Automatic detection of phrenic nerve stimulation

1. A system for ablating a subjects tissue while protecting a phrenic nerve of the subject, said system comprising:

a catheter comprising a distal tip having an ablation electrode, the distal tip movable within the subject's body;

a pacing stimulator configured to generate pacing currents from the catheter;

a radiofrequency generator configured to generate radio frequency ablation signals that are passed by the catheter into the subject's tissue;

a plurality of other electrodes other than the ablation electrode, said other electrodes configured to be electrically coupled to the body of the subject; and

a processor configured to:

navigate the catheter within the body of the subject to a particular location by ascertaining the current location of the catheter in response to the different respective impedances exhibited between the catheter and each of the other electrodes,

ascertain whether the distal tip of the catheter is near the phrenic nerve of the subject at the particular location when pacing current is passed from the catheter into the subject's tissue by detecting whether the phrenic nerve of the subject is stimulated by the pacing current,

acquire impedance signals that represent the impedance between one or more pairs of the other electrodes when such pacing current is passed from the catheter into the subject's tissue, and

determine whether the phrenic nerve of the subject was stimulated by the pacing current based on the impedance signals; wherein the impedance signals which determine whether the phrenic nerve of the subject was stimulated by the pacing current are effective to indicate whether damage to the phrenic nerve may result if ablating of the subject's tissue at the current location of the catheter is performed.

2. The system according to claim 1 , wherein the processor is configured to acquire the impedance signal by causing an electric current to be passed between a given pair of the other electrodes while the pacing current is passed into the tissue.

3. The system according to claim 2 , wherein the processor is configured to acquire the impedance signal by:

causing respective electric currents to be passed between a plurality of pairs of the other electrodes, including the given pair of the other electrodes, while the pacing current is passed into the tissue, and

deriving the impedance signal from respective impedances measured between the pairs of the other electrodes while the electric currents are passed.

4. The system according to claim 1 , wherein an output of said system includes a warning indicating that the tissue at the particular location should not be ablated.

5. The system according to claim 1 ,

wherein some of the other electrodes are configured to be coupled to a front of the subject and others of the other electrodes are configured to be coupled to a back of the subject, and

wherein a given pair of the other electrodes includes one of the other electrodes coupled to the front of the subject and one of the other electrodes coupled to the back of the subject.

6. The system according to claim 1 , wherein the processor is configured to navigate the catheter by repeatedly:

ascertaining the current location of the catheter, responsive to different respective impedances exhibited between the catheter and each of the other electrodes, and

generating an output that indicates the ascertained location.

7. The system according to claim 1 , wherein the processor is configured to identify a quivering pattern by identifying a longer amount of time between successive zero-crossings of a first portion of the impedance signal, relative to other portions of the impedance signal.

8. The system according to claim 1 , wherein the processor is configured to identify a quivering pattern by identifying a larger amplitude of peaks of a first portion of the impedance signal, relative to other portions of the impedance signal.

9. The system according to claim 1 ,

wherein the pacing current is passed into the tissue during a plurality of time periods, and

wherein the processor is configured to identify that the phrenic nerve was stimulated by identifying a quivering pattern in those portions of the impedance signal corresponding to the time periods, but not in other portions of the impedance signal.

10. A method for ablating a subject's tissue while protecting a phrenic nerve of the subject, said method comprising:

providing a catheter comprising a distal tip having an ablation electrode, the distal tip movable within the subject's body;

providing a pacing stimulator configured to generate pacing currents from the catheter;

providing a radiofrequency generator configured to generate radio frequency ablation signals that are passed by the catheter into the subject's tissue;

providing a plurality of other electrodes other than the ablation electrode which are coupled to the body of the subject;

navigating the catheter within the body of the subject to a particular location of the catheter in response to the different impedance exhibited between the catheter and each of the other electrodes;

passing a pacing current from the catheter into the subject's tissue;

ascertaining whether the distal tip of the catheter is near the phrenic nerve of the subject when the pacing current is passed from the catheter into the subject's tissue at the particular location by detecting whether the phrenic nerve of the subject is stimulated by the pacing current;

acquiring impedance signals that represent the impedance between one or more pairs of the other electrodes;

determining from said impedance signals whether the phrenic nerve of the subject was stimulated by the pacing current; and if so, generating an audio output and/or an output on a display to indicate that the phrenic nerve of the subject was stimulated by the pacing current;

wherein the impedance signals which determine whether the phrenic nerve of the subject was stimulated by the pacing current are effective to indicate whether damage to the phrenic nerve may result if ablating of the subject's tissue at the current location of the catheter is performed.

11. The method according to claim 10 , wherein acquiring the impedance signals comprises causing an electric current to be passed between a given pair of the other electrodes at least while the pacing current is passed into the tissue.

12. The method according to claim 11 , wherein acquiring the impedance signals comprises:

causing respective electric currents to be passed between a plurality of pairs of the other electrodes, including the given pair of the other electrodes, at least while the pacing current is passed into the tissue,

measuring respective impedances between the pairs of the other electrodes while the electric currents are passed, and

deriving the impedance signals from the respective impedances.

13. The method according to claim 10 , wherein generating an output comprises displaying a warning indicating that the tissue at the particular location should not be ablated.

14. The method according to claim 10 ,

wherein some of the other electrodes are coupled to a front of the subject and some of the other electrodes are coupled to a back of the subject, and

wherein the pair of the other electrodes includes one of the other electrodes coupled to the front of the subject and one of the other electrodes coupled to the back of the subject.

15. The method according to claim 10 , wherein navigating the catheter comprises repeatedly:

ascertaining the current location of the catheter, responsive to different respective impedances exhibited between the catheter and each of the other electrodes, and

generating an output that indicates the ascertained location.

16. The method according to claim 10 , further comprising identifying a quivering pattern in the impedance signal by identifying a longer amount of time between successive zero-crossings of a first portion of the impedance signal, relative to other portions of the impedance signal.

17. The method according to claim 10 , further comprising identifying a quivering pattern in the impedance signal by identifying a larger amplitude of peaks of a first portion of the impedance signal, relative to other portions of the impedance signal.

18. The method according to claim 10 ,

wherein the pacing current is passed into the tissue during a plurality of time periods, and

wherein identifying that the phrenic nerve was stimulated comprises identifying a quivering pattern in those portions of the impedance signal corresponding to the time periods, but not in other portions of the impedance signal.