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

1. A method of operating a pacing system to respond to a presence of electromagnetic interference tending to corrupt the features of an electrogram, comprising:

accumulating data from 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 a pacing mode of the pacing system 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 a 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.

2. The method of claim 1 , 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.

3. The method of claim 1 , 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.

4. A system that operates a pacing system to respond to a presence of electromagnetic interference tending to corrupt the features of an electrogram, comprising:

means for accumulating data from 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 a pacing mode of the pacing system 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 a 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.

5. The system of claim 4 , 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.

6. The system of claim 4 , 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.