implantable medical device according to claim 2 wherein the impedance determination unit is further adapted to determine said intrathoracic impedance values wherein:

said constant current source is connected to the IMD case and a right ventricular or a left ventricular ring electrode, respectively, such that a current pulse injection source is the respective ring electrode and the current pulse sink is the IMD case; and

,

wherein said impedance determination unit measures a voltage between the IMD case and a right ventricular or a left ventricular tip electrode, respectively.

5. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to determine said intracardiac impedance values to be an impedance between an electrode in a right ventricle of a heart and an electrode in a left ventricle of a heart.

6. The implantable medical device according to claim 5, wherein the impedance determination unit is further adapted to determine said intracardiac impedance values wherein:

said impedance determination unit is connected via the constant current source to a right ventricular ring electrode and a right ventricular tip electrode, such that a current pulse injection source is the right ventricular ring electrode and the current pulse sink is the right ventricular tip electrode; and

,

wherein said impedance determination unit measures a voltage between a left ventricular ring electrode and a left ventricular tip electrode.

7. The implantable medical device according to claim 1, wherein the impedance determination unit comprises a quantizer having an operating range and wherein the impedance determination unit is adapted to automatically adjust measurement gains of an amplifier to accomodate values outside the operating range of said quantizer.

8. The implantable medical device according to claim 7, wherein the impedance determination unit is further adapted to perform a bidirectional gain adjustment process that can adapt to large and small signals.

9. The implantable medical device according to claim 8, wherein the impedance determination unit is further adapted to use variance to compute a resolution of a sample average and to adjust the forcing function to make the resolution of the average value meet requirements for a test of statistical significance.

10. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to measure a Z_cardiac waveform from an atrial lead and to use said Z_cardiac waveform to verify the atrial contribution to cardiac output as an indicator of overall cardiac output.

11. The implantable medical device according to claim 1, wherein the implantable medical device comprises an activity sensor that is connected to said impedance measuring control and evaluation unit and wherein said impedance measuring control and evaluation unit is adapted to process an output value generated by said activity sensor for evaluating said sequence of consecutive impedance values.

12. The implantable medical device according to claim 2, wherein said impedance measuring control and evaluation unit is adapted to determine a respiratory rate from said intrathoracic impedance values and to use respiration rate values thus determined for further evaluating said sequence of consecutive impedance values.

13. The implantable medical device according to claim 5, wherein said impedance measuring control and evaluation unit is adapted to determine a heart rate from said intracardiac impedance values and to use heart rate values thus determined for further evaluating said sequence of consecutive impedance values.

14. The implantable medical device according to claim 1, wherein the implantable medical device comprises sensing means for sensing electric potential indicating excitation of myocardium and a heart rate detector connected to said sensing means for determining a heart rate, wherein said impedance measuring control and evaluation unit is connected to said heart rate detector and is adapted to use heart rate values received from said heart rate detector for further evaluating said sequence of consecutive impedance values.

15. The implantable medical device according to claim 1, wherein said impedance measuring control and evaluation unit is adapted to perform a population analysis based on sequences of consecutive impedance values.

16. The implantable medical device according to claim 1, wherein said impedance measuring control and evaluation unit is adapted to determine predetermined population properties.

17. The implantable medical device according to claim 16, wherein said impedance measuring control and evaluation unit is adapted to determine said predetermined population properties by analysing cluster of points in at least one dimensional space wherein a first dimension is given by a heart rate and a second dimension is given by an impedance and the third dimension is either a time of day or a respiratory rate and wherein point of a cluster is determined by a triplet formed by a heart rate value, an impedance value and either a time of day value or a respiratory rate value belonging together.

18. The implantable medical device according to claim 2, wherein said impedance measuring control and evaluation unit is adapted to compare a recent intrathoracic impedance value to a reference intrathoracic impedance value and to detect a decrease in thoracic fluid if said recent intrathoracic impedance value exceeds said reference intrathoracic impedance value.

19. The implantable medical device according to claim 2, wherein said impedance measuring control and evaluation unit is adapted to compare a recent intrathoracic impedance value to a reference intrathoracic impedance value and to detect a change in thoracic fluid if said recent intrathoracic impedance value shows a corresponding change in said reference intrathoracic impedance value.

20. The implantable medical device according to claim 5, wherein said impedance measuring control and evaluation unit is adapted to compare a recent intracardiac impedance value to a reference intracardiac impedance value and to detect a change in end-systolic volume if said recent intracardiac impedance value shows a corresponding change in said reference intracardiac impedance value.

21. The implantable medical device according to claim 5, wherein said impedance measuring control and evaluation unit is adapted to compare a recent intracardiac impedance value to a reference intracardiac impedance value and to detect an increase in end-systolic volume if said recent intracardiac impedance value is smaller than said reference intracardiac impedance value.

22. The implantable medical device according to claim 1, wherein said impedance measuring control and evaluation unit is adapted to analyse the modulation of a fiducial value in said intracardiac impedance Z_cardiac by said intrathoracic impedance Z_resp corresponding to the cardiac function indication of the Frank-Starling curve, thus enabling the discrimination of absence, decompensation to, presence of, or resolution from heart failure as a prodromal or other confirmatory sign of lung edema.

23. The implantable medical device according to claim 22, wherein said impedance measuring control and evaluation unit is adapted to determine a fiducial value of Z_cardiac versus the phase between a fiducial value of Z_resp and a fiducial value of Z_cardiac.

24. The implantable medical device according to claim 22, wherein said impedance measuring control and evaluation unit is adapted to detect a significant decrease in modulation amplitudes in a graph representing said fiducial value of Z_cardiac versus said phase between said fiducial value of Z_resp and said fiducial value of Z_cardiac.

25. The implantable medical device according to claim 1, wherein said constant current source is further adapted to generate monophasic measuring current pulse and to alternate polarity of the measuring current pulses from current pulse to current pulse.

26. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to vary the measurement pulse duration from pulse to pulse.

27. The implantable medical device according to claim 1, wherein said constant current source is further adapted to generate measuring current pulse having a pulse strength that is smaller than an excitation threshold of a myocardium.

28. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to modulate the sampling rate for intrathoracic impedance samples such that the sampling rate is not constant but varies about 10% or less.

29. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to limit each sequence of impedance values to represent a single full respiratory cycle.

30. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to initiate recording of a sequence of impedance values several times a day.

31. The implantable medical device according to claim 1, wherein the impedance determination unit is further adapted to initiate recording of a sequence of impedance values once an hour.

32. The implantable medical device according to claim 1, wherein the impedance determination unit comprises a switch matrix that is arranged between said constant current or constant voltage source and said measuring unit on one side and said electrodes on the other side and wherein the impedance determination unit is further adapted to alternate an electrode configuration from time to time by controlling said switch matrix such that connections between said constant current or constant voltage source and said measuring unit on one side and said electrodes on the other side are altered.

33. A method of monitoring intrathoracic fluid content, comprising the steps of:

measuring intrathoracic impedance between a first electrode and a second electrode continuously without regard to the cardiac cycle, achieving reduced amount of electrical noise due to filtering, averaging, and auto gain and auto current magnitude methods and providing an impedance output signal thereof,

storing the un-filtered impedance output signal;

performing steps for a predetermined number of measurement unit sample timing ticks to thereby generate a set of un-filtered impedance data; and

,

mathematically manipulating the sample of impedance data to render an estimate of

population statistics from said sample of impedance data.