Abstract:
A method for generating a cardiac electrical instability assesment is disclosed herein. The method includes obtaining a SDTWA measurement, obtaining a LDTWA measurement, and obtaining a cardiac electrical instability assessment based on both the SDTWA measurement and the LDTWA measurement.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This disclosure relates to method and system for detecting T-wave alternans. 
         [0002]    An electrocardiogram (ECG) of a single heartbeat is commonly referred to as a PQRST complex. The PQRST complex includes a P-wave that corresponds to activity in the atria, a QRS complex that represents the electrical activation of the ventricles, and a T-wave that represents the electrical recovery or recharge phase of the ventricles. The PQRST complex also includes an ST segment connecting the QRS complex and the T-wave. T-wave alternans (TWA) is an electrophysiological phenomenon that is evident in the ECG as an alternating pattern of ST segment and/or T-wave morphologies on successive beats. 
         [0003]    Clinical studies have demonstrated that TWA is an indicator of cardiac electrical instability. One problem is that it is difficult to identify and measure the specific TWA morphological patterns that are most indicative of cardiac electrical instability. 
       SUMMARY OF THE INVENTION 
       [0004]    The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
         [0005]    In an embodiment, a method includes obtaining a short duration T-wave altemans (TWA) measurement, obtaining a long duration TWA measurement, and obtaining a cardiac electrical instability assessment based on both the short duration TWA measurement and the long duration TWA measurement. 
         [0006]    In another embodiment, a method includes obtaining a short duration TWA differential measurement, and obtaining a long duration TWA differential measurement. The method also includes eliminating any data exceeding a first high differential limit from the long duration TWA measurement. Cardiac electrical instability is diagnosed if the short duration TWA differential measurement exceeds a second high differential limit or if the long duration TWA differential measurement exceeds a low differential limit. The method also includes performing a TWA burden analysis in order to obtain a degree of concern assessment. 
         [0007]    In yet another embodiment, a system includes a plurality of sensors; and a processor operatively connected to the plurality of sensors. The processor is configured to generate a cardiac electrical instability assessment based on a short duration TWA measurement and a long duration TWA measurement. 
         [0008]    Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic illustration of a cardiac diagnostic/monitoring system operatively connected to a patient via a twelve lead system in accordance with an embodiment; 
           [0010]      FIG. 2  is a PQRST complex of an electrocardiogram in accordance with an embodiment; 
           [0011]      FIG. 3  shows two consecutive PQRST complexes that have been superimposed in accordance with an embodiment; 
           [0012]      FIG. 4  is a flow chart in accordance with an embodiment; 
           [0013]      FIG. 5  is a flow chart in accordance with an embodiment; 
           [0014]      FIG. 6  is a flow chart in accordance with an embodiment; and 
           [0015]      FIG. 7  is a flow chart in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention. 
         [0017]    Referring to  FIG. 1 , a schematically represented cardiac diagnostic/monitoring system  6  is adapted measure an electrical signal generated by a patient&#39;s heart. The cardiac diagnostic/monitoring system  6  can be coupled to the patient  12  by an array of sensors or transducers. In the illustrated embodiment, the array of sensors include a right arm electrode RA; a left arm electrode LA; chest electrodes V 1 , V 2 , V 3 , V 4 , V 5  and V 6 ; a right leg electrode RL; and a left electrode leg LL for acquiring a standard twelve lead, ten-electrode electrocardiogram (ECG) signal. The twelve ECG leads include leads I, II, V 1 , V 2 , V 3 , V 4 , V 5  and V 6  which are acquired directly from the patient leads, and leads III, aVR, aVL and aVF which are derived using Einthoven&#39;s law. The cardiac diagnostic/monitoring system  6  comprises a processor  8  configured to generate a patient diagnosis based on the measured cardiac electrical signals as will be described in detail hereinafter. 
         [0018]    Referring to  FIG. 2 , an electrocardiogram (ECG) of a single heartbeat typically referred to as a PQRST complex is shown. The portion of the PQRST complex defined between reference points  14  and  16  is defined as the P-wave, and corresponds to activity in the atria. The portion of the PQRST complex defined between reference points  18  and  20  is defined as the QRS complex, and represents the electrical activation of the ventricles. The portion of the PQRST complex defined between reference points  22  and  24  is defined as the T-wave, and represents the electrical recovery or recharge phase of the ventricles. The portion of the PQRST complex defined between reference points  20  and  22  is defined as the ST segment. The portion of the PQRST complex defined between reference points  20  and  24  comprising both the ST segment and the T-wave will hereinafter be referred to as the ST-T segment. 
         [0019]    T-wave alternans (TWA) is an electrophysiological phenomenon that is evident in the ECG as an alternating pattern of ST-T segment morphologies on consecutive beats. Referring to  FIG. 3 , two consecutive PQRST complexes have been superimposed to illustrate TWA. More precisely, a first PQRST complex  30  illustrated with a solid line has been superimposed onto an immediately consecutive PQRST complex  32  illustrated with a dashed line. 
         [0020]    According to one embodiment, TWA is measured as the maximum differential between the ST-T segment of the PQRST complex  30  and the ST-T segment of the PQRST complex  32 . For purposes of this disclosure, the term differential refers to the difference between two or more data points and is typically measured in microvolts. As an example, if the ST segment of the PQRST complex  32  exceeds that of the PQRST complex  30  by a maximum amount of  3 . 0  microvolts, and the T-wave portion of the PQRST complex  30  exceeds that of the PQRST complex  32  by a maximum amount of 5.0 microvolts, the TWA measurement for the consecutive PQRST complexes  30 ,  32  may be defined as 5.0 microvolts. This TWA measurement can be compared with previously acquired research or test data in order to identify cardiac electrical instability. 
         [0021]    According to another embodiment, a user can identify specific portions of the PQRST complex  30  and the PQRST complex  32  to be evaluated. According to this embodiment, TWA is measured as the maximum differential between the PQRST complex  30  and the PQRST complex  32  as measured in the identified portions of the respective complexes  30  and  32 . It should be appreciated that the identified portions of the complexes  30  and  32  may comprise a specific point or a range of points to be evaluated. 
         [0022]    The preceding method for measuring TWA is merely illustrative, and TWA may alternatively be measured in a variety of different manners. Some methods for measuring TWA are more capable of identifying cardiac electrical instability than others. In an effort to develop and validate an improved TWA measurement adapted to identify the highest percentage of patients with cardiac electrical instability, a clinical study comprising 681 patients was conducted and the method  100  (shown in  FIG. 4 ) was validated. The clinical study will now be described in more detail. 
         [0023]    During the course of the clinical study, it was observed that a first subset of patients with cardiac electrical instability is identifiable with a short duration high differential TWA measurement, and a second generally distinct subset of patients with cardiac electrical instability is identifiable with a long duration low differential TWA measurement. As a result of this observation, the method  100  (shown in  FIG. 4 ) assesses cardiac electrical instability based on two distinct TWA measurements in order to identify the greatest percentage of patients with cardiac electrical instability. For purposes of this disclosure a short duration TWA (SDTWA) measurement is a TWA measurement derived from  16  or fewer consecutive heartbeats, and a long duration TWA (LDTWA) measurement is a TWA measurement derived from 64 or more consecutive heartbeats. Also for purposes of this disclosure, the term high differential should be defined to include differentials in excess of 40 microvolts, and the term low differential should be defined to include differentials below 10 microvolts. It will be appreciated by those skilled in the art that SDTWA is sometimes referred to as non-sustained TWA, and LDTWA is sometimes referred to as sustained TWA. 
         [0024]    Support for the previously described clinical study observation can be seen from the following data. Over a three-year period, approximately 3.0% of the clinical study patients having cardiac electrical instability resulting in sudden arrhythmic death were identifiable using either short duration high differential TWA measurements or long duration low differential TWA measurements. By generally simultaneously measuring both short duration high differential and long duration low differential TWA, approximately 6.0% of the clinical study patients having cardiac electrical instability resulting in sudden arrhythmic death were identifiable. In contrast, no patients died if neither the short duration high differential TWA or long duration low differential TWA was detected. It can be seen from this data that a combined short and long duration TWA measurement identified twice as many patients with cardiac electrical instability as compared to a measurement relying on either short or long duration TWA exclusively. 
         [0025]    Referring to  FIG. 4 , the method  100  will now be described in accordance with an embodiment. As shown, the method  100  comprises steps  102 - 107 . According to one embodiment, one or more of the steps  102 - 107  may be performed by the processor  8  of the cardiac diagnostic/monitoring system  6  (shown in  FIG. 1 ). 
         [0026]    At step  102 , a SDTWA measurement is obtained. At step  104 , a LDTWA measurement is obtained. At step  106 , a cardiac electrical instability assessment is obtained based on both the SDTWA measurement of step  102  and the LDTWA measurement of step  104 . At step  107 , a TWA burden analysis is performed. Having briefly described each step of the method  100 , the individual steps  102 - 107  will now be described in more detail. 
         [0027]    Referring to  FIG. 5 , step  102  of the method  100  (shown in  FIG. 4 ) will now be described in accordance with an embodiment. As shown, step  102  comprises steps  110 - 116 . According to one embodiment, one or more of the steps  110 - 116  may be performed by the processor  8  of the cardiac diagnostic/monitoring system  6  (shown in  FIG. 1 ). It should be appreciated that the steps  110 - 116  need not necessarily be performed in the order shown. 
         [0028]    At step  110 , PQRST complex data pertaining to a plurality of sequential heartbeats is bifurcated into even beat data and odd beat data. At step  112 , a predetermined portion of the even beat data is extracted and averaged to produce an even beat average. According to one embodiment, at step  112 , the ST-T segments are extracted from the even beat data and are thereafter averaged. At step  114 , a predetermined portion of the odd beat data is extracted and averaged to produce an odd beat average. According to one embodiment, at step  114 , the ST-T segments are extracted from the odd beat data and are thereafter averaged. At step  116 , the even beat averages are compared with the odd beat averages. According to one embodiment, the step  116  comparison includes identifying the maximum difference between the even beat averages and the odd beat averages. 
         [0029]    Referring to  FIG. 6 , step  104  of the method  100  (shown in  FIG. 4 ) will now be described in accordance with an embodiment. As shown, step  104  comprises steps  108 - 116 . According to one embodiment, one or more of the steps  108 - 116  may be performed by the processor  8  of the cardiac diagnostic/monitoring system  6  (shown in  FIG. 1 ). It should be appreciated that the steps  108 - 116  need not necessarily be performed in the order shown. 
         [0030]    Steps  110 - 116  were previously described with respect to  FIG. 5  and will therefore not be described again. Step  108  is an optional step that may be implemented to filter out or otherwise eliminate high differential data from the LDTWA measurement. The filtration may be performed in a variety of known ways such as with a low-pass filter or with an algorithm adapted to digitally eliminate any data exceeding a predefined high differential limit before the LDTWA measurement is performed. The process of filtering out the high differential data from the LDTWA measurement prevents this data from introducing imprecision into the LDTWA measurement. In other words, if the high differential data is not filtered, it could skew the resultant LDTWA measurement and yield an imprecise or misleading result. 
         [0031]    Referring to  FIG. 7 , step  106  of the method  100  (shown in  FIG. 4 ) will now be described in accordance with an embodiment. As shown, step  106  comprises steps  120 - 124 . According to one embodiment, one or more of the steps  120 - 124  may be performed by the processor  8  of the cardiac diagnostic/monitoring system  6  (shown in  FIG. 1 ). 
         [0032]    At step  120 , it is determined if the SDTWA measurement obtained at step  102  (shown in  FIG. 5 ) exceeds a predetermined high differential limit. According to one embodiment, step  120  determines if the SDTWA measurement exceeds  60  microvolts. At step  120 , it is also determined if the LDTWA measurement obtained at step  104  (shown in  FIG. 6 ) exceeds a low differential limit. According to one embodiment, step  120  determines if the LDTWA measurement exceeds  5  microvolts. If, at step  120 , the SDTWA measurement exceeds the high differential limit or the LDTWA measurement exceeds the low differential limit, the algorithm proceeds to step  122  at which the patient is positively diagnosed for cardiac electrical instability. If, at step  120 , the SDTWA measurement does not exceed the high differential limit and the LDTWA measurement does not exceed the low differential limit, the algorithm proceeds to step  124  at which the patient is negatively diagnosed for cardiac electrical instability. 
         [0033]    Referring again to  FIG. 4 , step  107  of the method  100  will now be described in more detail. The TWA burden analysis of step  107  is optional and is adapted to provide a quantitative assessment along with each cardiac electrical instability diagnosis. In other words, the TWA burden analysis provides a degree of concern assessment that is intended to convey the seriousness of a given cardiac electrical instability diagnosis. 
         [0034]    According to one embodiment, the TWA burden analysis of step  107  may be performed by calculating the number of times the SDTWA measurement of step  102  exceeds a first predefined threshold (e.g., 60 microvolts); and the number of times the LDTWA measurement of step  104  exceeds a second predefined threshold (e.g., 5 microvolts). The following will provide an example illustrating this embodiment. For purposes of this illustrative example, assume a first patient exceeds a 60 microvolt threshold three times during a SDTWA measurement and exceeds a 5 microvolt threshold two times during a LDTWA measurement. Also for purposes of this illustrative example, assume that a second patient does not exceeds the 60 microvolt threshold during a SDTWA measurement and exceeds the 5 microvolt threshold one time during a LDTWA measurement. Both patients would receive a positive diagnosis for cardiac electrical instability at step  106  described in detail hereinabove with  FIG. 7 ; however, the first patient would also receive a TWA burden analysis of five indicating a greater degree of concern as compared to the second patient having a TWA burden analysis of one. 
         [0035]    According to another embodiment, the TWA burden analysis of step  107  may be performed by measuring the duration or amount of time during which the SDTWA measurement of step  102  exceeds a first predefined threshold (e.g., 60 microvolts); and the amount of time during which the LDTWA measurement of step  104  exceeds a second predefined threshold (e.g., 5 microvolts). The following will provide an example illustrating this embodiment. For purposes of this illustrative example, assume a first patient exceeds a 60 microvolt threshold for a period of three seconds during a SDTWA measurement and exceeds a 5 microvolt threshold for a period of two seconds during a LDTWA measurement. Also for purposes of this illustrative example, assume that a second patient does not exceeds the 60 microvolt threshold during a SDTWA measurement and exceeds the 5 microvolt threshold for a period of one second during a LDTWA measurement. Both patients would receive a positive diagnosis for cardiac electrical instability at step  106  described in detail hereinabove with  FIG. 7 ; however, the first patient would also receive a TWA burden analysis of five seconds indicating a greater degree of concern as compared to the second patient having a TWA burden analysis of one second. 
         [0036]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.