Contractility modulation based on impedance signal

1. An implantable medical device, comprising:

edema detection circuitry configured to sense a condition correlative to edema associated with cardiac dysfunction in a subject; and

therapy delivery circuitry configured to deliver a cardiac therapy in response to said detection, wherein said therapy delivery circuitry provides non-excitatory electrical energy to the heart during a refractory period for the purpose of cardiac contractility modulation.

2. The device of claim 1 , wherein the edema detection circuitry comprises an impedance measurement circuit including:

an exciter configured to provide a high frequency test stimulus to the subject's thorax, the test stimulus being of a type that does not stimulate tissue or muscle contractions in the thorax;

a preamplifier configured to receive a thoracic signal responsive to the high frequency test stimulus, and to provide a preamplifier output as a function thereof; and a demodulator configured to demodulate the preamplifier output.

3. An implantable medical device according to claim 1 , wherein the edema detection circuit further comprises:

a plurality of electrodes configured to be electrically coupled to a patient's heart;

an impedance measurement circuit coupled to at least one of the electrodes and configured to measure transthoracic impedance; and

a processor configured to generate a first and a second transthoracic impedance signal based on the measured transthoracic impedance.

4. The device of claim 3 , wherein the first transthoracic impedance signal is long-term relative to the second transthoracic impedance signal and wherein the processor is further configured to provide an output based on a difference between the first and second transthoracic impedance signals.

5. The device of claim 4 , wherein the processor is further configured to compare the output to a threshold value.

6. The device of claim 5 , wherein the processor is further configured to provide an indicator based on the comparison, the indicator relating to fluid buildup in the patient's thorax.

7. The device of claim 3 , wherein the processor generates the first and second transthoracic impedance signals based on averaging the measured transthoracic impedance.

8. An implantable medical device according to claim 3 , wherein the processor generates the first and second transthoraric impedance signal responsive to a cardiac stroke of said heart.

9. An implantable medical device according to claim 3 , wherein the therapy delivery circuit is configured to modify one or more of a timing parameter, a pulse width parameter and a magnitude parameter of the delivery of cardiac therapy based on the first and second transthoracic impedance signals.

10. The device of claim 1 , further comprising:

a cardiac pacing circuit coupled to a plurality of electrodes and configured to deliver pacing therapy to the patient's heart.

11. The device of claim 10 , wherein the pacing therapy includes cardiac resynchronization therapy.

12. The device of claim 1 , further comprising: a housing adapted for implantation in the patient; wherein the impedance measurement circuit and the processor are disposed in the housing.

13. The device of claim 12 , further comprising:

a cardiac pacing circuit coupled to at least some of the plurality of electrodes and configured to deliver pacing therapy to the patient's heart;

wherein the cardiac pacing circuit is also disposed in the housing.

14. A device adapted to be implanted in a patient, comprising:

an implantable cardiac rhythm management apparatus including a circuit housing and a plurality of electrodes;

a cardiac pacing circuit disposed in the housing and coupled to at least some of the electrodes to deliver pacing therapy to the patient's heart;

an impedance measurement circuit disposed in the housing, coupled to at least some of the electrodes, and configured to measure a transthoracic impedance of the patient, the impedance measurement circuit including (a) an exciter circuit configured to apply a high frequency test stimulus to the patient's thorax, the test stimulus being of a type that does not stimulate tissue or muscle contractions in the thorax, (b) a preamplifier configured to receive a transthoracic response signal associated with the high frequency test stimulus, and configured to provide a preamplifier output as a function thereof, and (c) a demodulator configured to demodulate the preamplifier output;

a processor disposed in the housing and being configured to (a) receive the measured transthoracic impedance from the impedance measurement circuit, (b) generate first and second transthoracic impedance signals by averaging the measured transthoracic impedance, the first transthoracic impedance signal being long-term relative to the second transthoracic impedance signal, (c) compare the first and second thoracic impedance signals, and (d) provide an indicator based on the comparison, the indicator relating to fluid buildup in the patient's thorax and taking into account a cardiac stroke of said heart; and

therapy delivery circuitry configured to modify one or more of a timing parameter, a pulse width parameter and a magnitude parameter of the delivery of cardiac therapy based on the indicator, wherein said cardiac therapy includes the therapeutic delivery of non-excitatory electrical energy to the heart during a refractory period for the purpose of cardiac contractility modulation.

15. The device of claim 14 , wherein the first transthoracic impedance signal is a long-term value of the second transthoracic impedance signal.

16. The device of claim 14 , wherein the first transthoracic impedance signal is characterized by a delay relative to the second transthoracic impedance signal.

17. The device of claim 14 , wherein the processor generates a difference signal from the comparison of the first and second thoracic impedance signals.

18. The device of claim 17 , wherein the processor provides the indicator as a function of the difference signal and a threshold value.

19. A method of monitoring fluid buildup and providing corrective therapy to a patient, comprising:

measuring a transthoracic impedance of the patient;

determining edema associated with a cardiac dysfunction based on the measured transthoracic impedance; and

delivering a corrective cardiac therapy signal based on the determination, wherein said cardiac therapy includes the therapeutic delivery of non-excitatory electrical energy to the heart during a refractory period for the purpose of cardiac contractility modulation.

20. A method according to claim 19 , further comprising:

providing a plurality of electrodes for implantation in the patient;

delivering cardiac pacing therapy to the patient's heart using at least some of the electrodes,

wherein said measuring further comprises (a) applying a high frequency test stimulus to the patient's thorax using at least some of the electrodes, the test stimulus being of a type that does not stimulate tissue or muscle contractions in the thorax, (b) receiving a transthoracic response signal associated with the high frequency test stimulus, and (c) demodulating the response signal;

generating first and second transthoracic impedance signals by averaging the measured transthoracic impedance, the first transthoracic impedance signal being long-term relative to the second transthoracic impedance signal;

comparing the first and second thoracic impedance signals; and

providing an indicator based on the comparison, the indicator relating to fluid buildup in the patient's thorax.

21. The method of claim 20 , wherein the the first transthoracic impedance signal is a long-term value of the second transthoracic impedance signal.

22. The method of claim 20 , wherein the the first transthoracic impedance signal is delayed relative to the second transthoracic impedance signal.

23. The method of claim 20 , wherein the first and second thoracic impedance signals are compared by generating a difference signal from the two thoracic impedance signals.

24. The method of claim 23 , wherein the indicator provided is a function of the difference signal and a threshold value.

25. A method according to claim 20 , wherein the indicator is responsive to a cardiac stroke of said heart.

26. A method according to claim 20 , wherein one or more of a timing parameter, a pulse width parameter and a magnitude parameter of the corrective therapy signal is modified in response to said indicator.