Abstract:
Methods and implantable devices that detect cardiac events using dynamic filtering. Illustratively, default filtering is performed except for a predefined period of time following detection of cardiac events, during which post-beat filtering is performed instead. The example post-beat filtering applies a narrower pass-band to the signal than the default filtering in order to attenuate T-waves more greatly than the default filtering during a time period after a detected event that is expected to correspond to occurrence of T-waves.

Description:
FIELD 
     The present invention relates to the field of implantable cardiac devices, including monitoring and stimulus devices. More particularly, the present invention relates to beat detection in such devices. 
     SUMMARY 
     The present invention, in a first illustrative embodiment, includes a method of detecting cardiac events in which a first filtering approach is used as a default and, when a beat is detected, a second filtering approach is used during an interval following the detected beat. In some examples, a refractory period is defined around the detected beat, and the second filtering approach is used during an interval following the refractory period. The second filtering approach may include more aggressive and frequency specific filtering directed at eliminating certain cardiac artifacts such as T-waves, relative to the first filtering approach. In addition to methods, the present invention also includes embodiments in the form of systems and implantable devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the frequency content of typical signals that an implantable cardiac device encounters during operation; 
         FIG. 2  illustrates the application of filtering and refractory periods to a cardiac signal for an illustrative embodiment; 
         FIGS. 3A-3C  demonstrate three configurations for performing filtering and detection in illustrative embodiments; 
         FIG. 4  shows an illustrative subcutaneous implantable defibrillator; 
         FIG. 5  shows an illustrative transvenous implantable defibrillator; and 
         FIG. 6  shows frequency content and filter response for an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description should be read with reference to the drawings. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention. Any references to other patents or patent applications are intended as illustrative of useful methods or devices and are not intended to foreclose suitable alternatives. 
       FIG. 1  illustrates the frequency content of typical signals that an implantable cardiac device encounters during operation. The signals shown omit the potential impact of motion artifact, but cover many other potential system inputs. The height of each block generally corresponds to typical amplitude ranges. The horizontal axis represents frequency in log format. 
     Cardiac signal is characterized in the chart of  FIG. 1  by T-waves, R-waves and P-waves; other “waves” in the cardiac signal are typically of lesser amplitude and are not represented in the drawing. The QRS complex is often referred to as the heart “beat”. Non-cardiac sources of interference can include myopotentials, which are generated by any non-cardiac muscle in the body, external line noise and/or other sources of noise. The external line noise varies in frequency depending on geographic region. In the example shown, external line noise is shown as 60 Hz line noise, which would occur in the United States. As is known in the art, other geographies may have 50 Hz line noise instead. Other sources of interference, whether intermittent or pervasive, are omitted for simplicity. 
     As can be seen, T-wave and R-wave signals are relatively lower in frequency than the line noise and myopotentials, and T-waves typically have a lower frequency content than the R-waves. Thus, frequency selectivity can be used to eliminate certain non-cardiac signals. It has been known to use notch filtering to attenuate line noise, and bandpass filtering can also be used. For example, U.S. Pat. No. 6,754,528 suggests the use of a Narrow Band filter with corner frequencies at approximately 10 Hz and 30 Hz, with a parallel Wide Band filter having corner frequencies at approximately 1 Hz and 50 Hz. In U.S. Pat. No. 6,754,528, the outputs of the two filters may be used for different purposes, for example, with the Narrow Band filtered signal used for event detection and the Wide Band filtered signal used for beat morphology analysis. In additional examples, filters may be modified in response to detected conditions, such as in US Patent Application Publication Number 2007-0032829, wherein a high pass filter can be bypassed in response to high beat rate to avoid attenuating low frequency components of the signal. 
     Several illustrative embodiments perform a different process in which cardiac signal data is filtered according to its timing relative to detected events. In other illustrative embodiments, beat detection is performed using differently filtered signals at different times relative to previous detected events.  FIG. 2  provides a graphic illustration. 
       FIG. 2  illustrates the application of filtering and refractory periods to a cardiac signal for an illustrative embodiment. A refractory period is a time period during which additional cardiac events are not declared by the system; sensing input circuitry may be on or off during refractory, as desired. In some examples, signals are captured during the refractory period to support morphology analysis of detected events. 
     The captured cardiac signal trace is shown at  10  and includes repetitive signal features marked according to standard convention as P, Q, R, S and T waves. Detection of events for this signal can be performed using a detection profile, for example as set forth in commonly assigned US Patent Application Publication Number 2009-0228057, titled ACCURATE CARDIAC EVENT DETECTION IN AN IMPLANTABLE CARDIAC STIMULUS DEVICE, the disclosure of which is incorporated herein by reference. For example, the captured signal is compared to a detection threshold and when the amplitude of the captured signal exceeds the detection threshold, a detected event is declared. For simplicity of illustration, detection profiles are not shown in  FIG. 2 . Any other suitable methods of detection may be used instead. 
     Detections are indicated at the X-es shown at  12  and  14 . Upon each detection  12 ,  14 , corresponding refractory periods  16 ,  18  start. Each refractory period  16 ,  18  has a predetermined duration. As noted in the 2009-0228057 Publication, the refractory periods may vary in duration in response to detected conditions such as amplitude similarities and/or calculated event/beat rate. 
     Ordinarily the system uses a default filter noted as “Filter  1 ,” as indicated at  20 ,  24 . Filter  1  may use frequency selective filtering such as bandpass filtering and/or notch filtering, as desired and known in the art. Upon termination of the refractory periods  16 ,  18 , a time period is defined for use of “Filter  2 ”, as indicated at  22 ,  26 . In the illustrative example, Filter  2  provides different frequency selectivity to the detection circuitry/module of the system when compared to Filter  1 , which is applied during other time periods shown at  20 ,  24 . Filter  1  may be considered the default filter for the system, while Filter  2  is applied for a period of time following refractory. As can be seen from comparison to the signal  10 , the time periods during which Filter  2  is applied at  22 ,  26  correspond to T-waves occurring in the cardiac signal  10 . The use of Filter  1  and Filter  2  may parallel other steps in the overall detection method, such as the use of constant threshold periods as shown in the 2009-0228057 Publication. In another example, a third filter, Filter  3 , is applied during the refractory period or another predetermined period initially following the detection, where Filter  3  is designed to support morphology analysis of the system, as in U.S. Pat. No. 6,754,528. 
     In several examples, Filter  2  is designed to more greatly attenuate frequencies that correspond to T-waves. For example, Filter  2  may include additional attenuation for frequencies between 3-15 Hz. In one example, Filter  1  sets the high-pass frequency corner of its passband in the range of 1-5 Hz, while Filter  2  moves the high-pass frequency corner of its passband to a higher level in the range of 3-10 Hz. Following are some illustrative numeric examples: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Filter 1 High Pass 
                 Filter 2 High Pass 
                 Filters 1 and 2 Low Pass 
               
               
                   
               
             
             
               
                 1 Hz 
                  3 Hz 
                 40 Hz 
               
               
                 1 Hz 
                 10 Hz 
                 40 Hz 
               
               
                 3 Hz 
                 10 Hz 
                 50 Hz 
               
               
                 5 Hz 
                 12 Hz 
                 40 Hz 
               
               
                   
               
             
          
         
       
     
     As noted, the Filter  2  approach can be applied for a limited period of time following refractory. In one example, the refractory period is in the range of 100-250 milliseconds, and the time period for applying Filter  2  is in the range of 100-200 milliseconds. For example, the refractory period may be about 160 milliseconds and the time period for Filter  2  may be about 140 milliseconds. In another example, the refractory period may be variable depending on cardiac conditions such as rate, and the time period for Filter  2  may adjust such that the sum of the refractory plus Filter  2  timer periods is generally constant in the range of 250-450 milliseconds. The examples may help to attenuate the T-wave during a time period where the system is susceptible to R-wave double/triple detection and T-wave overdetection. 
       FIGS. 3A-3C  demonstrate three configurations for performing filtering and detection in illustrative embodiments. Each example references an input signal  50 ,  70 ,  90 . The input signals  50 ,  70 ,  90  may be analog pre-amplifier, analog post-amplifier, and/or digital signals. 
       FIG. 3A  shows an example in which the input signal  50  is fed to a cascade of filters, including Filter  1 , at  52 , which provides an input to a default detection block  54 , and Filter  2 , at  56 , which provides an input to a Post-Beat detection block  58 .  FIG. 3B  shows an example in which the input signal  70  is fed to Filter  1 , at  72 , in parallel with Filter  2 , at  74 , and detection block  76  selects which filter  72 ,  74  to use at any given time depending upon when the last beat was detected.  FIG. 3C  shows an example in which the input signal  90  is fed to Filter  1 , at  92 , and Filter  2 , at  94 , in parallel, and each of the filters  92 ,  94  is used by different detection blocks, the default detection block  96  or the post-beat detection block  98 . 
     Reviewing  FIGS. 3A-3C , it should be noted that depending upon the design of analog and/or digital filters in these systems, it can be difficult to turn on or turn off filters without introducing additional filter-related-artifacts to the signal. Thus,  FIGS. 3A and 3C  both show examples in which separate detection systems are applied to different filter outputs. In  FIG. 3B  it is assumed that filter switching can occur without creating additional noise. In addition, each example of  FIGS. 3A-3C  shows multiple filters and, if desired, one of the filters may be applied in the digital domain while the other is applied in the analog domain. In some examples, the system may simply switch additional filtering components in/out of the circuit during operation, without adding additional layers of detection circuits. While separate blocks are shown for the different filters, it should be understood that physically separate implementation is not necessary; separate data processing may occur within a single physical unit such as a microcontroller. The use of separate blocks is merely for illustrative purposes. 
       FIG. 4  shows an illustrative subcutaneous implantable defibrillator. The illustrative system is shown relative to a patient&#39;s heart  100  and includes a canister  102  coupled to a lead  104  having electrodes  106 ,  108 ,  110 . The canister  102  includes an electrode  112 , such that the implanted system provides multiple sensing vectors shown at A-Can (between electrodes  106  and  112 ), B-Can (between electrodes  110  and  112 ) and A-B (between electrodes  106  and  110 ). Additional sensing vectors may use electrode  108 , which is shown as a relatively larger electrode and may take the form of a coil, as desired. Various designs can be used. Stimulus delivery in the illustrative system may use any chosen pair or combination of three or more electrodes; in one example, stimulus is provided between electrodes  108  and  112 . The canister  102  is shown as having an isolated button electrode  112 ; in other embodiments much of the exterior of the canister, rather than an isolated portion, can be used as an electrode. A programmer  114  is also shown, and may be used as is known in the art to communicate with the implanted system to perform various diagnostic, programming, testing and other functions. A single vector may be selected for sensing, or multiple vectors may be used simultaneously. 
     The system of  FIG. 4  is shown as a subcutaneous-only system lacking transvenous, endocardial and/or epicardial electrodes. The location is illustrated with a parasternal lead  104  extending from a lateral canister approximately located at the left axilla of the patient, such that electrode  110  is near the xiphoid of the patient with electrodes  108  and  106  more superiorly located along the sternum. Other subcutaneous-only implant locations can also be used, including anterior-posterior placements, anterior only placement, and/or lateral-posterior placement. 
       FIG. 5  shows an illustrative transvenous implantable defibrillator. The transvenous system is shown relative to the patient&#39;s heart  130  and includes a canister  132  coupled to a lead  134  that extends transvenously into the heart  130  through the using venous access via the subclavian vein. The lead  134  includes electrodes  136 ,  138  that are disposed within the heart, and the canister  132  includes a canister electrode  140 . A programmer  142  is again provided for communication with the implanted system. 
     The implanted systems can use any suitable technology for such aspects as the lead design, electrodes, canister design, electronics, batteries, communication circuitry etc. In one illustrative example, the canister contains operational circuitry including input circuitry having passive filtering components, a vector selection switch array, one or more ECG amplifiers and analog-to-digital conversion circuitry. A microcontroller may receive signal from this input circuitry. Various battery chemistries can be used, such as lithium-magnesium battery cells. Illustrative output circuitry that can also be part of the operational circuitry may include an H-bridge-type system having multiple legs and high and low sides with high power switches that enable multi-phasic therapy delivery. Therapy may be delivered from capacitors that can be charged with a charging circuit (such as a flyback transformer circuit) taking current from the battery cells, each of which may also be part of the operational circuitry. The canister itself may be formed of titanium, stainless steel or other suitable material and may include coatings such as titanium nitride, iridium oxide, porous carbon, etc. The leads may be formed of suitable biocompatible materials such as silicone, polyurethane, polycarbonate, and/or blends thereof or other polymers, coated or uncoated. The leads may contain conductors made, for example, with stainless steel (including MP35N alloy), silver, etc., in various forms including drawn filled tube designs. The electrodes can be coated or uncoated and may also be formed of suitable materials such as MP35N and other stainless steels, platinum, gold, silver, or titanium, for example. 
       FIG. 6  shows frequency content and filter response for an illustrative embodiment. Although presented in a form similar to a Bode plot, the graphic in  FIG. 6  is merely illustrative and is not necessarily to scale. 
     The chart of typical amplitudes versus frequency, as shown in  FIG. 1 , is condensed for illustration as shown at  200 . Two filter gain profiles are shown at  202  and  204 . Filter  1 , shown at  202 , has a gain profile that allows maximum gain across frequencies from about 4 Hz to about 50 Hz, with a notch at 60 Hz and attenuation at higher frequencies. Filter  2 , shown at  204 , as a gain profile that allows maximum gain across a smaller range of frequencies, attenuating the T-waves occurring below 10 Hz to a greater extent than Filter  1 . As indicated, the High Pass corner frequency is moved out to about 10 Hz. While a relatively gradual slope is shown, those skilled in the art will recognize that digital filter designs in particular can provide steep gain dropoff at desired corner frequencies. 
     In some examples, rather than Notch filter at the line frequency, the system may use a low pass filter having a very steep profile in the range of 40 Hz or so, which will function to attenuate line frequencies in various geographies. 
     The following US Patents, application publications, and provisional applications are incorporated herein by reference as illustrative examples for design, operation and implantation of cardiac devices: U.S. Pat. No. 6,647,292, titled UNITARY SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER; U.S. Pat. No. 6,721,597, titled SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER; U.S. Pat. No. 6,754,528, titled APPARATUS AND METHOD OF ARRHYTHMIA DETECTION IN A SUBCUTANEOUS IMPLANTABLE CARDIOVERTER/DEFIBRILLATOR; U.S. Pat. No. 7,149,575, titled SUBCUTANEOUS CARDIAC STIMULATOR DEVICE HAVING AN ANTERIORLY POSITIONED ELECTRODE; U.S. Pat. No. 7,330,757, titled METHOD FOR DISCRIMINATING BETWEEN VENTRICULAR AND SUPRAVENTRICULAR ARRHYTHMIAS; U.S. Pat. No. 7,248,921, titled METHOD AND DEVICES FOR PERFORMING CARDIAC WAVEFORM APPRAISAL; U.S. Pat. No. 7,392,085, titled MULTIPLE ELECTRODE VECTORS FOR IMPLANTABLE CARDIAC TREATMENT DEVICES; US Patent Application Publication Number 2006-0122676, titled APPARATUS AND METHOD FOR SUBCUTANEOUS ELECTRODE INSERTION, now U.S. Pat. No. 7,655,014; U.S. Pat. No. 7,376,458, titled METHOD FOR DEFINING SIGNAL TEMPLATES IN IMPLANTABLE CARDIAC DEVICES; U.S. Pat. No. 7,477,935, titled METHOD AND APPARATUS FOR BEAT ALIGNMENT AND COMPARISON; US Patent Application Publication Number 2006-0167503, titled METHOD FOR ADAPTING CHARGE INITIATION FOR AN IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR, now U.S. Pat. No. 8,160,697; US Patent Application Publication Number 2009-0228057, titled ACCURATE CARDIAC EVENT DETECTION IN AN IMPLANTABLE CARDIAC STIMULUS DEVICE; US Patent Application Publication Number 2009-0259271, titled METHODS AND DEVICES FOR ACCURATELY CLASSIFYING CARDIAC ACTIVITY, now U.S. Pat. No. 8,160,686; U.S. Pat. No. 7,623,913, titled IMPLANTABLE MEDICAL DEVICES USING HEURISTIC FILTERING IN CARDIAC EVENT DETECTION; U.S. Pat. No. 7,623,909, titled IMPLANTABLE MEDICAL DEVICES AND PROGRAMMERS ADAPTED FOR SENSING VECTOR SELECTION; US Patent Application Publication Number 2009-0036944, titled ELECTROMAGNETIC INTERFERENCE SHIELDING IN AN IMPLANTABLE MEDICAL DEVICE, now U.S. Pat. No. 7,769,457; US Patent Application Publication Number 2009-0198296, titled ADAPTIVE SHOCK DELIVERY IN AN IMPLANTABLE CARDIAC STIMULUS DEVICE, now U.S. Pat. No. 8,244,349; US Patent Application Publication Number 2009-0187227, titled DATA MANIPULATION FOLLOWING DELIVERY OF A CARDIAC STIMULUS IN AN IMPLANTABLE CARDIAC STIMULUS DEVICE; U.S. Provisional Patent Application Ser. No. 61/221,316, titled CONFIRMATION OF TREATABLE ARRHYTHMIA IN IMPLANTABLE CARDIAC STIMULUS DEVICES; U.S. Provisional Patent Application Ser. No. 61/255,249, titled METHODS AND DEVICES FOR IDENTIFYING OVERDETECTION OF CARDIAC SIGNALS; and U.S. Provisional Patent Application Ser. No. 61/255,253, titled ADAPTIVE WAVEFORM APPRAISAL IN AN IMPLANTABLE CARDIAC SYSTEM. These patents and publications are incorporated for illustrative purposes and the present invention may be used in other implantable cardiac systems as well, including monitoring systems and/or transvenous or epicardial systems. 
     Those skilled in the art will recognize that the present invention may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departures in form and detail may be made without departing from the scope and spirit of the present invention.