Abstract:
An arrhythmia detection system provides automatic detection criteria adjustment in an implantable cardioverter-defibrillator that applies arrhythmia terminating electrical energy to a heart responsive to detection of the arrhythmic episode of the heart. A first detector detects arrhythmic episodes of the heart in accordance with detection criteria. A second detection confirms the detection of each arrhythmic episode by the first detector and a detection criteria regulator adjusts the detection criteria of the first detector responsive to confirmation results provided by the second detector.

Description:
FIELD OF THE INVENTION 
     The present invention is generally directed to an implantable cardioverter-defibrillator which applies arrhythmia-terminating electrical energy to a heart when an arrhythmic episode is detected. The present invention is more particularly directed to such a device, wherein arrhythmic episode detection criteria are adjusted in response to arrhythmic episode detection confirmation results. 
     BACKGROUND OF THE INVENTION 
     Implantable cardioverters-defibrillators, such as implantable ventricular defibrillators, are well known in the art. Such devices include an arrhythmia detector which detects an arrhythmic episode of the heart and an output circuit or generator which applies electrical energy to a heart when an arrhythmic episode is detected to terminate the detected arrhythmia. 
     The performance of an arrhythmia detector is generally measured by its sensitivity and specificity. Sensitivity is the measure of how well a detector detects all of the arrhythmic episodes. For example, a detector that has a sensitivity of 100% detects all arrhythmic episodes of the type intended to be detected which occur. Specificity, on the other hand, is the measure of how well the detector is able to distinguish or discriminate the arrhythmia intended to be detected from other arrhythmias. For example, an arrhythmia detector which has a specificity of 100% detects only the arrhythmia intended to be detected, and no others. 
     There are many different types of cardiac arrhythmias. Among these are, for example, ventricular fibrillation, ventricular tachycardia, atrial fibrillation, and atrial tachycardia or flutter. Hence, a ventricular fibrillation detector which is has a sensitivity of 100% and a specificity of 100% is able to detect all ventricular fibrillations episodes that occur(100% sensitive) while at the same time not mistaking any other form of arrhythmic episode for ventricular fibrillation (100% specific). 
     Although modern implantable cardiovertersdefibrillators employ arrhythmia detectors which have very high sensitivities and specificities, no arrhythmia detector has both a sensitivity and specificity of 100%. Ventricular fibrillation is an immediately life threatening arrhythmia. Hence, all ventricular fibrillation episodes must be detected and, when detected, terminated quickly. As a result, a ventricular fibrillation detector must be very sensitive. In fact, to assure such sensitivity, it is desirable to tolerate some arrhythmic episodes, other than ventricular fibrillation episodes, to be detected as ventricular fibrillation. These are known as false positives. 
     In order to reduce unneeded, attempted arrhythmic episode terminations, it is well known to provide confirmation of detections. When a ventricular fibrillation episode is initially detected, a storage capacitor which applies the arrhythmia terminating electrical energy to the heart begins charging. Either during or immediately after charging the initial ventricular fibrillation episode detection is confirmed. If confirmation is successful, the arrhythmia terminating electrical energy is immediately applied to the heart. However, if confirmation of the initial ventricular fibrillation episode detection is unsuccessful, the energy delivery is aborted. Such unsuccessful confirmation can, for example, be the result of a false positive in the initial detection or the heart spontaneously returning to a normal rhythm in the short time period between initial detection and unsuccessful confirmation. 
     The detection parameters or criteria used in the initial detection of ventricular fibrillation episodes represent a difficult tradeoff for the physician. If the number of heartbeats to be analyzed is set too high, there could be a significant delay in the detection of the ventricular fibrillation episodes. If the number of heartbeats to be analyzed is set too low, the confidence that a rhythm detected as ventricular fibrillation truly being ventricular fibrillation is reduced. If the criteria applied to the analyzed beats are set too high, the detection will be overly specific and relatively insensitive resulting in the potential that a ventricular fibrillation episode will go undetected. Lastly, if the criteria applied to the analyzed beats are too low, the opposite problem can occur and the detection will be overly sensitive and relatively unspecific resulting in inappropriate shocks being delivered to the heart. 
     The present invention addresses these issues. More particularly, as will be seen hereinafter, the confirmation results of the confirmation detection are utilized for the automatic adjustment or regulation of the initial arrhythmia detection criteria to assure maximum sensitivity with appropriate specificity in the initial arrhythmic episode detection parameters or criteria. 
     SUMMARY OF THE INVENTION 
     The invention provides an arrhythmia detection system that provides automatic detection criteria adjustment for use in an implantable cardioverter-defibrillator that applies arrhythmia terminating electrical energy to a heart responsive to detection of an arrhythmic episode of the heart. A first detector detects arrhythmic episodes of the heart in accordance with detection criteria. A second detector confirms the detection of each arrhythmic episode by the first detector and a detection criteria regulator adjusts the detection criteria of the first detector responsive to the second detector. 
     In accordance with one aspect of the present invention, the detection criteria regulator adjusts sensitivity and specificity detection criteria of the first detector. The second, or confirmation detector provides one of successful and unsuccessful confirmation results for each arrhythmic episode detected by the first detector. In accordance with a further aspect of the present invention, the detection system further includes a counter that counts consecutive confirmation results provided by the second detector. The detection criteria regulator adjusts the detection criteria of the first detector in response to the counter counting either at least two consecutive successful confirmation results or two consecutive unsuccessful confirmation results. 
     In accordance with a further aspect of the present invention the arrhythmia detection system includes a processor, coupled to a ventricular activation detector, that times successive time spans between successive detected ventricular activations and executes an X out of Y routine wherein Y is the total number of successive time spans to be timed for a ventricular arrhythmic episode to be detected and X is the number of time spans shorter than a predetermined time span of the successive number of total time spans required for a ventricular arrhythmic episode to be detected. The detection criteria regulator adjusts the values of X and y. 
     In accordance with a further aspect of the present invention, the detection system includes a ventricular activation rate variability factor calculator that determines the ventricular activation rate variability. When the ventricular activation rate variability exceeds a given factor, the detection criteria regulator causes the number of heartbeats to be analyzed to be increased or to remain constant. 
     In accordance with a still further aspect of the present invention, an abort stage causes the application of arrhythmia terminating electrical energy to be inhibited if an arrhythmic episode detection confirmation is unsuccessful. Moreover, the storage capacitor that stores the arrhythmia terminating electrical energy may be reformed when such reforming is required and if a confirmation of an arrhythmic episode detection is unsuccessful. 
     The invention further provides in an implantable cardioverter-defibrillator that applies arrhythmia detecting electrical energy to a heart responsive to detection of an arrhythmic episode of the heart, an arrhythmia detection system that provides automatic detection criteria adjustment. The system includes first detecting means for detecting arrhythmic episodes of the heart in accordance with detection criteria, second detecting means for confirming the detection of each arrhythmic episode of the first detecting means, and detection criteria regulating means for adjusting the detection criteria of the first detecting means responsive to the second detecting means. 
     The invention still further provides a method of adjusting arrhythmia episode detection criteria in an implantable cardioverter-defibrillator that applies arrhythmia terminating electrical energy to a heart responsive to detection of an arrhythmia episode of the heart. The method includes the steps of detecting arrhythmia episodes of the heart in accordance with detection criteria, confirming the detection of and providing a confirmation result for each arrhythmia episode detected and adjusting the detection criteria responsive to the confirmation results. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference characters identify identical elements, and wherein: 
     FIG. 1 is a schematic illustration of a human heart in need of ventricular arrhythmia cardioversion-defibrillation shown in association with an implantable ventricular cardioverter-defibrillator embodying the present invention; 
     FIG. 2 is a block diagram of the implantable ventricular cardioverter-defibrillator of FIG. 1; 
     FIG. 3 is a flow diagram illustrating operative steps that the ventricular fibrillation detection system embodying the present invention of the device of FIGS. 1 and 2 may implement in accordance with a preferred embodiment of the present invention for adjusting ventricular fibrillation detection criteria; 
     FIG. 4 is another flow diagram illustrating operative steps that the ventricular fibrillation detection system may implement in accordance with the preferred embodiment of the present invention; 
     FIG. 5 is a graph illustrating operative ventricular fibrillation detection criteria ranges in accordance with a further aspect of the present invention; and 
     FIG. 6 is a logic diagram illustrating the ventricular fibrillation detection criteria adjustments which may be made in accordance with the present invention in the presence of high and low ventricular rate variabilities. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now FIG. 1, it illustrates heart  10  in need of ventricular arrhythmia cardioversion-defibrillation and an associated implantable ventricular cardioverter-defibrillator  30  embodying the present invention. The portions of the heart  10  illustrated in FIG. 1 are the right ventricle  12 , the left ventricle  14 , the right atrium  16 , and the left atrium  18 . Also illustrated are the superior vena cava  20  and inferior vena cava  27 . As is well known in the art, the cardioverter-defibrillator  30  is arranged to be implanted in an upper left chest portion of a patient within a subcutaneous pocket. 
     The implantable device  30  includes a first endocardial lead  32  which is of the “single-pass” type. To that end, the lead  32  includes a first shock coil  34  arranged to be disposed within the right ventricle  12 , a second shock coil  36  proximal to the shock coil electrode  34  and arranged to be disposed within the right atrium  16  or superior vena cava  20 , and a distal tip pacing electrode  38 . The implantable device  30  further includes a second endocardial lead  42  having an electrode pair including a distal electrode  44  and a proximal electrode  46 . 
     The implantable cardioverter-defibrillator  30  includes a hermetically sealed, electrically conductive enclosure  50 . When an quantity of cardioverting or defibrillating electrical energy is applied to the heart  10 , in accordance with this preferred embodiment, the electrodes  34  and  36  are connected in parallel and the quantity of arrhythmia terminating electrical energy is applied between the parallel connection of electrode  36  and the electrically conductive enclosure  50  of the implantable device  30  and electrode  34 . Alternatively, the cardioverting or defibrillating quantity of electrical energy may be applied between electrode  34  and the electrically conductive enclosure  50  without employing electrode  36 . All such cardioverting and defibrillating methodologies apply cardioverting and defibrillating electrical energy to the heart and are thus deemed to be alternative structures and methods in practicing the present invention. Electrodes  44  and  46  of lead  42  support sensing of right atrial electrical activity and delivery of atrial pacing pulses to the right atrium  16 . 
     As illustrated in FIG. 2, the implantable cardioverter-defibrillator  30  includes within the enclosure  50  a ventricular sense channel  52 , an atrial sense channel  62 , and a pacing pulse generator  70  including a first or atrial pacing pulse generator  72  for providing atrial pacing pulses and a second or ventricular pacing pulse generator  74  for providing ventricular pacing pulses. The device  30  further includes a microprocessor  80 , a memory  110 , and a telemetry stage  150 . The device  30  still further includes and cardioversion-defibrillation generator  166  including a charging circuit  160 , a storage capacitor  162 , and a switch  164 . 
     The ventricular sense channel  52  includes a sense amplifier  54  and a threshold detector  56 . The sense amplifier  54  has an input coupled to electrode  38  of lead  32  by a conductor  138  of the lead  32 . The sense amplifier  54  has another input which is coupled to electrode  34  of lead  32  by another conductor  134  of the lead  32 . The sense amplifier  54  further includes an output which forms an input to the threshold detector  56 . As further illustrated, the threshold detector  56  has an output which is coupled to the microprocessor  80 . 
     The sense amplifier  54 , together with electrodes  38  and  34  sense electrical activity in the right ventricle  12 . When the output from the amplifier  54  transitions through a programmed threshold of the threshold detector  56 , the threshold detector  56  provides an input signal to the microprocessor  80  indicating that a ventricular activation or R wave has been detected. Such detection is well known in the art. 
     Similarly, the atrial sense channel  62  includes a sense amplifier  64  and a threshold detector  66 . The sense amplifier  64  has an input which is coupled to electrode  44  of lead  42  by a conductor  144  of lead  42 . The sense amplifier  64  has another input which is coupled to the electrode  46  of lead  42  by another conductor  146  of lead  42 . As further illustrated, the sense amplifier has an output which forms an input to the threshold detector  66  and the threshold detector  66  has an output which is coupled to the microprocessor  80 . 
     The sense amplifier  64 , together with electrodes  44  and  46 , sense electrical activity in the right atrium. When the output of the sense amplifier  64  transitions through a programmed threshold of the threshold detector  66 , the threshold detector  66  provides an input signal to the microprocessor  80  indicating that an atrial activation or P wave has been detected. Again, such detection is also well known in the art. 
     The first or atrial pulse generator  72  has outputs coupled to electrodes  44  and  46  of lead  42  by conductors  144  and  146  respectively of lead  42 . This permits atrial pacing pulses produced by the atrial pacer  72  to be applied to the right atrium  16 . The second or ventricular pulse generator  74  has outputs coupled to electrodes  34  and  38  of lead  32  by conductors  134  and  138  respectively of lead  32 . This permits ventricular pacing pulses produced by the ventricular pacer  74  to be applied to the right ventricle  12 . 
     The cardioversion-defibrillation generator  166  applies a quantity of arrhythmia terminating electrical energy to the heart  10 . To that end, the charging circuit  160  charges the storage capacitor  162  with the quantity of electrical energy to be applied to the heart upon the detection of a ventricular arrhythmia, such as ventricular fibrillation, as will be described subsequently. The switch  164  applies the quantity of electrical energy from the storage capacitor  162  to the heart. As can be seen in FIG. 2, the switch has an output coupled to electrode  34  of lead  32  by the conductor  134  of lead  32  and another output which is coupled to electrode  36  by a conductor  136  of lead  32 . Also, another output of the switch  164  is coupled to the electrically conductive enclosure  50 . As a result, when the arrhythmia terminating electrical energy is applied to the heart  10 , the electrode  36  is coupled in parallel with the electrical conductive enclosure  50  to provide a return path for current from electrode  34 . 
     The microprocessor  80  controls the overall functioning of the implantable cardioverter-defibrillator  30 . To implement such control, the microprocessor executes operating instructions stored in the memory  110  and utilizes various parameters also stored in memory  110 . For example, the memory  110  stores the operating instructions defining various pacing modalities which may be provided by the device  30  in a storage location  112 . Detection parameters such as the programmable thresholds of threshold detectors  56  and  66  may be stored in storage location  114 . As will be seen hereinafter, a ventricular fibrillation detector executes an X out of Y algorithm, and the values of X and Y may be stored in a storage location  116 . The operating instructions defining ventricular defibrillation therapy may be stored in a storage location  118 . Lastly, defibrillation parameters such as defibrillating energies may be stored in a storage location  120 . 
     The telemetry stage  150  permits modality selections and storage of detection parameters, X and Y values, and defibrillation parameters in the memory  110  to be made through the use of an external programmer (not shown) of the type well known in the art. The telemetry stage includes a receiver  152  which receives telemetry commands including mode selection and parameter commands from the programmer. The receiver  152  conveys the commands to the microprocessor  80  which then stores them in the memory  110 . The telemetry stage  150  also includes a transmitter  154 . The transmitter may be used for transmitting data to the programmer. The transmitted data may include sensed electograms or status information, for example, as is well known in the art. 
     The microprocessor  80  is coupled to the memory  110  by a multiple-bit address bus  120  and a bi-directional, multiple-bit data bus  122 . The microprocessor  80  uses the address bus  120  to fetch operating instructions or programmable parameters from the memory at address locations defined on the address bus  120 . The fetched instructions and parameters are conveyed to the microprocessor  80  over the data bus  122 . Similarly, the microprocessor  80  may store data in the memory  110  at memory locations defined on the address bus  120 . The microprocessor  80  conveys the data to the memory over the data bus  122 . Such microprocessor and memory operation are conventional in the art. 
     When executing the operating instructions stored in memory  110 , the microprocessor implements a number of functional stages in accordance with the present invention. These stages include a first detector  82  including a timer  84 , a counter  86 , and an X of Y stage  88 . The functional stages of microprocessor  80  further include a second detector  90  including a timer  92 , an averaging stage  94 , an abort stage  96 , and a capacitor reform timing stage  98 . In addition to the first detector  82  and the second detector  90 , the functional stages further include a detection criteria regulator  100  including a confirmation counter  102 , a variability factor determining stage  104 , a decrementer  106 , and incrementer  108 . Lastly, the functional stages include a charge control  105  and a synchronizing stage  107 . The first detector  82 , second detector  90  and detection criteria regulator  100  form an arrhythmia detection system  81  embodying the present invention. 
     In accordance with a primary aspect of the present invention, when the first detector  82  detects a ventricular fibrillation episode, the charge control  105  causes the charger  160  to begin charging the storage capacitor  162  with the arrhythmia terminating electrical energy. Also, as the storage capacitor  162  is being charged, the second detector  90  executes a ventricular fibrillation confirmation detection. The successful confirmation or unsuccessful confirmation results of the second detector  94  are counted by the confirmation counter  102  of the detection criteria regulator  100 . When a predetermined number of consecutive successful or unsuccessful confirmations have occurred, the detection criteria regulator  100  adjusts the sensitivity and specificity of the detection criteria of the first detector  82 . If the confirmation is successful, after the storage capacitor  162  is charged to a desired level under control of the charge control  105 , the synchronizing stage  107  causes the switch  164  to operate in synchronization with a detected R wave for applying the arrhythmia terminating electrical energy to the heart  10 . If the confirmation of the ventricular fibrillation detection is unsuccessful, the abort stage  96  of the second detector will cause the application of the arrhythmia terminating electrical energy to be aborted and hence not applied to the heart  10 . 
     In accordance with this preferred embodiment, the first detector  82  executes an X out of Y algorithm of the type well known in the art. As will be appreciated by those skilled in the art, other methodologies of initial ventricular fibrillation detection may be employed without departing from the present invention. In executing the X out of Y algorithm, the first detector  82  determines if X beats out of the last Y beats were fast. To that end, the timer  84  times time spans between ventricular activations detected by ventricular sense channel  52 . The counter  86  counts the time spans that are shorter than a predetermined time span. If X or more of the last Y beats are shorter than the predetermined time span, the first detector  82  will have detected a ventricular fibrillation. The short time spans may be on the order of 200-350 milliseconds. As an example of the above, if X is equal to 12 and Y is equal to 16, the detector  82  will determine if 12 out of the last 16 beats were shorter than the predetermined span time. If 12 or more of the beats of the last 16 beats were shorter than the predetermined time span, the detector  82  will consider ventricular fibrillation to have been detected. 
     Upon the initial detection of ventricular fibrillation, the charger  160  as previously described begins to charge storage capacitor  162 . Also during the charging time, the second or confirmation detector  90  performs another detection to confirm the initial ventricular fibrillation detection. To confirm the original detection, the second detector  94  may, for example, monitor 4 more beats. If the average of these 4 beats is still considered fast, then the original ventricular fibrillation detection will be successfully confirmed and the arrhythmia terminating electrical energy will be applied as previously described. If the average of those 4 beats is considered not to be fast, then the confirmation is unsuccessful and the abort stage  96  will abort the application of the arrhythmia terminating electrical energy. For timing the time spans between the ventricular activations of the 4 beats, the detector  90  includes a timer  92 . The averaging stage  94  averages the time spans between the ventricular activations comprising the last 4 beats to provide the average. 
     Referring now to FIG. 3, it illustrates a flow diagram of the operative steps which may be taken by the detection criteria regulator  100  if there is a successful confirmation of the original ventricular fibrillation episode detection. The first step in the process is step  170  wherein the original detection is confirmed. Next, in step  172 , the confirmation counter  102  is addressed to determine if the previous episode was also confirmed. If the previous episode detection was not confirmed, the current values of X and Y in storage location  116  will remain unchanged in accordance with step  174  and the process returns. 
     If in step  172  it is determined that the previous episode detection was confirmed, in addition to the delivery of the arrhythmia terminating electrical energy, the decrementing stage  106  in step  176  decrements both X and Y in storage location  116  by one. 
     Following step  176 , the process will return. 
     By decrementing X and Y by one, the detection criteria of detector  82  is adjusted such that the next time it detects an episode, one less heartbeat will be analyzed and one less heartbeat need be fast to satisfy the adjusted ventricular fibrillation detection criteria. This renders the new criteria more sensitive and less specific. 
     Referring now to FIG. 4, it illustrates the operative steps that the detection system may take upon an unsuccessful confirmation. In step  178  it is determined that there has been an unsuccessful confirmation. Next, in step  180  it is determined if the previous episode detection was also unconfirmed. If the previous episode detection was not confirmed, in step  182  the current values of X and Y in storage location  116  are left unchanged and the process returns. However, if in step  180  it is determined that the previous episode detection was also unconfirmed, the values of X and Y in storage location  116  are incremented by one by the incrementing stage  108  in step  182 . This causes the adjusted detection criteria of the first detector  82  to be less sensitive and more specific. 
     Referring now FIG. 5, it illustrates the present invention in a broader context. It will be noted that the embodiment of the invention described above has the X and Y values being adjusted in lock step. In FIG. 5 an irregular quadrilateral describes an acceptable region for the X and Y values. The X value must be less than a certain extreme to limit the overall delay for detection. In this case, it is shown as a relatively conservative number of 16. The Y value must also be greater than a minimum level to ensure that a certain robust detection takes place. Experience has shown that a value of 4 is probably a minimum. Since X will not be greater than Y, the Y equals X line  190  obtains which is the highest specificity. If one were to assume that at least 50% of the beats should be required to be fast, this results in a high sensitivity line  192  where X is equal to one-half Y. Line  194  is the line of the least detection delay requiring only four beats to be analyzed while line  196  is the line which obtains for the most delay when 16 beats are analyzed for detection. By placing limits on Y and the relationship between X and Y, an acceptable region  198  results for the operation of the first detector as determined by its detection criteria. 
     Referring now to FIG. 6, it illustrates in a logic diagram form the manner in which a detection criteria may be adjusted automatically using a “fuzzy” subset approach. Here it will be noted that a rate variability has been added. The rate variability may be, for example, a coefficient of a variability of the ventricular rate or a standard deviation. The rate variability may be determined by the variability factor determining stage  104  of the detection criteria regulator  100  of FIG.  2 . 
     The logic circuit  200  includes AND gates  202 ,  204 ,  206 , and  208 . AND gate  202  includes an inverting input  210  and an input  212 . AND gate  204  includes inputs  214  and  216 . AND gate  206  includes an inverting input  218  and inputs  220  and  222 . Lastly AND gate  208  includes inputs  224 ,  226 , and  228 . Inputs  210 ,  214 ,  218 , and  224  are coupled together and to a line  230  which is a logical high when the ventricular rate variability factor is high. For example, if the ventricular rate variability factor is a coefficient of a variability greater than, for example, 10%-20%, then the line  230  will be a logical high. Inputs  212  and  216  are coupled to another line  232  which is high when there have been two consecutive successful detection confirmations. Inputs  222  and  228  are coupled to a line  234  which is high when there are two consecutive unsuccessful detection confirmation. Lastly, inputs  220  and  226  are coupled to a line  236  which is high when X is less than Y−1. 
     As will be noted from FIG. 6, if the coefficient of variability of the rate is low and two detections have been successfully confirmed in a row, then the X and Y values are decremented by one. However, if the coefficient of variability of the rate is high, and two detections have been successfully confirmed, then only the X value is decremented while the Y value is not. In this case, Y is not decremented because the greater variability in the rate requires a larger statistical base to ensure that a representative average is calculated for the rate. 
     If the coefficient of variability of the rate is low and X is at least one less than Y, and two consecutive unsuccessful confirmations have occurred, than both the X and Y values are incremented. However, if the coefficient of variability of the rate is high, than the X and Y values are both incremented by two. This is to increase the statistical base of the rate to reduce the risk of the detections being aborted merely because of statistical while not raising the percentage of the beats that must be fast. 
     In accordance with a further aspect of the present invention, the second detector  90  includes a capacitor reform timer  98 . In accordance with this aspect of the present invention, if a confirmation is unsuccessful and the capacitor reform is due or almost due as evidenced by the condition of the timer  98 , the capacitor  162  would finish charging without being discharged into the heart to allow for the capacitor  162  to be reformed. 
     While particular embodiments of the present invention have been shown and described, modifications may be made. It is therefore intended in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.