Methods and apparatus for controlling a pacing system in the presence of EMI

• US 20030144705A1
• Filed: 05/10/2002
• Published: 07/31/2003
• Est. Priority Date: 01/29/2002
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Funke, Hermann D.
• Granted Patent: US 7,082,328 B2
• Time in Patent Office: Days
• US Class Current: 607/27
• CPC Class Codes: A61N 1/37 Monitoring; Protecting
• CPC Class Codes: A61N 1/3718 Monitoring of or protection against external electromagnetic fields or currents

Abstract

Pacing systems are disclosed including detectors for detecting the presence of electromagnetic interference and setting an interference state pacing mode and pacing rate. The interference state pacing mode and pacing rate are altered as a function of patient pacemaker dependency and the prevailing mean heart rate. When pacemaker dependency exists, the pacing rate is maintained and even increased from the prevailing mean heart for the duration of the interference state. When the patient is determined to not be pacemaker dependent, pacing is inhibited or suspended for the duration of the interference state.

Claims

1. In a cardiac pacing system operating in a pacing mode by:

timing out a pacing escape interval that establishes a pacing rate;

delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

restarting the pacing escape interval upon generation of a sense event,

a method of operating the pacing system to respond to the presence of electromagnetic interference tending to corrupt the features of the electrogram comprising;

accumulating data from the sense events and delivered pacing pulses over a predetermined time interval and deriving a mean heart rate from the accumulated data;

detecting electromagnetic interference tending to corrupt the features of the electrogram and declaring an interference state;

adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

2. The method of claim 1, wherein the detecting step comprises detecting strong magnetic fields, and declaring the interference state when the detected strong magnetic fields exceed a predetermined magnitude.

3. The method of claim 2, further comprising restoring a prevailing pacing mode and pacing rate when the magnetic field falls below the predetermined magnitude for a predetermined time duration.

4. The method of claim 1, wherein the detecting step comprises detecting high intensity radiation signals, and declaring the interference state when the detected high intensity radiation signals exceed a predetermined magnitude.

5. The method of claim 2, further comprising restoring a prevailing pacing mode and pacing rate when the high intensity radiation signals fall below the predetermined magnitude for a predetermined time duration.

6. In a cardiac pacing system operating in a pacing mode comprising:

means for timing out a pacing escape interval that establishes a pacing rate;

means for delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

means for sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

means for restarting the pacing escape interval upon generation of a sense event,

a system that operates the pacing system to respond to the presence of electromagnetic interference tending to corrupt the features of the electrogram comprising;

means for accumulating data from the sense events and delivered pacing pulses over a predetermined time interval and for deriving a mean heart rate from the accumulated data;

means for detecting electromagnetic interference tending to corrupt the features of the electrogram and declaring an interference state; and

means for adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment, whereby the patient'"'"'s heart is paced at an adjusted pacing rate while minimizing occurrences of parasystole.

7. The system of claim 6, wherein the detecting means comprises means for detecting strong magnetic fields, and declaring the interference state when the detected strong magnetic fields exceed a predetermined magnitude.

8. The system of claim 7, further comprising means for restoring a prevailing pacing mode and pacing rate when the magnetic field falls below the predetermined magnitude for a predetermined time duration.

9. The system of claim 6, wherein the detecting means comprises means for detecting high intensity radiation signals, and declaring the interference state when the detected high intensity radiation signals exceed a predetermined magnitude.

10. The system of claim 6, further comprising means for restoring a prevailing pacing mode and pacing rate when the high intensity radiation signals fall below the predetermined magnitude for a predetermined time duration.

11. In a cardiac pacing system operating in a pacing mode by:

timing out a pacing escape interval that establishes a pacing rate;

delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

restarting the pacing escape interval upon generation of a sense event,

a method of operating the pacing system to respond to the presence of electromagnetic interference tending to corrupt the features of the electrogram comprising;

accumulating data from the sense events and delivered pacing pulses over a predetermined time interval, deriving a mean heart rate from the accumulated data, and deriving a pacing dependency percentage representing the ratio of delivered pacing pulses to the sum of delivered pacing pulses and sensed events over the predetermined time interval;

declaring an interference state if electromagnetic interference tending to corrupt the features of the electrogram is detected;

determining whether the patient is pacemaker dependent or pacemaker independent from the pacing dependency percentage;

during the declared interference state;

setting the pacing mode to an inhibited pacing mode if the patient is determined to be pacemaker independent, whereby pacing pulses are not delivered to the heart chamber;

setting the pacing mode to a fixed rate pacing mode if the patient is determined to be pacemaker dependent, whereby the pacing escape interval is not restarted by a sense event and pacing pulses are delivered to the heart chamber at a fixed rate; and

adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

12. The method of claim 11, wherein the step of determining if the patient is pacemaker dependent or pacemaker independent comprises:

establishing a pacing dependency threshold;

comparing the pacing dependency percentage to the pacing dependency threshold;

declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

13. The method of claim 11, wherein:

the step of determining if the patient is pacemaker dependent or pacemaker independent comprises;

establishing a first pacing dependency threshold and a second pacing dependency threshold exceeding the first pacing dependency threshold;

comparing the pacing dependency percentage to the first and second pacing dependency thresholds;

declaring the patient pacemaker independent when the pacing dependency percentage falls below the first pacing dependency threshold;

declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the second pacing dependency threshold; and

declaring the patient moderately pacemaker dependent when the pacing dependency percentage falls between the first and second pacing dependency thresholds; and

the adjusting step comprises;

adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a first predetermined increment when the patient is declared pacemaker dependent, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole; and

adjusting the pacing escape interval to provide a second interference state pacing rate exceeding the mean heart rate by a second predetermined increment greater than the first predetermined increment when the patient is declared moderately pacemaker dependent, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

14. In a cardiac pacing system operating in a pacing mode by operation of:

means for timing out a pacing escape interval that establishes a pacing rate;

means for delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

means for sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

means for restarting the pacing escape interval upon generation of a sense event,

a system that operates the pacing system to respond to the presence of electromagnetic interference tending to corrupt the features of the electrogram comprising;

means for accumulating data from the sense events and delivered pacing pulses over a predetermined time interval, for deriving a mean heart rate from the accumulated data, and for deriving a pacing dependency percentage representing the ratio of delivered pacing pulses to the sum of delivered pacing pulses and sensed events over the predetermined time interval;

means for declaring an interference state upon detecting electromagnetic interference tending to corrupt the features of the electrogram;

means for determining whether the patient is pacemaker dependent or pacemaker independent from the pacing dependency percentage;

means for setting the pacing mode to an inhibited pacing mode if the patient is determined to be pacemaker independent, whereby pacing pulses are not delivered to the heart chamber;

means for setting the pacing mode to a fixed rate pacing mode if the patient is determined to be pacemaker dependent, whereby the pacing escape interval is not restarted by a sense event and pacing pulses are delivered to the heart chamber at a fixed rate; and

means for adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

15. The system of claim 14, wherein the means for determining if the patient is pacemaker dependent or pacemaker independent comprises:

means for establishing a pacing dependency threshold;

means for comparing the pacing dependency percentage to the pacing dependency threshold;

means for declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

means for declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

16. The system of claim 14, wherein:

the means for determining if the patient is pacemaker dependent or pacemaker independent comprises;

means for establishing a first pacing dependency threshold and a second pacing dependency threshold exceeding the first pacing dependency threshold;

means for comparing the pacing dependency percentage to the first and second pacing dependency thresholds;

means for declaring the patient pacemaker independent when the pacing dependency percentage falls below the first pacing dependency threshold;

means for declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the second pacing dependency threshold; and

means for declaring the patient moderately pacemaker dependent when the pacing dependency percentage falls between the first and second pacing dependency thresholds; and

the adjusting means comprises;

means for adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a first predetermined increment when the patient is declared pacemaker dependent, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole; and

means for adjusting the pacing escape interval to provide a second interference state pacing rate exceeding the mean heart rate by a second predetermined increment greater than the first predetermined increment when the patient is declared moderately pacemaker dependent, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

17. In a cardiac pacing system operating in a pacing mode comprising:

timing out a pacing escape interval that establishes a pacing rate;

delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

restarting the pacing escape interval upon generation of a sense event,

a method of operating the pacing system to respond to the presence of a magnetic field comprising;

accumulating data from the sense events and delivered pacing pulses over a predetermined time interval, deriving a mean heart rate from the accumulated data, and deriving a pacing dependency percentage representing the ratio of delivered pacing pulses to the sum of delivered pacing pulses and sensed events over the predetermined time interval;

detecting a magnetic field exceeding an interference threshold and declaring an interference state;

determining whether the patient is pacemaker dependent or pacemaker independent from the pacing dependency percentage;

if the patient is determined to be pacemaker independent;

comparing the magnet mode pacing rate to the sum of the mean heart rate and a first rate increment;

setting the pacing escape interval to establish an interference state pacing rate equal to the mean heart rate and the first rate increment if the sum of the mean heart rate and the first rate increment exceeds the magnet mode pacing rate; and

setting the pacing escape interval to establish the magnet mode pacing rate if the sum of the mean heart rate and the first rate increment falls below the magnet mode pacing rate; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

18. The method of claim 17, further comprising, if the patient is determined to be pacemaker independent:

comparing the magnet mode pacing rate to the sum of the mean heart rate and a second rate increment greater than said first rate increment;

setting the pacing escape interval to establish an interference state pacing rate equal to the mean heart rate and the second rate increment if the sum of the mean heart rate and the second rate increment exceeds the magnet mode pacing rate;

setting the pacing escape interval to establish the magnet mode pacing rate if the sum of the mean heart rate and a second rate increment falls below the magnet mode pacing rate; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

19. The method of claim 18, wherein the step of determining if the patient is pacemaker dependent or pacemaker independent comprises:

establishing a pacing dependency threshold;

comparing the pacing dependency percentage to the pacing dependency threshold;

declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

20. The method of claim 17, wherein the step of determining if the patient is pacemaker dependent or pacemaker independent comprises:

establishing a pacing dependency threshold;

comparing the pacing dependency percentage to the pacing dependency threshold;

declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

21. In a cardiac pacing system operating in a pacing mode comprising:

timing out a pacing escape interval that establishes a pacing rate;

delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

restarting the pacing escape interval upon generation of a sense event,

a method of operating the pacing system to respond to the presence of a magnetic comprising;

accumulating data from the sense events and delivered pacing pulses over a predetermined time interval, deriving a mean heart rate from the accumulated data, and deriving a pacing dependency percentage representing the ratio of delivered pacing pulses to the sum of delivered pacing pulses and sensed events over the predetermined time interval;

detecting a magnetic field exceeding an interference threshold and declaring an interference state;

determining whether the patient is pacemaker dependent or pacemaker independent from the pacing dependency percentage;

if the patient is determined to be pacemaker dependent;

comparing the magnet mode pacing rate to the sum of the mean heart rate and a rate increment;

setting the pacing escape interval to establish an interference state pacing rate equal to the mean heart rate and the rate increment if the sum of the mean heart rate and the rate increment exceeds the magnet mode pacing rate; and

setting the pacing escape interval to establish the magnet mode pacing rate if the sum of the mean heart rate and the rate increment falls below the magnet mode pacing rate; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

22. The method of claim 21, wherein the step of determining if the patient is pacemaker dependent or pacemaker independent comprises:

establishing a pacing dependency threshold;

comparing the pacing dependency percentage to the pacing dependency threshold;

declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

23. In a cardiac pacing system comprising:

means for timing out a pacing escape interval that establishes a pacing rate;

means for delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

means for sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

means for restarting the pacing escape interval upon generation of a sense event,

a system that alters the operation of the pacing system in an interference state comprising;

means for accumulating data from the sense events and delivered pacing pulses over a predetermined time interval, for deriving a mean heart rate from the accumulated data, and for deriving a pacing dependency percentage representing the ratio of paced events to the sum of paced and sensed events over the predetermined time interval;

means for detecting a magnetic field exceeding a magnetic field threshold and declaring an interference state;

means for determining whether the patient is pacemaker dependent or pacemaker independent from the pacing dependency percentage;

means operable if the patient is determined to be pacemaker independent for comparing the magnet mode pacing rate to the sum of the mean heart rate and a first rate increment;

means for setting the pacing escape interval to establish an interference state pacing rate equal to the mean heart rate and the first rate increment if the sum of the mean heart rate and the first rate increment exceeds the magnet mode pacing rate; and

means for setting the pacing escape interval to establish the magnet mode pacing rate if the sum of the mean heart rate and the first rate increment falls below the magnet mode pacing rate,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

24. The system of claim 23, further comprising:

means operable if the patient is determined to be pacemaker independent for comparing the magnet mode pacing rate to the sum of the mean heart rate and a second rate increment greater than said first rate increment, and either;

means for setting the pacing escape interval to establish an interference state pacing rate equal to the mean heart rate and the second rate increment if the sum of the mean heart rate and the second rate increment exceeds the magnet mode pacing rate; and

means for setting the pacing escape interval to establish the magnet mode pacing rate if the sum of the mean heart rate and a second rate increment falls below the magnet mode pacing rate,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

25. The method of claim 24, wherein the means for determining if the patient is pacemaker dependent or pacemaker independent comprises:

means for establishing a pacing dependency threshold;

means for comparing the pacing dependency percentage to the pacing dependency threshold;

means for declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

means for declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

26. The method of claim 23, wherein the means for determining if the patient is pacemaker dependent or pacemaker independent comprises:

means for establishing a pacing dependency threshold;

means for comparing the pacing dependency percentage to the pacing dependency threshold;

means for declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

means for declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

27. In a cardiac pacing system operating by operation of:

means for timing out a pacing escape interval that establishes a pacing rate;

means for delivering a pacing pulse to a heart chamber upon time-out of the pacing escape interval;

means for sensing a characteristic feature of the electrogram of the heart in the heart chamber during time-out of the pacing escape interval and generating a sense event; and

means for restarting the pacing escape interval upon generation of a sense event,

a system that operates the pacing system in an interference state pacing mode in the presence of a magnetic field exceeding the low strength magnetic field comprising;

means for accumulating data from the sense events and delivered pacing pulses over a predetermined time interval, for deriving a mean heart rate from the accumulated data, and for deriving a pacing dependency percentage representing the ratio of paced events to the sum of paced and sensed events over the predetermined time interval;

means for detecting a magnetic field exceeding a threshold and declaring an interference state;

means for determining whether the patient is pacemaker dependent or pacemaker independent from the pacing dependency percentage;

means operable if the patient is determined to be pacemaker dependent for comparing the magnet mode pacing rate to the sum of the mean heart rate and a rate increment;

means for setting the pacing escape interval to establish an interference state pacing rate equal to the mean heart rate and the rate increment if the sum of the mean heart rate and the rate increment exceeds the magnet mode pacing rate; and

means for setting the pacing escape interval to establish the magnet mode pacing rate if the sum of the mean heart rate and the rate increment falls below the magnet mode pacing rate,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

28. The pacing system of claim 27, wherein the means for determining if the patient is pacemaker dependent or pacemaker independent comprises:

means for establishing a pacing dependency threshold;

means for comparing the pacing dependency percentage to the pacing dependency threshold;

means for declaring the patient pacemaker dependent when the pacing dependency percentage exceeds the pacing dependency threshold; and

means for declaring the patient pacemaker independent when the pacing dependency percentage falls below the pacing dependency threshold.

29. In a dual chamber cardiac pacing system operating in a dual chamber pacing mode, the dual chamber pacing system having an atrial sensing channel including an atrial sense amplifier that generates an atrial sense event upon detection of a predetermined characteristic of the atrial electrogram, a ventricular sensing channel including a ventricular sense amplifier that generates a ventricular sense event upon detection of a predetermined characteristic of the ventricular electrogram, an atrial pacing pulse generator for delivering atrial pacing pulses to at least one atrial heart chamber, means for timing an AV delay from an atrial sense event or a delivered atrial pacing pulse, inhibiting means for stopping the time-out of the AV delay if a ventricular sense event is generated during time-out of the AV delay, and a ventricular pacing pulse generator for delivering ventricular pacing pulses to at least one ventricular heart chamber upon time-out of the AV delay, a method of operating the pacing system to respond to the presence of electromagnetic interference tending to corrupt the features of the ventricular electrogram comprising:

determining if electromagnetic interference is present in the ventricular sensing channel;

determining if electromagnetic interference is present in the atrial sensing channel; and

preventing the inhibiting means from stopping the time-out of the AV delay upon generation of a ventricular sense event when electromagnetic interference is determined to be present in the ventricular sensing channel and is not determined to be present in the atrial sensing channel,

whereby synchronized atrial and ventricular pacing pulses are delivered to the atrial and ventricular heart chambers while electromagnetic interference is present in the ventricular sensing channel.

30. The method of claim 29, wherein the dual chamber pacing mode is the DDD(R) pacing mode, and the preventing step comprises changing the pacing mode to the DDO*(R) interference state pacing mode.

31. The method of claim 29, further comprising:

enabling the inhibiting means when electromagnetic interference is determined to no longer be present in the ventricular sensing channel.

32. In a dual chamber cardiac pacing system operating in a dual chamber pacing mode, the dual chamber pacing system timing out a pacing escape interval and having an atrial sensing channel including an atrial sense amplifier that generates an atrial sense event upon detection of a predetermined characteristic of the atrial electrogram, a ventricular sensing channel including a ventricular sense amplifier that generates a ventricular sense event upon detection of a predetermined characteristic of the ventricular electrogram, an atrial pacing pulse generator for delivering atrial pacing pulses to at least one atrial heart chamber upon time-out of the pacing escape interval, means for timing an AV delay from an atrial sense event or a delivered atrial pacing pulse, inhibiting means for stopping the time-out of the AV delay if a ventricular sense event is generated during time-out of the AV delay and for inhibiting delivery of an atrial pacing pulse upon time-out of the escape interval if an atrial sense event is generated during time-out of the escape interval, and a ventricular pacing pulse generator for delivering ventricular pacing pulses to at least one ventricular heart chamber upon time-out of the AV delay,

a method of operating the pacing system in response to the presence of electromagnetic interference tending to tending to corrupt the features of the atrial and ventricular electrogram comprising:

during the dual chamber pacing mode, accumulating data from the atrial and ventricular sense events and delivered pacing pulses over a predetermined time interval;

deriving a mean heart rate from the accumulated data;

determining if electromagnetic interference is present in the ventricular sensing channel;

determining if electromagnetic interference is present in the atrial sensing channel;

determining whether the patient is pacemaker dependent or pacemaker independent in the ventricles when electromagnetic interference is determined to be present in the atrial and ventricular sensing channels, and if the patient is determined to be pacemaker dependent in the ventricles;

setting the pacing mode to a dual chamber interference state pacing mode preventing the inhibiting means from stopping the time-out of the AV delay upon generation of a ventricular sense event or stopping the time-out of the escape interval upon generation of an atrial sense event;

adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

33. The method of claim 32, wherein the adjusting step further comprises:

determining whether the patient is pacemaker dependent or pacemaker independent in the atria;

adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a first predetermined increment rack when the patient is declared pacemaker dependent in the atria; and

adjusting the pacing escape interval to provide a second interference state pacing rate exceeding the mean heart rate by a second predetermined increment greater than the first predetermined increment when the patient is declared pacemaker independent in the atria,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

34. The method of claim 32, further comprising, if the patient is determined to be pacemaker independent in the ventricles:

determining whether the patient is pacemaker dependent or pacemaker independent in the atria;

if the patient is determined to be pacemaker independent in the atria;

setting the pacing mode to an interference state pacing mode providing pacing to the atria and preventing the inhibiting means from stopping the time-out of the escape interval upon an atrial sense event; and

adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment; and

providing atrial pacing pulses at the escape interval established by an interference state pacing rate, whereby the patient'"'"'s heart is paced in the atria while minimizing occurrences of parasystole.

35. The method of claim 34, further comprising setting the pacing mode to a fully inhibited interference state pacing mode preventing the delivery of pacing pulses to the atria and ventricles if the patient is declared to be pacemaker independent in the atria.

36. In a dual chamber cardiac pacing system operating in a dual chamber pacing mode, the dual chamber pacing system timing out a pacing escape a interval and having an atrial sensing channel including an atrial sense amplifier that generates an atrial sense event upon detection of a predetermined characteristic of the atrial electrogram, a ventricular sensing channel including a ventricular sense amplifier that generates a ventricular sense event upon detection of a predetermined characteristic of the ventricular electrogram, an atrial pacing pulse generator for delivering atrial pacing pulses to at least one atrial heart chamber upon time-out of the pacing escape interval, means for timing an AV delay from an atrial sense event or a delivered atrial pacing pulse, inhibiting means for stopping the time-out of the AV delay if a ventricular sense event is generated during time-out of the AV delay and for inhibiting delivery of an atrial pacing pulse upon time-out of the escape interval if an atrial sense event is generated during time-out of the escape interval, and a ventricular pacing pulse generator for delivering ventricular pacing pulses to at least one ventricular heart chamber upon time-out of the AV delay,

a method of operating the pacing system to respond to the presence of electromagnetic interference tending to tending to corrupt the features of the atrial and ventricular electrogram comprising:

during the dual chamber pacing mode, accumulating data from the atrial and ventricular sense events and delivered pacing pulses over a predetermined time interval;

determining if electromagnetic interference is present in the ventricular sensing channel;

determining if electromagnetic interference is present in the atrial sensing channel;

determining whether the patient is pacemaker dependent or pacemaker independent in the ventricles when electromagnetic interference is determined to be present in the atrial and ventricular sensing channels, and if the patient is determined to be pacemaker independent in the ventricles;

determining whether the patient is pacemaker dependent or pacemaker independent in the atria;

if the patient is determined to be pacemaker dependent in the atria, setting the pacing mode to an interference state pacing mode preventing the inhibiting means from stopping the time-out of the escape interval and providing atrial pacing pulses at an escape interval established by an interference state pacing rate;

if the patient is determined to be pacemaker independent in the atria, setting the pacing mode to a fully inhibited interference state pacing mode preventing the delivery of pacing pulses to the atria and ventricles; and

operating the pacing system in the interference state pacing mode at the interference state pacing rate,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

37. The method of claim 36, wherein:

the step of determining whether the patient is pacemaker dependent or pacemaker independent in the atria further comprises;

deriving an atrial pacing dependency percentage representing the ratio of delivered atrial pacing pulses to the sum of delivered atrial pacing pulses and atrial sensed events over a predetermined time interval;

establishing a first atrial pacing dependency threshold and a second atrial pacing dependency threshold exceeding the first pacing a dependency threshold;

comparing the atrial pacing dependency percentage to the first and second atrial pacing dependency thresholds;

declaring the patient pacemaker independent in the atria when the atrial pacing dependency percentage falls below the first atrial pacing dependency threshold;

declaring the patient pacemaker dependent in the atria when the atrial pacing dependency percentage exceeds the second atrial pacing dependency threshold; and

declaring the patient moderately pacemaker dependent in the atria when the atrial pacing dependency percentage falls between the first and second atrial pacing dependency thresholds; and

;

if the patient is declared to be pacemaker dependent in the atria, setting the interference state pacing rate to a first interference state pacing rate; and

if the patient is declared to be moderately pacemaker dependent in the atria, setting the interference state pacing rate to a second interference state pacing rate exceeding the first interference state pacing rate.

38. The method of claim 37 further comprising:

if the patient is declared to be pacemaker independent in the atria, setting the pacing mode to a fully inhibited interference state pacing mode preventing the delivery of pacing pulses to the atria and ventricles.

39. The method of claim 36, further comprising:

deriving a mean heart rate from the accumulated data;

deriving an atrial pacing dependency percentage representing the ratio of delivered atrial pacing pulses to the sum of delivered atrial pacing pulses and atrial sensed events over a predetermined time interval;

determining whether the patient is pacemaker dependent or pacemaker independent in the atria from the atrial pacing dependency percentage if the patient is determined to be pacemaker independent in the ventricles by;

establishing a first atrial pacing dependency threshold and a second atrial pacing dependency threshold exceeding the first pacing dependency threshold;

comparing the atrial pacing dependency percentage to the first and second atrial pacing dependency thresholds;

declaring the patient pacemaker independent in the atria when the atrial pacing dependency percentage falls below the first atrial pacing dependency threshold;

declaring the patient pacemaker dependent in the atria when the atrial pacing dependency percentage exceeds the second atrial pacing dependency threshold; and

declaring the patient moderately pacemaker dependent in the atria when the atrial pacing dependency percentage fails between the first and second atrial pacing dependency thresholds;

if the patient is declared to be pacemaker dependent in the atria;

setting the pacing mode to an interference state pacing mode preventing the inhibiting means from stopping the time-out of the escape interval;

adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a first predetermined increment, and

providing atrial pacing pulses at the adjusted escape interval, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole; and

if the patient is declared to be moderately pacemaker dependent in the atria;

setting the pacing mode to an interference state pacing mode preventing the inhibiting means from stopping the time-out of the escape interval;

adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a second predetermined increment greater than the first predetermined increment, and

providing atrial pacing pulses at the adjusted escape interval, whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

40. The method of claim 39, further comprising setting the pacing mode to a fully inhibited interference state pacing mode preventing the delivery of pacing pulses to the atria and ventricles if the patient is declared to be pacemaker independent in the atria.

41. In a dual chamber cardiac pacing system operating in a dual chamber pacing mode, the dual chamber pacing system having an atrial sensing channel including an atrial sense amplifier that generates an atrial sense event upon detection of a predetermined characteristic of the atrial electrogram, a ventricular sensing channel including a ventricular sense amplifier that generates a ventricular sense event upon detection of a predetermined characteristic of the ventricular electrogram, an atrial pacing pulse generator for delivering atrial pacing pulses to at least one atrial heart chamber, means for timing an AV delay from an atrial sense event or a delivered atrial pacing pulse, inhibiting means for stopping the time-out of the AV delay if a ventricular sense event is generated during time-out of the AV delay, and a ventricular pacing pulse generator for delivering ventricular pacing pulses to at least one ventricular heart chamber upon time-out of the AV delay, means for responding to the presence of electromagnetic interference tending to corrupt the features of the ventricular electrogram comprising:

means for determining if electromagnetic interference is present in the ventricular sensing channel;

means for determining if electromagnetic interference is present in the atrial sensing channel; and

means for preventing the inhibiting means from stopping the time-out of the AV delay upon generation of a ventricular sense event when electromagnetic interference is determined to be present in the ventricular sensing channel and is not determined to be present in the atrial sensing channel,

whereby synchronized atrial and ventricular pacing pulses are delivered to the atrial and ventricular heart chambers while electromagnetic interference is present in the ventricular sensing channel.

42. The pacing system of claim 41, wherein the dual chamber pacing mode is the DDD(R) pacing mode, and the preventing means comprises means for changing the pacing mode to the DDO*(R) interference state pacing mode.

43. The pacing system of claim 41, further comprising means for enabling the inhibiting means when electromagnetic interference is determined to no longer be present in the ventricular sensing channel.

44. In a dual chamber cardiac pacing system operating in a dual chamber pacing mode, the dual chamber pacing system timing out a pacing escape interval and having an atrial sensing channel including an atrial sense amplifier that generates an atrial sense event upon detection of a predetermined characteristic of the atrial electrogram, a ventricular sensing channel including a ventricular sense amplifier that generates a ventricular sense event upon detection of a predetermined characteristic of the ventricular electrogram, an atrial pacing pulse generator for delivering atrial pacing pulses to at least one atrial heart chamber upon time-out of the pacing escape interval, means for timing an AV delay from an atrial sense event or a delivered atrial pacing pulse, inhibiting means for stopping the time-out of the AV delay if a ventricular sense event is generated during time-out of the AV delay and for inhibiting delivery of an atrial pacing pulse upon time-out of the escape interval if an atrial sense event is generated during time-out of the escape interval, and a ventricular pacing pulse generator for delivering ventricular pacing pulses to at least one ventricular heart chamber upon time-out of the AV delay, means for responding to the presence of electromagnetic interference tending to tending to corrupt the features of the atrial and ventricular electrogram comprising:

means for accumulating data from the atrial and ventricular sense events and delivered pacing pulses over a predetermined time interval;

means for deriving a mean heart rate from the accumulated data;

means for determining if electromagnetic interference is present in the ventricular sensing channel;

means for determining if electromagnetic interference is present in the atrial sensing channel;

means for determining whether the patient is pacemaker dependent or pacemaker independent in the ventricles when electromagnetic interference is determined to be present in the atrial and ventricular sensing channels;

means for setting the pacing mode to a dual chamber interference state pacing mode preventing the inhibiting means from stopping the time-out of the AV delay upon generation of a ventricular sense event or stopping the time-out of the escape interval upon generation of an atrial sense event if the patient is determined to be pacemaker dependent in the ventricles; and

means for adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment if the patient is determined to be pacemaker dependent in the ventricles,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

45. The pacing system of claim 44, wherein the adjusting means further comprises:

means operable if the patient is determined to be pacemaker dependent in the ventricles for determining whether the patient is pacemaker dependent or pacemaker independent in the atria;

means for adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a first predetermined increment when the patient is declared pacemaker dependent in the atria; and

means for adjusting the pacing escape interval to provide a second interference state pacing rate exceeding the mean heart rate by a second predetermined increment greater than the first predetermined increment when the patient is declared pacemaker independent, in the atria,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

46. The pacing system of claim 44, further comprising:

means operable if the patient is determined to be pacemaker independent in the ventricles for determining whether the patient is pacemaker dependent or pacemaker independent in the atria;

means for setting the pacing mode to an interference state pacing mode providing pacing to the atria and preventing the inhibiting means from stopping the time-out of the escape interval upon an atrial sense event if the patient is determined to be pacemaker independent in the atria; and

means for adjusting the pacing escape interval to provide an interference state pacing rate exceeding the mean heart rate by a predetermined increment event if the patient is determined to be pacemaker independent in the atria,

whereby the patient'"'"'s heart is paced in the atria while minimizing occurrences of parasystole.

47. The pacing system of claim 46, further comprising means for setting the pacing mode to a fully inhibited interference state pacing mode preventing the delivery of pacing pulses to the atria and ventricles if the patient is declared to be pacemaker independent in the atria.

48. In a dual chamber cardiac pacing system operating in a dual chamber pacing mode, the dual chamber pacing system timing out a pacing escape interval and having an atrial sensing channel including an atrial sense amplifier that generates an atrial sense event upon detection of a predetermined characteristic of the atrial electrogram, a ventricular sensing channel including a ventricular sense amplifier that generates a ventricular sense event upon detection of a predetermined characteristic of the ventricular electrogram, an atrial pacing pulse generator for delivering atrial pacing pulses to at least one atrial heart chamber upon time-out of the pacing escape interval, means for timing an AV delay from an atrial sense event or a delivered atrial pacing pulse, inhibiting means for stopping the time-out of the AV delay if a ventricular sense event is generated during time-out of the AV delay and for inhibiting delivery of an atrial pacing pulse upon time-out of the escape interval if an atrial sense event is generated during time-out of the escape interval, and a ventricular pacing pulse generator for delivering ventricular pacing pulses to at least one ventricular heart chamber upon time-out of the AV delay, means for operating the pacing system in response to the presence of electromagnetic interference tending to tending to corrupt the features of the atrial and ventricular electrogram comprising:

means for accumulating data from the atrial and ventricular sense events and delivered pacing pulses over a predetermined time interval;

means for determining if electromagnetic interference is present in the ventricular sensing channel;

means for determining if electromagnetic interference is present in the atrial sensing channel;

means for determining whether the patient is pacemaker dependent or pacemaker independent in the ventricles when electromagnetic interference is determined to be present in the atrial and ventricular sensing channels;

means for determining whether the patient is pacemaker dependent or pacemaker independent in the atria if the patient is determined to be pacemaker independent in the ventricles;

means for setting the pacing mode to an interference state pacing mode preventing the inhibiting means from stopping the time-out of the escape interval and providing atrial pacing pulses at the escape interval established by an interference state pacing rate if the patient is determined to be pacemaker dependent in the atria; and

means for setting the pacing mode to a fully inhibited interference state pacing mode preventing the delivery of pacing pulses to the atria and ventricles if the patient is determined to be pacemaker independent in the atria,

whereby the patient'"'"'s heart is paced while minimizing occurrences of parasystole.

49. The pacing system of claim 48, further comprising means for deriving an atrial pacing dependency percentage representing the ratio of delivered atrial pacing pulses to the sum of delivered atrial pacing pulses and atrial sensed events over a predetermined time interval and wherein:

the means for determining whether the patient is pacemaker dependent or pacemaker independent in the atria further comprises;

means for establishing a first atrial pacing dependency threshold and a second atrial pacing dependency threshold exceeding the first pacing dependency threshold; and

means for comparing the atrial pacing dependency percentage to the first and second atrial pacing dependency thresholds and declaring the patient pacemaker independent in the atria when the atrial pacing dependency percentage falls below the first atrial pacing dependency threshold or declaring the patient pacemaker dependent in the atria when the atrial pacing dependency percentage exceeds the second atrial pacing dependency threshold or declaring the patient moderately pacemaker dependent in the atria when the atrial pacing dependency percentage falls between the first and second atrial pacing dependency thresholds; and

;

the means for setting the pacing mode to an interference state pacing mode further comprises;

means for setting the interference state pacing rate to a first interference state pacing rate if the patient is declared to be pacemaker dependent in the atria; and

means for setting the interference state pacing rate to a second interference state pacing rate exceeding the first interference state pacing rate if the patient is declared to be moderately pacemaker dependent in the atria.

50. The pacing system of claim 48, further comprising:

means for deriving a mean heart rate from the accumulated data; and

means for deriving an atrial pacing dependency percentage representing the ratio of delivered atrial pacing pulses to the sum of delivered atrial pacing pulses and atrial sensed events over a predetermined time interval if the patient is determined to be pacemaker independent in the ventricles, and wherein;

the means for determining whether the patient is pacemaker dependent or pacemaker independent in the atria further comprises;

means for establishing a first atrial pacing dependency threshold and a second atrial pacing dependency threshold exceeding the first pacing dependency threshold;

means for comparing the atrial pacing dependency percentage to the first and second atrial pacing dependency thresholds; and

means for declaring the patient pacemaker independent in the atria when the atrial pacing dependency percentage falls below the first atrial pacing dependency threshold or declaring the patient pacemaker dependent in the atria when the atrial pacing dependency percentage exceeds the second atrial pacing dependency threshold or declaring the patient moderately pacemaker dependent in the atria when the atrial pacing dependency percentage falls between the first and second atrial pacing dependency thresholds; and

the means for setting the pacing mode to an interference state pacing mode further comprises;

means for setting the pacing mode to an interference state pacing mode preventing the inhibiting means from stopping the time-out of the escape interval if the patient is declared to be pacemaker dependent in the atria; and

means for adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a first predetermined increment if the patient is declared to be pacemaker dependent in the atria, and

means for setting the pacing mode to an interference state pacing mode preventing the inhibiting means from stopping the time-out of the escape interval if the patient is declared to be moderately pacemaker dependent in the atria; and

means for adjusting the pacing escape interval to provide a first interference state pacing rate exceeding the mean heart rate by a second predetermined increment greater than the first predetermined increment if the patient is declared to be moderately pacemaker dependent in the atria.