Safe-mode operation of an implantable medical device

1. A method for minimizing current in a lead circuit of an implantable medical device (IMD) system induced by coupled energy from an external source, comprising:

providing a first impedance between a first node and a second node of the IMD, the lead circuit comprising a first lead configured to couple the first node of the IMD to a first target tissue and a second lead configured to couple the second node of the IMD to a second target tissue;

receiving an indication of the presence of the coupled energy on the lead circuit;

determining a characteristic of the coupled energy;

selecting a second impedance from a plurality of impedances based at least in part on the characteristic of the coupled energy, the second impedance selected to minimize the current induced by the coupled energy; and

providing the selected second impedance between the first node and the second node in place of the first impedance, wherein the first impedance and the second impedance are not open circuit impedances.

2. The method of claim 1 , further comprising:

providing a stimulation signal from the IMD through the lead circuit to deliver a therapy in the presence of the coupled energy, the IMD having the second impedance between the first node and the second node.

3. The method of claim 2 , wherein the stimulation signal is provided to a vagus nerve.

4. The method of claim 1 , further comprising:

determining that the coupled energy between the first node and the second node has substantially diminished; and

providing the first impedance in place of the second impedance between the first node and the second node based upon determining that the coupled energy on the lead circuit has substantially diminished.

5. The method of claim 1 , wherein the characteristic of the coupled energy comprises a magnetic resonance imaging (MRI) signal level including one of 0.3 Tesla, 0.5 Tesla, 0.7 Tesla, 1.0 Tesla, 1.5 Tesla, 3.0 Tesla, 5.0 Tesla, and 7.0 Tesla.

6. The method of claim 1 , wherein the second impedance is selected to substantially reduce radio frequency (RF) heating of the lead circuit at one of about 128 MHz, about 64 MHz, and about 42.67 MHz.

7. The method of claim 1 , wherein the second impedance includes at least one of a capacitor and an inductor.

8. The method of claim 1 , wherein receiving the indication of the presence of the coupled energy comprises receiving at least one of an input from a source external to the IMD and a signal responsive to a detection of the coupled energy.

9. The method of claim 1 , further comprising:

determining that the coupled energy on the lead circuit has changed to a second coupled energy;

determining a second characteristic of the second coupled energy;

selecting a third impedance from the plurality of impedances based at least in part on the second characteristic of the second coupled energy, the third impedance selected to minimize current induced by the second coupled energy; and

providing the selected third impedance between the first node and the second node in place of the second impedance, wherein the third impedance is not an open circuit impedance.

10. The method of claim 1 , wherein determining the characteristic of the coupled energy comprises determining that the coupled energy is greater than a predetermined threshold.

11. The method of claim 1 , wherein determining the characteristic of the coupled energy comprises determining a frequency of the coupled energy.

12. The method of claim 11 , wherein the second impedance is selected based at least in part on the frequency of the coupled energy.

13. The method of claim 1 , wherein the plurality of impedances are provided by a programmable impedance array between the first node and the second node.

14. A method for minimizing current in a lead circuit of an implantable medical device (IMD) system induced by coupled energy from an external source, comprising:

providing a first impedance between a first node and a second node of the IMD, the lead circuit comprising a first lead configured to coupled the first node of the IMD to a first portion of a cranial nerve and a second lead configured to couple the second node of the IMD to a second portion of the cranial nerve;

detecting the coupled energy on the lead circuit at a detection unit of the IMD;

determining a characteristic of the coupled energy at the detection unit;

selecting a second impedance from at least three impedances based at least in part on the characteristic of the coupled energy, the second impedance selected to minimize the current induced by the coupled energy;

providing the selected second impedance between the first node and the second node in place of the first impedance, wherein the first impedance and the second impedance are not open circuit impedances; and

providing a stimulation signal from the IMD through the lead circuit to deliver a therapy to the cranial nerve in the presence of the coupled energy, the IMD having the second impedance between the first node and the second node.

15. The method of claim 14 , wherein the cranial nerve is a vagus nerve.

16. The method of claim 14 , further comprising:

determining that the coupled energy between the first node and the second node has substantially diminished; and

providing the first impedance in place of the second impedance between the first node and the second node based upon determining that the coupled energy on the lead circuit has substantially diminished.

17. The method of claim 14 , wherein the characteristic of the coupled energy comprises a magnetic resonance imaging (MRI) signal level including one of 0.3 Tesla, 0.5 Tesla, 0.7 Tesla, 1.0 Tesla, 1.5 Tesla, 3.0 Tesla, 5.0 Tesla, and 7.0 Tesla.

18. The method of claim 14 , wherein the second impedance is selected to substantially reduce radio frequency (RF) heating of the lead circuit at one of about 128 MHz, about 64 MHz, and about 42.67 MHz.

19. The method of claim 14 , wherein the second impedance includes at least one of a capacitor and an inductor.

20. The method of claim 14 , further comprising:

determining that the coupled energy on the lead circuit has changed to a second coupled energy;

determining a second characteristic of the second coupled energy;

selecting a third impedance from the plurality of impedances based at least in part on the second characteristic of the second coupled energy, the third impedance selected to minimize current induced by the second coupled energy; and

providing the selected third impedance between the first node and the second node in place of the second impedance, wherein the third impedance is not an open circuit impedance.

21. The method of claim 14 , wherein determining the characteristic of the coupled energy comprises at least one of determining that the coupled energy is greater than a predetermined threshold and determining the characteristic of the coupled energy comprises determining a frequency of the coupled energy.

22. The method of claim 14 , wherein the at least three impedances are provided by a programmable impedance array between the first node and the second node.

23. A method for minimizing current in a lead circuit of an implantable medical device (IMD) system induced by coupled energy from an external source, comprising:

providing a first impedance between a first node and a second node of the IMD, the lead circuit comprising a first lead configured to coupled the first node of the IMD to a first portion of a cranial nerve and a second lead configured to couple the second node of the IMD to a second portion of the cranial nerve;

detecting the coupled energy on the lead circuit at a detection unit of the IMD;

determining a characteristic of the coupled energy at the detection unit;

selecting a second impedance from at least three impedances in a programmable impedance array based at least in part on the characteristic of the coupled energy, the second impedance selected to minimize the current induced by the coupled energy;

providing the selected second impedance between the first node and the second node in place of the first impedance, wherein the first impedance and the second impedance are not open circuit impedances;

providing a stimulation signal from the IMD through the lead circuit to deliver a therapy to the cranial nerve in the presence of the coupled energy, the IMD having the second impedance between the first node and the second node;

determining that the coupled energy on the lead circuit has changed to a second coupled energy; and

dynamically adjusting the second impedance to minimize a second current induced by the second coupled energy.

24. The method of claim 23 , wherein the second impedance includes at least one of a capacitor and an inductor.

25. The method of claim 23 , wherein the second impedance is selected to substantially reduce radio frequency (RF) heating of the lead circuit at one of about 128 MHz, about 64 MHz, and about 42.67 MHz.