Abstract:
An implantable cardioverter/defibrillator includes a tachycardia detection system that detects one-to-one (1:1) tachycardia, which is a tachycardia with a one-to-one relationship between atrial and ventricular contractions. When the 1:1 tachycardia is detected, the system discriminates ventricular tachycardia (VT) from supraventricular tachycardia (SVT) based on analysis of a cardiac time interval. Examples of the cardiac time interval include an atrioventricular interval (AVI) and a ventriculoatrial interval (VAI). A template time interval is created during a known normal sinus rhythm. The system measures a tachycardia time interval after detecting the 1:1 tachycardia, and indicates a VT detection if the tachycardia time interval differs from the template time interval by at least a predetermined percentage of the template time interval.

Description:
This document is a continuation-in-part of, and claims priority to, U.S. patent application Ser. No. 09/283,159, filed Apr. 1, 1999, now issued as U.S. Pat. No. 6,179,865. 

   FIELD OF THE INVENTION 
   The present invention relates generally to implantable medical devices and more particularly to a system and method for classifying 1:1 atrial-to-ventricular cardiac rhythms. 
   BACKGROUND 
   The heart is generally divided into four chambers, two atrial chambers and the two ventricular chambers. As the heart beats, the atrial chambers and the ventricular chambers of the heart go through a cardiac cycle. The cardiac cycle consists of one complete sequence of contraction and relaxation of the chambers of the heart. The terms systole and diastole are used to describe the contraction and relaxation phases the chambers of the heart experience during a cardiac cycle. In systole, the ventricular muscle cells are contracting to pump blood through the circulatory system. During diastole, the ventricular muscle cells relax, causing blood from the atrial chambers to fill the ventricular chambers. After the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated. 
   Control over the timing and order of the atrial and ventricular contractions during the cardiac cycle is critical for the heart to pump blood efficiently. Efficient pumping action of the heart requires precise coordination of the contraction of individual cardiac muscle cells. Contraction of each cell is triggered when an electrical excitatory impulse (an “action potential”) sweeps over the heart. Proper coordination of the contractual activity of the individual cardiac muscle cells is achieved primarily by the conduction of the action potential from one cell to the next by gap junctions that connect all cells of the heart into a functional system. In addition, muscle cells in certain areas of the heart are specifically adapted to control the frequency of cardiac excitation, the pathway of conduction and the rate of impulse propagation through various regions of the heart. The major components of this specialized excitation and conduction system include the sinoatrial node (SA node), the atrioventricular node (AV node), the bundle of His, and specialized cells called Purkinje fibers. 
   The SA node is located at the junction of the superior vena cava and the right atrium. Specialized atrium muscle cells of the SA node spontaneously generate action potentials which are then propagated through the rest of the heart to cause cardiac contraction. This SA node region normally acts as the intrinsic cardiac pacemaker. The action potential generated by the SA node spreads through the atrial wall, causing the atrial chambers to contract and the P-wave of an electrocardiogram signal. 
   The AV node consists of small, specialized cells located in the lower portion of the atrial chamber. The AV node acts like a bridge for the action potential to cross over into the ventricular chamber of the heart. Once the action potential has crossed over to the ventricular chambers, the bundle of His carries the action potential to specialized cardiac fibers called Purkinje fibers. The Purkinje fibers then distribute the action potential throughout the ventricular chamber of the heart. This results in rapid, very nearly simultaneous excitation of all ventricular muscle cells. The conduction of the action potential through the AV node and into the ventricular chambers creates the QRS-complex of an electrogram signal. 
   During the cardiac cycle, the action potential moves in an antegrade direction, first causing the atrial chambers to contract and then causing the ventricle chambers to contract. When the action potential causes a single atrial contraction followed by a single ventricular contraction the heart is displaying a one-to-one atrial to ventricular response. In other words, for a given atrial contraction, the cardiac signal causing the atrial contraction subsequently causes a ventricle contraction. In this manner, there is a one-to-one atrial to ventricular response. Cardiac conditions also exist where the action potential moves in a retrograde direction, where the cardiac signal moves from the ventricular chamber up into the atrial chamber. 
   When a patient&#39;s heart rate increases to above 100 beats per minute, the patient is said to be experiencing a tachycardia. Many different types of tachycardias can exist. For example, a heart in a sinus tachycardia (heart rates between 100–180 beats per minute) exhibits a normal cardiac cycle, where action potential moves in the antegrade direction from the atrial chambers to the ventricular chambers to cause the contraction of the heart. The increased heart rate during the sinus tachycardia is a response to a stimulus, and not to a cause within the heart. For example, sinus tachycardia stimulus can include physiologic responses to maintain adequate cardiac output and tissue oxygenation during exercise. Unlike sinus tachycardia, a ventricular tachycardia (heart rates between 120–250) is caused by electrical disturbances within the heart, and not due to the physiological demands of the body. Ventricular tachycardias must be treated quickly in order to prevent the tachycardia from degrading into a life threatening ventricular fibrillation. 
   Distinguishing a ventricular tachycardia from a sinus tachycardia is important for diagnosing and properly treating the patient&#39;s cardiac condition. Misdiagnosis of a sinus tachycardia as a ventricular tachycardia can lead to inappropriate treatment. Difficulty in distinguishing among tachyarrhythmias increases when the heart is displaying a one-to-one atrial to ventricular rhythm. One reason for this difficulty is that the action potentials generated during the tachyarrhythmia can travel either in the antegrade direction, from the atria to the ventricles, or in a retrograde direction, from the ventricles into the atria. Tachyarrhythmias having action potentials conducted in an antegrade direction include sinus tachycardia and atrial tachycardia. Tachyarrhythmias having action potentials conducted in a retrograde direction include ventricular tachycardia with 1-to-1 retrograde conduction. Distinguishing the direction of the action potential (antegrade or retrograde) during a tachyarrhythmia is important in diagnosing and delivering the appropriate type of treatment to the patient. 
   Ways of classifying one-to-one tachyarrhythmias have been suggested. For example, Thompson et al. ( J. Of Electrocardiography  1998; 31:152–156) have suggested that VA intervals can be compared to a retrograde zone, where the retrograde zone is defined as a zone between a predetermined upper time bound and a lower time bound relative the ventricular contractions. A rhythm whose VA intervals fall inside the retrograde zone is classified as retrograde. Otherwise, the rhythm is classified as an antegrade rhythm. However, limitations to this suggested method exist. For example, the VA intervals can change with the heart rate. Also, patients with first degree heart block (PR&gt;200 milliseconds) may have short VA during sinus tachycardia or normal sinus rhythm. Thus, a need exists in the art for a reliable and convenient approach which can distinguish antegrade and retrograde action potentials during a one-to-one tachyarrhythmia episode. 
   SUMMARY OF THE INVENTION 
   The present subject matter provides a system and a method for distinguishing antegrade from retrograde action potentials during a 1:1 atrial-to-ventricular tachyarrhythmia episode. The classified action potentials are then used to classify the tachyarrhythmia episode as occurring in either a retrograde direction or an antegrade direction. Based on this classification it is then possible to determine an appropriate course of treatment. 
   Discriminating one-to-one atrial-to-ventricular rhythms conducted in an antegrade direction (e.g., sinus tachycardia, atrial tachycardia) from one-to-one rhythms conducted in a retrograde direction (e.g., VT with one-to-one retrograde conduction) is an important aspect of properly diagnosing a tachyarrhythmia episode. The present subject matter utilizes two or more sensed cardiac signals, where at least a first cardiac signal is sensed from the ventricular region of the heart and at least a second cardiac signal is sensed from a supraventricular region of the heart. Each cardiac signal includes indications of cardiac complexes, where the cardiac complexes are the electrical excitatory impulses, or action potentials, sensed as the heart goes through the cardiac cycle. Information derived from the cardiac complexes in the two or more cardiac signals is then used in classifying, or distinguishing, the conduction direction (e.g., antegrade or retrograde) of the cardiac action potential. 
   In one embodiment, a first cardiac signal and a second cardiac signal are sensed. In one embodiment, the first cardiac signal is sensed from a ventricular location and the second cardiac signal is sensed from a supraventricular location. Ventricular depolarizations are sensed, or detected, from the first cardiac signal and atrial depolarizations are sensed, or detected, from the second cardiac signal. The first and second cardiac signals are analyzed to detect the occurrence of a tachycardia episode having a one-to-one association of atrial depolarizations to ventricular depolarizations. In one embodiment, the association of atrial depolarizations to ventricular depolarizations are analyzed to determine if a one-to-one association of atrial depolarizations to ventricular depolarizations exists during the tachycardia episode. 
   Once a tachycardia episode having a one-to-one association of atrial depolarizations to ventricular depolarizations is detected, time intervals are measured between predetermined features on combinations of the first cardiac signal and the second cardiac signal. In one embodiment, first intervals are measured between ventricular depolarizations detected in the first cardiac signal and first predetermined cardiac events in either the first or second cardiac signal. Similarly, second intervals are measured between atrial depolarizations detected in the second cardiac signal and second predetermined cardiac events in either the first or second cardiac signal. 
   The values of the first intervals are then used to calculate, or determine, a first interval characteristic, or dispersion, of intervals from the first intervals and the second intervals are used to calculate, or determine, a second interval characteristic, or dispersion, of intervals from the second intervals. The values for the first interval characteristic and the second interval characteristic are then used to classify the tachycardia episode as either occurring in an antegrade direction or in a retrograde direction. In one embodiment, the first interval characteristic and the second interval characteristic are compared in classifying the tachycardia episode based on the first interval characteristic and the second interval characteristic. 
   In one embodiment, the first interval characteristic and the second interval characteristic are a first variance value and a second variance value, respectively. However, other first interval and second interval characteristics exist and can be used with the present subject matter. For example, the first interval and second interval characteristics can include calculating and using a first range and a second range of values which are then compared is classifying a tachycardia episode as either occurring in an antegrade or retrograde direction. 
   In one embodiment, a first predetermined series of the first intervals and a second predetermined series of the second intervals are used to calculate the first interval and the second interval characteristics, respectively. The values of the first interval and the second interval characteristics are then compared. In one embodiment, the comparison between the two characteristic values is between a first variance value and a second variance value, where the comparison is to determine which value is larger. Based on the comparison, the tachycardia episode is then classified as either occurring in an antegrade direction or is a retrograde direction. In one embodiment, the tachycardia episode is classified as an antegrade rhythm, or occurring in the antegrade direction, when the value of the second variance is less than or equal to the value of the first variance. Alternatively, the tachycardia episode is classified as an retrograde rhythm, or occurring in the retrograde direction, when the value of the second variance is greater than the value of the first variance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a heart; 
       FIG. 2  is a flow chart illustrating one embodiment of the present subject matter; 
       FIG. 3  is a schematic illustrating one embodiment of a first cardiac signal and a second cardiac signal; 
       FIG. 4  is a flow chart illustrating one embodiment of the present subject matter; 
       FIG. 5A  is a schematic view of one embodiment of an implantable medical device according to one embodiment of the present subject matter; 
       FIG. 5B  is a schematic view of one embodiment of an implantable medical device according to one embodiment of the present subject matter; and 
       FIG. 6  is a block diagram of one embodiment of an implantable medical device according to the present subject matter. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings which form a part hereof and in which is shown by way of illustration specific embodiments in which the invention can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice and use the invention, and it is to be understood that other embodiments may be utilized and that electrical, logical, and structural changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the present invention is defined by the appended claims and their equivalents. 
   The embodiments illustrated herein are demonstrated in an implantable cardiac defibrillator (ICD), which may include numerous defibrillation, pacing, and pulse generating modes known in the art. However, these embodiments are illustrative of some of the applications of the present system, and are not intended in an exhaustive or exclusive sense. The concepts described herein can be used in a variety of applications which will be readily appreciated by those skilled in the art upon reading and understanding this description. For example, the present system is suitable for implementation in a variety of implantable, such as an implantable pacemaker, and external medical devices. 
   As discussed above, discriminating one-to-one atrial-to-ventricular rhythms conducted in an antegrade direction (e.g., sinus tachycardia, atrial tachycardia) from one-to-one rhythms conducted in a retrograde direction (e.g., VT with one-to-one retrograde conduction) is an important aspect of properly diagnosing a tachyarrhythmia episode. The present subject matter utilizes two or more sensed cardiac signals, where at least a first cardiac signal is sensed from the ventricular region of the heart and at least a second cardiac signal is sensed from a supraventricular region of the heart. Each cardiac signal includes indications of cardiac complexes, where the cardiac complexes are the electrical excitatory impulses, or action potentials, sensed as the heart goes through the cardiac cycle. Information derived from the cardiac complexes in the two or more cardiac signals is then used in classifying, or distinguishing, the conduction direction of the cardiac action potential. 
   The present subject matter discriminates and classifies tachyarrhythmias displaying a one-to-one atrial to ventricular rhythm as either being conducted in an antegrade direction or in a retrograde direction. In one embodiment, intervals between predetermined points on an atrial cardiac signal and/or a ventricular cardiac signal are used in determining whether the tachyarrhythmia is being conducted in the antegrade or retrograde direction. In one embodiment, this determination is based on the variability between alike measurements on the atrial cardiac signal and/or the ventricular cardiac signal. The variability from measurements taken from the atrial cardiac signal and/or the ventricular cardiac signal is then used to classify the tachyarrhythmia as either occurring in the antegrade or the retrograde direction. 
   The present subject matter has a distinct advantage over previously described systems and methods for classifying one-to-one atrial-to-ventricular in that the cardiac complex detected during a tachycardia episode need not be compared to a “template” or a predetermined cardiac complex in order to classify the cardiac complex as occurring in either the antegrade or retrograde direction. Previous approaches have relied upon a “template” cardiac complex (i.e., an idealized cardiac complex) which was stored in the memory of the implantable device. Upon detecting a tachycardia episode, the system would retrieve the “template” cardiac complex and then proceed to compare the template cardiac complex to the cardiac complexes sensed during the tachycardia episode. This process takes time and energy, two factors which are reduced by the present subject matter as the need for a “template” cardiac complex has been eliminated. In place of a template cardiac complex the present subject matter utilizes characteristics of the intervals measured between predetermined features on cardiac complexes sensed in at least a first cardiac signal and a second cardiac signal during the tachycardia episode. No predetermined template or cardiac signals are required, thus saving both computational time and electrical energy. 
   Referring now to  FIG. 1 , there is shown a drawing of a human heart  100 . The heart  100  is divided into atrial chambers  110  and ventricular chambers  120 . When the heart  100  is in normal sinus rhythm or in sinus tachycardia the depolarization wave (action potential) for the heart beat originates in the SA node  130 , which is located in the atrial region of the heart  100 . In this situation the atrial chambers  110  can be thought of as originating the signals to start the contraction of the heart  100 . The depolarization wave moves from the SA node  130  across the atria and then to the AV node  140 . Lines  150  show the direction of the depolarization wave as it would move across the atrial chambers  110 . The AV node consists of small specialized cells located on the right side of the atrial septum just under the endocardium. The lower portion of the AV node consists of parallel fibers that form a “bridge” of contiguous cardiac cells crossing the cartilaginous structure that provides support for the cardiac valves and electrically separates atria from ventricles. Propagation of the impulse through this AV nodal region is typically very slow (approximately 0.05 m/s) and therefore a delay is imposed between excitation of the atria and the ventricles. The term AV delay is given to denote this delay. The action potential causing the depolarization wave then moves through the AV node and down into the ventricle chambers  120 . The depolarization wave is distributed quickly and essentially evenly through out the ventricle chambers  120  which allows for near simultaneous contraction of ventricles of the heart  100 . 
   When the depolarization wave originates in the SA node and moves through the AV node into the ventricles, the depolarization wave is said to be moving in an antegrade direction. Examples of when the depolarization wave is moving in the antegrade direction include when the heart is in normal sinus rhythm or when the heart is in sinus tachycardia. There are also cardiac conditions in which the depolarization wave, or action potential, can move in a retrograde direction. In this situation the cardiac signal moves from the ventricular chamber up into the atrial chamber. An example of a cardiac condition displaying a retrograde direction is ventricular tachycardia with 1-to-1 retrograde conduction. During a tachycardia episode, having a fast and reliable method and system for discriminating one-to-one atrial-to-ventricular rhythms conducted in an antegrade direction (e.g., sinus tachycardia, atrial tachycardia) from one-to-one rhythms conducted in a retrograde direction (e.g., VT with one-to-one retrograde conduction) is important for quickly and accurately diagnosing the tachyarrhythmia episode. 
   Referring now to  FIG. 2 , there is shown one embodiment of a method for the present subject matter. At  200 , ventricular depolarizations and atrial depolarizations are sensed. In one embodiment, the ventricular depolarizations are sensed, or detected, in a first cardiac signal and the atrial depolarizations are sensed, or detected, in a second cardiac signal. In one embodiment, the first cardiac signal is sensed from a ventricular location and the second cardiac signal is sensed from an supraventricular location. For example, the first cardiac signal is sensed across a ventricular region which includes at least a right ventricular chamber and the second cardiac signal is sensed across a right atrial chamber location. Alternatively, the second cardiac signal is sensed from a location within the coronary sinus vein or other coronary vein which would allow for the cardiac signal to be sensed from the supraventricular region of the heart. Additionally, the first and second cardiac signals are any combination of rate (near field) and/or morphology (far field) signals. In one embodiment, the first cardiac signal sensed from the ventricular region is a morphology signal and the second cardiac signal sensed from the supraventricular location is a rate signal. 
   At  210 , ventricular depolarizations are analyzed to detect the occurrence of a tachycardia episode. In one embodiment, the occurrence of a tachycardia episode is determined from time intervals between the sensed ventricular intervals. The atrial and ventricular depolarizations are then analyzed during a detected tachycardia episode to determine if there is a one-to-one association of atrial depolarizations to ventricular depolarizations at  220 . In the present subject matter, the occurrence of a tachycardia episode is defined generally as a heart rate in the range of 120–250 beats per minute which results from electrical disturbances within the heart, and not due to the physiological demands of the body. In one embodiment, the heart rate is determined from the ventricular depolarizations detected in the first cardiac signal. 
   Once a tachycardia episode having a one-to-one association of atrial depolarizations to ventricular depolarizations is detected, time intervals are measured between predetermined features on combinations of the atrial depolarizations and the ventricular depolarizations. For example, during the tachycardia episode that has the one-to-one association of atrial depolarizations to ventricular depolarizations, first intervals are measured between the ventricular depolarizations and first predetermined cardiac events and second intervals are measured between the atrial depolarizations and second predetermined cardiac events at  230 . 
   In one embodiment, values of the first and second intervals are analyzed to determine whether the value of a given interval falls outside of a predetermined threshold value. In one embodiment, interval values falling outside the predetermined threshold (e.g., intervals longer than the threshold and/or intervals shorter than the threshold) are not utilized in the present subject matter. In one embodiment, the predetermined threshold is a percentage, or ratio, of a predetermined number of the most current interval values. Alternatively, the morphology (i.e., shape) and/or direction (i.e., trajectory) of the sensed depolarization are used to determine whether the sensed wave is used in determining time intervals. 
   Referring now to  FIG. 3 , there are shown examples of various combinations of signal features used when measuring, or calculating, the first and second intervals.  FIG. 3  displays one embodiment of a first cardiac signal  300  and a second cardiac signal  310  being used in the present subject matter. The first cardiac signal  300  is sensed from a ventricular region of the heart and the second cardiac signal  310  is sensed from an atrial region of the heart. The first cardiac signal  300  includes indications of ventricular depolarizations which are sensed in any number of ways, including use of implanted intravascular leads having one or more electrodes for sensing a rate signal (near field signal) and/or a morphology signal (far field signal) in either unipolar or bipolar sensing mode. In one embodiment, the sensed ventricular depolarizations are the R-waves of a sensed electrocardiogram signal. Alternatively, the sensed ventricular depolarizations include QRS-complexes detected in the first cardiac signal. In the present embodiment, the sensed ventricular depolarizations in the first cardiac signal  300  are QRS-complexes which are shown generally at  320 . 
   The second cardiac signal  310  includes indications of atrial depolarizations which are sensed in any number of ways, including use of implanted intravascular leads having one or more electrodes for sensing a rate signal (near field signal) and/or a morphology signal (far field signal) in either unipolar or bipolar sensing mode. In one embodiment, the sensed atrial depolarizations are the P-waves of a sensed electrocardiogram signal. In the present embodiment, the sensed atrial depolarizations in the second cardiac signal  310  are P-waves which are shown generally at  330 . 
   The embodiment of the first and second cardiac signal shown in  FIG. 3  is an example of a one-to-one atrial-to-ventricular rhythm. As  FIG. 3  shows, for each sensed atrial depolarization  330  there is a subsequent ventricular depolarization  320  which occurs before the next atrial depolarization. As previously discussed, time intervals are measured between at least two different combinations of atrial and ventricular depolarizations. For example, first intervals are measured and calculated between pairs of detected ventricular depolarizations  320  and first predetermined cardiac events. Similarly, the second intervals are measured and calculated between pairs of detected atrial depolarizations  330  and second predetermined cardiac events. 
   In one embodiment, the first predetermined cardiac events are ventricular depolarizations  320  detected in the first cardiac signal  300  and the second predetermined cardiac events are atrial depolarizations  330  detected in the second cardiac signal  310 . Thus, the first intervals are measured between a ventricular depolarization  320  and a subsequent ventricular depolarization. This measurement is a ventricular—ventricular (VV)-interval measurement (also known as a ventricular cycle length) which is the time between successively sensed ventricular depolarizations. An example of the VV-interval measurement is shown at  340 . As for the second intervals, they are measured between an atrial depolarization  330  and a subsequent atrial depolarization. This measurement is an atrial—atrial (AA)-interval measurement (also known as atrial cycle lengths) which is the time between successively sensed atrial depolarizations. An example of the AA-interval measurement is shown at  350 . 
   In an alternative embodiment, the first predetermined cardiac events are atrial depolarizations  330  detected in the second cardiac signal  310  and the second predetermined cardiac events are ventricular depolarizations  320  detected in the first cardiac signal  300 . Thus, the first intervals are measured between a ventricular depolarization  320  and a subsequent atrial depolarization. These measurements are ventricular-atrial (VA)-interval measurements of the time between a ventricular depolarization  320  and a subsequent atrial depolarization  330 . An example of the VA-interval measurement is shown at  360 . The second intervals are measured between an atrial depolarization  330  and a subsequent ventricular depolarization. These measurements are atrial-ventricular (AV)-interval measurement of the time between an atrial depolarization  330  and a subsequent ventricular depolarization  320 . An example of the AV-interval measurement is shown at  370 . 
   In one embodiment, interval measurements made on the first cardiac signal and the second cardiac signal take place between predetermined points on the sensed cardiac complexes. In one embodiment, the predetermined points are repeatably identifiable portions of the cardiac complex in the cardiac signal. In one embodiment, the predetermined points are selected by a physician and are subsequently programmed into the medical device system (e.g., an ICD) for use with the present subject matter. Examples of repeatably identifiable portions of cardiac complexes include the maximum (or minimum) deflection point of the cardiac signal during the cardiac complex, the point of maximum slope of the cardiac signal during the cardiac complex, or the start or end of a cardiac complex detected in the cardiac signal. Other repeatably identifiable portion of cardiac complexes could also be used. 
   Examples of predetermined points are shown in  FIG. 3 .  FIG. 3  shows an example of a first predetermined point  380  located along the ventricular depolarizations. In the present embodiment, the first predetermined point  380  is a maximum deflection point of the first cardiac signal. Also shown in  FIG. 3  is an example of a second predetermined point  390  located along the atrial depolarizations. In the present embodiment, the second predetermined point  390  is a maximum deflection point of the second cardiac signal. 
   In one embodiment, the values of a plurality of first intervals and the values of a plurality of second intervals are used to determine, or calculate, characteristics of the first and second intervals (e.g., first characteristics and second characteristics). At  240 , the characteristics of the first and second intervals are then used to classify the tachycardia episode as either an antegrade rhythm or a retrograde rhythm based on characteristics of the first and second intervals. In one embodiment, the characteristics of the first and second intervals include first interval characteristics and second interval characteristics, where the values for the first interval characteristics and second interval characteristics are calculated from the first and second intervals. The tachycardia episode is then classified based on the first interval characteristic and the second interval characteristic. In one embodiment, characteristic means some metric of the interval set that quantifies its variability. For example, the first interval characteristic can be a first variance value, σ 2 (x), calculated from the first intervals, and the second interval characteristic can be a second variance value, σ 2 (y), calculated from the second intervals. In one embodiment, the tachycardia episode is then classified based on the first variance value and the second variance value. 
   In addition to variance (i.e., the second moment of the intervals), other examples of first characteristics and second characteristics that can be used to describe the variability of the first intervals and the second intervals respectively include, but are not limited to, other moments of the intervals, the size of the maximal range (i.e., the maximum interval size–minimum interval size), a percentile range (i.e., the size of the range that is centered on the average interval size and which includes a specified percentage of the intervals–such as range that includes the center 50% of the intervals), or a range that is based on the first (second, third, etc.) smallest intervals to the first (second, third, etc.) largest intervals. Alternate embodiments use these alternate first and second characteristics to classify the tachycardia episodes. 
   In one embodiment, a first predetermined series of the first intervals and a second predetermined series of the second intervals are used to calculate the first interval characteristic and the second interval characteristic, respectively. In one embodiment, the first and second predetermined series of intervals are programmable and have a value of at least five (5) intervals. In an alternative embodiment, the first and second predetermined series are programmable values in the range of between five (5) and fifty (50), five (5) and twenty five (25), ten (ten) and fifty (50), or ten (10) and twenty five (25), where ten is an acceptable value. 
   Referring now to  FIG. 4 , there is shown one embodiment of a method for the present subject matter. At  400 , a first cardiac signal and a second cardiac signal are sensed. In one embodiment, the first cardiac signal is sensed from a ventricular location and the second cardiac signal is sensed from an supraventricular location. For example, the first cardiac signal is sensed across a ventricular region which includes at least a right ventricular chamber and the second cardiac signal is sensed across a right atrial chamber location. Alternatively, the second cardiac signal is sensed from a location within the coronary sinus vein or other coronary vein which would allow for the cardiac signal to be sensed from the supraventricular region of the heart. Additionally, the first and second cardiac signals are any combination of rate (near field) and/or morphology (far field) signals. In one embodiment, the first cardiac signal sensed from the ventricular region is a morphology signal and the second cardiac signal sensed from the supraventricular location is a rate signal. 
   At  410 , ventricular depolarizations are sensed, or detected, from the first cardiac signal and atrial depolarizations are sensed, or detected, from the second cardiac signal. The first and second cardiac signals are analyzed to detect the occurrence of a tachycardia episode having a one-to-one association of atrial depolarizations to ventricular depolarizations at  420 . In one embodiment, the association of atrial depolarizations to ventricular depolarizations are analyzed to determine if a one-to-one association of atrial depolarizations to ventricular depolarizations exists during the tachycardia episode. In the present subject matter, the occurrence of a tachycardia episode is defined generally as a heart rate in the range of 120–250 beats per minute which results from electrical disturbances within the heart, and not due to the physiological demands of the body. In one embodiment, the heart rate is determined from the ventricular depolarizations detected in the first cardiac signal. 
   Once a tachycardia episode having a one-to-one association of atrial depolarizations to ventricular depolarizations is detected, time intervals are measured between predetermined features on combinations of the first cardiac signal and the second cardiac signal at  430 . In one embodiment, first intervals are measured between ventricular depolarizations detected in the first cardiac signal and first predetermined cardiac events in either the first or second cardiac signal. Similarly, second intervals are measured between atrial depolarizations detected in the second cardiac signal and second predetermined cardiac events in either the first or second cardiac signal. In one embodiment, values of the first and second intervals are analyzed to determine whether the value of a given interval falls outside of a predetermined threshold value. In one embodiment, interval values falling outside the predetermined threshold (e.g., intervals longer than the threshold and/or intervals shorter than the threshold). In one embodiment, the predetermined threshold is a percentage, or ratio, of a predetermined number of the most current interval values. Alternatively, the morphology (i.e., shape) and/or direction (i.e., trajectory) of the sensed depolarization are used to determine whether the sensed wave is used in determining time intervals. 
   At  440  the values of a plurality of first intervals are used to determine, or calculate, a first variance value, σ 2 (x), from the first intervals and the values of a plurality of second intervals are used to determine, or calculate, a second variance value, σ 2 (y), from the second intervals. In one embodiment, a first predetermined series of the first intervals and a second predetermined series of the second intervals are used to calculate the first variance and the second variance, respectively. In one embodiment, the first and second predetermined series of intervals are programmable and have a value of at least five (5) intervals. In an alternative embodiment, the first and second predetermined series are programmable values in the range of between five (5) and fifty (50), five (5) and twenty five (25), ten (ten) and fifty (50), or ten (10) and twenty five (25), where ten is an acceptable value. 
   The values of the first variance and the second variance are then compared at  450 . In one embodiment, the comparison between the two variance values is to determine which variance value is larger. Based on the comparison of the first variance value and the second variance value, the tachycardia episode is then classified as either occurring in an antegrade direction or is a retrograde direction at  460 . In one embodiment, the tachycardia episode is classified as an antegrade rhythm, or occurring in the antegrade direction, when the value of the second variance is less than or equal to the value of the first variance. Alternatively, the tachycardia episode is classified as an retrograde rhythm, or occurring in the retrograde direction, when the value of the second variance is greater than the value of the first variance. 
   Referring now to  FIG. 5A , there is shown one embodiment of a system according to the present subject matter. The system includes an implantable cardiac defibrillator  500  and at least one cardiac lead including at least three electrodes. In one embodiment, the at least one cardiac lead is a ventricular lead  504 . The ventricular lead  504  includes at least a first electrode  508  and a second electrode  512 .  FIG. 5A  shows an embodiment in which the first electrode  508  is a defibrillation coil electrode positioned along a peripheral surface of the ventricular lead  504 . The first electrode  508  is connected to the electronic circuitry within the implantable cardiac defibrillator  500  through a lead conductor housed and electrically insulated within the body of the ventricular lead  504 . The second electrode  512  is also a defibrillation coil electrode which is positioned along the peripheral surface of the ventricular lead  504 . The second electrode  512  is located at a position that is proximal to the first electrode  508  which allows for the ventricular lead  504  to be implanted within the vasculature with the first electrode  508  positioned in the right ventricle and the second electrode  512  positioned in either the right atrial chamber or a major vein leading to the right atrial chamber of the heart. In one embodiment, the first and second electrodes,  508  and  512 , are used to sense, or detect, a cardiac morphology signal from the heart. In one embodiment, the cardiac morphology signal sensed from the heart includes both atrial and ventricular signals. In an additional embodiment, the electrically conductive portion of the implantable cardiac defibrillator  500  housing is used in conjunction with the first and second electrodes  508 ,  512  to allow for a morphology signal to be sensed between three electrodes. 
   In addition to the first and second electrodes  508 ,  512 , the ventricular lead  504  is shown further including a pacing electrode  528  located at or adjacent a distal end  532  of the ventricular lead  504 . This allows for both rate and morphology signals to be sensed from the ventricular region of the heart using the supplied electrodes, where, for example, the rate signal is sensed between the pacing electrode  528  and the first electrode  508  and the morphology signal is sensed between the first and second electrodes  508 ,  512 . 
   Referring now to  FIG. 5B , there is shown an additional embodiment of the system according to the present subject matter. The system includes the implantable cardiac defibrillator  500 , the ventricular lead  504  and an atrial lead  536 . The atrial lead  536  includes a first atrial electrode  538 , which in  FIG. 5B  is shown positioned at a distal end  540  of the atrial lead  536 . The first atrial electrode  538  is connected to electronic circuitry within the implantable cardiac defibrillator  500  through a lead conductor housed and electrically insulated within the body of the atrial lead  536 . The lead conductor allows for cardiac signals sensed using the first atrial electrode  538  to be supplied to the electronic circuitry and for pacing pulses generated though the use of the electronic circuitry to be delivered to the first atrial electrode  538 . In the present embodiment, unipolar sensing and pacing is accomplished between the first atrial electrode  538  and an electrically conductive portion of the implantable cardiac defibrillator  500  housing. 
   In one embodiment, the atrial lead  536  and the ventricular lead  504  have elongated bodies made of one or more materials suitable for implantation in a human body, where such materials are known in the art. Additionally, the first and second electrodes  508 ,  512 , the pacing electrode  528  and the first atrial electrode  538  are constructed of electrically conductive materials, such as platinum, platinum-iridium alloys, or other alloys as are known. The lead conductors are also constructed of electrically conductive materials such as MP35N, an alloy of nickel, chromium, cobalt, and molybdenum. 
     FIG. 5  also shows a medical device programmer  544 . The medical device programmer  544  and the implantable cardiac defibrillator  500  include communication circuitry which allows for cardiac data to be to and from the implantable cardiac defibrillator  500 . In addition, command signals for controlling the operation of the implantable cardiac defibrillator  500  can also be sent between the medical device programmer  544  and the implantable cardiac defibrillator  500 . In one embodiment, communication between the medical device programmer  544  and the implantable cardiac defibrillator  500  is established over a radio frequency telemetry channel as is known in the art. 
   Referring now to  FIG. 6 , there is shown a block diagram of an implantable cardiac defibrillator (ICD)  600  according to one embodiment of the present subject matter. The ICD  600  includes control circuitry  602  which receives one or more cardiac signals and delivers electrical energy to electrodes positioned on the atrial and ventricular leads under predetermined conditions. 
   In one embodiment, the control circuitry  602  is a programmable microprocessor-based system, with a microprocessor  604  and a memory circuit  606 , which contains parameters for various pacing and sensing modes and stores data indicative of cardiac signals received by the control circuitry  602 . The control circuitry  602  includes terminals labeled with reference numbers  608 ,  610 ,  612 ,  614  and  616  for connection to the electrodes attached to the surface of a ventricular lead and an atrial lead. In the embodiment shown in  FIG. 5B , the first electrode  508  is coupled to terminal  608  through a first electrically insulated conductor provided within the ventricular lead  504 . The second electrode  512  is coupled to terminal  610  through a second electrically insulated conductor provided within the ventricular lead  504 . The pacing electrode  528  on the ventricular lead  504  is coupled to terminal  612  through a third electrically insulated conductor provided within the ventricular lead  504 . Finally, the first atrial electrode  538  is coupled to terminals  614  by electrically insulated conductors provided within the atrial lead  536 . 
   The control circuitry  602  is encased and hermetically sealed in a housing  620  suitable for implanting in a human body. In one embodiment, the housing  620  is made of titanium, however, other biocompatible housing materials as are known in the art may be used. A connector block  624  is additionally attached to the housing  620  to allow for the physical and the electrical attachment of the ventricular lead  504 , the atrial lead  536  and the electrodes to the ICD  600  and the encased control circuitry  602 . 
   Sense amplifiers  626  and  628  are coupled to the control circuitry  602 , and are electrically coupled to terminals  608 ,  610  and  612  to allow for a first cardiac signal to be sensed between the ventricular electrode  528  and first defibrillation electrode  508  and/or between the first electrode  508  and the second electrode  512 . The output of the sense amplifiers  626  and  628  are connected to a ventricular depolarization detector circuit  630  which is adapted to detect the occurrence of ventricular depolarizations in the first cardiac signal. In one embodiment, these components serve to sense near and/or far field ventricular cardiac signals and to amplify the signals indicating ventricular depolarizations, for example by sensing ventricular R-waves and or QRS-complexes, and apply signals indicative thereof to microprocessor  604 . Among other things, the microprocessor  604  responds to the ventricular depolarization detector  630  by providing pacing signals to a pace output circuit  632  via bus  634 , as needed according to the programmed pacing mode. In one embodiment, the pace output circuit  632  then provides output pacing signals to the ventricular electrode  528  and first defibrillation electrode  508  via terminals  610  and  612 . The first defibrillation electrode  508 , the second defibrillation electrode  512  and the housing  620  are also coupled to a cardioversion/defibrillation output circuit  650  to provide pulses of either cardioversion or defibrillation electrical energy to the terminals  610  or  608  and the housing  620  under the control of the microprocessor  604 . Power to the ICD  600  is supplied by an electrochemical battery  654  that is housed within the ICD  600 . 
   Sense amplifier  640  is coupled to the control circuitry  602 , and is electrically coupled to terminal  614  and  616  to sense a cardiac signal between the atrial electrode  538  and the housing  620 . In an alternative embodiment, a second atrial electrode (not shown) can be added to the atrial lead  536  and be coupled to sense amplifier  640  to allow for bipolar sensing and pacing. The output of the sense amplifier  640  is connected to an atrial depolarization detector  646  which is adapted to detect the occurrence of atrial depolarizations in a second cardiac signal. In one embodiment, these components serve to sense the second cardiac signal and to amplify the atrial depolarizations, for example by sensing atrial P-waves, and apply signals indicative thereof to microprocessor  604 . Among other things, the microprocessor  604  can respond to the atrial depolarization detector  646  by providing pacing signals to the pace output circuit  632  via bus  634 , as needed according to the programmed pacing mode. Pace output circuit  632  provides output pacing signals to terminals  614  and  616 . 
   The control circuitry  602  further includes a cardiac data analyzing circuit  660 , which is coupled to the ventricular depolarization detector circuit  630 , the atrial depolarization detector circuit  646 , the microprocessor  604  and the memory circuit  606  via bus  634 . In one embodiment, the cardiac data analyzing circuit  660  analyzes ventricular depolarizations for the occurrence of a tachycardia episode. When a tachycardia episode is detected, the cardiac data analyzing circuit  660  analyzes the ventricular depolarizations in the first cardiac signal and the atrial depolarizations in the second cardiac signal to determine whether a one-to-one association of atrial depolarizations to ventricular depolarizations exists. 
   When a tachycardia episode having a one-to-one association of atrial depolarizations to ventricular depolarizations is detected, a cycle length interval circuit  664 , coupled to the cardiac data analyzing circuit  660 , is used to calculate both the first intervals between detected ventricular depolarizations in the first cardiac signal and first predetermined cardiac events and the second intervals between detected atrial depolarizations in the second cardiac signal and second predetermined cardiac events. In one embodiment, the cycle length interval circuit  664  locates the predetermined points on the sensed cardiac complexes from which the interval measurements are made. In one embodiment, the predetermined points are repeatably identifiable portions of the cardiac complex in the cardiac signal which have been programmed into the memory  606  of the ICD  600  for use in the cycle length interval circuit  664 . 
   Once the intervals have been measured, the microprocessor  604  determines a first characteristic for a first predetermined series of the first intervals and a second characteristic for a second predetermined series the second intervals measured by the cycle length interval circuit. The microprocessor  604  then classifies the tachycardia episode as either occurring in an antegrade direction or a retrograde direction based on the first characteristic and the second characteristic. 
   In one embodiment, the microprocessor  604  calculates variance values (e.g., σ 2 (x), σ 2 (y)) from the intervals measured by the cycle length interval circuit  664 . The microprocessor  604  then calculates the variance values from predetermined series of measured intervals. For example, the microprocessor  604  is programmed to calculate the first variance value from the first predetermined series of the first intervals and to calculate the second variance value from the second predetermined series of the second intervals. The microprocessor  604  then compares the variance values and classifies the tachycardia episode as either occurring in an antegrade direction or a retrograde direction based on the values of the first variance and the second variance. In one embodiment, the microprocessor  604  classifies the tachycardia episode as occurring in an antegrade direction when the second variance is less than or equal to the first variance. Alternatively, the microprocessor  604  classifies the tachycardia episode as occurring in a retrograde direction when the second variance is greater than the first variance. 
   Electronic communication circuitry  668  is additionally coupled to the control circuitry  602  to allow the ICD  600  to communicate with an external controller  670 . In one embodiment, the electronic communication circuitry  668  includes a data receiver and a data transmitter to send and receive and transmit signals and cardiac data to and from an external programmer  670 . In one embodiment, the data receiver and the data transmitter include a wire loop antenna  672  to establish a radio frequency telemetric link, as is known in the art, to receive and transmit signals and data to and from the programmer unit  670 .