Multiple stage morphology-based system detecting ventricular tachycardia and supraventricular tachycardia

A system for detecting ventricular tachycardia and supraventricular tachycardia using a multiple stage morphology based system. Cardiac signals are sensed from a patient's heart and analyzed for the occurrence of a tachycardia event. When a tachycardia event is detected, the method and system analyzes a plurality of features of the sensed cardiac signals in two or more discrimination stages. Each of the two or more discrimination stages classify the tachycardia event as either a ventricular tachycardia or a candidate supraventricular tachycardia event. When a discrimination stage detects the occurrence of a ventricular tachycardia, therapy is delivered to the heart to treat the ventricular tachycardia.

FIELD OF THE INVENTION
 This invention relates generally to the field of medical devices, and more
 particularly to a method and system for discriminating and classifying
 supraventricular tachycardia and ventricular tachycardia events.
 BACKGROUND
 Recent prospective clinical trials have shown that
 cardioverter-defibrillators, such as implantable
 cardioverter-defibrillators (ICDs), reduce sudden arrhythmic death and
 favorably impact overall mortality in patients at risk for spontaneous
 ventricular tachyarrhythmia. Cardioverter-defibrillator systems are
 designed to provide therapy when rapid ventricular activation rates are
 sensed. However, rapid ventricular rhythms can occur in the presence of a
 supraventricular tachycardia (SVT). When therapy is applied in response to
 SVT (in absence of a ventricular tachycardia, VT, or ventricular
 fibrillation, VF), the therapy is classified as clinically
 "inappropriate", even though the cardioverter-defibrillator responded
 appropriately to an elevated ventricular rate.
 Cardioverter-defibrillators may deliver inappropriate ventricular therapy
 to patients afflicted with non-malignant SVTs. These inappropriate
 therapies may be delivered due to the device's inability to reliably
 discriminate SVT from malignant VT.
 For the reasons stated above, and for other reasons stated below which will
 become apparent to those skilled in the art upon reading and understanding
 the present specification, there is a need in the art for a system and a
 method of reliably and accurately discriminating between the occurrence of
 a SVT and a VT event during a detected tachyarrhythmia event which can
 reduce the frequency of inappropriate therapies delivered to
 cardioverter-defibrillator patients. Such a system may also be suitable
 for use with patients having implantable cardioverter-defibrillators.
 SUMMARY OF THE INVENTION
 The present system provides a means for discriminating, or classifying
 supraventricular tachycardias (SVT) from malignant ventricular
 tachycardias (VT). The present disclosure teaches a number of embodiments
 useful for, among other things, classifying a tachycardia or fast
 arrhythmia as either a SVT or a VT event. In one embodiment, the system
 utilizes a series of discrimination stages employing a plurality of
 methods for distinguishing and classifying VT and SVT. In one embodiment,
 stages are arranged so that the computationally more efficient stages are
 used initially in assessing and classifying the tachycardia event. In one
 embodiment, this multiple stage system allows for a more accurate
 assessment of the patient's condition before treatment is delivered.
 Furthermore, this multiple stage system allows for earlier (i.e.,faster)
 treatment of certain VT events, which, in the case of an implantable
 device, results in a more efficient use of the ICD's battery.
 In one embodiment, there is provided a system for classifying VT from SVT
 during a tachycardia event. Cardiac signals representative of electrical
 cardiac activity are sensed and analyzed of the occurrence of a
 tachycardia event. When a tachycardia event is detected, a plurality of
 features along the sensed cardiac signals are analyzed in two or more
 discrimination stages. In one embodiment, a discrimination stage is used
 to distinguish and classify the tachycardia event as either being a VT
 event or a candidate SVT event.
 In one embodiment, the first discrimination stage analyzes the width of
 repeatably identifiable features on the sensed cardiac signals and
 compares them to a template value to classify the tachycardia event. In
 one embodiment, the first discrimination stage acts to measure a width of
 sensed R-waves using the plurality of features from cardiac signals sensed
 during a tachycardia event. The width of each of the sensed R-waves is
 then compared to a template R-wave width. In one embodiment, the template
 R-wave width is determined from cardiac signals sensed during the
 patient's normal sinus rhythm.
 If the comparison of the sensed R-waves to the template R-wave width
 reveals that the width of a sensed R-wave is greater than or equal to a
 predetermined value of the template R-wave width, the cardiac signal is
 classified as a ventricular tachycardia complex. As the cardiac signals
 are classified, the number of ventricular tachycardia complexes are
 recorded, and when the number of ventricular tachycardia complexes reach a
 predetermined threshold a ventricular tachycardia is declared. Once a
 ventricular tachycardia is declared, therapy for treating the ventricular
 tachycardia is delivered to the patient.
 If in the first discrimination stage a ventricular tachycardia is not
 declared, a second discrimination stage is then used to assess and
 classify the tachycardia event. In one embodiment, the second
 discrimination stage includes the acts of determining values for each of
 the plurality of features of the cardiac signals sensed during a
 tachycardia event. In one embodiment, the plurality of features includes
 the value of maximum and minimum deflection points along the sensed
 cardiac signals.
 The values for the plurality of features for each of the cardiac signals
 sensed during the tachycardia event are then used in determining a
 similarity value and a dissimilarity value. In one embodiment, the
 similarity value and the dissimilarity value indicate the similarity of
 the sensed cardiac signal to cardiac signals sensed during normal sinus
 rhythm. As such, the similarity value and the dissimilarity value for the
 sensed cardiac complexes are assessed relative to a plurality of features
 on normal sinus rhythm signals.
 The similarity value and the dissimilarity value are then used to determine
 if each of the cardiac signals is a ventricular tachycardia complex. In
 one embodiment, this is accomplished by plotting the similarity value and
 the dissimilarity value on a discrimination plane. Based on where the
 cardiac signal is plotted on discrimination plane, the cardiac signal is
 either classified as a ventricular tachycardia complex or a candidate
 supraventricular tachycardia complex. In one embodiment, the candidate
 supraventricular tachycardia complex is also known as a non-ventricular
 tachycardia complex.
 As the cardiac signals are classified, the number of ventricular
 tachycardia complexes are recorded, and when the number of ventricular
 tachycardia complexes reach a predetermined threshold a ventricular
 tachycardia is declared. In one embodiment, once a ventricular tachycardia
 is declared, therapy for treating the ventricular tachycardia is delivered
 to the patient.
 In one embodiment, if in the first discrimination stage the tachycardia
 event is classified as a candidate supraventricular tachycardia (or a
 non-ventricular tachycardia) and the second discrimination stage also
 classifies the tachycardia event as a candidate supraventricular
 tachycardia, then the tachycardia event is declared a supraventricular
 tachycardia.
 These and other features and advantages of the invention will become
 apparent from the following description of the embodiments of the
 invention.

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.
 Some of the embodiments illustrated herein are demonstrated in an
 implantable cardiac defibrillator, 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. For example, the present system is suitable for implementation in a
 variety of implantable and external devices.
 One embodiment of the present system provides a means for discriminating,
 or classifying, supraventricular tachycardias (SVT) from malignant
 ventricular tachycardias (VT). The present disclosure provides a number of
 embodiments useful for, among other things, classifying a tachycardia or
 fast arrhythmia as either a SVT or a VT. 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.
 Embodiments of distinguishing of classifying VT and SVT are discussed
 herein, but other arrhythmic events (both ventricular and
 supraventricular) can also be distinguished using the teachings provided
 herein, and therefore, the express teachings of this disclosure are not
 intended in an exclusive or limiting sense.
 In one embodiment, the distinction, or classification, between VT and SVT
 events is accomplished through the use of a series of discrimination
 stages which utilize a plurality of methods for distinguishing and
 classifying VT and SVT. In one embodiment, the series of discrimination
 stages includes two or more discrimination stages, where each of the two
 or more discrimination stages classifies the tachycardia event as either a
 ventricular tachycardia or a candidate supraventricular tachycardia. By
 using two or more discrimination stages, the present system is able to
 take advantage of each stage's ability to differentiate between an SVT and
 a VT event. This series of discrimination stages, therefore, allows for
 the benefits or advantages of each stage in making the determination
 between VT and SVT. For example, some stages provide greater sensitivity
 to correctly classify VT episodes, while other stages allow for greater
 specificity in classifying SVT episodes correctly. In addition, some
 stages are more computationally efficient than others, which allows for
 VT/SVT classification that is accomplished more quickly while using less
 time and energy resources of the implantable system. Therefore, in one
 embodiment, the present method and system provides for a synergistic
 mechanism of making the VT and SVT distinction. This synergistic
 interaction allows for a more accurate assessment of the patient's cardiac
 condition which results in more effective treatment being delivered to the
 patient.
 A wide variety of methods or stages for distinguishing VT from SVT can be
 utilized in the present system. In one embodiment, the order in which the
 stages are applied, or used, affects the accuracy and the speed in making
 the distinction between VT and SVT. In one embodiment, the system utilizes
 a series of discrimination stages in which individual stages determine and
 classify the occurrence of VT and SVT based on sensed cardiac signals. In
 one embodiment, the sensed cardiac signals are representative of
 electrical cardiac activity. The embodiments provided herein classify VT
 from SVT during a tachycardia or fast arrhythmia based on signals sensed
 by a single chamber implantable cardiac defibrillator. In one embodiment,
 the single chamber implantable cardiac defibrillator has a multiple
 electrode, single endocardial lead which senses both ventricular
 near-field signals (ventricular rate signals) and ventricular far-field
 signals (ventricular morphology signals). In one embodiment, the
 implantable cardiac defibrillator employs an single body lead catheter
 sold under the trademark ENDOTAK (Cardiac Pacemaker, Inc./Guidant
 Corporation, St. Paul, Minn.) having a pacing tip electrode and two
 defibrillation coil electrodes. One example of such a system is shown in
 FIG. 7. ICD 700 is coupled to catheter 710, which is implanted to receive
 signals from heart 720. The catheter 710 also may be used for transmission
 of pacing and/or defibrillation signals to the heart 720. In an
 alternative embodiment, a three defibrillation electrode system is
 employed, wherein the housing of the implantable system is used as a third
 defibrillation electrode. In one embodiment, this configuration is known
 in the art as a "hot can" system.
 In an alternative embodiment, a dual chamber implantable cardiac
 defibrillator is used to classify VT from SVT based on sensed cardiac
 signals. In one embodiment, the dual chamber implantable cardiac
 defibrillator includes an ENDOTAK single body lead catheter implanted in
 the ventricular region of the heart and an atrial catheter implanted in a
 supraventricular region of the heart. This embodiment allows for
 ventricular near-field signals and ventricular far-field signals, along
 with atrial near-field signals to be sensed and analyzed by the
 implantable cardiac defibrillator.
 Other cardiac defibrillator systems and catheter configurations may be used
 without departing from the present system. In addition to implantable
 cardiac defibrillator systems, the present system may be utilized in
 external defibrillation systems and in external cardiac monitoring
 systems. In addition to employing endocardial leads, the present system
 can also utilize body surface leads.
 Current implantable cardioverter defibrillators frequently deliver
 inappropriate ventricular therapy to patients afflicted with non-malignant
 SVTs. These inappropriate therapies are usually delivered due to the
 device's inability to reliably discriminate SVT from malignant VT during a
 sensed tachycardia event. Referring to FIG. 1, there is shown one
 embodiment of a method for classifying VT from SVT during a tachycardia
 event. At 100, cardiac signals representative of electrical cardiac
 activity are sensed. In one embodiment, the cardiac signals are sensed by
 an endocardiac lead system of implantable cardiac defibrillator as
 previously described. The cardiac signals include cardiac complexes which
 are portions of the complete cardiac cycles. In one embodiment, the sensed
 cardiac complexes include the QRS-wave of a cardiac cycle. Included in the
 QRS-wave is an R-wave, which is produced by the contraction of the
 ventricle during systole. In one embodiment, the system detects a sensed
 R-wave for one or more complexes of the cardiac signals sensed by the
 implantable cardiac defibrillator. At 110, the system analyzes the sensed
 cardiac complexes to determine if a tachycardia event is occurring. In one
 embodiment, the system determines the occurrence of a tachycardiac event
 by analyzing the sensed cardiac rate. A cardiac rate that exceeds a
 predetermined threshold indicates the occurrence of a ventricular
 tachycardia. In one embodiment, the predetermined threshold is for cardiac
 rates of between 150-180 beats per minute. In an alternative embodiment,
 the predetermined threshold is a lower rate zone of multiple rate-zone
 device. Other methods of determining the occurrence of tachycardia episode
 which are known in the art may be used without departing from the present
 system.
 If the system determines that a tachycardia event is not occurring, the
 system takes path 120 back to 100 and continues to sense and analyze
 cardiac complexes for the occurrence of a tachycardia event. If a
 tachycardia event is detected at 110, the system proceeds via 130 to 140.
 At 140, the cardiac complexes sensed during the tachycardia event are
 analyzed by a series discrimination stages. In one embodiment, the
 discrimination stages are procedures which are implemented by an
 implantable cardiac defibrillator. In one embodiment, the series of
 discrimination stages are selected in such a way that the discrimination
 stages progress from stages that are the simplest in terms of
 implementation (ie., requiring less information or using fewer features
 extracted from the cardiac complexes and thereby being computationally
 less complicated) to progressively more complex discrimination stages. In
 one embodiment, the discrimination stages analyze a plurality of features
 of the sensed cardiac complexes in two or more discrimination stages,
 where each of the two or more discrimination stages classifies the
 tachycardia event as either a ventricular tachycardia or a candidate
 supraventricular tachycardia.
 In one embodiment, the initial stage used at 140 is intended to quickly
 assess and classify the most easily identifiable tachycardias. In one
 embodiment, the most easily identifiable tachycardias are those that have
 cardiac signals with distinctive morphological features which are useful
 in distinguishing a VT from an SVT episode. In one embodiment, the width
 of repeatably identifiable features on sensed cardiac complexes are used
 to distinguish VT from SVT. For example, the width of an R-wave sensed in
 a QRS-cardiac complex during a tachycardia event is measured and compared
 to a template R-wave width to distinguish the sensed cardiac complex as
 either a VT complex or a candidate SVT complex. In one embodiment, when
 the R-wave width is less than a predetermined value of the template R-wave
 width, the cardiac signal is classified as a candidate supraventricular
 tachycardia complex. Additionally, when the R-wave width is greater than
 or equal to the predetermined value of the template R-wave width, the
 cardiac signal is classified as a ventricular tachycardia complex. As the
 cardiac signals are classified, the number of VT complexes and candidate
 SVT complexes are recorded and analyzed at 150. At 150, when the number of
 VT complexes exceeds a predetermined threshold, a VT episode is declared.
 The system then follows path 160 to 170 where therapy is delivered to the
 patient's heart to treat the VT. Alternatively, when the number of
 candidate SVT complexes exceed the predetermined threshold, the system
 then declares a candidate SVT episode and proceeds to the next
 discrimination stage.
 In one embodiment, the next discrimination stage is a more computationally
 advanced discrimination stage. In one embodiment, the advanced
 discrimination stage is used on tachycardia events that are more difficult
 to assess. An example of a tachycardia event that has traditionally been
 difficult to assess has been narrow complex ventricular tachycardias, or
 tachycardias with any atrial to ventricular depolarization ratio
 (including, but not limited to, 1:1).
 The number and type of advanced stages used in assessing the tachycardia is
 a programmable feature of the implantable medical device. In one
 embodiment, the advanced stages utilize different morphological features
 from cardiac complexes sensed during the tachycardia event. Based on the
 morphological features of the cardiac signals, a determination of the
 origin of the tachycardia event is possible. In an additional embodiment,
 the advanced stages are weighed in terms of what type of therapy to
 provide to a patient when two or more advanced stages provide conflicting
 assessments of the tachycardia event. For example, the system is
 programmed to deliver therapy based on the determination of a second
 advanced stage, even though a first advanced stage determination provided
 an opposing assessment.
 As with the initial stages, if at 150 applying series of discrimination
 stages during the advanced stages results in the determination of a
 ventricular tachycardiac, the system follows path 160 to 170 where therapy
 for converting the ventricular tachycardia to normal sinus rhythm is
 delivered to the patient. Appropriate therapy for treating a ventricular
 tachycardia can include such therapy as overdrive pacing or delivering
 cardioversion shocks to the heart. Other types of therapy for treating a
 ventricular tachycardia are known in the art and considered within the
 scope of the present system.
 At 150, if a ventricular tachycardia is not determined using the series of
 discrimination stages, the system follows path 180 to 190 where a
 supraventricular tachycardia is declared. In one embodiment, therapy is
 delivered to the supraventricular region of the heart to treat the SVT. In
 an alternative embodiment, therapy is not delivered to the
 supraventricular region of the heart, but rather the system continues to
 monitor the cardiac condition and provides treatment only when a
 ventricular tachycardia is determined.
 Referring now to FIG. 2, there is shown an additional embodiment of the
 present system for distinguishing the nature of the tachycardia event
 occurring in the heart. At 200, the system senses cardiac signals
 representative of electrical cardiac activity. At 210, the system analyzes
 the sensed cardiac signals to determine if a tachycardia event is
 occurring. If a tachycardia event is not detected, the system takes path
 220 back to 200 and continues to sense and analyze cardiac signals for the
 occurrence of a tachycardia event or fast arrhythmic event. As previously
 mentioned, numerous methods, including the use of the cardiac rate, exist
 in the art for determining the occurrence of tachycardia events, and are
 considered to be within the scope of the present invention.
 If a tachycardia event is detected at 210, the system proceeds along path
 230 to 240. At 240 the cardiac signals are analyzed in a first
 discrimination stage. In one embodiment, the first discrimination stage
 determines the width of R-waves sensed from the cardiac signals sensed
 during the tachycardia event. The width of the sensed R-wave is useful in
 discriminating VT from SVT during a tachycardia event. In one embodiment,
 the width of the sensed R-waves changes due to differences in the
 conduction velocity of the hearts intrinsic contraction wave during VT as
 compared to SVT. During normal sinus rhythm and SVT, electrical stimuli
 propagate through the His-Purkinje System. This allows for rapid
 conduction of the electrical stimuli throughout a large portion of the
 ventricular cardiac tissue. During VT, electrical stimuli must propagate
 through the myocardium. The conduction velocity in myocardium is less than
 the conduction velocity in the His-Purkinje System. This difference in
 conduction velocity often translates into a wider R-wave during VT in both
 body surface and endocardial biopotentials. This difference allows for
 cardiac signals to be discriminated and classified by the system.
 Referring now to FIG. 3, there is shown one embodiment of a sensed cardiac
 complex 300. In one embodiment, the sensed cardiac complex 300 is an
 electrogram recording of the QRS-wave of the cardiac cycle. The sensed
 cardiac complex 300 displays a plurality of features. In one embodiment,
 the plurality of features are at major deflection points along the sensed
 cardiac complex. For example, in FIG. 3 four major deflection points are
 found at a first feature 302, a second feature 304, a third feature 306,
 and a fourth feature 308. Values for these major deflection points provide
 a four element feature vector. In one embodiment, feature vectors are
 extracted for each tachycardia complex that is sensed. The feature values
 are then used in measuring the width of the ventricular R-wave 310. In one
 embodiment, the width of the ventricular R-wave 310 is measured between
 the second feature 304 at approximately the start of the R-wave 310 and
 the fourth feature 308 at approximately the end of the R-wave 310.
 In one embodiment, the method of classifying VT and SVT by measuring the
 width of a patient's R-wave 310 involves measuring the width of the R-wave
 by first digitizing electrical signals from the ventricle to digital
 signals. The digitized signals are then analyzed to determine the second
 feature 304 and the fourth feature 308 of the sensed R-waves. The width of
 the R-wave is then defined as the interval between the second feature 304
 associated with a detected R-wave 310 and the fourth feature 308
 associated with the same detected R-wave 310.
 At 250, the system determines if a ventricular tachycardia is occurring. In
 one embodiment, the R-wave width is compared to a template R-wave width.
 In one embodiment, the template R-wave width is an average R-wave width of
 cardiac complexes sensed during normal sinus rhythm. In an alternative
 embodiment, the template R-wave width is a median R-wave width of cardiac
 complexes sensed during normal sinus rhythm. When the R-wave width is
 greater than or equal to a predetermined value of the template R-wave
 width, the cardiac complex is categorized as a VT complex. Accordingly,
 when the R-wave width is less than the predetermined value of the template
 R-wave width, the cardiac complex is categorized as a candidate SVT
 complex. In one embodiment, the predetermined value is a programmable
 value in the range of 20 to 50 percent, where 30 percent is an acceptable
 value.
 As the sensed cardiac complexes are categorized, the system records the
 number of VT complexes and candidate SVT complexes that have been
 categorized during the tachycardia event at 250. In one embodiment, the
 tachycardia event is classified as a VT when the number of VT complexes
 exceeds a predetermined threshold. In one embodiment, the predetermined
 threshold is an x out of the last y complexes counter. When x out of the
 last y complexes are not classified as VT complexes, the system classifies
 the tachycardia event as a candidate SVT. Candidate SVT events are then
 analyzed in at least a second discrimination stage to either confirm the
 presence of an SVT event or determine the presence of a VT event. In one
 embodiment, the values for x and y are programmable, where x has
 programmable integer values in the range of 3 to 10, where 5 is an
 acceptable value, and y has a programmable integer values in the range of
 8 to 30, where 10 is an acceptable value. In an alternative embodiment,
 the system determines a percentage of VT complexes during the tachycardia
 event. When the percentage of the VT complexes exceeds a predetermined
 percentage threshold, the system declares the occurrence of a ventricular
 tachycardia. In one embodiment, the predetermined percentage threshold is
 a programmable value in the range of 30 to 100 percent, where 50 percent
 is an acceptable value.
 When the number of VT complexes exceeds the predetermined threshold, a VT
 episode is declared. The system then delivers therapy to the patient's
 heart to treat the VT event at 260. If a VT episode is not declared, the
 system records the event as a candidate SVT episode and proceeds along
 path 270 to the next discrimination stage 280.
 In an alternative embodiment, the first arrhythmia discrimination
 procedures determines changes in the polarity of detected R-waves. The
 polarity of the detected R-waves is useful in determining VT from SVT. For
 example, the system records the sign of the largest amplitude of cardiac
 complexes sensed during normal sinus rhythm. During a tachycardia event,
 the sign of the cardiac complex feature having the largest amplitude is
 recorded and compared to that of the normal sinus rhythm. If the largest
 amplitude feature is different in sign between the normal sinus rhythm an
 the tachycardia event, the tachycardia event is determined to be a VT
 event.
 At 280, the cardiac signals are analyzed by a second discrimination stage.
 In one embodiment, the second discrimination stage is either a
 determination of polarity change in the R-wave or the width of the R-wave,
 which ever analysis was not utilized in the first discrimination stage. In
 an alternative embodiment, an advanced procedure is used to assess the
 tachycardia event. In one embodiment, advanced procedures performs a
 cardiac complex feature comparison on the sensed cardiac signals. In one
 embodiment, the cardiac signals feature comparison involves analyzing a
 morphological similarity of the cardiac signals to a normal sinus rhythm
 template complex. In one embodiment, analyzing the morphological
 similarity of the cardiac signals involves determining a similarity
 feature value and a dissimilarity feature value for each sensed cardiac
 signals. Based on the calculated feature values, the cardiac signal is
 classified as either being a ventricular tachycardia complex or a
 supraventricular tachycardia complex. In one embodiment, the cardiac
 signal is a far-field or morphology electrocardiogram signal. In an
 alternative embodiment, the cardiac signal is a near-field or rate
 electrocardiogram signal.
 In one embodiment, when the system proceeds to analyze the tachycardia
 event in the second discrimination stage, the cardiac signals used in the
 second discrimination stage are the cardiac signals classified in the
 first discrimination stage. So in one embodiment, the cardiac signals used
 in comparing the width of the sensed R-wave to the template R-wave width
 are the same cardiac signals used in the step of analyzing the
 morphological similarity of the cardiac signals. This allows the cardiac
 signals analyzed in the first discrimination stage to be re-evaluated
 before a decision as to whether the tachycardia is a VT or an SVT. In an
 alternative embodiment, the cardiac signals classified in the second
 discrimination stage are different cardiac signals than those classified
 in the first discrimination stage. So in one embodiment, additional
 cardiac signals for use in the step of analyzing the morphological
 similarity of the cardiac signals are sensed by the system.
 One example of determination of a similarity feature value and a
 dissimilarity feature value is discussed in U.S. Pat. No. 5,311,874 by
 Baumann et al., which is hereby incorporated by reference in its entirety.
 Values for the similarity feature value and the dissimilarity feature
 value distinguishes cardiac complexes as either being a ventricular
 tachycardiac complex or a supraventricular tachycardia complex. This is
 accomplished through a comparison of a feature vector, A, for a sensed
 cardiac complex and a feature vector, N, for cardiac complexes sensed
 during normal sinus rhythm. In one embodiment, the feature vector, A, and
 the feature vector, N, are four element feature vectors as previously
 described.
 In one embodiment, the feature vector, A, is generated for each cardiac
 signal sensed during a tachycardia event. In one embodiment, the feature
 vector, A, is determined from a plurality of features of the cardiac
 complexes sensed during a tachycardia event. Values for each of the
 plurality of features are determined by the system. In one embodiment, the
 morphological features acquired from sensed QRS-waves are used to
 determine the feature vector, A. The normal sinus rhythm vector, N, is
 also determined from a plurality of features of the cardiac complexes
 sensed during normal sinus rhythm. The feature vector, A, is then used
 with the normal sinus rhythm vector, N, to determine a similarity value
 and a dissimilarity value for each of the cardiac signals, where the
 similarity value and the dissimilarity value are assessed relative to a
 plurality of features on normal sinus rhythm signals.
 In one embodiment, feature vectors are derived from morphological features
 along the sensed cardiac complex waveform. In one embodiment, the
 morphological features are the extracted amplitude values of peaks and
 valleys (or maxima and minima) in the QRS wave of each arrhythmic complex
 through a process called feature extraction. Each arrhythmic complex is
 isolated according to a known morphological template. In one embodiment,
 the morphological template operates to detect the activation of an heart
 beat (such as the occurrence of an R-wave), at which point the electronic
 control circuitry of the implantable medical device analyzes the complex
 associated with the signal indicating the activation of the heart beat. In
 one embodiment, a threshold value or a detection criterion, as known in
 the art, is used to indicate the activation of the heart beat. The
 resulting feature vector, A, includes a set of numbers, each number
 associated with a particular morphological point of the complex.
 Each feature vector, A, is then compared with the feature vector, N,
 representing the patient's QRS complex during normal sinus rhythm. In one
 embodiment, the feature vector, N, is known as a normal rhythm vector. In
 one embodiment, the normal rhythm vector, N, is determined from
 predetermined waveform characteristics of cardiac QRS-waves recorded
 during normal sinus rhythm. This information is obtained from the normal
 sinus rhythm snapshot. The resulting normal rhythm vector, N, includes a
 set of numbers, each number associated with a particular morphological
 point of the normal sinus rhythm. The electronic control circuitry then
 compares each feature vector, A, with the normal rhythm vector, N, to
 calculate a similarity value and a dissimilarity value for each cardiac
 signal sensed during a tachycardia event.
 Referring now to FIG. 4A, there is shown one embodiment of an arrhythmic
 episode electrocardiogram 400. The typical cardiac arrhythmia comprises a
 series of arrhythmia complexes, or signals, 402(1), 402(2), . . . 402(N)
 as shown in FIG. 4A. In one embodiment, the implantable medical device 20
 determines a similarity value and a dissimilarity value for each of the
 arrhythmia signals by analyzing the individual QRS waves 404 of the
 arrhythmic signals relative the patient's normal sinus rhythm. An
 embodiment of an individual QRS wave 404 is shown in FIG. 4B. The
 tachycardia complexes are processed by the implantable medical device 20
 to determine the amplitudes of peaks 406 and valleys 408 in the QRS
 complex 404 of the arrhythmia complexes 402(1), 402(2) . . . 402(N). In
 one embodiment, the peaks 406 and valleys 408 are determined by
 determining major inflection points in the QRS complex as represented in
 FIG. 4B.
 The resulting values of the peaks 406 and valleys 408 provides a four
 dimensional feature vector, A=[A1, A2, A3, A4], representing each of the
 arrhythmic complexes. In one embodiment, the four dimensional feature
 vector, A, is the four element feature vector used in determining the
 width of the R-wave. In one embodiment, to align the complexes from
 different cardiac rhythms, the system 20 is programmed to set the
 deflection with the largest absolute value as A3. Values for A1 and A2 and
 A4 are chosen to be the relative extreme immediately before and after A3.
 If one of the relative extreme does not exist, a slope criterion is used
 to detect a decrease in slope below a set threshold.
 In an additional embodiment, the implantable medical device 20 analyzes the
 "snapshot" of normal sinus rhythm to determine average amplitudes of peaks
 and valleys for the QRS complex of the patient's normal sinus rhythm. From
 these values a four dimensional normal rhythm vector, N=[N1, N2, N3, N4],
 for normal sinus rhythm is determined. The two vectors A and N are then
 used to determine values for the similarity and dissimilarity for each
 tachycardia complex.
 The similarity feature value and dissimilarity feature value for the
 tachycardia complex is than mapped onto a discrimination plane 500 as
 shown in FIG. 5. In one embodiment, a discrimination plane is defined by
 the two-dimensional plane created by the vectors N/.vertline.N.vertline.
 and A/.vertline.N.vertline., where the orthogonal axises of the
 discrimination plane are defined by the similarity feature values
 (a.parallel.) and the dissimilarity feature values (a.perp.).
 Similarity and dissimilarity feature values are then calculated for the
 A/.vertline.N.vertline. vector, where the feature values designated as all
 and a.perp. are the components of the vector A/.vertline.N.vertline.
 parallel and perpendicular, respectively, to the N/.vertline.N.vertline.
 vector. The component a.parallel. represents the degree with which the
 arrhythmic vector A/.vertline.N.vertline. is similar to the baseline, or
 normal, vector N/.vertline.N.vertline.. This value is obtained by taking
 the projection (dot product) of the arrhythmic vector
 A/.vertline.N.vertline. onto the baseline, or normal, vector
 N/.vertline.N.vertline., which has the units of length. So, the similarity
 value, all, is determined by the equation [A.multidot.N]/[N.multidot.N].
 Thus, the feature value all is the similarity feature of the vector
 A/.vertline.N.vertline. with respect to the vector
 N/.vertline.N.vertline.. The component al represents the degree with which
 the arrhythmic vector A/.vertline.N.vertline. is dissimilar to the
 baseline, or normal, vector N/.vertline.N.vertline.. This value is
 obtained by taking the projection of the vector A/.vertline.N.vertline.
 onto the vector in the discrimination plane which has the unit of length,
 and which is perpendicular to the vector N/.vertline.N.vertline.. So, the
 dissimilarity value, a.perp., is determined by the equation
 SQRT[(A.multidot.A)/(N.multidot.N)-(a.parallel.).sup.2 ]. Thus, the value
 a.perp., is the dissimilarity feature of the vector
 A/.vertline.N.vertline. with respect to the vector
 N/.vertline.N.vertline..
 As previously stated the similarity/dissimilarity plane 500 is defined by
 the two-dimensional plane created by the vectors N/.vertline.N.vertline.
 and A/.vertline.N.vertline., where the orthogonal axises of the
 discrimination plane are defined by the similarity feature values
 (a.parallel.) and the dissimilarity feature values (a.perp.). In one
 embodiment, the similarity/dissimilarity plane is used to classify the
 arrhythmic episode as a ventricular tachycardia (VT) episodes or a non-VT
 episodes. FIG. 5, shows the similarity/dissimilarity plane 500 having
 orthogonal axes a.parallel. and a.perp., which are referred to as the
 similarity and dissimilarity coordinate axes.
 Next, the location in the discrimination plane of the feature values
 a.parallel. and a.perp. for the arrhythmic complex is examined to classify
 the complex as a VT complex or an SVT complex. Classification of the
 tachycardia complex is determined by the location of the point, termed the
 discrimination point, having coordinates equal to the similarity and
 dissimilarity feature values (a.parallel. and a.perp.) of the arrhythmic
 complex's vector. If the discrimination point (all, a.perp.) falls within
 a predetermined region surrounding the baseline point (1.0,0.0), then the
 tachycardia complex is classified as a SVT complex. Otherwise, if the
 discrimination point (a.parallel. a.perp.) falls outside of this region,
 the tachycardia complex is classified as a VT complex. The boundary
 separating the non-VT from the VT regions within the discrimination plane
 is predetermined by testing a population of patients. In one embodiment,
 the boundary separating the non-VT from the VT regions on the
 discrimination plane is a fixed boundary and does not change from patient
 to patient. In an alternative embodiment, the boundary separating the
 non-VT from the VT regions on the discrimination plane is a programmable
 boundary that is adapted to a patient's individual medical the therapeutic
 needs. In addition, the programmable boundary can be programmed with any
 number of shapes, including, but not limited to rectangular, circle
 segments, ellipse and ellipse segments, parabolic segments, triangular,
 parallelogram, or any shape defining an area (whether enclosed on not).
 FIG. 5 displays an example of a notice region 502 surrounding the baseline
 point (1.0, 0.0). In one embodiment, the notice region 502 is defined by
 the boundary defining the predetermined region. Tachycardia episodes which
 fall into the notice region 502 are morphologically similar to normal
 sinus rhythm, but have a cardiac rate that exceeds that of normal sinus
 rhythm. In one embodiment, tachycardia episodes that fall within notice
 region 502 are classified as supraventricular tachyarrhythmias. The area
 falling outside of the notice region 502 is considered to represent
 ventricular tachycardia activity, and tachycardia complexes falling in
 this area are considered to represent an ventricular tachycardia
 arrhythmic episode.
 Referring again to FIG. 2, as the sensed cardiac complexes are analyzed at
 290, the system records the number of VT complexes and candidate SVT
 complexes. In one embodiment, the tachycardia event is classified as a VT
 when the number of VT complexes exceeds the predetermined threshold. In
 one embodiment, the predetermined threshold is an x out of the last y
 complexes counter. When the number of VT complexes exceeds the
 predetermined threshold, a VT episode is declared. The system then
 delivers therapy to the patient's heart to treat the VT event at 292.
 When x out of the last y complexes are not classified as VT complexes, the
 system classifies the tachycardia event as a candidate SVT. In one
 embodiment, the values for x and y are programmable, where x has
 programmable integer values in the range of 3 to 10, where 5 is an
 acceptable value, and y has a programmable integer values in the range of
 8 to 30, where 10 is an acceptable value. In an alternative embodiment,
 the system determines a percentage of VT complexes and candidate SVT
 complexes during the tachycardia event. When the percentage of either the
 VT complexes or the candidate SVT complexes exceeds a predetermined
 percentage threshold, the system declares the occurrence of the
 tachycardia that exceeded the predetermined percentage threshold. In one
 embodiment, the predetermined percentage threshold is a programmable value
 in the range of 30 to 100 percent, where 50 percent is an acceptable
 value.
 If at 290 the system declares a candidate STV event, so that both the first
 discrimination stage and the second discrimination stage have declared
 candidate SVT events, the system follows path 294 and declares an SVT
 event at 296. In one embodiment, therapy is delivered to the
 supraventricular region of the heart to treat the SVT. In an alternative
 embodiment, therapy is not delivered to the supraventricular region of the
 heart, but rather the system continues to monitor the cardiac condition
 and provides treatment only when a ventricular tachycardia is determined.
 Referring now to FIG. 6, there is shown an additional embodiment of a
 method for classifying VT from SVT during a tachycardia event. Cardiac
 signals are sensed at 200 and analyzed at 210 as previously discussed.
 When a tachycardia event is detected at 210, the system proceeds to 600.
 At 600, the system analyzes the R-wave width of the sensed cardiac
 complexes as previously discussed. At 250, if a VT event is not declared,
 the system then proceeds to 610. At 610, the system determines a
 similarity value and a dissimilarity value for the cardiac signals sensed
 during the tachycardia event. Based on the system analysis of the
 similarity value and the dissimilarity value at 610 for the sensed cardiac
 signals, the system determines whether a ventricular tachycardia or a
 candidate supraventricular tachycardia is occurring. Based on the
 assessment at 290, the system either delivers ventricular tachycardia
 therapy at 292 or declares a supraventricular tachycardia at 296.
 In a further embodiment, additional discrimination stages are added to the
 system. In one embodiment, a third discrimination stage is added to the
 series of discrimination stages used in classifying a tachycardia event.
 The third discrimination stage allows for further assessment and
 discrimination of VT and candidate SVT events.
 The embodiments provided herein are intended to demonstrate only some of
 the embodiments of the present system. Other embodiments exist which are
 not described herein and which do not depart from the present system. For
 example, other stages may be added in varying orders without departing
 from the present system.