Impedance sensing

1. A medical device system comprising:

an electrode;

impedance measurement circuitry coupled to the electrode, the impedance measurement circuitry configured to generate an impedance signal indicating impedance proximate to the electrode;

therapeutic signal generation circuitry configured to generate and deliver an electrical stimulation; and

processing circuitry configured to:

identify a plurality of cardiac events within the impedance signal based on a determination that a measurement of the impedance signal satisfies at least one criterion;

determine a rate of the plurality of cardiac events identified within impedance signal;

detect a cardiac arrhythmia based on the rate of the plurality of cardiac events identified within the impedance signal; and

control the therapeutic signal generation circuitry to generate the electrical stimulation therapy in response to detection of the cardiac arrhythmia.

2. The medical device system of claim 1 , wherein the processing circuitry is further configured to:

determine one or more of a regularity of the identified plurality of cardiac events or a morphology of the identified plurality of cardiac events; and

detect the cardiac arrhythmia based on the one or more of the regularity or the morphology of the identified plurality of cardiac events.

3. The medical device system of claim 1 , wherein the processing circuitry is configured to:

detect a change in at least one of an amplitude or a slope in the impedance signal that exceeds a respective threshold; and

identify one of the plurality of cardiac events in response to detecting the change in at least one of the amplitude or the slope of the impedance signal that exceeds the respective threshold.

4. The medical device system of claim 1 , wherein the processing circuitry is configured to:

process the impedance signal using one or more of a filter, a transform function, or a derivative; and

analyze the processed impedance signal to detect the plurality of cardiac events.

5. The medical device system of claim 1 , further comprising sensing circuitry configured to sense an electrogram signal, wherein the rate of the cardiac events identified within the impedance signal is a first rate of first cardiac events, wherein the processing circuitry is configured to:

identify a second plurality of cardiac events within the electrogram signal;

detect a cardiac arrhythmia based on the second plurality of identified cardiac events; and

confirm the detected cardiac arrhythmia based on the first plurality of cardiac events within the impedance signal.

6. The medical device system of claim 5 , wherein the processing circuitry is further configured to:

determine a second rate based on the second plurality of identified cardiac events;

determine the second rate is greater than a threshold tachyarrhythmia rate; and

initiate the impedance measurement circuitry to begin obtaining the impedance signal in response to the rate being greater than the threshold tachyarrhythmia rate.

7. The medical device system of claim 5 , wherein the processing circuitry is configured to:

determine a second rate of the second cardiac events within the electrogram signal;

compare the first rate and the second rate;

confirm the detected cardiac arrhythmia based on the comparison of the first rate and the second rate; and

control the therapeutic signal generation circuitry to deliver the electrical stimulation in response to the confirmation.

8. The medical device system of claim 5 , wherein the processing circuitry is configured to:

determine a second rate based on the second plurality of identified cardiac events;

determine the second rate is greater than a threshold tachyarrhythmia rate;

initiate the impedance measurement circuitry to begin obtaining the impedance signal in response to the second rate being greater than the threshold tachyarrhythmia rate;

compare the first rate and the second rate;

confirm the detected cardiac arrhythmia based on the comparison of the first rate and the second rate; and

control the therapeutic signal generation circuitry to deliver the electrical stimulation in response to the confirmation.

9. The medical device system of claim 5 , wherein the processing circuitry is configured to confirm the detected cardiac arrhythmia based on one or more of the rate, a regularity or a morphology of the first plurality of cardiac events within the impedance signal.

10. The medical device system of claim 1 , wherein the processing circuitry is configured to:

determine a patient hemodynamic state;

control the therapeutic signal generation circuitry to deliver a shock when the determined patient hemodynamic state is non-stable; and

control the therapeutic signal generation circuitry to deliver anti-tachycardia pacing when the determined patient hemodynamic state is stable.

11. The medical device system of claim 10 , wherein the processing circuitry is configured to determine the patient hemodynamic state based on the impedance signal and at least one of a heart sound signal, an electrogram signal, or an accelerometer signal.

12. A method comprising:

generating an impedance signal indicating impedance proximate an electrode;

identifying a plurality of cardiac events within the impedance signal based upon a determination that a measurement of the impedance signal satisfies at least one criterion;

determining a rate of the plurality of cardiac events identified within the impedance signal;

detecting a cardiac arrhythmia based on the rate of the plurality of identified cardiac events within the impedance signal; and

controlling the therapeutic signal generation circuitry to generate the electrical stimulation therapy in response to detecting cardiac arrhythmia.

13. The method of claim 12 , wherein detecting the cardiac arrhythmia comprises:

determining one or more of a regularity of the identified plurality of cardiac events or a morphology of the identified plurality of cardiac events; and

detecting the cardiac arrhythmia based on the one or more of the regularity or the morphology of the identified plurality of cardiac events.

14. The method of claim 12 , wherein identifying the plurality of cardiac events within the impedance signal comprises:

detecting a change in at least one of an amplitude or a slope in the impedance signal that exceeds a respective threshold; and

identifying one of the plurality of cardiac events in response to detecting the change in at least one of the amplitude or the slope of the impedance signal that exceeds the respective threshold.

15. The method of claim 12 , wherein identifying a plurality of cardiac events comprises:

identifying the plurality of cardiac events by processing the impedance signal using one or more of a filter, a transform function, or a derivative; and

analyzing the processed impedance signal to detect the plurality of cardiac events.

16. The method of claim 12 , wherein the rate of the cardiac events identified within the impedance signal is a first rate of first cardiac events, the method further comprising:

identifying a second plurality of cardiac events within the electrogram signal; and

detecting a cardiac arrhythmia based on the second plurality of identified cardiac events;

wherein detecting a cardiac arrhythmia based on the plurality of identified cardiac events within the impedance signal comprises confirming the detected cardiac arrhythmia based on the first plurality of cardiac events within the impedance signal.

17. The method of claim 16 , further comprising:

determining a second rate based on the second plurality of identified cardiac events;

determining the second rate is greater than a threshold tachyarrhythmia rate; and

initiating the impedance measurement circuitry to begin obtaining the impedance signal in response to the second rate being greater than the threshold tachyarrhythmia rate.

18. The method of claim 16 , further comprising:

determining a second rate of the second cardiac events within the electrogram signal;

comparing the first rate and the second rate; and

confirming the detected cardiac arrhythmia based on the comparison of the first rate and the second rate;

wherein controlling the therapeutic signal generation circuitry to generate the electrical stimulation therapy in response to detecting cardiac arrhythmia comprises controlling the therapeutic signal generation circuitry to deliver the electrical stimulation in response to the confirmation.

19. The method of claim 16 , further comprising:

determining a second rate based on the second plurality of identified cardiac events;

determining the second rate is greater than a threshold tachyarrhythmia rate;

initiating the impedance measurement circuitry to begin obtaining the impedance signal in response to the second rate being greater than the threshold tachyarrhythmia rate;

comparing the first rate and the second rate; and

confirming the detected cardiac arrhythmia based on the comparison of the first rate and the second rate;

wherein controlling the therapeutic signal generation circuitry to generate the electrical stimulation therapy in response to detecting cardiac arrhythmia comprises controlling the therapeutic signal generation circuitry to deliver the electrical stimulation in response to the confirmation.

20. The method of claim 16 , wherein confirming the detected cardiac arrhythmia based on the first plurality of cardiac events within the impedance signal comprises confirming the detected cardiac arrhythmia based on one or more of the first rate, a regularity or a morphology of the first plurality of cardiac events within the impedance signal.

21. The method of claim 12 , further comprising:

determining a patient hemodynamic state;

wherein controlling the therapeutic signal generation circuitry to generate the electrical stimulation therapy in response to detecting cardiac arrhythmia comprises:

controlling the therapeutic signal generation circuitry to deliver a shock when the determined patient hemodynamic state is non-stable; and

controlling the therapeutic signal generation circuitry to deliver anti-tachycardia pacing when the determined patient hemodynamic state is stable.

22. The method of claim 21 , wherein determining the patient hemodynamic state comprises determining the patient hemodynamic state based on the impedance signal and at least one of a heart sound signal, an electrogram signal, or an accelerometer signal.

23. A medical device system comprising:

an electrode;

impedance measurement circuitry coupled to the electrode, the impedance measurement circuitry configured to generate an impedance signal indicating impedance proximate to the electrode; and

processing circuitry configured to:

analyze at least one of an amplitude and a slope in the impedance signal;

detect a change in at least one of the amplitude or the slope in the impedance signal that exceeds a respective threshold; and

detect a cardiac event in response to detecting the change in at least one of the amplitude or the slope of the impedance signal that exceeds the respective threshold.