Patent Publication Number: US-2021186406-A1

Title: Atrial arrhythmia episode detection in a cardiac medical device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 16/291,349, Filed Mar. 4, 2019, which is a Continuation of U.S. patent application Ser. No. 14/520,938, (now U.S. Pat. No. 10,219,718), filed Oct. 22, 2014, the content of which is incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to cardiac medical devices and, in particular, to methods for detecting atrial arrhythmia episodes during ventricular pacing in a cardiac medical device. 
     BACKGROUND 
     During normal sinus rhythm (NSR), the heart beat is regulated by electrical signals produced by the sino-atrial (SA) node located in the right atrial wall. Each atrial depolarization signal produced by the SA node spreads across the atria, causing the depolarization and contraction of the atria, and arrives at the atrioventricular (A-V) node. The A-V node responds by propagating a ventricular depolarization signal through the bundle of His of the ventricular septum and thereafter to the bundle branches and the Purkinje muscle fibers of the right and left ventricles. 
     Atrial tachyarrhythmia includes the disorganized form of atrial fibrillation and varying degrees of organized atrial tachycardia, including atrial flutter. Atrial fibrillation (AF) occurs because of multiple focal triggers in the atrium or because of changes in the substrate of the atrium causing heterogeneities in conduction through different regions of the atria. The ectopic triggers can originate anywhere in the left or right atrium or pulmonary veins. The AV node will be bombarded by frequent and irregular atrial activations but will only conduct a depolarization signal when the AV node is not refractory. The ventricular cycle lengths will be irregular and will depend on the different states of refractoriness of the AV-node. 
     As more serious consequences of persistent atrial arrhythmias have come to be understood, such as an associated risk of relatively more serious ventricular arrhythmias and stroke, there is a growing interest in monitoring and treating atrial arrhythmias. 
     Methods for discriminating arrhythmias that are atrial in origin from arrhythmias originating in the ventricles have been developed for use in dual chamber implantable devices wherein both an atrial EGM signal and a ventricular EGM signal are available. Discrimination of arrhythmias can rely on event intervals (PP intervals and RR intervals), event patterns, and EGM morphology. Such methods have been shown to reliably discriminate ventricular arrhythmias from supra-ventricular arrhythmias. In addition, such methods have been developed for use in single chamber implantable devices, subcutaneous implantable devices, and external monitoring devices, where an adequate atrial EGM signal having acceptable signal-to-noise ratio is not always available for use in detecting and discriminating atrial arrhythmias. However, such single chamber devices have been designed to monitor AF during non-paced ventricular rhythm. What is needed, therefore, is a method for monitoring atrial arrhythmias during a ventricular paced rhythm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary medical device for detecting arrhythmia during ventricular pacing according to an embodiment of the present disclosure. 
         FIG. 2  is a functional block diagram of an IMD according to one embodiment. 
         FIG. 3  is flowchart of a method for detecting atrial arrhythmias during ventricular pacing in a cardiac medical device according to an embodiment of the present disclosure. 
         FIG. 4  is a schematic diagram illustrating detecting atrial arrhythmias during ventricular pacing in a cardiac medical device according to an embodiment of the present disclosure. 
         FIG. 5  is a schematic diagram illustrating detecting atrial arrhythmias during ventricular pacing in a cardiac medical device according to another embodiment of the present disclosure. 
         FIG. 6  is a schematic diagram of classifying of cardiac events in a cardiac medical device according to an embodiment of the present disclosure. 
         FIG. 7  is a diagram of an exemplary two-dimensional histogram representing a Lorenz plot area for identifying cardiac events. 
         FIG. 8  is a flowchart of classification of an arrhythmia according to an embodiment of the disclosure. 
         FIG. 9  is a schematic diagram of determination of a cardiac event, according to the present disclosure. 
         FIG. 10  is a flowchart of determination of a cardiac episode, according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, references are made to illustrative embodiments for carrying out the methods described herein. It is understood that other embodiments may be utilized without departing from the scope of the disclosure. 
     In various embodiments, ventricular signals are used for determining successive ventricular cycle lengths for use in detecting atrial arrhythmias. The atrial arrhythmia detection methods do not require an atrial signal source. The methods presented herein may be embodied in software, hardware or firmware in implantable or external medical devices. Such devices include implantable monitoring devices having cardiac EGM/ECG monitoring capabilities and associated EGM/ECG sense electrodes, which may be intracardiac, epicardial, or subcutaneous electrodes. 
     The methods described herein can also be incorporated in implantable medical devices having therapy delivery capabilities, such as single chamber or bi-ventricular pacing systems or ICDs that sense the R-waves in the ventricles and deliver an electrical stimulation therapy to the ventricles. The atrial arrhythmia detection methods presently disclosed may also be incorporated in external monitors having ECG electrodes coupled to the patient&#39;s skin to detect R-waves, e.g. Holter monitors, or within computerized systems that analyze pre-recorded ECG or EGM data. Embodiments may further be implemented in a patient monitoring system, such as a centralized computer system which processes data sent to it by implantable or wearable monitoring devices. 
       FIG. 1  is a schematic diagram of an exemplary medical device for detecting arrhythmia during ventricular pacing according to an embodiment of the present disclosure. As illustrated in  FIG. 1 , a medical device according to an embodiment of the present disclosure may be in the form of an implantable cardioverter defibrillator (ICD)  10  a connector block  12  that receives the proximal ends of a right ventricular lead  16 , a right atrial lead  15  and a coronary sinus lead  6 , used for positioning electrodes for sensing and stimulation in three or four heart chambers. Right ventricular lead  16  is positioned such that its distal end is in the right ventricle for sensing right ventricular cardiac signals and delivering pacing or shocking pulses in the right ventricle. For these purposes, right ventricular lead  16  is equipped with a ring electrode  24 , an extendable helix electrode  26  mounted retractably within an electrode head  28 , and a coil electrode  20 , each of which are connected to an insulated conductor within the body of lead  16 . The proximal end of the insulated conductors are coupled to corresponding connectors carried by bifurcated connector  14  at the proximal end of lead  16  for providing electrical connection to the ICD  10 . It is understood that although the device illustrated in  FIG. 1  is a dual chamber device, other devices such as single chamber devices may be utilized to perform the technique of the present disclosure described herein. 
     The right atrial lead  15  is positioned such that its distal end is in the vicinity of the right atrium and the superior vena cava. Lead  15  is equipped with a ring electrode  21  and an extendable helix electrode  17 , mounted retractably within electrode head  19 , for sensing and pacing in the right atrium. Lead  15  is further equipped with a coil electrode  23  for delivering high-energy shock therapy. The ring electrode  21 , the helix electrode  17  and the coil electrode  23  are each connected to an insulated conductor with the body of the right atrial lead  15 . Each insulated conductor is coupled at its proximal end to a connector carried by bifurcated connector  13 . 
     The coronary sinus lead  6  is advanced within the vasculature of the left side of the heart via the coronary sinus and great cardiac vein. The coronary sinus lead  6  is shown in the embodiment of  FIG. 1  as having a defibrillation coil electrode  8  that may be used in combination with either the coil electrode  20  or the coil electrode  23  for delivering electrical shocks for cardioversion and defibrillation therapies. In other embodiments, coronary sinus lead  6  may also be equipped with a distal tip electrode and ring electrode for pacing and sensing functions in the left chambers of the heart. The coil electrode  8  is coupled to an insulated conductor within the body of lead  6 , which provides connection to the proximal connector  4 . 
     The electrodes  17  and  21  or  24  and  26  may be used as true bipolar pairs, commonly referred to as a “tip-to-ring” configuration. Further, electrode  17  and coil electrode  20  or electrode  24  and coil electrode  23  may be used as integrated bipolar pairs, commonly referred to as a “tip-to-coil” configuration. In accordance with the invention, ICD  10  may, for example, adjust the electrode configuration from a tip-to-ring configuration, e.g., true bipolar sensing, to a tip-to-coil configuration, e.g., integrated bipolar sensing, upon detection of oversensing in order to reduce the likelihood of future oversensing. In other words, the electrode polarities can be reselected in response to detection of oversensing in an effort to reduce susceptibility of oversensing. In some cases, electrodes  17 ,  21 ,  24 , and  26  may be used individually in a unipolar configuration with the device housing  11  serving as the indifferent electrode, commonly referred to as the “can” or “case” electrode. 
     The device housing  11  may also serve as a subcutaneous defibrillation electrode in combination with one or more of the defibrillation coil electrodes  8 ,  20  or  23  for defibrillation of the atria or ventricles. It is recognized that alternate lead systems may be substituted for the three lead system illustrated in  FIG. 1 . While a particular multi-chamber ICD and lead system is illustrated in  FIG. 1 , methodologies included in the present invention may adapted for use with any single chamber, dual chamber, or multi-chamber ICD or pacemaker system, subcutaneous implantable device, or other internal or external cardiac monitoring device. 
       FIG. 2  is a functional schematic diagram of the medical device of  FIG. 1 . This diagram should be taken as exemplary of the type of device with which the invention may be embodied and not as limiting. The disclosed embodiment shown in  FIG. 2  is a microprocessor-controlled device, but the methods of the present invention may also be practiced with other types of devices such as those employing dedicated digital circuitry. 
     With regard to the electrode system illustrated in  FIG. 1 , ICD  10  is provided with a number of connection terminals for achieving electrical connection to the leads  6 ,  15 , and  16  and their respective electrodes. A connection terminal  311  provides electrical connection to the housing  11  for use as the indifferent electrode during unipolar stimulation or sensing. The connection terminals  320 ,  313 , and  318  provide electrical connection to coil electrodes  20 ,  8  and  23  respectively. Each of these connection terminals  311 ,  320 ,  313 , and  318  are coupled to the high voltage output circuit  234  to facilitate the delivery of high energy shocking pulses to the heart using one or more of the coil electrodes  8 ,  20 , and  23  and optionally the housing  11 . 
     The connection terminals  317  and  321  provide electrical connection to the helix electrode  17  and the ring electrode  21  positioned in the right atrium. The connection terminals  317  and  321  are further coupled to an atrial sense amplifier  204  for sensing atrial signals such as P-waves. The connection terminals  326  and  324  provide electrical connection to the helix electrode  26  and the ring electrode  24  positioned in the right ventricle. The connection terminals  326  and  324  are further coupled to a ventricular sense amplifier  200  for sensing ventricular signals. 
     The atrial sense amplifier  204  and the ventricular sense amplifier  200  preferably take the form of automatic gain controlled amplifiers with adjustable sensitivity. In accordance with the invention, ICD  10  and, more specifically, microprocessor  224  automatically adjusts the sensitivity of atrial sense amplifier  204 , ventricular sense amplifier  200  or both in response to detection of oversensing in order to reduce the likelihood of oversensing. Ventricular sense amplifier  200  and atrial sense amplifier  204  operate in accordance with originally programmed sensing parameters for a plurality of cardiac cycles, and upon detecting oversensing, automatically provides the corrective action to avoid future oversensing. In this manner, the adjustments provided by ICD  10  to amplifiers  200  and  204  to avoid future oversensing are dynamic in nature. Particularly, microprocessor  224  increases a sensitivity value of the amplifiers, thus reducing the sensitivity, when oversensing is detected. Atrial sense amplifier  204  and ventricular sense amplifier  200  receive timing information from pacer timing and control circuitry  212 . 
     Specifically, atrial sense amplifier  204  and ventricular sense amplifier  200  receive blanking period input, e.g., ABLANK and VBLANK, respectively, which indicates the amount of time the electrodes are “turned off” in order to prevent saturation due to an applied pacing pulse or defibrillation shock. As will be described, the blanking periods of atrial sense amplifier  204  and ventricular sense amplifier  200  and, in turn, the blanking periods of sensing electrodes associated with the respective amplifiers may be automatically adjusted by ICD  10  to reduce the likelihood of oversensing. The general operation of the ventricular sense amplifier  200  and the atrial sense amplifier  204  may correspond to that disclosed in U.S. Pat. No. 5,117,824, by Keimel, et al., incorporated herein by reference in its entirety. Whenever a signal received by atrial sense amplifier  204  exceeds an atrial sensitivity, a signal is generated on the P-out signal line  206 . Whenever a signal received by the ventricular sense amplifier  200  exceeds a ventricular sensitivity, a signal is generated on the R-out signal line  202 . 
     Switch matrix  208  is used to select which of the available electrodes are coupled to a wide band amplifier  210  for use in digital signal analysis. Selection of the electrodes is controlled by the microprocessor  224  via data/address bus  218 . The selected electrode configuration may be varied as desired for the various sensing, pacing, cardioversion and defibrillation functions of the ICD  10 . Specifically, microprocessor  224  may modify the electrode configurations based on detection of oversensing due to cardiac or non-cardiac origins. Upon detection of R-wave oversensing, for example, microprocessor  224  may modify the electrode configuration of the right ventricle from true bipolar sensing, e.g., tip-to-ring, to integrated bipolar sensing, e.g., tip-to-coil. 
     Signals from the electrodes selected for coupling to bandpass amplifier  210  are provided to multiplexer  220 , and thereafter converted to multi-bit digital signals by A/D converter  222 , for storage in random access memory  226  under control of direct memory access circuit  228  via data/address bus  218 . Microprocessor  224  may employ digital signal analysis techniques to characterize the digitized signals stored in random access memory  226  to recognize and classify the patient&#39;s heart rhythm employing any of the numerous signal processing methodologies known in the art. An exemplary tachyarrhythmia recognition system is described in U.S. Pat. No. 5,545,186 issued to Olson et al, incorporated herein by reference in its entirety. 
     Upon detection of an arrhythmia, an episode of EGM data, along with sensed intervals and corresponding annotations of sensed events, are preferably stored in random access memory  226 . The EGM signals stored may be sensed from programmed near-field and/or far-field sensing electrode pairs. Typically, a near-field sensing electrode pair includes a tip electrode and a ring electrode located in the atrium or the ventricle, such as electrodes  17  and  21  or electrodes  26  and  24 . A far-field sensing electrode pair includes electrodes spaced further apart such as any of: the defibrillation coil electrodes  8 ,  20  or  23  with housing  11 ; a tip electrode  17  or  26  with housing  11 ; a tip electrode  17  or  26  with a defibrillation coil electrode  20  or  23 ; or atrial tip electrode  17  with ventricular ring electrode  24 . The use of near-field and far-field EGM sensing of arrhythmia episodes is described in U.S. Pat. No. 5,193,535, issued to Bardy, incorporated herein by reference in its entirety. Annotation of sensed events, which may be displayed and stored with EGM data, is described in U.S. Pat. 4,374,382 issued to Markowitz, incorporated herein by reference in its entirety. 
     The telemetry circuit  330  receives downlink telemetry from and sends uplink telemetry to an external programmer, as is conventional in implantable anti-arrhythmia devices, by means of an antenna  332 . Data to be uplinked to the programmer and control signals for the telemetry circuit are provided by microprocessor  224  via address/data bus  218 . EGM data that has been stored upon arrhythmia detection or as triggered by other monitoring algorithms may be uplinked to an external programmer using telemetry circuit  330 . Received telemetry is provided to microprocessor  224  via multiplexer  220 . Numerous types of telemetry systems known in the art for use in implantable devices may be used. 
     The remainder of the circuitry illustrated in  FIG. 2  is an exemplary embodiment of circuitry dedicated to providing cardiac pacing, cardioversion and defibrillation therapies. The pacer timing and control circuitry  212  includes programmable digital counters which control the basic time intervals associated with various single, dual or multi-chamber pacing modes or anti-tachycardia pacing therapies delivered in the atria or ventricles. Pacer circuitry  212  also determines the amplitude of the cardiac pacing pulses under the control of microprocessor  224 . 
     During pacing, escape interval counters within pacer timing and control circuitry  212  are reset upon sensing of R-waves or P-waves as indicated by signals on lines  202  and  206 , respectively. In accordance with the selected mode of pacing, pacing pulses are generated by atrial pacer output circuit  214  and ventricular pacer output circuit  216 . The pacer output circuits  214  and  216  are coupled to the desired electrodes for pacing via switch matrix  208 . The escape interval counters are reset upon generation of pacing pulses, and thereby control the basic timing of cardiac pacing functions, including anti-tachycardia pacing. 
     The durations of the escape intervals are determined by microprocessor  224  via data/address bus  218 . The value of the count present in the escape interval counters when reset by sensed R-waves or P-waves can be used to measure R-R intervals and P-P intervals for detecting the occurrence of a variety of arrhythmias. 
     The microprocessor  224  includes associated read-only memory (ROM) in which stored programs controlling the operation of the microprocessor  224  reside. A portion of the random access memory (RAM)  226  may be configured as a number of recirculating buffers capable of holding a series of measured intervals for analysis by the microprocessor  224  for predicting or diagnosing an arrhythmia. 
     In response to the detection of tachycardia, anti-tachycardia pacing therapy can be delivered by loading a regimen from microprocessor  224  into the pacer timing and control circuitry  212  according to the type of tachycardia detected. In the event that higher voltage cardioversion or defibrillation pulses are required, microprocessor  224  activates the cardioversion and defibrillation control circuitry  230  to initiate charging of the high voltage capacitors  246  and  248  via charging circuit  236  under the control of high voltage charging control line  240 . The voltage on the high voltage capacitors is monitored via a voltage capacitor (VCAP) line  244 , which is passed through the multiplexer  220 . When the voltage reaches a predetermined value set by microprocessor  224 , a logic signal is generated on the capacitor full (CF) line  254 , terminating charging. The defibrillation or cardioversion pulse is delivered to the heart under the control of the pacer timing and control circuitry  212  by an output circuit  234  via a control bus  238 . The output circuit  234  determines the electrodes used for delivering the cardioversion or defibrillation pulse and the pulse wave shape. 
     In one embodiment, the ICD  10  may be equipped with a patient notification system  150 . Any patient notification method known in the art may be used such as generating perceivable twitch stimulation or an audible sound. A patient notification system may include an audio transducer that emits audible sounds including voiced statements or musical tones stored in analog memory and correlated to a programming or interrogation operating algorithm or to a warning trigger event as generally described in U.S. Pat. No. 6,067,473 issued to Greeninger et al., incorporated herein by reference in its entirety. 
       FIG. 3  is flowchart of a method for detecting atrial arrhythmias during intermittent instances of ventricular pacing in a cardiac medical device according to an embodiment of the present disclosure. Flow chart  200  and other flow charts presented herein are intended to illustrate the functional operation of the device, and should not be construed as reflective of a specific form of software or hardware necessary to practice the invention. It is believed that the particular form of software will be determined primarily by the particular system architecture employed in the device and by the particular detection and therapy delivery methodologies employed by the device. Providing software to accomplish the present invention in the context of any modern IMD, given the disclosure herein, is within the abilities of one of skill in the art. 
     Methods described in conjunction with flow charts presented herein may be implemented in a computer-readable medium that includes instructions for causing a programmable processor to carry out the methods described. A “computer-readable medium” includes but is not limited to any volatile or non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flash memory, and the like. The instructions may be implemented as one or more software modules, which may be executed by themselves or in combination with other software. 
     As illustrated in  FIG. 3 , during detection of atrial arrhythmias, the device senses events, such as ventricular events, for example, Block  300 , and identifies the sensed ventricular event as being either an intrinsic sensed event Vs or a paced event Vp resulting from pacing being delivered by the device. Depending upon the number of RR intervals chosen for determining RR interval differences, the device determines whether a predetermined number of sensed events, either a ventricular pacing event Vp or intrinsic ventricular sensed event VS, have been sensed, Block  302 . For example, according to one embodiment, if the desired number of RR intervals for RR interval differences is three, the predetermined number of sensed events utilized in Block  302  would be four sensed events, with the four sensed events forming a sensing window, as will be illustrated below. If the predetermined number of sensed events have not been sensed, the device determines the next sensed event, Block  300 , and the process is repeated. 
     Once the predetermined number of events are sensed, Yes in Block  302 , a sensed event window is identified based on the four events, Block  304 , and a determination is made as to whether the number of the sensed events in the sensed event window that are ventricular pace Vp events is less than or equal to a predetermined pacing event threshold, Block  306 . For example, according to one embodiment, the pacing event threshold is set as one so that the device determines whether one or less of the sensed events in the sensed event window are ventricular pace events. If the number of the sensed events in the sensed event window that are ventricular pace Vp events is not less than or equal to, i.e., is greater than the predetermined pacing event threshold, No in Block  306 , the device determines the next sensed event, Block  300 , and the process is repeated. 
     If the number of the sensed events in the sensed event window that are ventricular pace Vp events is less than or equal to the predetermined pacing event threshold, Yes in Block  306 , the device determines whether each of the RR intervals associated with the sensed events in the current sensed event window are greater than a predetermined interval threshold, Block  308 . For example, according to one embodiment the device determines whether each of the RR intervals associated with the sensed events in the sensed event window are greater than 220 milliseconds. If each of the RR intervals associated with the sensed events in the sensed event window are not greater than 220 milliseconds, No in Block  308 , the device determines the next sensed event, Block  300 , and the process is repeated using the next sensed event and the resulting next sensed event window. 
     If each of the RR intervals associated with the sensed events in the sensed event window are greater than  220  milliseconds, Yes in Block  308 , the device determines differences or variability of the RR intervals associated with the sensed events in the sensed event window, Block  310 , as will be described below. Once the RR intervals differences for the current sensed event window have been determined in Block  308 , the device determines whether a predetermined cardiac event timer has expired, Block  312 . If the event timer has not expired, No in Block  312 , the device determines the next sensed event, Block  300 , and the process is repeated using the next sensed event and the resulting next sensed event window. According to one embodiment, the cardiac event timer is set as two minutes so that once the event timer has expired, Yes in Block  312 , the device determines an atrial fibrillation AF score, Block  314 , based on the determined RR interval differences, Block  310 , resulting from multiple sensed event windows occurring during the predetermined time period, Block  312 , i.e., two minutes for example. The determination of the AF score is described below, with the device making a determining of either an atrial fibrillation AF event or a non- atrial fibrillation event occurring based on a comparison of the AF score to an AF detection threshold. The stored differences are then cleared, Block  316 , and the device determines the next sensed event, Block  300 , and the process is repeated for the next time period using the next sensed events and the resulting next sensed event windows. 
       FIG. 4  is a schematic diagram illustrating detecting atrial arrhythmias during ventricular pacing in a cardiac medical device according to an embodiment of the present disclosure. As illustrated in  FIGS. 3 and 4 , according to one embodiment, once the device senses the predetermined number of sensed events  320 - 326 , Yes in Block  302 , a sensed event window  332  is formed, Block  304 , based on the current four sensed events  320 - 326 . The device determines whether only one or less of the sensed events  320 - 326  are ventricular paced events, Block  306 , and whether the RR intervals  338  formed between the sensed events  320 - 326  are greater than the interval threshold, Block  308 . In the example illustrated in  FIG. 4 , all of sensed events  320 - 326  are ventricular sensed Vs events, and assuming all of the intervals  338  formed by the sensed events  320 - 326  are greater than the interval threshold, Yes in Block  308 , the device determines and stores an interval difference factor associated with the intervals  338  of the current sensed events  320 - 326 , Block  310 . If all of the intervals  338  formed by the sensed events  320 - 326  are not greater than the interval threshold, No in Block  308 , the device determines the next sensed event, Block  300 , and the process is repeated using the next sensed event and the resulting next sensed event window. 
     Assuming the cardiac event timer has not yet expired, No in Block  312 , the device senses the next event  328 , Block  300 , and a sensed event window  334  is formed, Block  304 , based on the current four sensed events  322 - 328 . The device determines whether only one or less of the sensed events  322 - 328  are ventricular paced events, Block  306 , and whether the RR intervals  338  formed between the sensed events  322 - 328  are greater than the interval threshold, Block  308 . In the example illustrated in  FIG. 4 , since only one sensed event  338  of sensed events  322 - 328  is a ventricular paced Vp event, and assuming all of the intervals  338  formed by the sensed events  322 - 328  are greater than the interval threshold, Yes in Block  308 , the device determines and stores an interval difference factor associated with the intervals  338  of the current sensed events  322 - 328 , Block  310 . 
     Assuming the cardiac event timer has not yet expired, No in Block  312 , the device senses the next event  330 , Block  300 , and a sensed event window  336  is formed, Block  304 , based on the current four sensed events  324 - 330 . The device determines whether only one or less of the sensed events  324 - 330  are ventricular paced events, Block  306 , and whether the RR intervals  338  formed between the sensed events  324 - 330  are greater than the interval threshold, Block  308 . In the example illustrated in  FIG. 4 , since two sensed events  338  and  340  of sensed events  324 - 330  are ventricular paced Vp events, and therefore the number of sensed events in the sensed event window  336  that are ventricular paced Vp events is not less than or equal to the pacing event threshold, No in Block  306 , an RR interval difference factor is not determined for that sensed event window  336 , and the device determines the next sensed event, Block  300 , and the process is repeated using the next sensed event and the resulting next sensed event window, and so on until the timer has expired, Yes in Block  312 . Once the timer has expired, Yes in Block  312 , the atrial fibrillation AF score for that time period is determined based on the currently stored interval difference factors, as described below. 
       FIG. 5  is a schematic diagram illustrating detecting atrial arrhythmias during ventricular pacing in a cardiac medical device according to another embodiment of the present disclosure. As illustrated in  FIGS. 3 and 5 , according to another embodiment, once the device senses the predetermined number of sensed events  340 - 346 , Yes in Block  302 , a sensed event window  356  is formed, Block  304 , based on the current four sensed events  340 - 346 . The device determines whether only one or less of the sensed events  340 - 346  are ventricular paced Vp events, Block  306 , and whether the RR intervals  366  formed between the sensed events  340 - 346  are greater than the interval threshold, Block  308 . In the exemplary embodiment illustrated in  FIG. 5 , during the determination as to whether only one or less of the sensed events  340 - 346  are ventricular paced Vp events, Block  306 , rather than making the determination based on all of the sensed events  340 - 346  in the sensed event window  356 , the device determines whether one or more of a predetermined number of the sensed events  340 - 346  are ventricular pace Vp events. For example, according to one embodiment, the device may determine whether only one or less of the most recent sensed event  346  in the sensed event window  356  and the previous two sensed events  342  and  344  are ventricular sensed Vp events, Block  306 . 
     In the example illustrated in  FIG. 5 , the most recent sensed event  346  in the sensed event window  356  is a ventricular pace Vp event, and the two previous sensed events  342  and  344  are both ventricular sense VS events, resulting in there being only one ventricular pace Vp event. Therefore, the number of ventricular pace VP events is determined to be less than or equal to the ventricular pace Vp event threshold, i.e., one ventricular pace Vp event, Yes in Block  306 . As a result, similar to above, the device determines whether the RR intervals  366  associated with the current sensed events  340 - 346  are greater than an interval threshold, Block  308 , such as  220  milliseconds, for example. If the RR intervals  366  are not greater than the interval threshold, No in Block  308 , the device does not store an interval difference factor, Block  310 , for the intervals  366  associated with the current sensed events  340 - 346 , and the process is repeated using the next sensed event  348  and the resulting next sensed event window  358 . 
     If each of the RR intervals  366  are greater than the interval threshold, Yes in Block  308 , the device stores an interval difference factor, Block  310 , associated with the intervals  366  formed between the current sensed events  340 - 346 , described below, and, assuming the timer has not expired, No in Block  312 , the process is repeated using the next sensed event  348  and the resulting next sensed event window  360 . If the timer has expired, Yes in Block  312 , the device determines an atrial fibrillation AF score, Block  314 , based on the determined RR interval difference factors, Block  310 , resulting from multiple sensed event windows over the predetermined time period of Block  312 , such as two minutes, for example. The determination of the AF score is described below, with the device making a determining of either an atrial fibrillation AF event or a non- atrial fibrillation event occurring based on a comparison of the AF score to an AF detection threshold. The current counters are then cleared, Block  316 , and the device determines the next sensed event, Block  300 , and the process is repeated for the next time period using the next sensed events and the resulting next sensed event windows. 
     As described above, if the RR intervals are not greater than the interval threshold, No in Block  310 , or if the cardiac event timer has not yet expired, No in Block  312 , the device senses the next cardiac event  348 , Block  300 , and a sensed event window  358  is formed, Block  304 , based on the most current four sensed events  342 - 348 . The device determines whether only one or less of the most recent sensed event  348  in the sensed event window  358  and the previous two sensed events  344  and  346  are ventricular sensed Vp events, Block  306 . In the example illustrated in  FIG. 5 , the most recent sensed event  348  and one sensed event  346  of the two previous sensed events  344  and  346  are ventricular pace Vp events, and the other previous sensed event  344  is a ventricular sense VS event, resulting in there being two ventricular pace Vp events. Therefore, since the number of ventricular pace VP events is not less than or equal to the ventricular pace Vp event threshold, No in Block  306 , the device does not determine and store an interval difference factor, Block  310 , for the intervals  366  formed by the current sensed events  342 - 348 , and the process is repeated using the next sensed event  350  and the resulting next sensed event window  360 . 
     In particular, the device determines whether only one or less of the most recent sensed event  350  in the sensed event window  360  and the previous two sensed events  346  and  348  are ventricular sensed Vp events, Block  306 . In the example illustrated in  FIG. 5 , the most recent sensed event  350  is a ventricular sense Vs event and both of the previous two sensed events  346  and  348  are ventricular pace Vp events, resulting in there being two ventricular pace Vp events occurring during the sensed event window  360 . As a result, the number of ventricular pace Vp events is not less than or equal to the ventricular pace Vp event threshold, No in Block  306 , and therefore the device does not store an interval difference factor associated with the intervals  366  formed by the current sensed events  344 - 350 , and the process is repeated using the next sensed event  352  and the resulting next sensed event window  362 . 
     In particular, the device determines whether only one or less of the most recent sensed event  352  in the sensed event window  362  and the previous two sensed events  348  and  350  are ventricular sensed Vp events, Block  306 . In the example illustrated in  FIG. 5 , the most recent sensed event  352  and one sensed event  350  of the two previous sensed events  348  and  350  are ventricular sense Vs events, and the other previous sensed event  348  is a ventricular pace Vp event, resulting in only one ventricular pace Vp event occurring during the sensed event window  362 . As a result, the number of ventricular pace VP events is less than or equal to the ventricular pace Vp event threshold, Yes in Block  306 , and therefore the device determines whether the RR intervals  366  associated with the current sensed events  346 - 352  are greater than the interval threshold, Block  308 . If the RR intervals  366  are not greater than the interval threshold, No in Block  308 , the device does not store an interval difference factor, Block  310 , associated with the intervals  366  formed by the current sensed events  346 - 352 , and the process is repeated using the next sensed event  354  and the resulting next sensed event window  364 . 
     If each of the RR intervals  366  are greater than the interval threshold, Yes in Block  308 , the device stores an interval difference factor, Block  310 , associated with the intervals  366  formed between the current sensed events  346 - 352 , described below. Assuming the timer has not expired, No in Block  312 , the process is then repeated using the next sensed event  354  and the resulting next sensed event window  364 . If the timer has expired, Yes in Block  312 , the device determines an atrial fibrillation AF score, Block  314 , based on the determined RR interval difference factors, Block  310 , resulting from multiple sensed event windows over the predetermined time period of Block  312 , i.e., two minutes for example. The determination of the AF score is described below, with the device making a determining of either an atrial fibrillation AF event or a non- atrial fibrillation event occurring based on a comparison of the AF score to an AF detection threshold. The counters are then cleared, Block  316 , and the device determines the next sensed event, Block  300 , and the process is repeated for the next time period using the next sensed events and the resulting next sensed event windows, and so on. 
     In the example illustrated in  FIG. 5 , the most recent sensed event  354  and both of the two previous sensed events  350  and  352  are ventricular sense Vs events, resulting in the number of ventricular pace VP events being less than or equal to the ventricular pace Vp event threshold, Yes in Block  306 . Assuming that each of the RR intervals  366  are greater than the interval threshold, Yes in Block  308 , the device determines and stores an interval difference factor, Block  310 , associated with the intervals  366  formed between the current sensed events  348 - 354 , described below. In this way, assuming the timer has expired, Yes in Block  312 , in the example of  FIG. 5 , the device determines and stores an interval difference factor, Block  310 , only for intervals formed in sensed event windows  356 ,  362  and  364 , and not for intervals formed in sensed event windows  358  and  360 . The determination of the atrial fibrillation AF score, Block  314 , described below, is therefore made based on the interval difference factor, Block  310 , determined only for intervals formed in sensed event windows  356 ,  362  and  364 , and therefore does not include intervals formed in sense event windows  358  and  360  having more than the predetermined number of ventricular pace Vp events therein. 
       FIG. 6  is a schematic diagram of classifying of cardiac events in a cardiac medical device according to an embodiment of the present disclosure. As illustrated in  FIG. 6 , according to one embodiment, in order to determine the atrial fibrillation AF score based on the determined RR intervals difference factors resulting from multiple sensed event windows, described above, the determined RR interval difference factors calculated for the RR intervals formed by the sensed events for each sensed event window, described above, are used to plot single points on a Lorentz plot  14 . 
     The Lorenz plot  14  is a Cartesian coordinate system defined by δRR i  along the x-axis  18  and δRR i−1  along the y-axis  16 . As such, each plotted point in a Lorenz plot is defined by an x-coordinate equaling δRR i  and a y-coordinate equaling δRR i−1 . δRR i  is the difference between the i th  RR interval and the previous RR interval, RRI i−1 . δRR i−1  is the difference between RRI i−1  and the previous RR interval, RRI i−2 . As such, each data point plotted on the Lorenz plot  14  represents a ventricular cycle length VCL pattern relating to three consecutive VCLs: RRI i , RRI i−1  and RRI i−2 , measured between the four consecutively sensed R-waves associated with a sensing event window. 
     In order to plot each point on the Lorenz plot area  14 , a (δRR i , δRR i−1 ) point is identified based on the RR interval difference determined for the intervals formed by the sensed events in each single sensed event window during the two minute time period having one or less ventricular pace Vp events, described above. The atrial fibrillation AF score for each two minute time period is then determined based on the relative position of the resulting plotted points on the plot area  14 . For example, using the example illustrated in  FIG. 5 , a first data point  23  is plotted based on the RR interval difference factor determined for intervals  366  formed in sensed event window  356 , a second data point  25  is plotted based on the RR interval difference factor determined for intervals  366  formed in sensed event window  362 , and a third data point  27  is plotted based on the RR interval difference factor determined for intervals  366  formed in sensed event window  364 , and so forth. 
     In particular, for example, δRR i  for the first data point  23  is determined as the difference between the RR interval  366  between sense  346  and sense  344  and the RR interval  366  between sense  344  and sense  342 , and δRR i−1  is determined as the difference between the RR interval  366  between sense  344  and sense  342  and the RR interval  366  between sense  342  and sense  340 . In the same way, the corresponding (δRR i , δRR i−1 ) point is identified for sensed event windows  362  and  364 , and so on until the timer has expired. 
     The plotted (δRR i , δRR i−1 ) points over a two minute time period are then used to identify the event as either an atrial fibrillation event or a non-atrial fibrillation. Methods have been developed for detecting atrial arrhythmias based on the irregularity of ventricular cycles measured by RR intervals that exhibit discriminatory signatures when plotted in a Lorenz scatter plot such as the plot shown in  FIG. 6 . One such method is generally disclosed by Ritscher et al. in U.S. Pat. No. 7,031,765, or in U.S. Pat. No. 8,639,316 to Sarkar, both incorporated herein by reference in their entireties. Other methods are generally disclosed by Sarkar, et al. in U.S. Pat. No. 7,623,911 and in U.S. Pat. No. 7,537,569 and by Houben in U.S. Pat. No. 7,627,368, all of which patents are also incorporated herein by reference in their entirety. 
       FIG. 7  is a diagram of an exemplary two-dimensional histogram representing a Lorenz plot area for identifying cardiac events. Generally, the Lorenz plot area  14  shown in  FIG. 7  is numerically represented by a two-dimensional histogram  160  having predefined ranges  166  and  164  in both positive and negative directions for the δRR i  and δRR i−1  coordinates, respectively. The two-dimensional histogram is divided into bins  168  each having a predefined range of δRR i  and δRR i−1  values. In one example, the histogram range might extend from −1200 ms to +1200 ms for both δRR i  and δRR i−1  values, and the histogram range is divided into bins extending 7.5 ms in each of the two dimensions resulting in a 160 bin×160 bin histogram. The successive RRI differences determined over a detection time interval are used to populate the histogram  160 . Each bin stores a count of the number of (δRR i , δRR i−1 ) data points falling into the bin range. The bin counts may then be used in determining RRI variability metrics and patterns for determining a cardiac rhythm type. 
     An RRI variability metric is determined from the scatter plot. Generally, the more histogram bins that are occupied, i.e. the more sparse the distribution of (δRR i , δRR i−1 ) points, the more irregular the VCL during the data acquisition time period. As such, a metric of the RRI variability can be used for detecting atrial fibrillation, which is associated with highly irregular VCL. In one embodiment, an RRI variability metric for detecting AF, referred to as an AF score is computed as generally described in the above-incorporated &#39;911 patent. Briefly, the AF score may be defined by the equation: 
         AF  Evidence=Irregularity Evidence−Origin Count− PAC  Evidence
 
     wherein Irregularity Evidence is the number of occupied histogram bins outside a Zero Segment defined around the origin of the Lorenz plot area. During normal sinus rhythm or highly organized atrial tachycardia, nearly all points will fall into the Zero Segment because of relatively small, consistent differences between consecutive RRIs. A high number of occupied histogram bins outside the Zero segment is therefore positive evidence for AF. 
     The Origin Count is the number of points in a “Zero Segment” defined around the Lorenz plot origin. A high Origin Count indicates regular RRIs, a negative indicator of atrial fibrillation, and is therefore subtracted from the Irregularity Evidence term. In addition, a regular PAC evidence score may be computed as generally described in the above-incorporated &#39;911 patent. The regular PAC evidence score is computed based on a cluster signature pattern of data points that is particularly associated with PACs that occur at regular coupling intervals and present regular patterns of RRIs, e.g. associated with bigeminy (short-short-long RRIs) or trigeminy (short-short-short-long RRIs). 
     In other embodiments, an AF score or other RRI variability score for classifying an atrial rhythm may be computed as described in any of the above-incorporated &#39;765, &#39;316, &#39;911, &#39;569 and &#39;368 patents. 
     The AF score is compared to an AF threshold for detecting atrial fibrillation to determine whether the AF score corresponds to an AF event. The AF threshold may be selected and optimized based on historical clinical data of selected patient populations or historical individual patient data, and the optimal threshold setting may vary from patient to patient. If the metric crosses a detection threshold, AF detection occurs. A response to AF detection is made, either in response to a classification of a single two second time interval as being AF, i.e., being greater than the AF threshold, or in response to a predetermined number of two second intervals being classified as being an AF event by each being greater than the AF threshold. Such response to the AF detection may include withholding or altering therapy, such as a ventricular therapy, for example, storing data that can be later retrieved by a clinician, triggering an alarm to the patient or that may be sent remotely to alert the clinician, delivering or adjusting a therapy, and triggering other signal acquisition or analysis. 
     The RRI measurements may continue to be performed after an AF detection to fill the histogram during the next detection time interval. After each detection time interval, the RRI variability metric is determined and the histogram bins are re-initialized to zero for the next detection time interval. The new RRI variability metric determined at the end of each data acquisition interval may be used to determine if the AF episode is sustained or terminated. 
       FIG. 8  is a flowchart of classification of an arrhythmia according to an embodiment of the disclosure. According to another embodiment, once the two minute time period has expired and the plot has been populated with a point associated with each determined RR interval difference factor determined based on the intervals in each sensed event window occurring during the two minute time period, the device determines whether to classify the event during that two minute time period as being either an atrial fibrillation AF event, a non-atrial fibrillation event, or an unclassified event. For example, the device may look at any one or more of several factors, in any combination or particular order, to determine that the event should be determined to be an unclassified event, i.e., that the event can neither be classified as an AF event or a non-AF event. Therefore, as illustrated in  FIGS. 3 and 8 , the device may determine the number of valid RR interval pairs, i.e., three valid consecutive RR intervals, formed between the predetermined number of sensed events that were generated during the two minute time period, Block  400 . In particular, the device determines the number of sensed event windows containing three consecutive RR intervals that were formed in Block  304  during the two minute time period, Block  312 . 
     A determination is then made as to whether the total number of valid three consecutive RR intervals formed during the two minute time period is greater than an interval pair threshold, Block  402 . According to an embodiment, the predetermined number of valid interval pairs is set as 30 valid interval pairs. If the number of sensed event windows that are formed during the two minute time period is less than the interval pair threshold, Yes in Block  402 , meaning less than 30 sensed event windows were determined during the two minute time period, and therefore less than 30 interval difference factors were determined in Block  310  during the two minute time interval, resulting in the plot being populated with less than 30 points (assuming the RR interval threshold is also satisfied in Block  308 ), the two minute time period is determined to be unclassified, Block  404 . 
     If the number of sensed event windows that are formed during the two minute time period is not less than 30, No in Block  402 , meaning 30 or more sensed event windows were determined during the two minute time period, and therefore 30 or more interval difference factors were determined in Block  310  during the two minute time interval, resulting in the plot being populated with 30 or more points, the interval pairs factor is determined not to be satisfied as an indication of the two minute time interval being unclassified. 
     According to another embodiment, the device may determine the total number of RR intervals that were determined to be less than the interval threshold in Block  308  during the two minute time period, Block  410 , either alone or in combination with one or more other factors. The device determines whether a predetermined number of RR intervals were determined to be greater than the RR interval threshold during the two minute time period, Block  412 , such as 10 RR intervals, for example. 
     If more than 10 RR intervals of the total number of RR intervals that were determined during the two minute time period were greater than the RR interval threshold, Yes in Block  412 , the interval length factor is determined to be satisfied and the two minute time period is determined to be unclassified, Block  404 . If 10 RR intervals or less were greater than the RR interval threshold during the two minute time period, No in Block  412 , the interval length factor is determined not to be satisfied as an indication of the two minute time interval being unclassified. 
     In order to classify the two minute time period, the device may also determine a short interval count of the total number of RR intervals from all of the sensing windows obtained during the two minute time period that were less than or equal to a predetermined short interval rate, Block  414 , such as 120 milliseconds or 130 milliseconds, for example. The device determines whether the short interval count is greater than a short interval rate threshold, Block  416 , such as 5 short intervals for example. 
     If the determined short interval count is greater than the short interval rate threshold, Yes in Block  416 , the short interval count factor is satisfied as an indication of the two minute time interval being unclassified and therefore the two minute time interval is determined to be unclassified, Block  404 . On the other hand, if the determined short interval count is not greater than the short interval rate threshold, No in Block  416 , the short interval count is determined not to be satisfied as an indication of the two minute time interval being unclassified. 
     The device may also determine the number of sensed events, sensed during the total two minute time period within all of the sensed event windows that were determined to be ventricular pace Vp events, Block  418 . A determination is made as to whether the determined number of ventricular pace Vp sensed during the two minute time interval is greater than a total ventricular pace Vp event threshold, Block  420 . According to one embodiment, the total ventricular pace Vp threshold is set as 30 ventricular pace Vp events, for example. 
     If the number of ventricular pace Vp sensed during the two minute time interval is greater than the total ventricular pace Vp event threshold, Yes in Block  420 , the ventricular pace factor is satisfied as an indication of the two minute time interval being unclassified and therefore the two minute time interval is determined to be unclassified, Block  404 . On the other hand, if the determined short interval count is not greater than the short interval rate threshold, No in Block  416 , the short interval count is determined not to be satisfied as an indication of the two minute time interval being unclassified. 
     The device may also determine whether a determination of oversensing caused by noise was met, or in process during the two minute time period, Block  422 . The determination of oversensing may be performed by the device using any known oversensing determination scheme, such as the oversensing determination describe in U.S. Pat. No. 7,333,855 to Gunderson et. al., incorporated herein by reference in it&#39;s entirety. If a determination of oversensing was met or was in process during the two minute time period, Yes in Block  422 , the oversensing factor is satisfied as an indication of the two minute time interval being unclassified and therefore the two minute time interval is determined to be unclassified, Block  404 . If a determination of oversensing was not met or was not in process during the two minute time period, No in Block  422 , the oversensing factor is not satisfied as an indication of the two minute time interval being unclassified. 
     Finally, the device may determine whether a determination of T-wave oversensing was met or in process during the two minute time period, Block  424 . The determination of T-wave oversensing may be performed by the device using any known T-wave oversensing determination scheme, such as the T-wave oversensing determination describe in U.S. Pat. No. 7,831,304 to Gillberg, et al., incorporated herein by reference in it&#39;s entirety. If a determination of T-wave oversensing was met or was in process during the two minute time period, Yes in Block  424 , the T-wave oversensing factor is satisfied as an indication of the two minute time interval being unclassified and therefore the two minute time interval is determined to be unclassified, Block  404 . If a determination of T-wave oversensing was not met or was not in process during the two minute time period, No in Block  424 , the T-wave oversensing factor is not satisfied as an indication of the two minute time interval being unclassified. 
     In this way, the device may use one or more of the described factors, which if satisfied would cause the device to determine the two minute time period as being unclassified, and if at least one the described factors for identifying the two minute time period as being unclassified are met, the two minute time period is identified as being unclassified, Block  426 . If none of the factors are satisfied, the AF score is determined based on the populated plot, and a determination is made as to whether the AF score is greater than an AF threshold, Block  426 , as described above. If the AF score is greater than the AF threshold, Yes in Block  426 , the event for the two minute period is classified as an AF event, Block  406 . On the other hand, if the AF score is not greater than the AF threshold, No in Block  426 , the event for the two minute period is classified as a non AF event, Block  408 . 
     It is understood that the determination of whether the event is an unclassified event, Block  404 , or an atrial fibrillation event, Block  406  or a non-atrial fibrillation event, Block  408 , may be made in any order, or at the same time, so that the determination of the two minute time period as being an unclassified event may be used to override an initial determination of the two minute time period as being either an atrial fibrillation event or a non-atrial fibrillation event, or made prior to determining the two minute time period as an atrial fibrillation event or a non-atrial fibrillation event. 
       FIG. 9  is a schematic diagram of determination of a cardiac event, according to the present disclosure. As illustrated in  FIG. 9 , the device identifies each two minute interval as being either an AF event, a non AF event or an unclassified event using the method above in  FIG. 8 , and utilizes the identifications resulting for the two minute time periods to detect an AF episode. For example, once a predetermined number of two second windows, such as three, for example, have been identified as an AF event, the device determines that an atrial fibrillation episode is occurring. Therefore, as illustrated in the scenario of timing diagram (a) of  FIG. 9 , once the three two minute time intervals,  500 - 504 , are classified as an AF event, the device determines that atrial fibrillation is detected. However, in the scenario illustrated in timing diagram (b), two consecutive two minute intervals  506  and  508  are identified as being AF events, a subsequent two minute time interval  510  is identified as being unclassified, which is followed by a subsequent interval  512  being identified as an AF event. According to an embodiment, the device may ignore the unclassified two minute time interval  510  and determine an AF episode once the third time interval  512  identified as AF occurs, so that an AF episode may be identified despite intermittent unclassified two minute time intervals occurring. 
     As illustrated in the timing diagram of scenario (c), during the determination of whether the predetermined number of two second windows are identified as AF events, the device updates an AF event counter each time an AF event is determined. For example, at the identification of two minute interval  514 , the AF event counter is increment to one, and at the identification of subsequent two minute interval  514 , the AF event counter is incremented to two. If two minute interval  518  were also identified as an AF event, the device would determine an AF episode, since three two minute intervals identified as an AF event would have occurred. However, since two minute interval  518  was identified as a non-AF event, the episode is determined to have terminated, and the AF counter is reset to zero, and the process continues with the next two second time interval  520 . In the timing diagram of scenario (c), at the identification of two minute interval  506 , the AF event counter is increment to one, at the identification of subsequent two minute interval  508 , the AF event counter is incremented to two, at the identification of subsequent two minute interval  510 , since the event was determined to be unclassified, the event counter remains as being equal to two, and at the identification of subsequent two minute interval  512 , the AF event counter is incremented to three, and an AF episode is determined. 
     Similarly, in the timing diagram of scenario (d), at the identification of two minute interval  522 , the AF event counter is increment to one, at the identification of subsequent two minute interval  524 , the AF event counter is incremented to two, at the identification of subsequent two minute intervals  526  and  528 , since the event was determined to be unclassified, the unclassified event count is incremented and the AF event count remains as being equal to two, and at the identification of subsequent two minute interval  530 , the AF event count is incremented to three, and an AF episode is determined. 
     Had any of intervals  524 - 530  been identified as non- AF, indicating the termination of the AF episode, the AF event counter would have been set to zero and the process repeated starting with the next classified two minute interval. However, in addition to a two minute interval being identified as a non-AF event, the episode may also be determined to have terminate and the AF count is reset to zero if a predetermined number of two minute time periods are identified as unclassified, such as five two minute time periods, for example. Therefore, as illustrated in the timing diagram of scenario (e), at the identification of two minute interval  532 , the AF event count is increment to one, at the identification of subsequent two minute interval  534 , the AF event count is incremented to two, at the identification of the four subsequent two minute intervals  536 - 542 , since the event was determined to be unclassified, the AF event count remains equal to two. If the subsequent two minute interval  544  is determined to either unclassified or a non-AF event, the AF episode would be determined to have terminated and the AF event counter would be set to zero and the process repeated starting with the next classified two minute interval. If two minute time period  546  had been classified as an AF event, an AF episode would have been identified. 
       FIG. 10  is a flowchart of determination of a cardiac episode, according to an embodiment of the disclosure. As illustrated in  FIGS. 9 and 10 , during identification of two minute intervals as being either an AF event, a non AF event or an unclassified event using the method described above in  FIG. 8 , once the identification of a two minute time interval as an AF event occurs, Block  600 , and the AF event counter is incremented, Block  602 , and the device determines whether the AF event count is equal to an AF event count threshold, Block  604 , such as three AF events, for example. Once the AF event count is equal to the AF event count threshold, Yes in Block  604 , an AF episode is identified, Block  606 . If the AF event count is not equal to the AF event count threshold, No in Block  604 , the device determines, based on the next two minute time interval classification, Block  608 , whether the next two minute time interval is classified as a non-AF event, Block  612 . If the next two minute time interval is classified as a non-AF event, Yes in Block  612 , the AF event counter and the unclassified event counter are both set to zero, Block  614 , and the process repeated starting with the next classified two minute interval, Block  600 . 
     If the next two minute time interval is not classified as a non-AF event, No in Block  612 , the device determines whether the unclassified event counter is equal to the unclassified event count threshold, such as five unclassified events, for example, Block  616 . If the unclassified event counter is not equal to the unclassified event count threshold, No in Block  616 , the process is repeated based on the next subsequent two minute time interval classification, Block  608 . If the unclassified event counter is equal to the unclassified event count threshold, Yes in Block  616 , the AF event counter and the unclassified event counter are both set to zero Block  614 , and the process repeated starting with the next classified two minute interval, Block  600 . 
     According an embodiment of the disclosure, once the AF event count is equal to the AF event threshold, Yes in Block  604 , and therefore an AF episode is identified in Block  606 , the device may determine the end of the specific AF episode. For example, at identification of an AF episode, Block  606 , the unclassified event counter is set to zero, Block  618 , the device determines, based on the next two minute time period classification, Block  620 , whether the next two minute time interval is classified as a non-AF event, Block  622 . If the next two minute time interval is classified as a non-AF event, Yes in Block  622 , the episode is determine to have terminated, Block  624 , and the AF event counter and the unclassified event counter are both set to zero Block  614 , and the process is repeated starting with the next classified two minute interval, Block  600 . If the next two minute time interval is not classified as a non-AF event, No in Block  622 , the device determines whether the unclassified event counter is equal to the unclassified event count threshold, Block  626 . If the unclassified event counter is not equal to the unclassified event count threshold, No in Block  626 , the process is repeated based on the next subsequent two minute time period classification, Block  620 . If the unclassified event counter is equal to the unclassified event count threshold, Yes in Block  626 , the AF episode is determined to have terminated, Block  624 , the AF event counter and the unclassified event counter are both set to zero Block  614 , and the process repeated starting with the next classified two minute interval, Block  600 . 
     According to an embodiment of the disclosure, the identification of the AF episode, along with the classification of the two minute time intervals as being either an AF event, a non-AF event, or an unclassified event are stored and may be later retrieved by a clinician, either remotely or through interrogation of the device. AF burden (e.g., an AF daily burden) may be calculated using a single two minute AF classification or a predetermined number of two minute AF classifications, such as three two minute AF classifications, for example. An alarm may be sent remotely to alert the clinician if the AF burden exceeds a predetermined threshold (e.g., one hour for example), or may be sent to notify the clinician or patient of the one or more two minute time interval classifications and/or the identification of an AF episode, when termination of the episode occurs and how it was determined to have terminated, i.e., by a non-AF two minute time period or the predetermined number of unclassified two minute time periods. 
     Thus, an apparatus and method have been presented in the foregoing description with reference to specific embodiments. It is appreciated that various modifications to the referenced embodiments may be made without departing from the scope of the invention as set forth in the following claims.