System and method for classifying cardiac complexes

A system and a method for classifying cardiac complexes based on cardiac information derived from two or more cardiac signals. Two or more cardiac signals containing cardiac complexes are monitored. A cardiac complex in the two or more cardiac signals is isolated in an analysis window. The morphology of the cardiac complex in each of the two or more cardiac signals is then compared to the morphology of a template cardiac complex representing a predetermined cardiac condition. Based on this comparison, the cardiac complex is classified as either belonging or not belonging to the predetermined cardiac condition.

FIELD OF THE INVENTION
 This invention relates generally to the field of medical devices and more
 particularly to a system and method for classifying sensed cardiac
 complexes.
 BACKGROUND
 Effective, efficient ventricular pumping action depends on proper cardiac
 function. Proper cardiac function, in turn, relies on the synchronized
 contractions of the heart at regular intervals. When normal cardiac rhythm
 is initiated at the sinoatrial node, the heart is said to be in sinus
 rhythm. However, when the heart experiences irregularities in its
 coordinated contraction, due to electrophysiological disturbances caused
 by a disease process or from an electrical disturbance, the heart is
 denoted to be arrhythmic. The resulting cardiac arrhythmia impairs cardiac
 efficiency and can be a potential life threatening event.
 Cardiac arrhythmias occurring in the atrial of the heart are called
 supraventricular tachyarrhythmias (SVTs). Cardiac arrhythmias occurring in
 the ventricular region of the heart are called ventricular
 tachyarrhythmias (VTs). SVTs and VTs are morphologically and
 physiologically distinct events. VTs take many forms, including
 ventricular fibrillation and ventricular tachycardia. Ventricular
 fibrillation is a condition denoted by extremely rapid, nonsynchronous
 contractions of the ventricles. This condition is fatal unless the heart
 is returned to sinus rhythm within a few minutes. Ventricular tachycardia
 are conditions denoted by a rapid heart beat, 150 to 250 beats per minute,
 that has its origin in some abnormal location within the ventricular
 myocardium. The abnormal location is typically results from damage to the
 ventricular myocardium from a myocardial infarction. Ventricular
 tachycardia can quickly degenerate into ventricular fibrillation.
 SVTs also take many forms, including atrial fibrillation and atrial
 flutter. Both conditions are characterized by rapid uncoordinated
 contractions of the atria. Besides being hemodynamically inefficient, the
 rapid contractions of the atria can also adversely effect the ventricular
 rate. This occurs when the aberrant contractile impulse in the atria are
 transmitted to the ventricles. It is then possible for the aberrant atrial
 signals to induce VTs, such as a ventricular tachycardia.
 Implantable cardioverter/defibrillators (ICDs) have been established as an
 effective treatment for patients with serious ventricular
 tachyarrhythmias. ICDs are able to recognize and treat tachyarrhythmias
 with a variety of tiered therapies. These tiered therapies range from
 providing antitachycardia pacing or cardioversion energy for treating
 tachycardia to defibrillation energy for treating ventricular
 fibrillation. To effectively deliver these treatments, the ICD must first
 identify the what type of tachyrhythmia is occurring in the heart.
 Attempts at identifying tachyarrhythmias have included comparing the
 morphologies of individual cardiac complexes to model or template cardiac
 complexes. Template cardiac complex morphologies are created from cardiac
 complexes sensed from a signal channel electrogram. Once created, the
 template cardiac complex morphologies are integrated into morphology
 algorithms programmed into the ICD. As the ICD encounters a tachycardia
 episode, cardiac complexes sensed on the single channel electrogram are
 compared to the template cardiac complex morphologies in the morphology
 algorithms. Comparing the morphologies of each cardiac complex to the
 template cardiac complex requires the signals of the two complexes to be
 positioned over each other. The morphologies of the signals are then
 analyzed in a time and energy intensive signal shape analysis to determine
 whether the cardiac complex should be classified as a template cardiac
 complex.
 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 providing a
 reliable system and method for analyzing and classifying cardiac complexes
 in a more time and energy efficient manner.
 SUMMARY OF THE INVENTION
 As explained in detail below, the present subject matter utilizes cardiac
 information from two or more cardiac signals to analyze and classify
 sensed cardiac complexes in a more time and energy efficient manner. The
 two or more vardiac signals are sensed from two or more locations within,
 or on, the heart of the patient. A cardiac complex sensed using two or
 more cardiac signals includes the relative time at which the cardiac
 complex is sensed in each of the two or more cardiac signals. This
 additional factor allows for a more efficient comparison to be made
 between a sensed cardiac complex and a template cardiac complex
 representing a predetermined cardiac condition. Based on the comparison,
 the cardiac complex is classified and this information is used to either
 determine what type of therapy to deliver to a patient or to assess the
 occurrence of a variety of predetermined cardiac conditions which may be
 helpful in providing treatment to the patient.
 In one embodiment, the present subject matter provides for the monitoring
 of two or more cardiac signals representative of a patient's cardiac
 activity. A cardiac complex is detected in each of the two or more cardiac
 signals, where the cardiac complex in each of the two or more cardiac
 signals represents at least a portion of the heart's cardiac cycle. The
 morphology of the cardiac complex in each of the two or more cardiac
 signals is then compared to the morphology of a template cardiac complex.
 Based on the morphology comparison, the cardiac complex is then classified
 as either being associated with the template cardiac complex or not being
 associated with the template cardiac complex.
 In one embodiment, a first signal and a second signal representative of
 electrical cardiac activity are monitored. As the first signal is being
 monitored it is analyzed to detect the onset of a tachycardia episode.
 When a tachycardia episode is detected, individual cardiac complexes in
 the first signal and the second signal are detected and windowed for
 analysis. Predetermined features are then located in the cardiac complex
 detected in the first signal and in a first normal sinus rhythm
 representative complex. In one embodiment, the predetermined features
 include repeatably identifiable complex sections common to the cardiac
 complex detected in the first signal and the first NSR representative
 complex.
 The predetermined features located in the first signal and in the first NSR
 representative complex are then aligned within the analysis window. The
 cardiac complex detected in the second signal is then compared to a second
 NSR representative complex to determine whether the cardia complex is an
 arrhythmic cardiac complex. In one embodiment, the morphology of the
 second NSR representative complex and the morphology of the cardiac
 complex detected in the second signal to determine whether the cardia
 complex is an arrhythmic complex.
 In an alternative embodiment, as a cardiac complexes are detected in the
 first signal and the second signal they are windowed for analysis.
 Predetermined features are then located in the cardiac complex detected in
 the first signal and in a first normal sinus rhythm representative
 complex. Scalar values are then generated as a function of the position,
 or location, of the predetermined features in each of the cardiac signals.
 The scalar values are then used to create a cardiac vector which
 represents the cardiac complex.
 The cardiac vector is compared to one or more classification vectors, where
 each of the one or more classification vectors represents a predetermined
 cardiac condition. A similarity coefficient is determined for the
 comparison between the cardiac vector and each of the one or more
 classification vectors. When a similarity coefficient for the comparison
 of the cardiac vector and a classification vector exceeds a predetermined
 threshold, the cardiac complex is classified as the predetermined cardiac
 condition represented by the classification vector.
 Additionally, the cardiac vector can be aligned with a classification
 vector prior to comparing the two vectors. In one embodiment, the process
 of aligning includes adjusting each scalar value in the cardiac vector so
 one of the element positions of the cardiac vector equals a scalar value
 in a corresponding element position in a classification vector.
 In an alternative embodiment, as a cardiac complexes are detected in the
 first signal and the second signal they are windowed for analysis. The
 morphology of the cardiac complex in the first cardiac signal and the
 second cardiac signal is then compared to a first template morphology and
 a second template morphology, respectively. The cardiac complex is then
 classified based on the comparison of the morphology of the cardiac
 complex in the first cardiac signal and the second cardiac signal to the
 first template morphology and the second template morphology.
 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 may 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 of the present subject matter illustrated herein are
 described as being included in an implantable cardiac defibrillator, which
 may include numerous pacing modes known in the art and an external medical
 device programmer. In one embodiment, the implantable cardiac
 defibrillator is a single chamber defibrillator. In an alternative
 embodiment, the implantable cardiac defibrillator is a dual chamber
 defibrillator. Examples of both single and dual chamber implantable
 cardiac defibrillators are known in the art. However, the present medical
 system can also be implemented in an external cardioverter/monitor system
 as are known in the art. Also, the present medical system can also be
 implemented in an implantable atrial cardioverter-defibrillator, which may
 include numerous pacing modes known in the art. Furthermore, although the
 present system is described in conjunction with an implantable cardiac
 defibrillator having a microprocessor based architecture, it will be
 understood that the implantable cardiac defibrillator (or other implanted
 device) may be implemented in any logic based custom integrated circuit
 architecture, if desired.
 With respect to the present subject matter, accurate classification of
 sensed cardiac complexes is important to an overall diagnosis and
 treatment of a patient's cardiac condition. In an effort to classify and
 catagorize sensed cardiac complexes, the morphology of individual cardiac
 complexes is compared to template cardiac complexes. The template cardiac
 complexes are derived from cardiac complexes sensed during arrhythmic or
 non-arrhythmic cardiac episodes.
 The cardiac signals used in creating the template cardiac complex and
 classifying sensed cardiac complexes have typically been detected using a
 signal channel electrogram. Single channel electrograms, however, limit
 the amount of cardiac information available to use in classifying cardiac
 complexes. In contrast, one unique aspect of the present subject matter is
 that cardiac information from two or more cardiac channels is used to
 classify sensed cardiac complexes. Using two or more cardiac channels
 provides cardiac information from two or more locations within, or on, the
 heart of the patient. By utilizing multiple cardiac channels, cardiac
 information (e.g., sensed cardiac complexes) from the two or more cardiac
 regions and/or locations provide a larger overall and more complete "view"
 of the sensed cardiac complexes.
 The present subject matter uses two or more electrogram channels in a
 process of analyzing and classifying cardiac complexes. Each electrogram
 channel is used to sense a cardiac signal, where the cardiac signal
 includes cardiac complexes representative of at least a portion of a
 cardiac cycle. By way of example, and not by way of limitation, portions
 of the cardiac cycle sensed can include, but are not limited to, P-waves,
 QRS-cardiac complexes, and R-waves. Other portions of the cardiac cycle,
 including sensed signals of the entire cardiac complex are considered
 useful and with the scope of the present subject matter.
 In one embodiment, the present subject matter provides for two or more
 electrogram channels to simultaneously record cardiac complexes as they
 occur in the heart. As the cardiac complexes occur, each electrogram
 channel detects cardiac complexes representing portions of the cardiac
 cycle. The two or more electrogram channels are being sensed from sensors
 in or on different cardiac locations. While sensors may detect the
 electrical activity during the same portion of the cardiac cycle, having
 the sensors in different cardiac locations allows for cardiac information
 about the same cardiac complex to be gathered from a different "view" of
 the cardiac cycle. Each cardiac complex sensed with the two or more
 electrogram channels is placed in an analysis window, where the cardiac
 complex is isolated for analysis. In one embodiment, each cardiac complex
 is windowed by isolating and plotting the portions of the simultaneously
 sensed cardiac complex in the two or more electrogram channels as they
 occurred in time.
 FIG. 1 shows one embodiment of a windowed cardiac complex 100. In the
 present example, the windowed cardiac complex 100 has a first cardiac
 channel 104, a second cardiac channel 108 and a third cardiac channel 112.
 Cardiac complexes are sensed over the first cardiac channel 104, the
 second cardiac channel 108 and the third cardiac channel 112, where each
 cardiac channel may sense a different portion of the cardiac complex
 (e.g., R-wave, QRS-cardiac complex, P-wave, etc.). In the present example,
 the windowed cardiac complex 100 has three views of a cardiac complex as
 sensed over the three cardiac channels. A first cardiac complex 116 is
 shown in cardiac signal sensed in the first cardiac channel 104. A second
 cardiac complex 120 is shown in cardiac signal sensed in the second
 cardiac channel 108. Finally, a third cardiac complex 124 is shown in
 cardiac signal sensed in the third cardiac channel 112. The first, second
 and third cardiac complex 116, 120 and 124 are representative of at least
 a portion of a single cardiac cycle. In one embodiment, the first, second
 and third cardiac complex 116, 120 and 124 are snapshots of the single
 cardiac cycle taken either at different locations within or on the heart
 and/or taken using different electrode configurations (e.g., far-field,
 near-field).
 Cardiac information is derived from the windowed cardiac complexes. In one
 embodiment, information derived from the windowed cardiac complexes
 includes values derived or taken from one or more predetermined features
 on the cardiac complex sensed in each of the two or more cardiac channels.
 By way of example, and not by limitation, predetermined features of the
 cardiac complexes that are useful in deriving information include a
 maximum deflection of the cardiac complex, a beginning of a cardiac
 complex as indicated by a predetermined deviation of the cardiac signal
 from a baseline signal, an ending of a cardiac complex as indicated by a
 return of the first cardiac signal to a baseline signal and a fiducial
 point (the point of greatest slope along the cardiac complex signal).
 Other features of the cardiac complex signal are also useful for deriving
 information. In one embodiment, the selection criteria for these
 additional features of a cardiac complex is that the feature be a
 repeatably identifiable portion of the cardiac complex. In one embodiment,
 the features are selectively programmed into the medical device system.
 FIG. 1 shows several examples of predetermined features of the sensed
 cardiac complex that are useful in deriving information. In one
 embodiment, a beginning of the first, second and third cardiac complex
 (116, 120 and 124) sensed in the first, second and third cardiac channel
 (104, 108 and 112), respectively, is generally shown at 128. A maximum
 deflection point of the first, second and third cardiac complex (116, 120
 and 124) is shown generally at 132. Finally, an ending point of the first,
 second and third cardiac complex (116, 120 and 124) is shown generally at
 136.
 In one embodiment, the cardiac complexes sensed in the two or more cardiac
 channels are windowed with each of the signals of the cardiac complexes
 represented as a voltage at a function of time. FIG. 1 shows an example of
 the first, second and third cardiac complex (116, 120 and 124) sensed in
 the first, second and third cardiac channel (104, 108 and 112) being
 plotted as voltage as a function of time. Once the cardiac complex sensed
 in the two or more cardiac channels is represented in this fashion,
 information can be derived from the specific features of the cardiac
 complexes.
 In one embodiment, the information derived from the cardiac complexes in
 each of the cardiac channels is the time the repeatably identifiable
 feature of the cardiac complex occurred. Alternatively, the information
 derived is a time difference between pairs of repeatably identifiable
 features on a cardiac complex sensed in one of the two or more electrogram
 channels. In one embodiment, a first time difference 140 is between the
 beginning 128 of the first cardiac complex 116 and the end 136 of the
 first cardiac complex. In an additional embodiment, the information
 derived is a time difference between pairs of repeatably identifiable
 features on two different cardiac complexes sensed in two electrogram
 channels. In a further embodiment, the information derived is a
 combination of the time difference between pairs of repeatably
 identifiable features on two different cardiac complexes sensed in two
 electrogram channels and the time differences from pairs of repeatably
 identifiable features on a cardiac complex sensed in one of the two or
 more electrogram channels.
 Information from multiple cardiac channels can then be used to represent
 the cardiac complex. In one embodiment, the cardiac complex sensed in
 multiple cardiac channels can be numerically represented. One way of
 numerically representing the cardiac complex is to use scalar values
 derived from the cardiac signals of the cardiac complex. In one
 embodiment, the scalar values derived from the cardiac signals of the
 cardiac complex are the times repeatably identifiable features of the
 cardiac complex occur. Alternatively, time differences between the
 repeatably identifiable features, as previously described, are used to
 derive the scalar values. In an additional embodiment, the scalar values
 can be derived from a magnitude of the position of the predetermined
 feature for each of the two or more cardiac signals. In one embodiment,
 the magnitude of the position of the predetermined feature is the voltage
 measurement of the maximum deflection position along the cardiac signal.
 The values derived for a cardiac complex are then used to create an "N"
 dimensional cardiac complex vector. In one embodiment, the cardiac complex
 vector has the form: A=[A1, A2, A3, . . . An], where each of the values
 A1-An represent scalar values derived from the repeatably identifiable
 features of the cardiac complex. The cardiac complex vector represents the
 cardiac complex, and is used to identify and classify the cardiac complex.
 Once the cardiac complex vector has been derived from cardiac complex
 information taken from the two or more cardiac channels, the cardiac
 complex vector is compared to one or more classification vectors. In one
 embodiment, a classification vector represent a typical cardiac complex of
 a specific cardiac arrhythmia or cardiac state. For example, a
 classification vector can represent a ventricular tachycardia (VT)
 complex, where there can be one or more classification vectors to
 represent one or more different manifestations of ventricular tachycardia
 complexes. Additionally, classification vectors can represent
 supraventricular tachycardia (SVT) complexes, premature ventricular
 contraction (PVC) complexes, complexes indicating ischemic events, and any
 predefined representation of a cardiac complex.
 In one embodiment, classification vectors are predefined classification
 vectors derived from cardiac complexes sensed over two or more cardiac
 channels for a population of patients. In one embodiment, the population
 of patients chosen to determine a classification vector display or
 manifest the cardiac condition of interest (e.g., VT, SVT, PVC). Cardiac
 complexes of the cardiac conditions of interest are recorded over two or
 more cardiac channels from the patients. Using repeatably identifiable
 features of the multiple cardiac signals representing each cardiac
 complex, classification vectors for a cardiac condition are developed. In
 one embodiment, cardiac complexes are sensed and recorded during Holter
 monitoring.
 In one embodiment, the classification vectors has the form: C=[C1, C2, C3,
 . . . Cn], where each of the values C1-Cn represent scalar values derived
 from the repeatably identifiable features of the cardiac complexes. In one
 embodiment, each of the values C1-Cn are average values derived from
 repeatably identifiable features of the cardiac complexes for the patient
 population. In an alternative embodiment, each of the values C1-Cn are
 median values derived from repeatably identifiable features of the cardiac
 complexes for the patient population. One or more of the predetermined
 classification vectors can then be used in the implantable medical device
 to classify sensed cardiac complexes.
 In an alternative embodiment, the classification vectors are derived from
 cardiac complexes sensed from the patient. In one embodiment, two or more
 cardiac signals are recorded from the patient for a predetermined length
 of time. In one embodiment, sensing and recording the cardiac complexes is
 accomplished through Holter monitoring. From the recorded cardiac
 complexes, classification vectors are developed as previously discussed.
 In an additional embodiment, classification vectors represent predefined
 deviations between two or more repeatably identifiable features of a
 cardiac complex in the two or more cardiac signals. In this embodiment,
 the classification vector is not representing a particular cardiac
 condition (e.g., VT, SVT, PVC complex) but rather is representative of a
 very specific cardiac condition or occurrence that the physician want to
 pay particular attention to. By developing and programming this type of
 classification vector, the physician can determine how many cardiac
 complexes of this particular type are occurring.
 In an alternative embodiment, classification vectors are created after the
 medical device system has been implanted into the patient. One way to
 create patient specific classification vectors is to derive scalar values
 from predetermined features of the cardiac complexes sensed in the two or
 more cardiac channels. As the scalar values are determined from the first
 sensed cardiac complex, the medical device system determines the cardiac
 complex vector. The cardiac complex vector for the first cardiac complex
 is then classified as a first class of sensed cardiac complexes. As the
 second cardiac complex is sensed, the medical device system determines the
 cardiac complex vector A for the second cardiac complex. The cardiac
 complex vector A for the second cardiac complex is then compared to the
 cardiac complex vector A for the first cardiac complex to determine
 whether the second cardiac complex should be classified in the first class
 of sensed cardiac complexes or classified in a second class of sensed
 cardiac complexes.
 In one embodiment, determining whether a cardiac complex should be grouped
 with one or more classified cardiac complexes is accomplished through a
 process of comparing the complex vector A of the cardiac complex to be
 classified with the complex vector A of the cardiac complex that founded
 the class of cardiac complexes. FIG. 2 shows an embodiment of cardiac
 complex classes. FIG. 2 shows cardiac complexes being plotted in a
 Cartesian coordinate system with three-dimensions 200. Cardiac vectors
 plotted in 200 have three values A=[A1, A2, A3]. As the first cardiac
 complex of a patient is sensed, the cardiac vector is determined. As
 previously discussed, the first cardiac complex becomes the first class of
 sensed cardiac complexes. The cardiac vector of the first cardiac complex
 is shown at 204.
 The cardiac vector of the first cardiac complex not only establishes the
 first class of sensed cardiac complexes, but it is also used to determined
 whether subsequently sensed cardiac complexes should be included in the
 first class of sensed cardiac complexes or excluded to join another class
 of sensed cardiac complex or form a new class of sensed cardiac complexes.
 In one embodiment, to determine whether a second cardiac complex is
 classified with the first class of sensed cardiac complexes the cardiac
 vector of the second cardiac complex is compared to the cardiac vector of
 the first cardiac complex.
 In one embodiment, a mean square error calculation is used to determine if
 the scalar values of the cardiac vectors are sufficiently close, or within
 a region, or "neighborhood", to classify the second cardiac complex in the
 first class of cardiac complexes. In one embodiment, the mean square error
 is represented as .SIGMA. (Ai.sub.(n class) -Ai).sup.2.ltoreq.T, where
 Ai.sub.(n class) represents the ith scalar value (A1-An) of the n class
 (first class, second class, third class, . . . , nth class) of sensed
 cardiac complexes and T is a predetermined threshold value. In one
 embodiment, the predetermined threshold value T defines the region around
 the cardiac complex representing each of the classes of sensed cardiac
 complexes.
 FIG. 2 shows one example of a first region 208 surrounding the cardiac
 vector of the first cardiac complex 204. As previously discussed, after
 the second cardiac complex is sensed, the cardiac vector representing the
 second cardiac complex is compared to the cardiac vector representing the
 first class of cardiac complexes, which in this case is the first cardiac
 complex 204. In one embodiment, if the cardiac vector representing the
 second cardiac complex falls on or within the first region 208, the second
 cardiac complex is classified as part of the first class of cardiac
 complexes.
 In an alternative embodiment, if the cardiac vector representing the second
 cardiac complex falls outside the first region 208, the second cardiac
 complex is used to create a second class of cardiac complexes. In one
 embodiment, the cardiac vector of the second cardiac complex is shown at
 212. A second region 216 surrounds the second cardiac complex 212. As
 previously discussed, the size of the second region 216 surrounding the
 second cardiac complex is dependent on the predetermined threshold value
 T.
 As a subsequent cardiac complex is sensed a cardiac vector representing the
 cardiac complex is determined. The cardiac vector is then compared to each
 established class of cardiac complex. If the cardiac vector is found to
 fall within the region around a cardiac vector representing a class of
 cardiac complexes, the cardiac vector is classified with that class of
 cardiac complexes. So, if the cardiac complex is found to fall into the
 region representing a third class of cardiac complexes, the cardiac
 complex would be classified and counted with the third class of cardiac
 complexes. If, however, the cardiac complex does not fall into a region
 surrounding one or more classes of cardiac complexes, the cardiac vector
 of the cardiac complex is used to create a new class (an "nth" cardiac
 class) of cardiac complexes.
 The cardiac vectors that establish a class of cardiac complexes are
 electronically stored in the implantable medical device. In addition to
 electronically storing the cardiac vectors, the medical device system also
 stores at least the electrocardiogram signal of the cardiac complex that
 established the class of cardiac complexes. In an additional embodiment,
 values for the cardiac vectors classified in each of the classes of
 cardiac complexes are also electronically stored in the medical device
 system. In one embodiment, it is possible that a cardiac complex is
 classified into two or more classes of cardiac complexes. In one
 embodiment, the cardiac complex falling into two or more classes of
 cardiac complexes will be reported in each class of cardiac complexes. In
 an alternative embodiment, cardiac complexes falling into two or more
 classes will classified and reported in the class that they were
 originally classified.
 The patient cardiac complexes are sensed and classified by the implantable
 medical device for a predetermined period of time. In one embodiment, the
 predetermined period of time over which cardiac complexes are sensed and
 used to create the classes of cardiac complexes is programmed by the
 patient's attending physician. In one embodiment, cardiac complexes are
 sensed over a continuous period of time. Alternatively, the implantable
 medical device is programmed to classify sense cardiac complexes during
 predetermined time segments over the course of 24 hours. Additionally, the
 implantable medical device is programmed to classify sense cardiac
 complexes only after the cardiac rate (ventricular and/or atrial rate) has
 reached or exceeded a predetermined threshold (e.g., known cardiac rate
 threshold indicative of ventricular tachycardia, atrial tachycardia,
 etc.).
 Upon a follow-up visit to the patient's physician, the cardiac information
 contained in the implantable medical device is down-loaded to a medical
 device programmer unit. In one embodiment, information relating to cardiac
 complexes used to establish classes of cardiac complexes is down-loaded
 and displayed on a display screen of the medical device programmer.
 Additionally, the number of cardiac complexes classified into each of the
 classes of cardiac complexes can be displayed. Electrocardiogram signals
 representative of each class of cardiac complex can also be displayed. The
 physician then uses the classes of cardiac complexes to define or
 designate classification vectors which are subsequently used to identify
 cardiac complexes and classify a cardiac arrhythmia that the patient may
 experience.
 FIG. 3 shows one embodiment of a method of the present subject matter. At
 300, two or more cardiac signals representative of electrical cardiac
 activity are monitored. The cardiac signals are monitored to detect
 cardiac complexes occurring in the two or more cardiac signals. In one
 embodiment, an implantable medical device, such as an implantable
 cardioverter defibrillator, is used to detect cardiac complexes in the two
 or more cardiac signals. In one embodiment, the implantable medical device
 has two or more cardiac electrodes which allow for near-field and/or
 far-field cardiac signals to be sensed from a heart.
 At 310, the cardiac complex present in the two or more cardiac signals is
 isolated in an analysis window (or "windowed" for analysis). As previously
 discussed, windowing isolates a section, or portion, of the two or more
 cardiac signals which each contain a view of the cardiac complex that is
 being classified. After the cardiac signals containing the cardiac complex
 have been isolated, at least one predetermined feature is located in the
 cardiac complex present in each of the two or more cardiac signals.
 Once the cardiac signals containing the cardiac complex have been windowed,
 scalar values from predetermined features of the cardiac complex are
 generated. In one embodiment, the scalar values are generated as a
 function of the position of the predetermined feature, or features, on
 each of the two or more cardiac signals. The scalar values are then used
 to create a cardiac vector, where the cardiac vector represents the
 cardiac complex.
 FIG. 4 shows one embodiment of cardiac complexes of a cardiac cycle
 windowed for analysis and classification. A first cardiac complex 400 is
 present in the first cardiac signal 404 and a second cardiac complex 408
 is present in a second cardiac signal 412. As previously discussed, the
 two or more cardiac signals are sensed simultaneously from the heart, and
 so the first cardiac complex 400 and the second cardiac complex 408 are
 representative of portions of a complete cardiac cycle occurring in the
 heart.
 Predetermined features of the cardiac complex in each cardiac signal are
 used to derive a cardiac vector. In the present example, the cardiac
 vector is programmed to have four values, such that A=[A1, A2, A3, A4].
 Each of the values A1-A4 represent scalar values derived from the
 repeatably identifiable features of the cardiac complex. In one
 embodiment, A1 is programmed to be the beginning time 416 of the first
 cardiac complex 400, A2 is programmed to be the time of the maximum
 deflection point 420 of the first cardiac complex 400, A3 is programmed to
 be the time difference 424 between the time of the maximum deflection
 point 420 and the time of the maximum deflection point 428 of the second
 cardiac complex 408, and A4 is programmed to be the beginning time 432 of
 the second cardiac complex 408.
 Referring again to FIG. 3, after a cardiac vector has been determined for
 the cardiac complex, the cardiac vector is compared to one or more
 classification vectors at 330. In one embodiment, the one or more
 classification vectors represents a predetermined cardiac condition. For
 example, the predetermined cardiac conditions can include, but are not
 limited to, ventricular and supraventricular tachycardia, PVCs, ischemia,
 or other predetermined cardiac conditions designated by a physician. In
 addition, the cardiac vector has the same vector dimension as the
 classification vectors. In one embodiment, the scalar values making up the
 cardiac vector are determined from the same relative predetermined
 features of the cardiac signals sensed in the same general cardiac
 location using the same general cardiac electrode configuration.
 In one embodiment, the cardiac vector is compared to the classification
 vectors to determine whether the cardiac vector similar enough to one or
 more classification vectors to classify the cardiac complex. In one
 embodiment, a similarity coefficient is determined from each comparison of
 the cardiac vector and the one or more classification vectors. Based on
 this similarity coefficient, the cardiac complex is classified as the
 predetermined cardiac condition when the cardiac complex similarity
 coefficient exceed a predetermined threshold. In one embodiment, a mean
 square error is used to determine the similarity coefficient. In one
 embodiment, the mean square error value is represented as .SIGMA.
 (Ai-Ci).sup.2.ltoreq.X, where Ai represents the ith scalar value (A1-An)
 of the cardiac vector, Ci represents the ith scalar value (C1-Cn) of the
 classification vector and X is a predetermined threshold value. In one
 embodiment, the predetermined threshold value X defines the region or a
 neighborhood around the classification vector which is deemed to be
 sufficiently close to permit classifying the cardiac complex represented
 by the cardiac vector as a general class of cardiac complexes represented
 by the classification vector. Based on the comparison, the cardiac complex
 is classified at 340 as being a member of the group represented by one or
 more classification vectors if possible.
 In an additional embodiment, prior to comparing the cardiac vector to the
 classification vectors, the cardiac vector is aligned, or coordinated
 with, each of the classification vectors. In one embodiment, the cardiac
 vector and the classification vectors include element positions (A1 and C1
 are in the first element position, A2 and C2 are in the second element
 position, etc.) which are occupied by the scalar values. The cardiac
 vector and a classification vector are aligned around the scalar values in
 the same element position (i.e., a coordinating element position) in both
 the cardiac vector and the classification vector.
 In one embodiment, the process of aligning the cardiac vector and the
 classification vector involves adjusting each scalar value of the cardiac
 vector so one of the element positions of the cardiac vector equals a
 scalar value in a corresponding element position in a classification
 vector. So, when the scalar values in the vectors are the time of
 occurrence of features in different channels, the vectors are aligned by
 add or subtract an appropriate numerical value to all the elements of the
 cardiac vector (e.g., the scalar values in the cardiac vector) so that the
 scalar value in the coordinating element position of the cardiac vector
 has the same numerical value as the coordinating element position of the
 classification vector.
 In one embodiment of aligning a cardiac vector and a classification vector,
 a cardiac complex is sensed two or more cardiac channels, from which a
 cardiac vector A=[A1, A2, A3, A4] is determined. In this embodiment, the
 elements of the vector are times at which the repeatably identifiable
 features of the cardiac complex occurred in the two or more cardiac
 channels. One or more classification vectors are provided having the
 general form C=[C1, C2, C3, C4]. In one embodiment, the cardiac vector is
 aligned, or coordinated, around the same element position (the
 coordinating element) with each classification vector the cardiac vector
 is compared with. In one embodiment, the coordinating element is the first
 position in the cardiac vector and the classifying vector, A1 and C1. A
 numerical value Y is added to, or subtracted from, the value of A1 so that
 A1 equals C1. So, A1+Y=C1, where Y can have either a positive or a
 negative numerical value. In addition to modifying the value of A1 with Y,
 Y is also added to the remaining elements of the cardiac vector. So, a
 cardiac vector aligned with a classification vector has the form A.sub.a
 =[A1+Y, A2+Y, A3+Y, A4+Y], where A1+Y=C1 of the classification vector
 C=[C1, C2, C3, C4]. After the cardiac vector has been aligned with the
 classification vector, the elements of the vectors, excluding the
 coordinating elements, are compared. This means that for the present
 example, the classification vector A.sub.a =[A2+Y, A3+Y, A4+Y] would be
 compared to the classification vector C=[C2, C3, C4]. Finally, the cardiac
 vector can be aligned with a classification vector around any element
 (e.g., A1/C1, A2/C2, etc.) in the pair of vectors.
 As previously discussed, the present subject matter uses two or more
 electrogram channels in a process of analyzing and classifying cardiac
 complexes. In one embodiment, a first cardiac signal and a second cardiac
 signal are sensed over a first electrogram channel and a second
 electrogram channel, respectively. During a tachycardia episode, a cardiac
 complex is detected in the first cardiac signal and the second cardiac
 signal. The morphology of the first cardiac signal and the second cardiac
 signal representing the cardiac complex can then be compared to template
 cardiac complexes. Based on this comparison, the cardiac complex can be
 classified as being representative of one or more predetermined cardiac
 rhythm states. Predetermined cardiac rhythm states can include, but are
 not limited to, arrhythmic or non-arrhythmic, normal sinus rhythm (NSR),
 ventricular tachycardia and/or supraventricular tachycardia cardiac rhythm
 states.
 In an additional embodiment, the process of comparing the morphology of the
 cardiac complex to a template cardiac complex includes the act of
 aligning, or coordinating, the cardiac complex sensed in a first cardiac
 signal of the two or more cardiac signals with a template cardiac complex
 having been derived from cardiac complexes sensed in the first cardiac
 signal. Alternatively, the morphology of the cardiac complex sensed in the
 two or more cardiac channels is compared to the morphology of the template
 cardiac complex which was derived from cardiac complexes sensed in the two
 or more cardiac channels.
 In one embodiment, a first cardiac signal and a second cardiac signal are
 sensed over a first electrogram channel and a second electrogram channel,
 respectively. During a tachycardia episode, a cardiac complex is detected
 in the first cardiac signal and the second cardiac signal. In one
 embodiment, the cardiac complex detected in the first cardiac signal is
 aligned, or coordinated, with a first normal sinus rhythm (NSR)
 representative complex. In one embodiment, the first NSR representative
 complex is the template cardiac complex which, in the present embodiment,
 was derived from normal sinus rhythm cardiac complexes detected in the
 first cardiac signal.
 Once the cardiac complex from the first cardiac signal has been aligned
 with the first NSR representative complex, the cardiac complex from the
 second cardiac signal is morphologically compared to the second NSR
 representative complex. In the present embodiment, this comparison is then
 used to classify the cardiac complex as either a complex representative of
 an arrhythmic episode or of a non-arrhythmic episode.
 Through the process of first aligning the cardiac complex sensed in the
 first cardiac signal with the first NSR representative complex, the
 comparison of the cardiac complex as sensed in the second cardiac signal
 to the second NSR representative complex not only uses the relative
 morphologies of the cardiac complexes in classifying the cardiac complex,
 but also takes into consideration the relative time the cardiac complex
 detected in the first cardiac signal occurred with respect to the cardiac
 complex detected in the second cardiac signal. The relative time between
 the sensed cardiac complex in the first cardiac signal and the second
 cardiac signal helps to further accentuates morphological differences in
 the cardiac complex and template cardiac complex.
 Referring now to FIG. 5 of the drawings, there is shown one embodiment of a
 medical device system suitable for implementing the present subject
 matter. By way of example only, and not by limitation, the medical device
 system includes an implantable cardiac defibrillator 500 electrically and
 physically coupled to at least one catheter 502. In one embodiment, the
 catheter 502 is an intracardiac catheter which includes at least a first
 cardiac electrode and a second cardiac electrode.
 The catheter 502 is implanted in a human body 504 with portions of the
 catheter 502 inserted into a heart 506 to detect and analyze electric
 cardiac signals produced by the heart 506 and to provide electrical energy
 to the heart 506 under certain predetermined conditions to treat cardia
 arrhythmias, including ventricular fibrillation, of the heart 506.
 In one embodiment, the catheter 502 is an endocardial lead adapted to be
 releasably attached to the cardiac defibrillator 500. The catheter 502 has
 an elongate body with a proximal end 508 and a distal end 510, and
 includes at least a first cardiac electrode and a second cardiac
 electrode. In one embodiment, the catheter 502 has a pacing electrode 512
 located at, or adjacent, the distal end 510 of the catheter 502.
 Additional pacing electrodes can also be included on the catheter 502 to
 allow for bipolar sensing and pacing with the pacing electrode 512. In
 addition, other pacing and sensing electrode configurations are also
 possible. The catheter 502 also includes one or more defibrillation
 electrodes. In one embodiment, the catheter 502 has a first defibrillation
 electrode 514 and a second defibrillation electrode 516, where the
 defibrillation electrodes have a coil construction as is known.
 In addition to the catheter configuration shown in FIG. 5, it is considered
 within the scope of the present subject matter that additional catheters
 positioned in or around the ventricular and/or the atrial chambers can be
 added to the cardiac defibrillator 500 to allow for additional cardiac
 signals to be sensed (e.g., two or more cardiac signals). Thus, it would
 be possible to sense cardiac signals from any combination or
 subcombination of ventricular, right ventricular, left ventricular,
 atrial, right atria and/or left atria locations. These signals can
 include, but are not limited to, far-field and/or near-field cardiac
 signals.
 Referring now to FIG. 6, there is shown an embodiment of a block diagram of
 a cardiac defibrillator 500. The cardiac defibrillator 500 includes
 control system circuitry 600 for receiving cardiac signals from a heart
 506 and delivering electrical energy to the heart 506. The control system
 circuitry 600 includes a sensing system 602 attached to at least one
 catheter. The sensing system 602 includes terminals labeled with reference
 numbers 604, 608, and 612 for connection to electrodes attached to the
 surface of the catheter 502. In one embodiment, the pacing electrode 512
 is electrically connected to terminal 604 and to the control system
 circuitry 600 through an electrically insulated conductor provided within
 the elongate body of the catheter 502. The first defibrillation electrode
 514 and the second defibrillation electrode 516 are connected to terminals
 608 and 612, respectively, and to the control system circuitry 600 through
 electrically insulated conductors provided within the elongate body of the
 catheter 502.
 In one embodiment, the control system circuitry 600 of the cardiac
 defibrillator 500 is encased and hermetically sealed in a housing 530
 suitable for implanting in a human body as are known in the art. A
 connector block 532 (FIG. 5) is additionally attached to the housing 530
 of the cardiac defibrillator 500 to allow for the physical and the
 electrical attachment of the catheter 502 and the electrodes to the
 cardiac defibrillator 500 and the encased control system circuitry 600.
 In one embodiment, the control system circuitry 600 of the cardiac
 defibrillator 500 is a programmable microprocessor-based system, with a
 microprocessor 616 and a memory circuit 620 which contains parameters for
 various pacing, defibrillation, and sensing modes and stores data
 indicative of cardiac signals received by the control system circuitry
 600. A transmitter circuit 624 is additionally coupled to the control
 system circuitry 600 and the memory circuit 620 to allow the cardiac
 defibrillator 500 to communicate with and receive programming instructions
 and transmit data to and from a programmer unit 228 as is known in the
 art. In one embodiment, the transmitter circuit 624 and the programmer
 unit 628 use a wire loop antenna 632 and a radio frequency telemetric
 link, as is known in the art, to receive and transmit signals and data to
 and from the programmer unit 628 and the control system circuitry 600. In
 this manner, programming commands or instructions are transferred to the
 microprocessor 616 of the cardiac defibrillator 500 after implant, and
 stored cardiac data pertaining to sensed arrhythmic episodes within the
 heart 506 and subsequent therapy, or therapies, applied to correct the
 sensed arrhythmic event are transferred to the programmer unit 628 from
 the cardiac defibrillator 500.
 In the present embodiment, the medical device system senses at least a
 first signal and a second signal, both of which are representative of
 electrical cardiac activity. The first signal and the second signal each
 contain cardiac complexes which represent at least a portion of the
 cardiac cycle. Based on the embodiment shown in FIG. 5, the first signal
 is a rate signal sensed within the right ventricle chamber of the heart
 and the second signal is a morphology signal sensed across at least a
 portion of the ventricular region of the heart. In an alternative
 embodiment, the first and second signals can be rate and morphology
 signals sensed in a supraventricular region of the heart. Alternatively,
 cardiac electrodes and/or catheters can be provided to allow for the first
 signal to be a morphology signal and the second signal to be a rate
 signal.
 In the embodiment shown in FIG. 5, the pacing electrode 512 is included in
 a near-field channel which senses near-field signals or rate-signals of
 the heart 506. The near-field sensing channel includes the pacing
 electrode 512 which is coupled to a sense amplifier 630 within the sensing
 system 602. In one embodiment, the housing 530 of the cardiac
 defibrillator 500 is coupled to the sense amplified 630 at 634 to allow
 for unipolar cardiac rate sensing between the pacing electrode 512 and the
 housing 530 of the cardiac defibrillator 500. In an alternative
 embodiment, the cardiac rate signal is sensed using pacing electrode 512
 and the first defibrillation electrode 514.
 The output of the sense amplifier 630 is shown connected to an R-wave
 detector 636. The R-wave detector 636 serves to sense and amplify cardiac
 signals, including cardiac complexes, sensed from the heart, and apply
 signals indicative thereof to a signal feature comparison circuit 638. The
 signal feature comparison circuit 638 is coupled to the microprocessor
 616. Among other things, microprocessor 616 responds to signals from the
 R-wave detector 636 by providing pacing signals to a pace output circuit
 640, as needed according to the programmed pacing mode. In one embodiment,
 the pace output circuit 640 provides output pacing signals to terminals
 604 and 634, which connect to the pacing electrode 512 and the housing 530
 of the cardiac defibrillator 500, for cardiac pacing.
 Referring again to the embodiment in FIG. 5, far-field or morphology
 signals are sensed through the first defibrillation electrode 514 and the
 second defibrillation electrode 516. The first defibrillation electrode
 514 and the second defibrillation electrode 516 are used in a far-field
 sensing channel, where the first defibrillation electrode 514 and the
 second defibrillation electrode 516 are coupled to a sense amplifier 644
 to sense far-field signals, which include cardiac complexes, from the
 heart. In an alternative embodiment, far-field signals are sensed between
 the first defibrillation electrode 514, the second defibrillation
 electrode 516 and the housing 530. The output of the sense amplifier 644
 is coupled to a morphology analyzer circuit 648.
 The morphology analyzer circuit 648 receives and processes the cardiac
 complexes detected within the cardiac signals. In one embodiment, the
 morphology analyzer circuit 648 receives cardiac signals, including
 cardiac complexes representative of the cardiac cycle from the sensing
 system. Cardiac complexes analyzed by the morphology analyzer circuit 648
 can include detected P-waves, QRS-complexes, and R-waves. In one
 embodiment, the morphology analyzer circuit 648 includes an analog filter
 for filtering cardiac signal noise sensed by the electrodes. The cardiac
 signals are then bandlimited before arriving at an analog-to-digital
 filter which converts the analog signals into digital signals suitable for
 processing. In an alternative embodiment, the cardiac signals are filtered
 through an analog peak detector to extract the maximum and minimum cardiac
 signal values for each sensed cardiac complex.
 In processing sensed cardiac complexes, the morphology analyzer circuit 648
 windows a cardiac complex sensed in two or more cardiac signals. In one
 embodiment, the morphology analyzer circuit 648 locates and extracts
 information from one or more predetermined features of sensed cardiac
 complexes. As previously discussed, the type of information extracted by
 the morphology analyzer circuit 648 can include the time of occurrence of
 a predetermined feature and the amplitude value of a predetermined
 feature. In one embodiment, the predetermined features include repeatably
 identifiable portions of cardiac complexes which are repeatably
 identifiable in subsequent cardiac complexes. For example, features
 include a maximum deflection of the cardiac complex, a beginning of a
 cardiac complex as indicated by a predetermined deviation of the cardiac
 signal from a baseline signal, and an ending of a cardiac complex as
 indicated by a return of the first cardiac signal to a baseline signal. In
 one embodiment, the features are selectively programmed into the medical
 device system.
 A template generator circuit 650 is coupled to the sensing system 602. The
 template generator circuit 650 receives information from the morphology
 analyzer 648. In one embodiment, the information received from the
 morphology analyzer 648 includes the information extracted from the two or
 more cardiac signals by the morphology analyzer circuit 648. In one
 embodiment, the template generator circuit 650 creates the numerical
 representation of sensed cardiac complexes. The template generator circuit
 650 then creates the "N" dimensional cardiac complex vector from these
 values as previously described. In addition, the template generator
 circuit 650 is also used to create the one or more classification vectors
 from a patient's own cardiac complexes as previously described.
 In an alternative embodiment, the template generator circuit 650 receives
 at least the first signal and the second signal sensed from the heart 506.
 In one embodiment, the template generator circuit 650 determines a first
 normal sinus rhythm (NSR) representative complex and a second NSR
 representative complex from a plurality cardiac complexes sensed during
 normal sinus rhythm. In one embodiment, the first NSR representative
 complex and the second NSR representative complex are averages of cardiac
 complexes sensed in the first signal and the second signal during a
 patient's normal sinus rhythm. In an alternative embodiment, the first NSR
 representative complex and the second NSR representative complex are
 median values of the cardiac complexes sensed in the first signal and the
 second signal during a patient's normal sinus rhythm.
 After the cardiac signals and information relating to the cardiac complexes
 in the cardiac signals have been processed by the morphology analyzer
 circuit 648 and the template generator circuit 650, the signals are
 received by a signal feature comparison circuit 638. The signal feature
 comparison circuit 638 uses information contained in the cardiac signals
 to analyze and classify sensed cardiac complexes.
 In one embodiment, the signal feature comparison circuit 638 uses the one
 or more classification vectors which are stored in memory 620. The signal
 feature comparison circuit 638 compares the cardiac complex vector to the
 one or more classification vectors, as previously described, to classify
 the cardiac complex represented by the cardiac complex vector. In an
 additional embodiment, the signal feature comparison circuit 638 can also
 align the cardiac vectors with classification vectors prior to comparing
 the vectors when programmed to perform this function.
 In an alternative embodiment, as cardiac complexes are sensed using the
 first cardiac signal and the second cardiac signal, the signal feature
 comparison circuit 638 aligns the predetermined feature in the cardiac
 complex monitored in the first signal with the predetermined feature on
 the first NSR representative complex. Once the predetermined features in
 the cardiac complex from the first signal is aligned with the
 corresponding predetermined feature in the first NSR representative
 complex, the morphology analyzer circuit 648 compares the cardiac complex
 monitored in the second signal to the second NSR representative complex to
 determine whether the cardiac complex is an arrhythmic complex. In one
 embodiment, the morphology analyzer circuit 648 compares the morphology of
 the cardiac complex monitored in the second signal to the second NSR
 representative complex to determine whether the cardiac complex is an
 arrhythmic complex.
 In an alternative embodiment, the morphology analyzer 648 can also compare
 the morphology of a cardiac complex sensed in two or more cardiac channels
 to one or more template cardiac complexes to classify the cardiac complex.
 In one embodiment, the template cardiac complexes are determined from
 cardiac complexes sensed in the two or more cardiac channels which are
 subsequently used to sense cardiac complexes to be classified.
 Alternatively, the template cardiac complexes are determined from cardiac
 complexes sensed from a patient population experiencing the cardiac state
 for which template cardiac complexes are developed. In one embodiment, the
 cardiac states for which template cardiac complexes can be developed
 include cardiac rhythms where the left chamber and the right chamber of
 the ventricles or atria differ by about 20 milliseconds or more, premature
 ventricular contractions, ischemia, predetermined or custom patterns or
 classes of complexes predetermined by a physician, arrhythmia,
 non-arrhythmia, ventricular or supraventricular tachycardia.
 In one embodiment, as sensed cardiac complexes are sensed and classified,
 the microprocessor 616 determines the percentage of classified cardiac
 complexes for a plurality of sensed cardiac complexes. In one embodiment,
 the microprocessor 616 determines the percentage of cardiac complexes
 classified as arrhythmic complexes for a plurality of cardiac complexes
 sensed during an arrhythmic episode. In one embodiment, once the
 percentage of classified cardiac complexes reaches a predetermined
 threshold for any one of the cardiac conditions or states programmed into
 the medical device system, the microprocessor 616 responds by providing
 signals to cardioversion/defibrillation output circuitry 652 to deliver
 either cardioversion or defibrillation therapy to the heart 506. Power to
 the cardiac defibrillator 500 is supplied by an electrochemical battery
 656 that is housed within the cardiac defibrillator 500.
 In addition to the catheter 502, it is possible to add additional
 electrodes, catheters and the accompanying required circuitry to the
 medical device system. For example, the cardiac defibrillator 500 can be
 equipped with electrodes on the surface of the housing 530 to sense
 surface like cardiac signals. In addition, the medical device system can
 further include an additional intracardiac catheter implanted in the
 supraventricular region of the heart. The additional intracardiac catheter
 includes at least one pacing electrode from which rate signals, or near
 field signals, from the atria are sensed and pacing pulses are delivered
 to pace the atrial chamber of the patient's heart. In an additional
 embodiment, the additional intracardiac catheter is implanted through the
 coronary sinus vein and down the great cardiac vein to position an
 electrode, such as a pacing electrode, adjacent to the left ventricular
 chamber of the heart.
 In an alternative embodiment, the catheter 502 is implanted the
 supraventricular region of the heart for sensing or monitoring cardiac
 signals from the patient's atrial regions. In one embodiment, a pacing
 electrode at or adjacent the distal end of the intracardiac catheter is
 implanted in the coronary sinus vein to allow for rate signals to be
 sensed from the left atrium. In addition, far-field signals, or morphology
 signals, are sensed from the supraventricular region of the heart through
 the first defibrillation electrode and the second defibrillation
 electrode. In addition to implanting the catheter 502 in the
 supraventricular region of the heart, an additional atrial catheter can be
 implanted into the supraventricular region of the heart to allow for
 additional rate signals, or near-field signals, to be sensed along with
 the rate signals and morphology signals sensed with the catheter 502.
 Other intracardiac catheter arrangements and configurations known in the
 art are also possible and considered to be within the scope of the present
 system.
 Referring now to FIG. 7, there is shown an additional embodiment of the
 present subject matter. At step 700, a cardiac complex is detected during
 a tachycardia episode in the first signal and the second signal. In one
 embodiment, tachycardia episodes include, but are not limited to,
 ventricular tachycardias and supraventricular tachycardias. In one
 embodiment, the medical device system analyzes at least the cardiac
 complexes detected in the first signal to determined the occurrence of a
 tachycardia episode which will be classified by the present subject
 matter.
 In one embodiment, the first signal and the second signal detect the
 occurrence of cardiac cycles through either a near-field sensing channel
 or a far-field sensing channel as previously discussed. Cardiac complexes
 sensed in the first signal and the second signals can include the
 QRS-wave, the R-wave and/or the P-wave of the cardiac complex. This list,
 however, is not to be take in a limiting sense as it is recognized that
 other portions of the cardiac complex can also be sensed and used with the
 present subject matter.
 As the cardiac complex is detected in the first signal and the second
 signal during the tachycardia episode, the first and second signal of the
 cardiac complex are windowed together so the cardiac complex in the first
 signal is positioned relative the cardiac complex in the second signal. In
 other words, the first signal and the second signal for each sensed
 tachycardiac cardiac complex are positioned relative each other according
 to their occurrence in time.
 FIG. 8B shows an example of a cardiac complex having been windowed at 800.
 In the example, 804 shows a tachycardiac cardiac complex monitored on a
 near-field sensing channel, while 808 shows the cardiac complex monitored
 on a far-field sensing channel.
 After the cardiac complex has been windowed, the first and second signals
 of the cardiac complex are positioned or aligned relative the first NSR
 representative complex and the second NSR representative complex. In one
 embodiment, the first and second NSR representative complexes are
 determined from a plurality of NSR cardiac complexes detected in the first
 signal and the second signal, respectively, using the same medical device
 system that is subsequently used to senses the cardiac complexes. In one
 embodiment, the first NSR representative complex and the second NSR
 representative complex are derived by averaging cardiac complexes detected
 in the first and second signals for the plurality of NSR cardiac complexes
 sensed from the patient. In an alternative embodiment, the first NSR
 representative complex and the second NSR representative complex are
 derived by taking a median of cardiac complexes detected in the first and
 second signals for the plurality of NSR cardiac complexes sensed from the
 patient.
 In addition, the first NSR representative complex and the second NSR
 representative complex can be updated, either manually or automatically,
 to reflect changes in a patient's cardiac condition or in the medical
 device system. For example, the first NSR representative complex and the
 second NSR representative complex could change due to the type of drug or
 dosage of drugs being delivered to the patient and the cardiac disease
 state of the patient. Therefore, the system can recompute the first NSR
 representative complex and the second NSR representative complex at
 regular intervals based either on the physician's judgement or on the
 implantable medical devices assessment of the template. Additionally, a
 safe-check algorithm can be used in conjunction with any automatic
 updating procedure to ensure that only normal sinus rhythm complexes are
 used in updating the template.
 At 720, the cardiac complex detected in the first signal is aligned with
 the first NSR representative complex. In one embodiment, the cardiac
 complex in the first signal and the first NSR representative complex are
 aligned around a predetermined feature located in the cardiac complexes.
 In one embodiment, the predetermined feature includes a repeatably
 identifiable section, or portion, of the detected cardiac complex that is
 common to the cardiac complex detected in the first signal and the first
 NSR representative complex. In one embodiment, the repeatably identifiable
 complex section common to the cardiac complex detected in the first signal
 and the first NSR representative complex is a maximum deflection of the
 first signal. In one embodiment, the maximum deflection is along an R-wave
 sensed in the cardiac complex detected in the first signal and in the
 first NSR representative complex.
 In an alternative embodiment, the repeatably identifiable complex section
 common to the cardiac complex detected in the first signal and the first
 NSR representative complex is a predetermined deviation of the first
 signal from a baseline signal indicating a beginning of the cardiac
 complex and the first NSR representative complex. In an additional
 embodiment, the repeatably identifiable complex section common to the
 cardiac complex detected in the first signal and the first NSR
 representative complex is a return of the first signal to a baseline
 signal for each of the cardiac complex and the first NSR representative
 complex for a predetermined time period indicating an ending of the
 cardiac complex and the first NSR representative complex.
 After the cardiac complex detected in the first signal has been aligned
 with the first NSR representative complex, the cardiac complex detected in
 the second signal is compared to the second NSR representative complex to
 determine whether the cardia complex is an arrhythmic complex. One way of
 determining whether the cardia complex is an arrhythmic complex is to
 compare the morphology of the second signal to the second NSR
 representative complex. In one embodiment, the second NSR representative
 complex has a second NSR morphology and the cardiac complex detected in
 the second signal has a second signal morphology. The second NSR
 morphology of the second NSR representative complex is then compared to
 the second signal morphology of the second signal to determine whether the
 cardia complex is an arrhythmic complex. A variety of techniques exist for
 morphologic analysis of sensed cardiac complexes. In one embodiment, the
 morphology of the cardiac complex sensed in second signal and the
 morphology of the second NSR representative complex are compared using a
 correlation waveform analysis, as is known in the art. In addition to
 correlation waveform analysis, other morphology comparison methods or
 methods of classifying cardiac complexes sensed during an arrhythmic
 episode. Additional morphology analysis techniques include amplitude
 distribution analysis and spectral analysis. Other morphology analysis
 techniques are known and are considered to be with the scope of the
 present subject matter.
 By initially aligning the cardiac complex detected in the first signal with
 the first NSR representative complex, the time the cardiac complex
 detected in the second signal and the second NSR representative complex
 occurred relative the first signal with the first NSR representative
 complex become a factor in determining what portion of the second NSR
 representative complex is compared to the cardiac complex in the second
 signal.
 FIGS. 8A amd 8B show one embodiment of aligning the cardiac complex
 detected in the first signal with the first NSR representative complex. In
 800 of FIG. 8B there is shown a window containing a cardiac complex
 detected in a first signal 804 and a second signal 808. In one embodiment,
 the first signal 804 is a near-field signal of an R-wave, and the second
 signal 808 is a far-field signal of a QRS-cardiac complex. In 812 of FIG.
 8A there is shown a window containing a first NSR representative complex
 816 and a second NSR representative complex 820. In the present example,
 the first NSR representative complex 816 is a near-field signal of an
 R-wave, and the second NSR representative complex 820 is a far-field
 signal of a QRS-cardiac complex.
 As previously discussed, the morphology analyzer circuit 648 locates the
 predetermined feature in the cardiac complex detected in the first signal
 804 and in the first NSR representative complex 816. The signal feature
 comparison circuit 638 then aligns the predetermined feature in the
 cardiac complex monitored in the first signal 804 with the predetermined
 feature on the first NSR representative complex 816. In FIGS. 8A and 8B,
 the cardiac complex 800 and the first NSR representative complex 812 are
 aligned around a maximum deflection 824 of the cardiac complexes 804 and
 816. The signal feature comparison circuit 638 then compares the cardiac
 complex monitored in the second signal 808 to the second NSR
 representative complex 820 to determine whether the cardiac complex is an
 arrhythmic complex. As previously discussed, because the predetermined
 feature in the cardiac complex monitored in the first signal 804 has been
 aligned with the predetermined feature on the first NSR representative
 complex 816, the time the second signal 808 occurs relative the first
 signal 804 and the time the second NSR representative complex 820 occurs
 relative the first NSR representative complex 816 can be used to
 accentuate morphological differences between the complexes. So in one
 embodiment, the QRS-cardiac complex 828 in the second signal 808 is
 morphologically compared to a region 832 along the second NSR
 representative complex 820. Alternatively, the QRS-cardiac complex 834 in
 the second NSR representative complex 820 is morphologically compared to a
 region 836 along the econd signal 808.
 FIG. 9A amd 9B show an alternative embodiment of aligning a cardiac complex
 sensed during a tachycardia episode with a normal sinus rhythm template
 cardiac complex. In 900 of FIG. 9A there is shown a window of a first NSR
 representative complex 904 and a second NSR representative complex 908. In
 the present example, the first NSR representative complex 904 is a
 far-field signal of a QRS-cardiac complex, and the second NSR
 representative complex 908 is a near-field signal of an R-wave cardiac
 complex. In 912 of FIG. 9B there is shown a window of a cardiac complex
 sensed during a tachycardia episode, where the first signal 916 is a
 far-field signal of a QRS-cardiac complex and the second signal 920 is a
 near-field signal of an R-wave cardiac complex.
 In the present embodiment, the cardiac complex 912 and the representative
 NSR complex 900 are aligned around the beginning, or the start, of the
 cardiac complexes 904 and 916. After the representative NSR complex and
 the tachycardiac cardiac complex have been aligned, the morphology of the
 second signal 920 is compared to the morphology of the second NSR
 representative complex 908 to determine whether the cardiac complex is an
 arrhythmic complex. In one embodiment, the R-wave cardiac complex 924 in
 the second signal 920 is morphologically compared to a region 928 along
 the second NSR representative complex 908. In an alternative embodiment,
 the R-wave cardiac complex 930 in the second NSR representative complex
 908 is morphologically compared to a region 932 along the second signal
 920.
 In one embodiment, the present subject matter can classify a variety of
 cardiac complexes, including VT-complexes, SVT-complexes and
 NSR-complexes. Other classifications for the tachycardiac cardiac complex
 are known and are considered within the scope of the present invention.
 FIG. 10 shows an additional embodiment of the present subject matter. At
 1000, a first signal and a second signal representative of electrical
 cardiac activity are sensed from a patient. In one embodiment, each of the
 signals include cardiac complexes representative of cardiac cycles. At
 1010, a plurality normal sinus rhythm (NSR) cardiac complexes are detected
 in the first signal and the second signal. In one embodiment, the medical
 device system determines the first NSR representative complex from the
 plurality of NSR cardiac complexes detected in the first signal and
 determines the second NSR representative complex from the plurality of NSR
 cardiac complexes detected in the second signal. The first and second NSR
 representative complexes are then stored in the memory of the medical
 device system.
 At 1020, cardiac complexes monitored in at least the first signal are
 analyzed by the medical device system to detect the onset of a tachycardia
 episode.
 If no tachycardia episode is detected, the system continues to sense
 cardiac signals and analyzes them for the occurrence of a tachycardia
 episode. In one embodiment, the occurrence of a tachycardia episode is
 based on the cardiac rate, where a tachycardia episode is declared when
 the cardiac rate exceeds a predetermined threshold. In one embodiment, the
 predetermined threshold is a cardiac rate of between 150 and 180 beats per
 minute. Other systems of determining the occurrence of a tachycardia
 episode are known and are considered to be within the scope of the present
 system.
 When a tachycardia episode is detected the system then proceeds to 1030. At
 1030, the first signal and the second signal are monitored for the
 occurrence of cardiac complexes. The detected cardiac complexes are
 analyzed, or processed, so that each cardiac complex is classified as
 either an arrhythmic complex or a non-arrhythmic complex. In processing
 each cardiac complex at 1030, the predetermined feature is located in the
 cardiac complex detected in the first signal and the first NSR
 representative complex. As previously discussed, the predetermined feature
 includes a repeatably identifiable portion of cardiac complexes.
 At 1040, the predetermined feature on the cardiac complex sensed in the
 first signal is then aligned with the corresponding first feature found on
 the first NSR representative complex. In one embodiment, the predetermined
 feature on the first NSR representative complex has been previously
 located and stored in the memory of the medical device system. After the
 predetermined feature on both the cardiac complex sensed in the first
 signal and the first NSR representative complex have been aligned, the
 cardia complex sensed in the second signal is compared to the second NSR
 representative complex at 1050. In one embodiment, the second NSR
 representative complex has a second NSR morphology and the cardiac complex
 detected in the second signal has a second signal morphology, where the
 second NSR morphology is compared to the second signal morphology to
 determine whether the cardia complex is an arrhythmic complex. Based on
 the comparison, the cardiac complex is classified as either an arrhythmic
 complex or as a non-arrhythmic complex at 1060.
 After making a determination as to whether a cardiac complex is an
 arrhythmic complex or a non-arrhythmic complex, a percentage of arrhythmic
 complexes is determined at 1070. At 1080, the calculated percentage of
 arrhythmic complexes is compared to a predetermined percentage threshold.
 In one embodiment, therapy for treating the condition associated with the
 arrhythmic complexes being sensed is applied to the patient's heart at
 1090 when the percentage of arrhythmic complexes exceeds the predetermined
 percentage threshold. If the percentage of arrhythmic complexes does not
 exceed the predetermined percentage threshold, the system returns to 1030.
 In one embodiment, the predetermined percentage threshold is a
 programmable value in the range of 40 to 60 percent, where a value of
 approximately 50 percent is an acceptable value.
 FIG. 11 shows an additional embodiment of the present subject matter. At
 1100, a first signal and a second signal representative of electrical
 cardiac activity are sensed from a patient, where the first signal and the
 second signal each include at least a portion of a QRS-complex. At 1110, a
 plurality normal sinus rhythm (NSR) cardiac complexes are detected in the
 first signal and the second signal. The medical device system determines
 the first NSR representative complex from the at least a portion of the
 QRS-complex in each of the plurality of NSR cardiac complexes detected in
 the first signal and the second NSR representative complex from the at
 least a portion of the QRS-complex in each of the plurality of NSR cardiac
 complexes detected in the second signal. The first and second NSR
 representative complexes are then stored in the memory of the medical
 device system.
 At 1120, cardiac complexes monitored in at least the first signal are
 analyzed by the medical device system to detect the onset of a tachycardia
 episode.
 If no tachycardia episode is detected, the system continues to sense
 cardiac signals and analyzes them for the occurrence of a tachycardia
 episode. In one embodiment, the occurrence of a tachycardia episode is
 based on the ventricular cardiac rate, where a tachycardia episode is
 declared when the ventricular cardiac rate exceeds a predetermined
 threshold. In one embodiment, the predetermined threshold is a ventricular
 cardiac rate of between 150 and 180 beats per minute. Other systems of
 determining the occurrence of a tachycardia episode are known and are
 considered to be within the scope of the present system.
 When a tachycardia episode is detected the system then proceeds to 1130. At
 1130, the first signal and the second signal are monitored for the
 occurrence of cardiac complexes. The detected cardiac complexes are
 analyzed, or processed, so that each cardiac complex is classified as
 either a ventricular tachycardia complex or a non-ventricular tachycardia
 complex. In processing each cardiac complex at 1130, the predetermined
 feature is located in the cardiac complex detected in the first signal and
 the first NSR representative complex. As previously discussed, the
 predetermined feature includes a repeatably identifiable portion of
 cardiac complexes.
 At 1140, the predetermined feature on the cardiac complex sensed in the
 first signal is then aligned with the corresponding first feature found on
 the first NSR representative complex. In one embodiment, the predetermined
 feature on the first NSR representative complex has been previously
 located and stored in the memory of the medical device system. After the
 predetermined feature on both the cardiac complex sensed in the first
 signal and the first NSR representative complex have been aligned, the
 cardia complex sensed in the second signal is compared to the second NSR
 representative complex at 1150. In one embodiment, the second NSR
 representative complex has a second NSR morphology and the cardiac complex
 detected in the second signal has a second signal morphology, where the
 second NSR morphology is compared to the second signal morphology to
 determine whether the cardia complex is a ventricular tachycardia complex
 or a non-ventricular tachycardia complex. Based on the comparison, the
 cardiac complex is classified as either a ventricular tachycardia complex
 or a non-ventricular tachycardia complex at 1160.
 After making a determination as to whether a cardiac complex is a
 ventricular tachycardia complex or a non-ventricular tachycardia complex,
 a percentage of ventricular tachycardia complexes is determined at 1170.
 At 1180, the calculated percentage of ventricular tachycardia complexes is
 compared to a predetermined percentage threshold. In one embodiment,
 therapy for treating a ventricular tachycardia is applied to the patient's
 heart at 1190 when the percentage of ventricular tachycardia complexes
 exceeds the predetermined percentage threshold. If the percentage of
 ventricular tachycardia complexes does not exceed the predetermined
 percentage threshold, the system returns to 1130. In one embodiment, the
 predetermined percentage threshold is a programmable value in the range of
 40 to 60 percent, where a value of approximately 50 percent is an
 acceptable value.
 FIG. 12 shows an additional embodiment of the present subject matter. At
 1200, a first cardiac signal and a second cardiac signal representative of
 electrical cardiac activity are sensed and monitored from a patient. In
 one embodiment, the first cardiac signal and the second cardiac signal are
 monitored from a first cardiac region and a second cardiac region,
 respectively. At 1210, cardiac complexes are detected in the first cardiac
 signal and the second cardiac signal. As the cardiac complexes are sensed,
 template morphologies are developed for cardiac conditions exhibited by
 the patient. In one embodiment, as the patient experiences a cardiac
 condition (such as a ventricular tachycardia or supraventricular
 tachycardia) the cardiac complexes are recorded. In one embodiment, the
 system can then use the recorded cardiac complexes to develop a template
 morphology for the cardiac condition.
 In one embodiment, the template morphology is comprised of a first template
 morphology and a second template morphology. In one embodiment, the first
 template morphology is derived from cardiac complexes monitored in the
 first cardiac signal sensed from approximately the first cardiac region
 and the second template morphology is derived from cardiac complexes
 monitored in the second cardiac signal sensed from approximately the
 second cardiac region. In an alternative embodiment, the first template
 morphology and the second template morphology for each cardiac condition
 of interest are developed through groups of patient data and programmed
 into the memory of the medical device system. The template cardiac
 complexes are then stored in the memory of the medical device system.
 In one embodiment, the patient's cardiac complexes are monitored and
 analyzed by the medical device system. In one embodiment, the cardiac
 complexes are monitored for the onset of a tachycardia event. In an
 alternative embodiment, the cardiac complexes are monitored to detect the
 occurrence of a predetermined class or type of cardiac complex (e.g.,
 PVCs, ischemic complexes). In one embodiment, to determine the occurrence
 of the predetermined class of cardiac complexes, the cardiac complexes
 sensed in the cardiac signals are morphologically compared to the template
 cardiac complexes.
 For the present embodiment, the morphology of the cardiac complex in the
 first cardiac signal is compared to the first template morphology at 1220.
 At 1230, the morphology of the cardiac complex in the second cardiac
 signal is compared to the second template morphology of a cardiac
 condition template morphology. Based on the comparison, the sensed cardiac
 complexes are then classified as either belonging to or not belonging to
 one or more of the classes of cardiac complexes represented by the
 template cardiac complexes at 1240. After classifying each cardiac
 complex, the medical device system can retain the sensed cardiac complexes
 in memory. In addition, a percentage of the classified complexes is
 determined at 1070.
 In an additional embodiment, a third cardia signal containing the cardiac
 complex can be monitored. A third template morphology can be developed in
 the same manner as described for the first and second template
 morphologies. The morphology of the cardiac complex in the third cardiac
 signal is then compared to a third template morphology. So, the cardiac
 complex can be classified based on the comparison of the morphology of the
 cardiac complex in the first cardiac signal, the second cardiac signal and
 the third cardiac signal to the first template morphology, the second
 template morphology and the third template morphology.