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Claims  |
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What is claimed is:
1. A method of operating an implantable medical device in one of a main operating mode or a preemptive tachyarrhythmia preventive pacing operating mode, the implantable
medical device including pulse generating means for generating pacing pulses and directing such pulses to cardiac tissue of a patient, the pulse generating means being controlled by a processor, the processor being coupled to a memory wherein an
operating program for controlling the operating modes may be stored, the method comprising the steps of:
(a) sensing at least one parameter associated with the physiology of the patient's heart for an indication that a tachyarrhythmia may soon occur;
(b) determining when the at least one parameter exceeds a prescribed threshold, thereby indicating that a tachyarrhythmia may soon occur;
(c) automatically operating the implantable medical device in accordance with the main operating mode whenever the at least one parameter suggests that a tachyarrhythmia may not soon occur, wherein the main operating program is aimed at
controlling the implantable medical device so that it performs its intended function; and
(d) automatically operating the implantable medical device in accordance with the preemptive tachyarrhythmia preventive pacing operating mode whenever the at least one parameter exceeds the prescribed threshold thereby indicating that a
tachyarrhythmia may soon occur, wherein the preemptive tachyarrhythmia preventive pacing operating mode is aimed at preventing a tachyarrhythmia from occurring.
2. The method, as set forth in claim 1, wherein the step of sensing the at least one parameter for an indication that a tachyarrhythmia may soon occur comprises:
sensing a first value of a first parameter at a time when a tachyarrhythmia is not present, the first value being indicative of a normal, non-tachyarrhythmia, condition; and
storing the first value of the first parameter indicative of a normal, non-tachyarrhythmia, condition as a reference value; and
sensing a second value of the first parameter during subsequent cardiac cycles of the patient; and
wherein the determining step comprises:
comparing the second value of the first parameter to the reference value;
defining the patient's rhythm as one in which a tachyarrhythmia may soon occur when the first parameter has changed by at least a prescribed amount from the reference value; and
defining the patient's rhythm as one in which a tachyarrhythmia may not soon occur when the first parameter has not changed by at least a prescribed amount from the reference value.
3. The method, as set forth in claim 1, wherein the step of sensing the at least one parameter for an indication that a tachyarrhythmia may soon occur comprises:
sensing a first set of values for a first set of parameters at a time when a tachyarrhythmia is not present, the first set values being indicative of a normal, non-tachyarrhythmia, condition;
storing the first set of values of the first set of parameters at the time when a tachyarrhythmia is not present as a reference template; and
sensing a second set of values for the first set of parameters during subsequent cardiac cycles of the patient; and
wherein the determining step comprises:
comparing the first set of values of the first set of parameters during a subsequent cardiac cycle to the reference template;
defining the patient's rhythm as one in which a tachyarrhythmia may soon occur when the first set of parameters has changed by at least a prescribed amount from the reference template; and
defining the patient's rhythm as one in which a tachyarrhythmia may not soon occur when the first set of parameters has not changed by at least a prescribed amount from the reference template.
4. The method, as set forth in claim 1, wherein the preemptive tachyarrhythmia pacing operating mode comprises operating in an overdrive pacing mode.
5. The method, as set forth in claim 4, wherein operating in an overdrive pacing mode comprises setting a pacing rate based on a predetermined diurnal rate.
6. The method, as set forth in claim 4, further comprising automatically determining a diurnal rate, and operating in an overdrive pacing mode by setting a pacing rate based on the automatically determined diurnal rate.
7. The method, as set forth in claim 6, wherein automatically determining the diurnal rate comprises gathering heart rate data over at least a 24-hour period, filtering the heart rate data with a digital recursive low-pass filter using a delay
of at least 20 seconds, and using the resulting filtered output as an indicator of the diurnal rate.
8. The method, as set forth in claim 7, wherein the diurnal rate comprises a diurnal base rate (DBR), and wherein determining the diurnal base rate comprises:
setting a desired "Sleeprate" parameter as a desired heart rate to be allowed when sleeping;
computing a running average of the gathered heart rate data at prescribed increments of "Delay2" seconds;
finding a difference, "DIFF2(t)", between the running average after each prescribed increment of "Delay2" seconds;
determining an effective time constant, "ACT.sub.-- SD2(t)", after each prescribed increment of "Delay2" seconds, as
ACT.sub.-- SD2(t)=(1/j) [DIFF2(t)]+[(j-1)/j]ACT.sub.-- SD2(t-Delay2),
where j is an integer having a value of at least 30, and ACT.sub.-- SD2(t-Delay2) is the most recent value of the effective time constant determined during the most-recent increment prior to the present increment;
computing the diurnal base rate (DBR) corresponding to the prescribed increment as DBR(t)=Slope2[ACT.sub.-- SD2(t)]+Sleeprate,
where "Slope2" is a predetermined parameter.
9. The method, as set forth in claim 8, further comprising selecting the value of "Delay2" to be at least 25 seconds, and setting the value of "j" to be at least 60.
10. The method, as set forth in claim 8, further comprising selecting the value of "Delay2" to be a prescribed number, n2, of cardiac cycles, where n2 is at least 30.
11. The method, as set forth in claim 4, wherein the preemptive tachyarrhythmia pacing operating mode comprises operating in an atrial overdrive pacing mode.
12. The method, as set forth in claim 11, wherein operating in the atrial overdrive pacing mode comprises setting a pacing rate based on an automatic stepped increase over an average atrial rate.
13. The method, as set forth in claim 12, further comprising:
(a) determining the average atrial rate by monitoring at least three consecutive P-to-P intervals;
(b) setting the pacing rate to a new base rate value that is a prescribed step increase greater than the average atrial rate;
(c) pacing the atria at a rate faster than the new base rate value for a prescribed number n.sub.P of cardiac cycles; and
(d) returning to the new base rate value after the prescribed number n.sub.P of cardiac cycles.
14. The method, as set forth in claim 13, wherein the number of cardiac cycles n.sub.P that are paced faster than the new base rate value comprises at least three.
15. The method, as set forth in claim 13, wherein step (c) comprises pacing the atria at a rate that is both faster than the new base rate value and that varies from cycle to cycle for each of the n.sub.P cardiac cycles.
16. The method, as set forth in claim 15, further comprising pacing the atria at a rate that is increasingly faster than the new base rate value for each of the n.sub.P cardiac cycles.
17. The method, as set forth in claim 15, further comprising pacing the atria at a rate that is decreasingly faster than the new base rate value for each of the n.sub.P cardiac cycles.
18. The method, as set forth in claim 15, further comprising pacing the atria at a rate that is randomly faster than the new base rate value for each of the n.sub.P cardiac cycles.
19. The method, as set forth in claim 13, wherein the prescribed step increase above the average atrial rate to which the new base rate value is set comprises at least 5 bpm.
20. The method, set forth in claim 13, further comprising periodically decreasing the atrial pacing rate to test for the occurrence of P-waves; and if P-waves are detected, determining the average atrial rate by monitoring at least three
consecutive P-to-P intervals and resetting the pacing rate to yet a newer base rate value that is a prescribed step increase greater than the most-recently determined average atrial rate, whereby the base rate value is dynamically reset as needed as a
function of the most-recently determined average atrial rate.
21. The method, as set forth in claim 11, wherein operating in the atrial overdrive pacing mode comprises setting a pacing rate using a negative hysteresis scheme.
22. The method, as set forth in claim 21, further comprising:
setting the atrial pacing rate at a programmed base rate or a sensor indicated rate, whichever is faster;
increasing the atrial pacing rate to an atrial hysteresis rate, where the hysteresis rate is a programmed amount delta greater than the faster of the programmed base rate or the sensor-indicated rate, whenever an intrinsic P-wave is sensed within
an atrial escape interval of the current cardiac cycle so that the next cardiac cycle is subjected to the atrial hysteresis rate;
maintaining the atrial hysteresis rate for at least one cardiac cycle; and
returning the atrial pacing rate to the faster of the programmed base rate or the sensor-indicated rate.
23. The method, as set forth in claim 22, further comprising returning the atrial pacing rate to the faster of the programmed base rate or the sensor-indicated rate in one step so that the following cardiac cycle is subjected to the faster of
the programmed base rate or the sensor-indicted rate.
24. The method, as set forth in claim 22, further comprising returning the atrial pacing rate to the faster of the programmed base rate or the sensor-indicated rate gradually over several cardiac cycles.
25. The method, as set forth in claim 1, wherein the preemptive tachyarrhythmia preventive pacing operating mode aimed at preventing a tachyarrhythmia from occurring comprises introducing randomicity into a base rate of the implantable medical
device.
26. The method, as set forth in claim 25, further comprising:
gathering heart rate data;
processing the gathered heart rate data to determine a correlation dimension, D.sub.2 ; and
varying the pacing rate of the pacing pulses applied to the cardiac tissue to maintain D.sub.2 above a prescribed correlation dimension threshold level.
27. The method, as set forth in claim 26, further comprising adjusting the pacing rate as required to maintain correlation dimension D.sub.2 above a value of about 2.0.
28. The method, as set forth in claim 26, further comprising adjusting the pacing rate as required to maintain overdrive pacing and to maintain D.sub.2 above a value of about 2.0.
29. The method, as set forth in claim 1, further comprising switching the operating mode of the implantable medical device from its main operating mode to a preemptive operating mode whenever a tachyarrhythmia is determined to soon occur.
30. The method, as set forth in claim 29, wherein the at least one parameter that suggests a tachyarrhythmia may soon occur is selected from a group of physiological parameters that includes: blood pressure, contractility, pre-ejection interval,
intra-chamber impedance, stroke volume, and oxygen saturation.
31. The method, as set forth in claim 29, wherein sensing the at least one parameter which suggests a tachyarrhythmia may soon occur comprises sensing the patient's heart rate variability; and
wherein the determining step comprises determining a marked reduction in heart rate variability (HRV).
32. The method, as set forth in claim 29, wherein sensing the at least one parameter which suggests a tachyarrhythmia may soon occur comprises sensing the patient's right ventricular electrogram through a single endocardial lead; and
wherein the determining step comprises determining high frequency cardiac activity signals above the prescribed threshold thereby indicating the presence of ischemia.
33. The method, as set forth in claim 29, wherein sensing the at least one parameter which suggests a tachyarrhythmia may soon occur comprises sensing the patient's right ventricular electrogram through a single endocardial lead; and
wherein the determining step comprises determining electrical alternans of the QT interval above the prescribed threshold.
34. A method of operating an implantable tissue-stimulating medical device in a preemptive tachyarrhythmia pacing mode for the purpose of preventing the occurrence of a tachyarrhythmia, said method comprising:
sensing a value of at least one physiological condition capable of predicting the occurrence of a tachyarrhythmia;
detecting when the value of the at least one physiological condition provides an indication that a pre-tachycardia condition exists; and
when such pre-tachyarrhythmia condition is detected, modifying the behavior of the implantable tissue-stimulating medical device so as to minimize the likelihood of occurrence of a tachyarrhythmia.
35. The method, as set forth in claim 34, further including returning the behavior of the implantable tissue-stimulating medical device to what it was before being modified in response to detecting the pre-tachyarrhythmia condition upon the
occurrence of a prescribed event.
36. The method, as set forth in claim 35, further comprising returning the behavior of the implantable tissue-stimulating medical device to what it was initially upon a timing out of a prescribed period of time, whereby the behavior of the
implantable tissue-stimulating medical device is modified for only the prescribed period of time.
37. The method, as set forth in claim 35, further comprising returning the behavior of the implantable tissue-stimulating medical device to what it was initially upon the completion of a prescribed number of consecutive cardiac cycles.
38. The method, as set forth in claim 35, further comprising returning the behavior of the implantable tissue-stimulating medical device to what it was initially upon detecting the absence of the pre-tachyarrhythmia condition.
39. The method, as set forth in claim 35, further comprising returning the behavior of the implantable tissue-stimulating medical device to what it was initially upon detecting the return of a sensed parameter back to a value it had before the
pre-tachyarrhythmia condition was detected.
40. The method, as set forth in claim 34, wherein the step of modifying the behavior of the implantable tissue-stimulating medical device comprises introducing a degree of randomicity into the stimulation rate of the device so that a minimum
specified correlation dimension is maintained.
41. The method, as set forth in claim 34, wherein the step of modifying the behavior of the implantable tissue-stimulating medical device comprises switching its operating mode.
42. The method, as set forth in claim 34, wherein the implantable tissue-stimulating device comprises a device adapted to stimulate cardiac tissue, and wherein the step of modifying the behavior of the implantable tissue-stimulating medical
device comprises operating the device in an overdrive pacing mode which provides stimulation pulses at a rate faster than a natural underlying heart rate.
43. The method, as set forth in claim 42, wherein overdrive pacing comprises pacing at a rate that is a prescribed amount faster than an automatically determined diurnal rate.
44. The method, as set forth in claim 42, wherein overdrive pacing comprises pacing at a rate that is an automatic stepped increase over an average heart rate.
45. The method, as set forth in claim 42, wherein overdrive pacing comprises pacing every cardiac cycle using a negative hysteresis scheme.
46. The method, as set forth in claim 34, wherein the implantable tissue-stimulating medical device comprises a device adapted to stimulate cardiac tissue of a patient in order to maintain a desired heart rhythm, and wherein the step of
detecting a pre-tachyarrhythmia condition comprises:
monitoring, during a time when the patient is at rest and the cardiac rhythm is stable, cardiac events of the patient for a sufficiently long period of time to learn a particular sequence and rate of cardiac events that can thereafter be used to
represent a normal condition for the patient; and
detecting, after learning the sequence and rate of cardiac events that represent a normal condition for the patient, the pre-tachyarrhythmia condition as any cardiac events that vary a prescribed amount from the learned sequence and rate of
cardiac events representing a normal condition for the patient.
47. The method, as set forth in claim 34, wherein the implantable tissue-stimulating medical device comprises a device adapted to stimulate cardiac tissue of a patient in order to maintain a desired heart rhythm, and wherein the step of
detecting a pre-tachyarrhythmia condition comprises:
monitoring, during a time when the patient is not experiencing a tachyarrhythmia, at least one physiological parameter of the patient and saving the value of such at least one physiological parameter as a threshold reference; and
detecting, after saving the threshold reference, the pre-tachyarrhythmia condition by continuing to monitor the at least one physiological parameter and detecting when its monitored value differs from the threshold reference by more than a
prescribed amount.
48. The method, as set forth in claim 47, wherein the at least one physiological parameter is selected from a group of cardiac-related physiological parameters comprising: blood pressure, heart rate, blood temperature, blood O.sub.2 saturation,
contractility, pre-ejection interval, intra-chamber impedance, stroke volume, heart rate variability, and detection of ischemia.
49. A method of operating an implantable tissue-stimulating device, the device having generating means for generating electrical stimulating pulses of specified energies and applying the pulses to body tissue at specified times, and means for
sensing natural muscle contractions of the body tissue, the method comprising:
predicting when a tachyarrhythmia may occur; operating in a preemptive tachyarrhythmia pacing mode whenever a tachyarrhythmia is predicted to occur, the preemptive tachyarrhythmia pacing mode triggering the generation of electrical stimulating
pulses to the tissue at controlled times and energy levels designed to minimize the occurrence of a tachyarrhythmia in the tissue being stimulated; and
operating the tissue-stimulating device in a non-preemptive tachyarrhythmia pacing mode whenever a tachyarrhythmia is not predicted to occur.
50. The method, as set forth in of claim 49, further comprising
sensing at least one body parameter over a specified period of time;
determining whether the at least one body parameter satisfies specific criteria, the specific criteria defining a threshold condition which, when exceeded, is indicative of a body tissue condition conducive to the occurrence of a tachyarrhythmia;
operating the tissue-stimulating device in the preemptive tachyarrhythmia pacing mode when the at least one body parameter satisfies the specific criteria; and
operating the tissue-stimulating device in the non-preemptive tachyarrhythmia pacing mode when the at least one body parameter does not satisfy the specific criteria.
51. An implantable cardiac tissue-stimulating device comprising:
pulse generating means for generating electrical stimulating pulses of specified energies;
lead/electrode means for delivering the stimulating pulses to cardiac tissue;
sensing means for sensing natural muscle contractions of the cardiac tissue;
means for determining when a pre-tachycardia condition exists; and
processing/control means for operating in a preemptive tachyarrhythmia pacing mode when the pre-tachyarrhythmia condition exists and in a non-preemptive mode when the pre-tachyarrhythmia does not exist, the preemptive tachyarrhythmia pacing mode
comprising means for triggering the pulse generating means of the tissue-stimulating device to apply electrical stimulating pulses to the cardiac tissue at controlled times and energy levels designed to minimize the occurrence of a tachyarrhythmia in the
tissue being stimulated.
52. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein:
the means for determining when a pre-tachycardia condition includes:
means for sensing at least one body parameter over a specified period of time; and
means for determining whether the at least one body parameter satisfies specific criteria, the specific criteria defining a threshold condition which when exceeded is indicative of a body tissue condition conducive to the occurrence of a
tachyarrhythmia; and
the processing/control means includes:
means for operating the tissue-stimulating device in accordance with the preemptive tachyarrhythmia pacing mode when the at least one body parameter satisfies the specific criteria; and
means for not operating the tissue-stimulating device in accordance with the non-preemptive tachyarrhythmia pacing mode when the at least one body parameter does not satisfy the specific criteria.
53. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein the means for determining when a pre-tachycardia condition includes:
means for learning a normal sequence and rate of cardiac events at a time when the patient's cardiac rhythm is stable;
storing the normal sequence and rate of cardiac events; and
means for detecting as the pre-tachyarrhythmia condition any cardiac events that vary a prescribed amount from the normal sequence and rate of cardiac events.
54. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein the means for determining when a pre-tachycardia condition includes:
means for learning a normal heart rate variability during a time when the patient's cardiac rhythm is stable;
storing the normal heart rate variability; and
means for detecting as the pre-tachyarrhythmia condition a heart rate variability that varies a prescribed amount from the normal heart rate variability.
55. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein the means for determining when a pre-tachycardia condition includes:
means for learning a normal correlation dimension during a time when the patient's cardiac rhythm is stable;
storing the normal correlation dimension; and
means for detecting as the pre-tachyarrhythmia condition a correlation dimension that varies a prescribed amount from the normal correlation dimension.
56. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein the means for determining when a pre-tachycardia condition includes:
means for learning a normal a QT interval during a time when the patient's cardiac rhythm is stable;
storing the normal QT interval; and
means for detecting as the pre-tachyarrhythmia condition alternans which occur in the QT interval that differ by a prescribed amount from the normal QT interval.
57. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein the means for determining when a pre-tachycardia condition includes:
means for learning normal frequency components of a ventricular cardiac signal during a time when the patient's cardiac rhythm is stable;
storing the normal frequency components of the ventricular cardiac signal; and
means for detecting as the pre-tachyarrhythmia condition high frequency cardiac signal components which differ by a prescribed amount from the normal frequency components of the ventricular cardiac signal, the high frequency cardiac signal
components indicating the presence of ischemia.
58. The implantable cardiac tissue-stimulating device, as set forth in claim 51, wherein the processing/control means for operating in a preemptive tachyarrhythmia pacing mode comprises:
means for operating in an overdrive mode which provides stimulation pulses at a rate faster than a natural underlying heart rate.
59. The implantable cardiac tissue-stimulating device, as set forth in claim 58, wherein the means for operating in the overdrive mode comprises:
means for providing stimulation pulses at a varying rate faster than a natural underlying heart rate.
60. The implantable cardiac tissue-stimulating device, as set forth in claim 58, wherein the means for operating in the overdrive mode comprises:
means for providing stimulation pulses at a randomly-varying rate faster than a natural underlying heart rate.
61. The implantable cardiac tissue-stimulating device, as set forth in claim 58, wherein:
the means for determining when a pre-tachycardia condition exists includes means for determining a normal correlation dimension during a time when the patient's rhythm is stable; and
the means for operating in the overdrive mode includes means for providing stimulation pulses at a rate which maintains correlation dimension to within a prescribed range about the normal correlation dimension.
62. The implantable cardiac tissue-stimulating device, as set forth in claim 58, wherein:
the means for determining when a pre-tachycardia condition exists includes means for determining a normal heart rate variability during a time when the patient's rhythm is stable; and
the means for operating in the overdrive mode includes means for providing stimulation pulses at a rate which mimics the normal heart rate variability. |
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