Abstract:
This document discusses, among other things, systems, devices, and methods for detecting or classifying tachyarrhythmias or making a therapy decision. In one example, a rate-dependent threshold is used for comparing atrial and ventricular rates for classifying a tachyarrhythmia as a ventricular tachyarrhythmia (VT) or a supraventricular tachyarrhythmia (SVT). In another example, the classification uses an atrial rate cutoff value, a ventricular rate cutoff value, or both. In another example, a tachyarrhythmia detection is tested over a time window with a duration that is automatically adjusted as a substantially continuously monotonically decreasing function of duration vs. rate. These techniques improve the specificity of arrhythmia detection or classification, allow anti-tachyarrhythmia therapy to be better tailored to the particular tachyarrhythmia, or provide more automatic operation making it easier for a physician to use an implantable device.

Description:
TECHNICAL FIELD 
   This patent application pertains generally to cardiac rhythm management and more particularly, but not by way of limitation, to systems, devices, and methods for discriminating between ventricular and supraventricular tachyarrhythmias. 
   BACKGROUND 
   Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a patient. For example, a tachyarrhythmia includes a too-fast heart rhythm. A tachyarrhythmia may be caused by an improper positive-feedback-like reentry of intrinsic electrical signals that control heart contractions. A tachyarrhythmia may result in inefficient pumping of blood. Fibrillation is a particularly severe tachyarrhythmic episode. While ventricular fibrillation (“VF”) can have immediate life-threatening consequences, the adverse effects of atrial fibrillation (“AF”) are typically less immediate or severe. Atrial tachyarrhythmias (i.e., “AT”s, including AF) may call for a different therapy than ventricular tachyarrhythmias (i.e., “VT”s). For example, a VF may call for delivering a painful defibrillation shock to interrupt the VF, while an AF may call for delivering a painless anti-tachyarrhythmia pacing to interrupt the AF. Therefore, to promote efficacy or patient comfort, it is useful to know whether a particular tachyarrhythmia originates in the ventricle (i.e., is a VT) or above the ventricle (i.e., is a supraventricular tachyarrhythmia (“SVT”), such as an AT). 
   However, it is sometimes difficult to know where the tachyarrhythmia originates. A SVT may conduct its too-fast heart rhythm through the atrioventricular (AV) node to the ventricle, resulting in a fast ventricular heart rate. Similarly, a VT may exhibit retrograde conduction of its too-fast heart rhythm back to the atrium, resulting in a fast atrial heart rate. Thus, discriminating between the different origins of VTs and SVTs may not be an easy task. Accomplishing this VT/SVT discrimination task may require a physician to program a complicated set of parameters to achieve the intended result. The present inventors have recognized an unmet need for automatically or otherwise providing improved sensitivity and specificity of discriminating between VTs and SVTs, such as to avoid unneeded defibrillation shocks and to more effectively treat the particular tachyarrhythmia. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
       FIG. 1  is a graph of data illustrating tachyarrhythmia episodes from different patients. 
       FIG. 2  is a graph illustrating conceptually one example of using a fixed rate threshold for comparing atrial and ventricular rates, such as for classifying a tachyarrhythmia as a ventricular tachyarrhythmia. 
       FIG. 3  is a graph illustrating a rate-dependent comparison threshold, such as illustrated by a bilinear, piecewise linear, curvilinear, or other nonlinear threshold boundary. 
       FIG. 4  is a graph that illustrates an alternative example in which a bilinear threshold boundary includes a line segment that has a slope that is less than 0.5, and line segment that has a slope that is greater than 0.5. 
       FIG. 5  is a graph illustrating an alternative example of a bilinear threshold boundary comprising a lower rate line segment, and a higher rate line segment that has substantially infinite slope, such as to implement an atrial rate cutoff value. 
       FIG. 6  is a graph illustrating an alternative example in which the rate-dependent threshold boundary is piecewise linear, such as by including more than two line segments. 
       FIG. 7  is a graph illustrating an alternative example in which the rate dependent threshold boundary is curvilinear. 
       FIG. 8  is a graph illustrating an alternative example in which the rate dependent threshold boundary implements both an atrial rate cutoff and a ventricular rate cutoff. 
       FIG. 9  is a graph illustrating an example of a rate dependent VT threshold boundary that is separate or different from the rate dependent SVT threshold boundary. 
       FIG. 10  is a block diagram illustrating generally one example of a system providing VT/SVT discrimination. 
       FIG. 11  is a graph of a duration interval function, in which the y-axis represents the value of the duration interval and the x-axis represents a ventricular rate. 
       FIG. 12  is a flow chart illustrating generally one example of using at least one rate-dependent tachyarrhythmia detection criterion. 
       FIG. 13  is a flow chart illustrating generally one example of tachyarrhythmia classification. 
       FIG. 14  is a flow chart illustrating generally one example of a technique of classifying a tachyarrhythmia using a rate cutoff value. 
       FIG. 15  is a flow chart illustrating generally an example of a technique of classifying a tachyarrhythmia using ventricular and atrial rate cutoff values. 
   

   DETAILED DESCRIPTION 
   This detailed description includes references to accompanying drawings, which form a part of the detailed description. The drawings illustrate specific embodiments of practicing the invention. These embodiments, which are also referred to herein as “examples,” are described in enough detail to enable those skilled in the art to practice the invention. The embodiments may be combined, other embodiments may be utilized, or structural, logical, and electrical changes may be made without departing from the 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. 
   In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one. In this document, the term “or” is used to refer to a nonexclusive or, unless otherwise indicated. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     FIG. 1  is a graph  100  of data illustrating tachyarrhythmia episodes from different patients, as collected and analyzed by the present inventors. The graph of  FIG. 1  includes a y-axis  102  that illustrates ventricular rate (in beats per minute), and an x-axis  104  that illustrates atrial rate (in beats per minute). In  FIG. 1 , each VT (including VF) episode is indicated by a bullet (•) and each SVT (including AF) episode is indicated by a plus (+). Each polymorphic VT episode is illustrated by a box (?) around the corresponding bullet. A VT episode indicated by a bullet without a corresponding box is a monomorphic VT episode. A monomorphic VT episode has a more regular morphology (i.e., shape) of intrinsic heart signal than a polymorphic VT episode. A monomorphic VT episode may call for a different anti-tachyarrhythmia therapy than a polymorphic VT episode. 
   In  FIG. 1 , a line with a slope of +0.5 and intersecting the (extrapolated) y-axis  102  at y=0 defines an atrial rate (“AR”) that is equal to a ventricular rate (“VR”). As seen in  FIG. 1 , most VT episodes correspond to VR&gt;AR. Similarly, most SVT episodes correspond to AR&gt;VR. Therefore, one way to distinguish between a VT episode and an SVT episode in an implantable medical device is to include an algorithm that compares AR and VR. If VR&gt;AR by a desired threshold value (e.g., 10 bpm), then the algorithm deems the detected arrhythmia to be a VT. In  FIG. 1 , this corresponds to episodes to the left of line  106 . In this example, if AR&gt;VR by the same or a different threshold value (e.g., 10 bpm), then the algorithm deems the detected arrhythmia to be an SVT. In  FIG. 1 , this corresponds to episodes to the right of line  108 . If both desired threshold values are set to zero, this reduces to classifying episodes to the left of the AR=VR line as VTs and classifying the episodes to the right of the AR=VR line as SVTs. If desired, anti-tachyarrhythmia therapy can be tailored to the particular tachyarrhythmia using this information, and delivered to the patient. 
   However, in  FIG. 1 , there are fewer VTs at lower ventricular rates, such as in region  110 , than at higher ventricular rates. Also, in  FIG. 1 , there are fewer VTs, and more SVTs where the atrial rate exceeds an atrial rate cutoff value (e.g., at an AR that is somewhere between about 100 bpm and 200 bpm), such as in region  112 . Among other things, the present inventors have recognized that using a substantially larger fixed threshold for the comparison (e.g., VR&gt;&gt;AR by a fixed threshold value of at least about 40 bpm to about 60 bpm, instead of the 10 bpm depicted by the line  106  in  FIG. 1 ) would improve the specificity of classifying a tachyarrhythmia as VT. This is illustrated by the boundary line  200  in the graph of  FIG. 2 . The threshold value for the comparison is shown as the distance between the boundary line  200  in  FIG. 2  and a VR=AR line  202  having slope=0.5 and y-intercept=0. 
   Moreover, the present inventors have recognized that instead of using a fixed threshold for comparing AR and VR (e.g., a threshold represented on  FIG. 1  by a boundary line having a fixed distance from an AR=VR line  107  having slope of 0.5 and y-intercept of 0), using an atrial or ventricular rate dependent or other variable threshold may add power to the VT/SVT discrimination and classification algorithm, thereby improving its sensitivity or specificity. 
     FIG. 3  is a graph illustrating a rate-dependent comparison threshold, such as illustrated by a bilinear, piecewise linear, curvilinear, or other nonlinear threshold boundary  300  in the context of the graph of  FIG. 3 . (In examples illustrated in graphs such as shown in  FIG. 3 , the actual threshold value for comparing AR and VR is the distance between the threshold boundary  300  and the AR=VR line  301  illustrated in  FIG. 3 .) 
   In the example of  FIG. 3 , the threshold boundary  300  is such that, for an arrhythmia to be classified as a VT, VR must exceed AR by a greater threshold amount at lower values of VR than at higher values of VR. In other words, the distance between the threshold boundary  300  and the AR=VR line  301  is greater at lower values of VR than at higher values of VR. Similarly, the distance between the threshold boundary  300  and the AR=VR line  301  is greater at lower values of AR than at higher values of AR. 
   The example of  FIG. 3  depicts a bilinear threshold boundary  300 , formed by the line segments  302  and  304 , which are joined at breakpoint  306 . In this example, the line segment  302 , at lower values of VR and AR, is rate dependent (because its slope is not equal to 0.5) and the line segment  304 , at higher values of VR and AR is rate independent (because its slope is equal to 0.5). Therefore, in its entirety, the threshold boundary  300  can be considered rate dependent because at least a portion of it (i.e., line segment  302 ) is rate dependent. 
   In the example of  FIG. 3 , the breakpoint  306  is located at about VR=180 bpm and AR=170 bpm, however,  FIG. 3  is merely exemplary and is drawn to emphasize the conceptual nature of the rate dependent threshold as represented by a threshold boundary. The exact location of the breakpoint  306  or the slope of line segment  302  is typically determined using data (such as shown in  FIG. 1 ) along with a desired specificity of classifying the arrhythmia as a VT. Moreover, the line segment  304  need not be rate independent (e.g., slope=0.5), but may also be rate dependent.  FIG. 4  is a graph that illustrates an alternative example in which a bilinear threshold boundary  400  includes a line segment  402  that has a slope that is less than 0.5, and line segment  404  that has a slope that is greater than 0.5. 
     FIG. 5  is a graph illustrating an alternative example of a bilinear threshold boundary  500  comprising a lower rate line segment  502  and a higher rate line segment  504 . In this example, the higher rate line segment  504  has substantially infinite slope, as illustrated in  FIG. 5 . This effectively implements an atrial rate cutoff value, such as by extrapolating the line segment  504  to the corresponding atrial rate on the x-axis  104 . In this example, an arrhythmia occurring at an observed AR greater than the atrial rate cutoff value (e.g., about 110 bpm, in the example illustrated in  FIG. 4 ) will not be classified as a VT, regardless of the VR value observed during that arrhythmia. Although the line segment  502  is shown in  FIG. 5  as being rate independent (i.e., slope=0.5), it could also be made rate dependent (for example, slope less than 0.5). 
     FIG. 6  is a graph illustrating an alternative example in which the rate-dependent threshold boundary  600  is piecewise linear, such as by including more than two line segments. In the example of  FIG. 6 , the rate dependent threshold boundary  600  includes three line segments  602 ,  604 , and  606 , having slopes of 0, 0.5, and 8, respectively, although other slopes or breakpoints are also contemplated. 
     FIG. 7  is a graph illustrating an alternative example in which the rate dependent threshold boundary  700  is not piecewise linear, but is instead curvilinear. 
     FIG. 8  is a graph illustrating an alternative example in which the rate dependent threshold boundary  800  implements both an atrial rate cutoff (AR co )  802  and a ventricular rate cutoff (VR co )  804 . In this example, the ventricular rate cutoff has priority over the atrial rate cutoff. That is, if the tachyarrhythmia is observed at a VR that exceeds the ventricular rate cutoff  804 , then the tachyarrhythmia is classified as a VT regardless of the AR. Otherwise, if tachyarrhythmia is observed at an AR that exceeds the atrial rate cutoff  802 , the tachyarrhythmia is classified as an SVT regardless of the VR. Otherwise, the tachyarrhythmia is classified as a VT if the VR exceeds the AR by the threshold value (i.e., by the distance between the threshold boundary and the AR=VR line). 
   Although the above examples have been discussed with respect to classifying a tachyarrhythmia as a VT, similar examples also apply to classifying a tachyarrhythmia as SVT. In one example, the above described techniques may classify as an SVT any tacharrhythmia that is not deemed a VT. In another example, however, the SVT classification uses a separate test. That separate test may be individually tailored to classify the SVT with greater specificity than would be the case if a single test were used to classify a detected arrhythmia as a VT or an SVT. 
     FIG. 9  is a graph illustrating an example of a rate dependent VT threshold boundary  900  that is separate or different from the rate dependent SVT threshold boundary  902 . Because using separate boundaries may result in one or more indeterminate regions (either because the tachyarrhythmia is not classified as either a VT or an SVT, or because the tachyarrhythmia is classified as both a VT or SVT), it may be desirable to use the rate dependent threshold techniques described in this document together with one or more other VT/SVT discrimination techniques. Examples of other VT/SVT discrimination techniques include, by way of example, but not by way of limitation, stability, onset, vector timing, or correlation. The particular classification may be made by weighting or otherwise combining the results of more than one discrimination technique, either for the case of separate VT and SVT threshold boundaries as shown in  FIG. 9 , or for the other examples such as illustrated in  FIGS. 1–8 . Moreover, the examples shown in  FIGS. 1–9  or elsewhere in this document can be used in combination with each other, or in combination with other VT/SVT discrimination techniques. 
     FIG. 10  is a block diagram illustrating generally one example of a system  1000  providing the VT/SVT discrimination techniques described above. In  FIG. 10 , the system  1000  includes a cardiac rhythm management (CRM) or other implantable device  1002 , which may be accompanied by an external transceiver  1004  of an external programmer, a repeater, or other communication device. The implantable device  1002  is coupled to a patient&#39;s heart  1006 , such as by one or more intravascular or other leads carrying electrodes or the like for sensing heart signals or providing anti-tachyarrhythmia or other therapy to the heart  1006 . 
   In the example of  FIG. 10 , the implantable device  1002  includes an atrial heart contraction detector circuit  1008 A and a ventricular heart contraction detector circuit  1008 B. The heart contraction detector circuits  1008 A–B detect heart contractions associated with a respective atrium or ventricle of the heart  1006 , such as by sensing the intrinsic electrical heart signals from the heart chamber or by detecting triggering signals from contraction-evoking pulses delivered by a therapy circuit  1010  to the heart chamber. 
   The atrial contraction detector circuit  1008 A includes a sense amplifier  1012 A providing an output signal representative of the intrinsic atrial heart signal. This output signal includes electrical depolarizations (called “P-waves”) representing successive atrial heart contractions. The output signal is received by an atrial rate detector circuit  1014 A, which measures a time between successive atrial heart contractions to provide an output indication of the atrial rate (“AR”). 
   Similarly, the ventricular contraction detector circuit  1008 B includes a sense amplifier  1012 B providing an output signal representative of the intrinsic ventricular heart signal. This output signal includes electrical depolarizations (called “QRS-complexes”) representing successive ventricular heart contractions. The output signal is received by a ventricular rate detector circuit  1014 B, which measures a time between successive ventricular heart contractions to provide an output indication of the ventricular rate (“VR”). 
   The therapy delivery circuit  1010  typically includes one or more of: a pace pulse delivery circuit, an anti-tachyarrhythmia therapy circuit, a cardiac resynchronization therapy circuit, a cardiac contractility modulation (CCM) circuit, or any other therapy delivery circuit. The anti-tachyarrhythmia therapy circuit typically includes at least one defibrillation circuit or anti-tachyarrhythmia pacing (ATP) circuit or the like. 
   In the example of  FIG. 10 , the implantable device  1002  also includes a transceiver  1016  for wirelessly communicating with the external transceiver  1004 . The implantable device  1002  also includes a processor  1018 . The processor  1018  is coupled to the other circuits of the implantable device  1002  by at least one bus  1020  or the like. The processor  1018  is implemented as any controller or other circuit that is capable of sequencing through various control states such as, for example, by using a digital microprocessor having executable instructions stored in an associated instruction memory circuit, a microsequencer, or a state machine. 
   In the example of  FIG. 10 , the processor  1018  includes a tachyarrhythmia detection circuit  1022 . The tachyarrhythmia detection circuit  1022  processes signals received from the atrial contraction detector circuit  1008 A or the ventricular contraction detector circuit  1008 B. In response, the tachyarrhythmia detection circuit  1022  provides one or more indications  1024 A–N that a tachyarrhythmia is present. As one illustrative example, an a first indication  1024 A (sometimes referred to as an “Onset” indication) deems three consecutive “fast” (for example, at a rate greater than about 165 bpm) intervals between contractions of the same heart chamber as providing a first indication  1024  of an onset of a tachyarrhythmia. 
   In this same example, if the first indication  1024 A indicates an onset of a tachyarrhythmia, then this triggers a second test for a second indication  1024 N (sometimes referred to as a “Duration” indication). This second test looks for the presence of three of ten fast intervals occuring during a time period referred to as the “duration” period. If this condition is met, then the second indication  1024 N of a tachyarrhythmia is also present. In this way, a desired number of tachyarrhythmia indications can be used conjunctively to increase the specificity of a tachyarrhythmia detection before anti-tachyarrhythmia therapy is delivered. 
   The example of  FIG. 10  also includes a tachyarrhythmia classification circuit  1026 . In one example, the tachyarrhythmia classification circuit  1026  performs the VT/SVT discrimination, such as discussed above. Therefore, in one example, the tachyarrhythmia classification circuit  1026  includes a rate-dependent threshold  1028  (such as discussed above). The rate-dependent threshold  1028  is provided to a comparator  1030  that compares atrial and ventricular rates, using the rate-dependent threshold, to classify the tachyarrhythmia as VT or SVT. The rate-dependent threshold  1028  can be stored in one or more memory locations in various different forms, such as an equation, a lookup table, or in any other desired form. 
   The comparator  1030  compares the atrial rate and the ventricular rate using the rate-dependent threshold  1028 . In one example of classifying a tachyarrhythmia as VT, the tachyarrhythmia classification circuit uses a ventricular rate (or atrial rate) received from the ventricular rate detector circuit  1014 B (or the atrial rate detector  1014 A) as an index into a rate-dependent function that yields a threshold value for comparing AR and VR. If VR exceeds AR by at least the threshold value, then the tachyarrhythmia classification circuit deems the tachyarrhythmia to be a VT instead of an SVT. 
   In the example of  FIG. 10 , the processor  1018  also includes a therapy triggering circuit  1032  that triggers an appropriate anti-tachyarrhythmia therapy in response to the tachyarrhythmia detection indication(s) from the tachyarrhythmia detection circuit  1022  and the tachyarrhythmia classification from the tachyarrhythmia classification circuit  1026 . As an illustrative example, a detected tachyarrhythmia that is classified as VT may be treated with a defibrillation shock, while a detected tachyarrhythmia that is classified as an SVT may be treated by an anti-tachyarrhythmia pacing (ATP) pulse sequence. In general, there may be many different therapy responses, with the particular therapy response depending on the tachyarrhythmia classification or the particular tachyarrhythmia detection indication(s) that are present. 
   In one example, at least one of the tachyarrhythmia detection indications  1024 A–N is rate-dependent. In one example, the “duration” time interval discussed above is also rate dependent, as illustrated conceptually in  FIG. 11 .  FIG. 11  is a graph of a duration interval function  1100 , in which the y-axis  1102  represents the value of the duration interval and the x-axis  1104  represents a ventricular rate. In this example, the duration interval function  1100  automatically substantially continuously decreases monotonically with increasing ventricular rate. In the example illustrated in  FIG. 11 , a test for “X” of “Y” fast intervals is carried out over a shorter duration interval period at a higher ventricular rate than for a lower ventricular rate. The actual numbers for “X” and “Y” may also typically vary as a function of the ventricular rate. The example discussed earlier tested for X=6 of Y=10 fast intervals occurring during a duration period (e.g., 2.5 seconds). In one rate-dependent duration interval period example, this duration period corresponds to a VR=160 bpm. As one illustrative example, at a lower VR=130, a duration period of about 5 seconds is used, and the corresponding tachyarrhythmia detection test looks for X=12 of Y=20 fast R—R intervals between successive ventricular contractions. Continuing with this illustrative example, at a higher VR=240, a duration period of about 1 second is used, and the corresponding tachyarrhythmia detection test looks for X=3 of Y=5 fast R—R intervals. These values are provided for illustrative purposes only, the exact values may be programmed as desired. In one example, such programming is performed by the manufacturer, so that the physician need not program various durations corresponding to various ventricular rates. Such automaticity increases the ease of use of the implantable device  1002 . 
     FIG. 12  is a flow chart illustrating generally one example of using at least one rate-dependent tachyarrhythmia detection criterion. In the example of  FIG. 12 , at  1200 , heart contractions and heart rate are detected. In one example this includes detecting ventricular heart contractions and ventricular heart rate. At  1202 , a first test is performed to determine if a tachyarrhythmia is present. In one illustrative example, if three consecutive fast intervals between successive ventricular contractions is detected, an “onset” of a tachyarrhythmia is deemed present, and process flow continues at  1204 ; otherwise process flow returns to  1200 . At  1204 , a “duration period” parameter of a second tachyarrhythmia detection test corresponding to a particular heart rate is established. In one example, a substantially continuously decreasing duration vs. ventricular rate function, as illustrated in  FIG. 11 , is used to automatically set the duration period at  1204 . At  1206 , a second test is performed to confirm that the tachyarrhythmia is present. In one illustrative example, if three of ten fast intervals (intervals shorter than a threshold interval value) between successive ventricular contractions are detected during the duration period that was selected using the ventricular rate, then the tachyarrhythmia is deemed to be present. In a further example, the second test determines if X of Y fast intervals is present during the automatically selected duration period, where X or Y is also selected using the rate. If the second test deems a tachyarrhythmia to be present, then process flow continues to  1208 , and anti-tachyarrhythmia therapy is delivered to the heart. 
   In the above example, the rate-dependent duration period can alternatively be used as a single tachyarrhythmia detection test (e.g., without a first tachyarrhythmia detection criterion, such as the onset), or could be used in conjunction with one or more additional tachyarrhythmia detection criteria. Also, the above example could also be used in conjunction with a tachyarrhythmia classification before anti-tachyarrhythmia therapy is delivered. This permits the particular anti-tachyarrhythmia therapy to be tailored using the classification or the tachyarrhythmia detection indication(s). The rate-dependent duration period can be used with a rate-dependent threshold for arrhythmia detection, as discussed above, or with a rate-independent threshold for arrhythmia detection, if desired. 
     FIG. 13  is a flow chart illustrating generally one example of tachyarrhythmia classification. In the example of  FIG. 13 , at  1300 , atrial and ventricular contractions and rate are detected. At  1300 , atrial and ventricular contractions and corresponding rates are detected. At  1302 , a tachyarrhythmia is detected, such as by using one or more tachyarrhythmia detection criteria (e.g., onset test, duration test, etc.), examples of which are discussed above. At  1304 , atrial rate and ventricular rate are compared using a bilinear, piecewise linear, curvilinear or other rate-dependent threshold, as discussed above. The particular threshold value used for the comparison is selected using one of ventricular rate or atrial rate. At  1306 , if VR exceeds AR by the threshold value corresponding to the observed heart rate, then at  1308 , the tachyarrhythmia is classified as a VT. Otherwise, at  1310 , the tachyarrhythmia is either classified as an SVT, or a separate SVT classification routine is initiated at  1310 . In one example, after the classification is made, an anti-tachyarrhythmia therapy is then delivered. In another example, after the classification is made, one or more classification-specific tachyarrhythmia detection criteria is then applied to further enhance the specificity of the detection. In yet a further example, the anti-tachyarrhythmia therapy is tailored using one of the classification or the tachyarrhythmia detection criteria. 
     FIG. 14  is a flow chart illustrating generally one example of a technique of classifying a tachyarrhythmia using a rate cutoff value. In the example of  FIG. 14 , at  1400 , atrial and ventricular contractions and rates are detected. At  1402 , a tachyarrhythmia is detected using one or more tachyarrhythmia detection indications. In one example, at least one of these tachyarrhythmia detection indications uses a rate-dependent duration period, as discussed above. At  1404 , the atrial rate is compared to a cutoff value. At  1406 , if the atrial rate exceeds the cutoff value, then, at  1408 , the detected arrhythmia is deemed not a VT. Otherwise, at  1410 , atrial rate and ventricular rates are compared. In one example, this comparison includes using a bilinear, piecewise linear, curvilinear, or other rate-dependent threshold value. In another example, this comparison includes using a rate-independent threshold value. At  1412 , if the ventricular rate exceeds the atrial rate by at least the threshold value, then, at  1414 , the tachyarrhythmia is classified as a VT. Otherwise, at  1416 , the tachyarrhythmia is classified as not VT. In one example, after the classification is made, an anti-tachyarrhythmia therapy is then delivered. In another example, after the classification is made, one or more classification-specific tachyarrhythmia detection criteria is then applied to further enhance the specificity of the detection. In yet a further example, the anti-tachyarrhythmia therapy is tailored using one of the classification or the tachyarrhythmia detection criteria. Alternatively, the example illustrated in  FIG. 14  is used to implement a ventricular rate cutoff instead of an atrial rate cutoff, in which a ventricular rate exceeding the corresponding ventricular rate cutoff results in the detected tachyarrhythmia being classified as a VT. 
     FIG. 15  is a flow chart illustrating generally an example of a technique of classifying a tachyarrhythmia using ventricular and atrial rate cutoff values. In the example of  FIG. 15 , at  1500 , atrial and ventricular contractions and rates are detected. At  1502 , a tachyarrhythmia is detected using one or more tachyarrhythmia detection indications. In one example, at least one such tachyarrhythmia detection indication uses a rate-dependent duration period, as discussed above. At  1504 , the ventricular rate is compared to a cutoff value. At  1506 , if the ventricular rate exceeds the cutoff value, then, at  1508 , the detected arrhythmia is deemed a VT. Otherwise, at  1510 , the atrial rate is compared to a cutoff value. At  1512 , if the atrial rate exceeds the cutoff value, then, at  1514 , the detected arrhythmia is deemed not a VT. Otherwise, at  1516 , atrial rate and ventricular rates are compared. In one example, this comparison includes using a bilinear, piecewise linear, curvilinear, or other rate-dependent threshold value, as discussed above. In another example, this comparison includes using a rate-independent threshold value. At  1518 , if the ventricular rate exceeds the atrial rate by at least the threshold value, then, at  1520 , the tachyarrhythmia is classified as a VT. Otherwise, at  1522 , the tachyarrhythmia is classified as not VT. In one example, after the classification is made, an anti-tachyarrhythmia therapy is then delivered. In another example, after the classification is made, one or more classification-specific tachyarrhythmia detection criteria is then applied to further enhance the specificity of the detection. In yet a further example, the anti-tachyarrhythmia therapy is tailored using one of the classification or the tachyarrhythmia detection criteria. 
   The above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.