Patent Publication Number: US-9848833-B2

Title: Detection and display of irregular periodic wave forms

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 14/571,708, filed on Dec. 16, 2014 and now U.S. Pat. No. 9,681,818. Priority and benefit of that filing are claimed herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to displays of a signal, and specifically to displays of an irregular, periodic signal. 
     BACKGROUND OF THE INVENTION 
     During a cardiac procedure, the condition of the heart may be monitored, inter alia, by observing electrophysiological data in a graphical form, typically as potential vs. time graphs. While the heart is in a sinus condition, the periodic electrocardiogram (ECG) waveforms generated by the heart are relatively uncomplicated. However, in some non-sinus conditions of the heart, such as during atrial fibrillation, each periodic ECG signal may include one or more features that may only occur in some of the periodic signals, but not in every signal. Because of the irregularity of the signals, it is difficult to distinguish these features. 
     PCT Application WO 2010/054409, to Ramanathan et al., whose disclosure is incorporated herein by reference, describes a method for visualization of electrophysiology data. Electroanatomic data representing electrical activity on a surface of an organ over a time period is stored. An interval within the time period is selected in response to a user selection. Responsive to the user selection of the interval, a visual representation of physiological information for the user selected interval is generated by applying at least one method to the electroanatomic data. The visual representation is spatially represented on a graphical representation of a predetermined region of the surface of the organ. 
     U.S. Patent Application 2013/0245476, to Takizawa et al., whose disclosure is incorporated herein by reference, describes a cardiac muscle excitation waveform detector. The detector includes a waveform acquisition section that acquires, in a preset period, a waveform from an intracardiac electrocardiogram measured in the middle of occurrence of atrial fibrillation. The disclosure is stated to show a result of a mean conduction time that is calculated under an analysis technique described in the disclosure. 
     U.S. Patent Application 2007/0208260, to Afonso, whose disclosure is incorporated herein by reference, describes a system for presenting information representative of patient electrophysiological activity, such as complex fractionated electrogram information. The disclosure describes a presentation device that presents electrogram information as associated with a location at which it was measured on a model of the patient&#39;s heart. 
     Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that, to the extent that any terms are defined in these incorporated documents in a manner that conflicts with definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a method for display, including: 
     acquiring an electrical signal from a heart of a subject over multiple heart cycles; 
     partitioning the signal into a succession of synchronized segments having respective start times at a selected annotation point in the heart cycles; 
     overlaying respective graphical representations of the synchronized segments in the succession on a display screen, such that each segment is first presented on the display screen with an initial display intensity at a respective display time corresponding to a respective start time of the segment; and 
     gradually decreasing a display intensity of each segment overlaid on the display screen as a decaying function of a time elapsed since the respective display time. 
     Typically, overlaying the respective graphical representations includes summing the display intensity of the synchronized segments at overlapping points of the synchronized segments. Summing the display intensity may include summing the display intensity for a selected portion of the synchronized segments, wherein the selected portion is smaller than the synchronized segments. 
     In a disclosed embodiment overlaying the respective graphical representations includes summing the display intensity of the synchronized segments at points, of the synchronized segments, within a selected number of pixels of the display screen from each other. 
     In a further disclosed embodiment the selected annotation point is derived from a QRS complex of the heart signal. 
     In a yet further disclosed embodiment overlaying the respective graphical representations includes aligning respective baselines of the synchronized segments. 
     In an alternative embodiment overlaying the respective graphical representations includes aligning the selected annotation point of each of the synchronized segments with one another. 
     There is further provided, according to an embodiment of the present invention, apparatus, including: 
     a display screen; and 
     a processor, which is configured to: 
     acquire an electrical signal from a heart of a subject over multiple heart cycles, 
     partition the signal into a succession of synchronized segments having respective start times at a selected annotation point in the heart cycles, 
     overlay respective graphical representations of the synchronized segments in the succession on the display screen, such that each segment is first presented on the display screen with an initial display intensity at a respective display time corresponding to a respective start time of the segment, and 
     gradually decrease a display intensity of each segment overlaid on the display screen as a decaying function of a time elapsed since the respective display time. 
     The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a waveform display system, according to an embodiment of the present invention; 
         FIG. 2  is a schematic potential vs. time graph of an ECG signal derived from a heart, according to an embodiment of the present invention; 
         FIG. 3  is a flowchart of steps performed in implementing the waveform display system, according to an embodiment of the present invention; and 
         FIG. 4  shows schematic graphs illustrating some of the actions executed in the steps of the flowchart, according to an embodiment of the present invention; and 
         FIG. 5  illustrates different segments of a waveform as they are overlaid on a display screen, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     In certain conditions of the heart, such as some types of atrial fibrillation, each periodic ECG (electrocardiogram) signal may include one or more waveform features that only occur in some of the signals, but not in every signal. These irregularly recurring waveforms are periodic, in the sense that when they do recur, the recurrence is at approximately the same position (in time) in the signal. Because of the irregularity, the waveforms are difficult to distinguish. 
     Embodiments of the present invention provide a method for viewing the ECG signals so that the irregularly recurring waveforms are made more visible. A set of signals is acquired from the heart of a subject, the set being taken over multiple heart cycles of the subject. A processor partitions the signals into a succession of synchronized segments, using a common annotation point as a start time for each of the segments. A typical segment uses a part of the QRS complex of the ECG signal as the start time, and the succeeding QRS complex as the end time, to define the segments. 
     Graphical representations of the synchronized segments are overlaid on each other on a display screen, using the common annotation point as a registration point for the segments. The overlaid segments may also be overlaid on each other so that their baselines are aligned. The segments are presented on the display screen at a respective display time corresponding to a respective start time of the segment. In addition each segment is displayed with a common initial display intensity, and the display intensity of each segment is decreased as a decaying function of time elapsed since the initial presentation of the segment on the display screen. There are thus typically a number of different signals with different intensities on the display screen. 
     Where points or regions of the segments overlap, the display intensities of the overlapping points are summed. The summing emphasizes the irregularly recurring waveforms in the signals, making the waveforms significantly more visible than in prior art signal displays. 
     System Description 
     Reference is now made to  FIG. 1 , which is a schematic illustration of a waveform display system  20 , according to an embodiment of the present invention. System  20  is typically used during a medical procedure on a body organ, and in the description herein the body organ, by way of example, is assumed to comprise the heart, wherein the system is applied to display electrocardiogram (ECG) signals. Typically the system is used when the heart is undergoing atrial fibrillation, and the ECG signals may be intra-cardiac signals or signals derived from a location external to the heart, such as from a patient&#39;s skin. However, it will be understood that system  20  may be used for other states of the heart, including a sinus rhythm state, or for other signals, such as electroencephalograph (EEG) signals. 
     For clarity, except where otherwise stated, in the following description the signals displayed by system  20  are assumed to be ECG signals. 
     The description herein assumes that system  20  senses intra-cardiac ECG signals from a heart  22 , using a probe  24 . A distal end  26  of the probe is assumed to have an electrode  28  for sensing the signals. Typically, probe  24  comprises a catheter which is inserted into the body of a subject  30  during a cardiac procedure performed by a user  32  of system  20 . In the description herein user  32  is assumed to be a medical professional. 
     System  20  may be controlled by a system processor  40 , comprising a processing unit  42  communicating with an ECG module  44 . Module  44  in turn comprises a waveform display module  46 . Processor  40  may be mounted in a console  50 , which comprises operating controls which typically include a pointing device such as a mouse or trackball. Professional  32  uses the pointing device to interact with the processor, which, as described below, may be used to present results produced by system  20  to the professional on a display screen  54 . 
     The screen displays results of analysis and processing of ECG signals by ECG module  44 . Typically, the resultant ECG signals are presented on screen  54  in the form of a potential vs. time graph, and a schematic example  60  of such a graph is illustrated in  FIG. 1 . (A more detailed graph is shown in  FIG. 2 , described below.) However, the resultant ECG signals may also be used by processor  40  to derive other results associated with the ECG signals, such as a local activation time (LAT). These results are typically presented on screen  54  in the form of a three-dimensional (3D) map  64  of the internal surface of heart  22 . 
     Processor  40  uses software stored in a memory of the processor to operate system  20 . The software may be downloaded to processor  40  in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. 
     Processor  40  typically comprises other modules, such as a probe tracking module, a force module that measures a force on distal end  26 , and an ablation module that provides regulated to power to electrode  28 , or another electrode in the distal end. For simplicity, such modules are not shown in  FIG. 1 . The Carto® system produced by Biosense Webster, of Diamond Bar, Calif., uses such modules. 
       FIG. 2  is a schematic potential vs. time graph  100  of an ECG signal derived from heart  22 , according to an embodiment of the present invention. By way of example, graph  100  illustrates six QRS complexes occurring in the ECG signal, the complexes repeating at substantially regular intervals, and dividing the signal into five segments  102 ,  104 ,  106 ,  108 , and  110 . A heart beating in sinus rhythm, i.e., a “normal” heart, generates ECG signals that, except for minor variations between successive QRS complexes, comprise regular repeating substantially invariant segments. However, while graph  100  is periodic, having repeating QRS complexes, successive segments are irregular. Thus segments  102 ,  106 , and  110  have respective peaks  120 ,  122 , and  124  in the segments, whereas segments  104  and  108  have no such peaks. Such an irregular yet periodic waveform is typical of signals produced during some types of atrial fibrillation. 
       FIG. 3  is a flowchart of steps performed by processor  40 , together with modules  44  and  46 , in implementing system  20 , and  FIG. 4  shows schematic graphs illustrating some of the actions executed in the steps, according to embodiments of the present invention.  FIG. 2  is also used to illustrate some of the actions of the flowchart steps. 
     In an initial step  200 , processor  40  receives a series of heart signals, such as is illustrated by graph  100 . 
     In a partition step  202 , the processor partitions the series into a succession of segments, using a selected annotation point as a fiducial start time for each of the segments. Typically, the annotation point is derived from the signal received by electrode  28 . Alternatively, the annotation point is derived from another ECG signal, which may be acquired by another electrode on the catheter, or alternatively as a body surface signal from a body surface electrode. By way of example, the selected annotation points for the segments of graph  100  are assumed to be the peak of the QRS complex, and each segment is assumed to terminate at the next successive QRS complex. The succession of segments  102 ,  104 ,  106 ,  108 , and  110 , after they have been partitioned, is illustrated in  FIG. 4 . 
     In a synchronization step  204 , the succession of segments is synchronized to the selected annotation point. The synchronization is illustrated in  FIG. 4  by the peaks of the initial QRS complex of each segment being located on a vertical line  114 . 
     In a display step  206  graphs of the successive synchronized segments are presented on display screen  54 , the segments being aligned and overlaid with each other according to the selected annotation point. In some embodiments professional  32  selects the signal (different possible signals are described above in step  202 ) to be used for the selected annotation point, typically by preliminary observation of sets of overlaid segments, so as to determine the signal giving the most stable synchronization. Typically the alignment also includes aligning an estimated baseline of each segment. Each segment is first presented with an initial display intensity at a display time corresponding to the start time of the segment. 
     After initial presentation, the display intensity of each segment is allowed to decay, according to a predetermined decay function of a time elapsed since the initial presentation. A typical decay function reduces the intensity linearly by 15% for the time period between adjacent segments, so that after seven time periods, the display intensity is zero, i.e., the segment is not displayed. However, any other convenient function may be used for the reduced intensity. 
       FIG. 4  illustrates the different display intensities of the five segments of graph  100 , according to the presentation times of the segments shown in  FIG. 2 . Thus, in  FIG. 2 , the most recent segment is segment  110 , and the elapsed time from presentation of each of the segments, compared to the presentation time for segment  110 , increases monotonically through segments  108 ,  106 ,  104 , and  102 , so that segment  102  has the largest elapsed time. For clarity,  FIG. 4  shows the different segments as being separated vertically. Since the elapsed times for the respective segments  108 ,  106 ,  104 , and  102  increases, the display intensities of the segments decreases. By way of example, assuming the linear intensity reduction of 15% described above, and an intensity if  1  (measured in arbitrary units) for segment  110 , segments  108 ,  106 ,  104 , and  102  have respective intensities of 0.85, 0.7, 0.55, and 0.4. 
       FIG. 5  illustrates the different segments as they are overlaid on display screen  54 , according to an embodiment of the present invention. As the segments are overlaid, processor  40  sums the intensities of overlapping points of the graph. The summing enhances the visibility of regions of the graphs that repeat on an irregular basis. Thus, as illustrated, in overlaid segments  102 ,  104 ,  106 ,  108 , and  110  peaks  120 ,  122 , and  124  overlap, so that the intensity of the overlapped points is increased compared to the intensity of non-overlapped sections of the graphs. As shown in  FIG. 4 , peaks  120 ,  122 , and  124  have respective intensities 0.4, 0.7, and 1,so that the overlapped peaks have a nominal intensity of 2.1.  FIG. 5  illustrates the overlapped peaks as having an intensity on screen  54  of 2. 
     The embodiments described above have assumed that each segment is assigned an intensity according to the time elapsed on the display screen since its first presentment on the screen, and that where the segments overlap the intensities are summed. In an alternative embodiment of the present invention, rather than the intensity summation being applied to complete segments, the intensity summation is only applied to a portion of each of the segments. Typically, the portion to which the summation applies is selected by professional  32 . 
     For example, the professional may choose that the summation is only applied to the central 75% of each segment. This type of limitation allows system  20  to only enhance, by increasing their intensity, selected sections of the segments that are of interest to the professional, while not enhancing other sections, such as the start QRS complex and/or the end QRS complex, of the segments. 
     In a further alternative embodiment, rather than summing the intensities where segments overlap, e.g., where two or more segments have portions occupying the same pixels on screen  54 , the intensities may be configured to be summed if two or more segments have portions occupying pixels that are close to each other. The degree of closeness may be selected by professional  32 , and may be, for example where portions are within a selected number, for example 2, of pixels of each other. Allowing this type of summation provides the professional with the ability to sum sections that are of interest, even if there are slight variations, typically due to noise, between the sections of interest. The result of the summation may be presented on screen  54  as one or more pixels, with the summed intensity, at a mean location of the pixels that are close to each other. 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.