Systems and methods for adjusting electrical therapy based on impedance changes

1. A patient therapy system, comprising:

an implantable signal delivery device having an electrical contact positionable to deliver therapy signals to a target neural population of a patient; and

an implantable pulse generator electrically coupleable to the implantable signal delivery device and configured to generate the therapy signals, wherein the therapy signals include a signal parameter, and wherein the implantable pulse generator includes a computer-readable medium having instructions that, when executed,

determine a proximity of the electrical contact to the patient's cerebral spinal fluid, and

in response to a change in the proximity of the electrical contact to the patient's cerebral spinal fluid, adjust the signal parameter.

2. The system of claim 1 wherein determining a proximity of the electrical contact to the patient's cerebral spinal fluid includes determining an impedance of an electrical circuit that includes the electrical contact.

3. The system of claim 2 wherein determining an impedance includes determining a capacitive component of the impedance.

4. The system of claim 2 wherein determining an impedance includes determining a change in impedance as a function of time.

5. The system of claim 4 , further comprising generating a predicted future value of impedance.

6. The system of claim 1 wherein adjusting the signal parameter includes adjusting a compliance margin of the therapy signals.

7. The system of claim 1 wherein adjusting the signal parameter includes adjusting a frequency of the therapy signals.

8. The system of claim 1 wherein adjusting the signal parameter includes adjusting a signal strength of the therapy signals.

9. The system of claim 1 , wherein the implantable pulse generator generates therapy signals in accordance with multiple signal parameters and in response to a change in the proximity of the electrical contact to the patient's cerebral spinal fluid, the instructions adjust one or more of the multiple signal parameters.

10. A patient therapy system, comprising:

an implantable signal delivery device having a first electrode and a second electrode, wherein the first electrode is positionable to deliver first therapy signals to a target neural population of a patient via a first electrical circuit that includes the first electrode, and the second electrode is positionable to deliver second therapy signals to the target neural population via a second electrical circuit that includes the second electrode; and

an implantable pulse generator electrically coupleable to the implantable signal delivery device and having a computer-readable medium including instructions that, when executed,

initiate delivery of the first therapy signals to the target neural population via the first electrode;

measure an impedance of the first electrical circuit to determine if the first electrical circuit is an open circuit, and

in response to a determination that the first electrical circuit is an open circuit, initiate delivery of the second therapy signals to the target neural population via the second electrode.

11. The system of claim 10 wherein measuring an impedance of the first electrical circuit includes measuring a capacitive component of the impedance and a resistive component of the impedance.

12. The system of claim 10 , further comprising:

determining that the first electrical circuit is a closed circuit; and

re-initiating delivery of the first therapy signals to the target neural population via the first electrode.

13. The system of claim 12 wherein re-initiating delivery of the first therapy signals includes ramping up a signal strength of the first therapy signals.

14. The system of claim 10 wherein measuring an impedance of the first electrical circuit includes generating a non-therapeutic electrical signal at the implantable pulse generator and transmitting the non-therapeutic electrical signal via the first electrode.

15. The system of claim 10 , further comprising reducing the signal strength of the first therapy signal to a non-zero value.

16. The system of claim 10 wherein determining that the first electrical circuit is an open circuit includes determining that the impedance is greater than a threshold value.

17. A method for delivering therapy signals to a target neural population of a patient, the method comprising:

generating a therapy signal at an implantable pulse generator, wherein the therapy signal includes a signal parameter;

transmitting the therapy signal to the target neural population via an electrode of a signal delivery device coupled to the implantable pulse generator;

determining a proximity of the electrode to the patient's cerebral spinal fluid; and

in response to a change in the proximity of the electrode to the patient's cerebral spinal fluid, adjusting the signal parameter.

18. The method of claim 17 wherein adjusting the signal parameter includes adjusting a signal strength of the therapy signal.

19. The method of claim 17 wherein determining a proximity of the electrode to the patient's cerebral spinal fluid includes measuring an impedance of an electrical circuit that includes the electrode and the target neural population.

20. The method of claim 19 wherein measuring an impedance includes delivering a non-therapeutic electrical signal via the electrode.

21. The method of claim 17 , further comprising adjusting a compliance value of the therapy signal.