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Method and system for verifying the integrity of normal sinus rhythm templates Number:6,996,434 from the United States Patent and Trademark Office (PTO) owispatent

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Title: Method and system for verifying the integrity of normal sinus rhythm templates

Abstract: A method and system for verifying the integrity of normal sinus rhythm (NSR) templates and updating the NSR template after selected time intervals. At selected time intervals after establishing a NSR template, cardiac complexes are sensed and values for one or more cardiac parameters are measured. The values of the cardiac parameters are compared to predetermined value ranges for NSR cardiac complexes. When the values of the cardiac parameters fall within the predetermined value ranges, values for the differences between the values of the cardiac parameters for the cardiac complexes and the values for the cardiac parameters of the NSR cardiac complexes are calculated. When the values of the differences are greater than one or more threshold values, the NSR template is updated as a function of the sensed cardiac complexes.

Patent Number: 6,996,434 Issued on 02/07/2006 to Marcovecchio,   et al.

Inventors: Marcovecchio; Alan F. (Minneapolis, MN); Hsu; William (Circle Pines, MN)
Assignee: Cardiac Pacemakers, Inc. (St. Paul, MN)
Appl. No.: 921348
Filed: August 2, 2001

Current U.S. Class: 600/509
Current Intern'l Class: A61B 5/04     (20060101)
Field of Search: 600/513,509

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Primary Examiner: Bockelman; Mark
Attorney, Agent or Firm: Schwegman, Lundberg, Woessner & Kluth, P.A.
Parent Case Text


CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 09/267,306, filed on Mar. 12, 1999, now issued as U.S. Pat. No. 6,312,388, the specification of which is incorporated herein by reference.
Claims


What is claimed is:

1. A method, comprising:

comparing one or more sensed cardiac complex parameter values to one or more predetermined cardiac complex parameter values;

determining whether the one or more sensed cardiac complex parameter values and the one or more predetermined cardiac complex parameter values have differences greater than a threshold; and

updating a normal sinus rhythm (NSR) template upon determining that the one or more sensed cardiac complex parameter values and the one or more predetermined cardiac complex parameter values have differences that are greater than the threshold.

2. The method of claim 1, further comprising retaining the NSR template upon determining that the one or more sensed cardiac complex parameter values and the one or more predetermined cardiac complex parameter values do not have differences that are greater than the threshold.

3. The method of claim 1, further comprising determining whether the one or more sensed cardiac complex parameter values are within one or more ranges of normal values prior to comparing the one or more sensed cardiac complex parameter values to the one or more predetermined cardiac complex parameter values, wherein the one or more sensed cardiac complex parameter values are compared upon determining that the one or more sensed cardiac complex parameter values are within the one or more ranges of normal values.

4. The method of claim 3, further comprising:

upon determining that the one or more sensed cardiac complex parameter values are not within the one or more ranges of normal values, determining whether a maximum number of attempts has been made to evaluate the one or more sensed cardiac complex parameter values; and

upon determining that the maximum number of attempts has been made, retaining the NSR template.

5. The method of claim 1, further comprising waiting a selected time period before comparing the one or more sensed cardiac complex parameter values to one or more predetermined cardiac complex parameter values and determining whether the one or more sensed cardiac complex parameter values and the one or more predetermined cardiac complex parameter values have differences greater than a threshold.

6. The method of claim 5, wherein comparing one or more sensed cardiac complex parameter values to one or more predetermined cardiac complex parameter values and determining whether the one or more sensed cardiac complex parameter values and the one or more predetermined cardiac complex parameter values have differences greater than a threshold are automatically performed after the selected time period.

7. The method of claim 1, wherein comparing one or more sensed cardiac complex parameter values and determining whether the one or more sensed cardiac complex parameter values and the one or more predetermined cardiac complex parameter values have differences greater than a threshold are performed after receiving a physician-initiated command.

8. The method of claim 1, wherein comparing one or more sensed cardiac complex parameter values to one or more predetermined cardiac complex parameter values includes comparing one or more sensed cardiac complex parameter values to one or more physician-programmed values.

9. The method of claim 1, wherein comparing one or more sensed cardiac complex parameter values to one or more predetermined cardiac complex parameter values includes comparing one or more sensed cardiac complex parameter values to one or more values derived from a current NSR template.

10. The method of claim 1, further comprising creating an initial NSR template which is updated when the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences that are greater than the threshold.

11. The method of claim 10, wherein creating an initial NSR template includes:

determining whether one or more sensed cardiac complex parameter values are within one or more ranges of normal parameter values; and

setting the initial NSR template based on the one or more sensed cardiac complex parameter values.

12. The method of claim 11, further comprising displaying a proposed NSR template, and determining whether the proposed NSR template is approved prior to setting the initial NSR template.

13. The method of claim 11, further comprising:

upon determining that the one or more sensed cardiac complex parameter values are not within one or more ranges of normal parameter values, determining whether a maximum number of attempts has been made to evaluate the one or more sensed cardiac complex parameter values; and

upon determining that a maximum number of attempts has been made, redefining one or more ranges of acceptable parameter values.

14. The method of claim 1, wherein updating a NSR template comprises setting an updated NSR template based on the sensed cardiac complex parameter values.

15. The method of claim 14, further comprising displaying a proposed NSR template, and determining whether the proposed NSR template is approved prior to setting the updated NSR template.

16. A method, comprising:

creating a normal sinus rhythm (NSR) template;

determining whether sensed cardiac complex parameter values are within a range of normal values;

upon determining that the sensed cardiac complex parameter values are within a range of normal values, comparing the sensed cardiac complex parameter values to predetermined cardiac complex parameter values;

determining whether the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences greater than a threshold; and

updating the NSR template upon determining that the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences that are greater than the threshold.

17. The method of claim 16, further comprising:

upon determining that the sensed cardiac complex parameter values are not within a range of normal values, determining whether a maximum number of attempts has been made to evaluate the sensed cardiac complex parameter values; and

upon determining that the maximum number of attempts has been made, retaining the NSR template.

18. The method of claim 16, further comprising waiting a selected time period before comparing sensed cardiac complex parameter values to predetermined cardiac complex parameter values and determining whether the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences greater than a threshold.

19. The method of claim 18, wherein comparing sensed cardiac complex parameter values to predetermined cardiac complex parameter values and determining whether the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences greater than a threshold are automatically performed after the selected time period.

20. The method of claim 18, wherein comparing sensed cardiac complex parameter values to predetermined cardiac complex parameter values and determining whether the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences greater than a threshold are performed after receiving a physician-initiated command.

21. The method of claim 16, wherein creating a NSR template includes:

determining whether sensed cardiac complex parameter values are within a range of normal parameter values;

displaying a proposed NSR template based on the sensed cardiac complex parameter values;

determining whether the proposed NSR template is approved; and

setting the proposed NSR template as the NSR template.

22. The method of claim 16, wherein updating the NSR template upon determining that the sensed cardiac complex parameter values and the predetermined cardiac complex parameter values have differences that are greater than the threshold includes:

displaying a proposed updated NSR template;

determining whether the proposed updated NSR template is approved; and

upon determining that the proposed updated NSR template is approved, setting the proposed updated NSR template an updated NSR template.

23. The method of claim 16, wherein comparing the sensed cardiac complex parameter values to predetermined cardiac complex parameter values includes comparing sensed cardiac complex parameter values to physician-programmed values.

24. The method of claim 16, wherein comparing the sensed cardiac complex parameter values to predetermined cardiac complex parameter values includes comparing sensed cardiac complex parameter values to values derived from the NSR template.
Description


FIELD OF THE INVENTION

This invention relates generally to medical devices, and more particularly to a system and method for verifying the integrity of normal sinus rhythm templates.

BACKGROUND

The heart is divided into four chambers, the left and right atrial chambers and the left and right ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells contract to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chamber to fill the ventricular chamber. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.

Through the cardiac cycle, the heart pumps blood through the circulatory system. Effective pumping of the heart depends upon five basic requirements. First, the contractions of cardiac muscle must occur at regular intervals and be synchronized. Second, the valves separating the chambers of the heart must fully open as blood passes through the chambers. Third, the valves must not leak. Fourth, the contraction of the cardiac muscle must be forceful. Fifth, the ventricles must fill adequately during diastole.

When the contractions of the heart are not occurring at regular intervals or are unsynchronized the heart is said to be arrhythmic. During an arrhythmia, the heart's ability to effectively and efficiently pump blood is compromised. Many different types of arrhythmias have been identified. Arrhythmias can occur in either the atrial chambers or in the ventricular chambers of the heart.

Ventricular tachycardia is an arrhythmia that occurs in the ventricular chambers of the heart. Ventricular tachycardias are typified by ventricular rates between 120-250 and are caused by disturbances (electrical or mechanical) within the ventricles of the heart. During a ventricular tachycardia, the diastolic filling time is reduced and the ventricular contractions are less synchronized and therefore less effective than normal. Ventricular tachycardias must be treated quickly in order to prevent the tachycardia from degrading into a life threatening ventricular fibrillation.

Arrhythmias that occur in the atrial chambers of the heart are referred to generally as supraventricular tachycardias. Supraventricular tachycardias include atrial tachycardias, atrial flutter and atrial fibrillation. During certain supraventricular tachycardias, aberrant cardiac signals from the atria drive the ventricles at a very rapid rate. Such a situation occurs during paroxysmal atrial tachycardia. This condition begins abruptly, lasts for a few minutes to a few hours, and then, just as abruptly, disappears and the heart rate reverts back to normal.

Cardioverter-defibrillators, such as implantable cardioverter-defibrillators (ICDs), have been shown to be effective in reducing the incidence of sudden cardiac death. Sudden cardiac death is typically caused by either ventricular tachycardia or ventricular fibrillation. Cardioverter-defibrillator systems operate by sensing and analyzing cardiac signals and applying electrical energy to the heart when either a ventricular tachycardia or ventricular fibrillation is detected.

One common way cardioverter-defibrillators detect cardiac arrhythmias is to sense and analyze the rate of ventricular contractions. When the ventricular rate exceeds a programmed threshold value, the cardioverter-defibrillator applies electrical energy in one or more specific patterns to treat either the ventricular tachycardia or ventricular fibrillation.

An additional method cardioverter-defibrillators use to detect cardiac arrhythmias is to compare the morphology of sensed cardiac complexes to template cardiac complexes representative of specific cardiac rhythms. As each cardiac complex is sensed, it is compared to the template cardiac complexes in an effort to identify and classify the sensed cardiac complex. Template cardiac complexes can be representative of a variety of cardiac complexes, including ventricular tachycardias and normal sinus rhythm.

Template cardiac complexes are typically programmed into an implantable medical device shortly before or after the device has been implanted into the patient. Once the implantable medical device has been implanted into the patient, however, the physiologic environment in which cardiac electrodes are placed (i.e., the heart) begins to change. These changes can include an inflammatory response, localized fibrosis around the implanted electrode and cardiac disease progression. These physiological changes lead to a deterioration, or a change in the strength and the morphology of the signal sensed by the implanted medical device. Additionally, changes in a patient's medication regimen can also change the sensing of cardiac signals by the implanted medical device. Therefore, cardiac complex templates developed before or soon after implanting the medical device can become less useful, or reliable, in the process of assessing and classifying unknown cardiac complexes.

Therefore, a need exists for addressing the changes in sensed cardiac signals as the physiological environment surrounding implanted cardiac electrodes changes.

SUMMARY OF THE INVENTION

The present subject matter provides a system and method to verify sensed normal sinus rhythm (NSR) cardiac complexes and to use the NSR cardiac complexes to update a NSR template. The system and method can either function automatically after a selected time interval has expired, or after commands have been delivered by a physician. As a result of updating, cardiac complexes being compared to the NSR template can be classified more accurately than if the cardiac complexes were compared to a NSR template that had not been updated.

Initially, a NSR template is created. In one embodiment, the NSR template is created by an implantable medical device, such as an implantable cardioverter defibrillator, under the control of a patient's attending physician. In creating a NSR template, cardiac complexes are sensed from a patient's heart. Values of one or more cardiac parameters are measured from each of the sensed cardiac complexes. In one embodiment, an implantable cardioverter defibrillator is used to sense cardiac complexes and to measure the values of the cardiac parameters. Cardiac parameters can include, but are not limited to, ventricular and atrial cycle lengths, widths of ventricular depolarizations, atrioventricular conduction times, and R-wave amplitudes.

The values of the cardiac parameters measured from the cardiac complexes are then compared to predetermined ranges for the values of the cardiac parameters for normal sinus rhythm (NSR) complexes. Based on this comparison, the cardiac complexes can be determined to be, or not to be, NSR cardiac complexes. When the cardiac complexes are determined to be NSR complexes, a NSR templates is calculated as a function of these NSR complexes.

After a selected time interval, the NSR template is examined to determine if it continues to accurately reflect the NSR cardiac complexes being sensed from the patient's heart. In one embodiment, values of cardiac parameters are measured from sensed cardiac complexes in a predetermined set of cardiac complexes. The values are then compared to predetermined value ranges. In one embodiment, the predetermined value ranges are individually established and programmed for each of the cardiac parameters. Values for each cardiac parameter measured are then compared to the corresponding predetermined value range established for that particular cardiac parameter.

When the values of the cardiac parameters are found to be within the predetermined value ranges, values for cardiac signal parameter differences are then calculated. The cardiac signal parameter difference values are calculated by taking the difference of the values of the cardiac parameters from each of the sensed cardiac complexes and the values of the cardiac parameters for the NSR template complexes. The values of the cardiac signal parameter differences are then compared to threshold values. In one embodiment, the threshold values for each of the cardiac signal parameter differences are calculated by multiplying a predetermined deviation percentage and each value of the cardiac parameters for the NSR cardiac complexes used to calculate the NSR template.

Based on the comparison, when the values for the cardiac signal parameter differences are found to be greater than the threshold values, the NSR template is updated as a function of the cardiac complexes. Alternatively, when the values for the cardiac signal parameter differences are found to be less than or equal to the threshold values, the NSR template is not updated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating one embodiment of the present subject matter;

FIG. 2 is a flow chart illustrating one embodiment of the present subject matter;

FIG. 3 is a schematic of an implantable medical device; and

FIG. 4 is one embodiment of a block diagram of an implantable medical device according to the present subject matter.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof and in which is shown by way of illustration specific embodiments in which the invention can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice and use the invention, and it is to be understood that other embodiments may be utilized and that electrical, logical, and structural changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the present invention is defined by the appended claims and their equivalents.

The embodiments illustrated herein are demonstrated in an implantable cardiac defibrillator (ICD), which may include numerous defibrillation, pacing, and pulse generating modes known in the art. However, these embodiments are illustrative of some of the applications of the present system, and are not intended in an exhaustive or exclusive sense. The concepts described herein can be used in a variety of applications which will be readily appreciated by those skilled in the art upon reading and understanding this description. For example, the present system is suitable for implementation in a variety of implantable and external devices.

The present subject matter allows for medical devices to examine and to recalculate or update a cardiac complex template. As previously discussed, medical device systems can use a cardiac complex template to assess and classify sensed cardiac complexes. Based on the classification of cardiac complexes, the medical device system can generate commands to cause the delivery of therapy to a patient's heart.

The physiologic environment in which cardiac electrodes are placed (i.e., the heart) changes from the moment the electrodes are implanted. Changes in the physiologic environment can include an inflammatory response and localized fibrosis around the implanted electrode due to the presence of the implanted electrode. These physiological changes, and other changes in cardiac disease, can lead to deterioration in sensing by the implanted medical device. Additionally, changes in a patient's medication regimen can also change the sensing of cardiac signals by the implanted medical device. Therefore, cardiac complex templates developed soon after implanting the medical device can become less reliable in the process of assessing and classifying unknown cardiac complexes.

The present subject matter allows for cardiac complex templates to be examined and, if certain predetermined conditions are met, to be recalculated. Recalculating cardiac complex templates can occur based on a determination that cardiac signals sensed during a cardiac state for which there is a cardiac complex template are no longer within a predetermined range of acceptability. In one embodiment, recalculation of the cardiac complex template is performed at specific time intervals over the life of the implantable medical device. Alternatively, the cardiac complex template is recalculated, or updated, at the direction of an attending physician when the physician determines the sensed cardiac signals have deviated sufficiently from the cardiac complex template signals.

In one embodiment, the present subject matter is useful for recalculating normal sinus rhythm (NSR) templates or models, such as the NSR template previously discussed. NSR templates are used in systems and methods where morphological features from cardiac signals sensed during a tachycardia event are compared to morphological features for cardiac signals sensed during normal sinus rhythm (e.g., the NSR templates). In one embodiment, procedures that compare values derived from NSR cardiac complexes are referred to as NSR template based procedures. In one embodiment, NSR template based procedures require that the NSR template be calculated, or derived at, from a patient's NSR cardiac complexes. However, as discussed above, as the cardiac tissue surrounding the cardiac leads change (e.g., as the tissue in which the lead is embedded changes) sensing of the cardiac complexes changes. Thus, there is a need for the NSR template to be updated after the medical device has been implanted.

Referring now to FIG. 1, there is shown an embodiment for creating a normal sinus rhythm (NSR) template. The procedure for setting the NSR template is started at 100. In one embodiment, the procedure for setting the NSR template is initiated by the patient's attending physician. Once the procedure is started at 100, values for one or more NSR parameters representative of the patient's normal sinus rhythm are set at 104. In one embodiment, the values of the NSR parameters are set by and programmed into the implantable medical device by the attending physician. In one embodiment, the values of the NSR parameters are based on a patient's previously recorded cardiac data and/or the patient's cardiac history. Alternatively, the values for the NSR parameters are a set of typical values for the NSR parameters.

In one embodiment, the values for the NSR parameters are derived from, but not limited to, the following cardiac parameters: ventricular cycle lengths (e.g., time between consecutively sense R-waves), atrial cycle lengths (e.g., time between consecutively sense P-waves), standard deviation of a plurality of ventricular cycle lengths, standard deviation of a plurality of atrial cycle lengths, width of ventricular depolarizations as manifested in the ventricular electrogram, atrioventricular conduction times, standard deviation of a plurality of atrioventricular conduction times, and standard deviation of a plurality of R-wave amplitudes as manifested in the ventricular electrogram. In one embodiment, the NSR parameters for which values are derived are selected and programmed into the medical device system through the use of a medical device programmer. In addition, the values for the NSR parameters are also programmed into the medical device system through the use of the medical device programmer.

After the values for the NSR parameters are set, these initial values must be checked against the values for the cardiac parameters measured from the patient's NSR cardiac complexes. At 108, one or more cardiac complexes are sensed from cardiac signals sensed from the patient's heart. In one embodiment, the cardiac signals are electrogram signals sensed through the use of the implanted medical device, such as an ICD, and include cardiac complexes representative of at least a portion of the cardiac cycle.

In one embodiment, the cardiac signals are viewed in real time on the display screen of the external medical device programmer. As the cardiac signal scrolls across the display screen, the user can select a series of cardiac complexes which he or she believes to be NSR complexes. The selected cardiac complexes are then tested against the initial values of the NSR parameters. This procedure saves valuable time by allowing only those cardiac complexes believed to be NSR complexes to be checked against the initial cardiac parameter values.

The implanted medical device measures and stores values for the one or more cardiac parameters from each of the sensed cardiac complexes. In one embodiment, the values for the one or more cardiac parameters are taken, or derived, from the cardiac parameters whose values were initially set at 104. At 112, the values of the one or more cardiac parameters are used to determine whether the sensed cardiac complexes are normal sinus rhythm (NSR) complexes. In one embodiment, the values of the one or more cardiac parameters are compared to values of the corresponding NSR parameters. In one embodiment, the values of the NSR parameters are those initially set at 104. In an alternative embodiment, the values of the NSR parameters are values of one or more cardiac parameters acquired during the process of setting the NSR template, an embodiment which will be described in greater detail below.

At 112, if the values of the cardiac signal parameters fall within a predetermined range of the values for the NSR parameters, then the NSR template and the values for the NSR parameters are displayed at 116. In one embodiment, displaying the NSR template and the values of the NSR parameters is done on the screen of the medical device programmer. Alternatively, the NSR template and the values of the NSR parameters are displayed on any suitably enabled display screen from which this information can be viewed.

The NSR template is calculated as a function of sensed NSR cardiac complexes. In one embodiment, after qualified NSR complexes have been identified, a single NSR complex can be formed to represent multiple NSR complexes. This is done by taking a sample by sample median, mean, or other statistic of one through n sensed NSR complexes, where n is an integer value representing the number of sensed NSR complexes. In one embodiment, n is an integer value in the range of 2 to 20 NSR complexes.

As the one through n NSR complexes are sensed, they are aligned around a common feature of the cardiac signal. In one embodiment, the common feature is a maximum deflection point of the cardiac signal as sensed during the occurrence of the ventricular R-wave. Other common features of the cardiac complexes can also be used to align the sensed NSR complexes. Once the NSR complexes have been aligned, median or mean values of the cardiac complexes are used to calculate the NSR template. Once the NSR template has been created from the sample by sample process, values for the NSR features are extracted from the NSR template and stored for use with the present subject matter. In an alternative embodiment, values for NSR features are measured for each of the sensed one through n NSR complexes. Median or mean values for the NSR feature values are calculated and the median or mean values are used to calculate the NSR template.

Once the NSR template and the values for the NSR parameter have been displayed, the attending physician can review the information. At 120, the physician can either approve of or not approve of the NSR template and the values for the NSR parameters proposed for use in the medical device. If the NSR template and the values for the NSR parameters are approved at 120, the NSR template and the values for the NSR parameters are programmed into the medical device system at 124 for subsequent use in discrimination procedures which rely on NSR templates and values of NSR parameters to distinguish one arrhythmic event from another.

If, however, the values of the cardiac signal parameters do not fall within values of the NSR parameters at 112 or the user does not approve of the NSR template and/or the values of the NSR parameters at 120, the system proceeds to 128. At 128, the number of attempts at setting the NSR template is determined. In one embodiment, when 128 is reached a value of one is added to the va


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