Abstract:
A method for evaluating an electrocardiogram is disclosed herein. The method includes measuring an electrical activity of a patient, processing the measured electrical activity to form a multi-lead signal, and extracting a segment of the multi-lead signal. The method for evaluating an electrocardiogram also includes transforming the segment of the multi-lead signal into a synthesized signal that is most representative of the patient&#39;s electrical activity, and evaluating the synthesized signal. A corresponding system for evaluating an electrocardiogram is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from provisional application Ser. No. 60/871,495, filed Dec. 22, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention pertains generally to a system and method for the evaluation of an electrocardiogram. 
       BACKGROUND OF THE INVENTION 
       [0003]    An electrocardiogram (ECG) of a single heartbeat is commonly referred to as a PQRST complex. The PQRST wave 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 QT interval is the time between the QRS onset and the end of the T-wave, and is commonly measured for purposes of evaluating cardiac electrical stability and thereby predicting potentially life threatening medical conditions such as cardiac arrhythmia. Some pharmaceuticals have side affects that increase the QT interval of an otherwise healthy patient and induce unstable cardiac electrical activity. Therefore, the FDA has begun to perform a drug-induced QT study on new pharmaceuticals prior to their approval. One problem is that an increase in QT interval is not directly correlated with cardiac electrical instability and another problem is that the QT interval is difficult to precisely measure. Therefore, if we only rely on the QT interval, there is the potential for the exclusion of beneficial pharmaceuticals based on the erroneous assumption that they may cause electrical instability and malignant cardiac arrhythmias. 
         [0004]    Studies have shown that an increase of heterogeneity in the re-polarization of the heart is directly linked to cardiac electrical instability. Accordingly, an attempt has been made to find the ECG features which have a higher correlation with the heterogeneity of the re-polarization as an indicator of cardiac electrical instability. More recently, it has been determined that the shape of the T-wave is an ECG feature that can be evaluated to more accurately asses cardiac electrical stability. As an example, T-wave flatness, asymmetry, and the presence of a “notch” in the T-wave have been correlated with unstable cardiac electrical activity. The problem is that, although T-wave shape observation has the potential to more accurately assess cardiac electrical stability, its consistency relies on the quality of data defining the T-wave. For example, a twelve lead ECG provides 12 separate T-waves representing different electrical views/projections of cardiac re-polarization. Therefore, if the optimal and most consistent T-wave representation is not implemented for observation, it will be difficult to assess a re-polarization abnormality based on any particular lead. 
         [0005]    Another problem relates to the placement of the sensors or transducers on a patient for the purpose of monitoring the electrical activity of the patient&#39;s heart. The conventional process involves placing multiple sensors at a variety of locations selected to optimally monitor the electrical activity. As the optimal sensor placement location varies during the course of the electrical cycle, and varies from patient to patient, it is currently not feasible to ensure that a sensor is placed at every optimal location for every patient. 
       SUMMARY OF THE INVENTION 
       [0006]    The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
         [0007]    In an embodiment, a method for evaluating an electrocardiogram includes measuring an electrical activity of a patient, processing the measured electrical activity to form a multi-lead signal, and extracting a segment of the multi-lead signal. The method for evaluating an electrocardiogram also includes transforming the segment of the multi-lead signal into a synthesized signal that is most representative of the patient&#39;s electrical activity, and evaluating the synthesized signal. 
         [0008]    In another embodiment, a method for evaluating an electrocardiogram includes measuring an electrical activity of a patient, processing the measured electrical activity to form a multi-lead signal, converting the multi-lead signal into a multi-dimensional representation, and extracting a segment from the multi-dimensional representation. The method for evaluating an electrocardiogram also includes performing a principal component analysis on the multi-dimensional representation to provide a synthesized signal that is most representative of the patient&#39;s electrical activity, and evaluating the synthesized signal. 
         [0009]    In yet another embodiment, a system for evaluating an electrocardiogram includes a plurality of leads attachable to a patient. The plurality of leads are configured to measure an electrical activity of the patient. The system for evaluating an electrocardiogram also includes a diagnostic system operatively connected to the plurality of leads. The diagnostic system is configured to form a multi-lead signal based on the measured electrical activity, extract a segment of the multi-lead signal, and transform the segment of the multi-lead signal into a synthesized signal that most accurately reflects the patient&#39;s electrical activity. 
         [0010]    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 
         [0011]      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; 
           [0012]      FIG. 2  is an electrocardiogram; and 
           [0013]      FIG. 3  is a block diagram illustrating a method in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    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. 
         [0015]    Referring to  FIG. 1 , a schematically represented cardiac diagnostic/monitoring system  10  is adapted measure an electrical signal generated by a patient&#39;s heart. The cardiac diagnostic/monitoring system  10  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. In other embodiments, alternative configurations of sensors and sensor locations can be used to acquire a standard or non-standard ECG signal. For example, an expanded fifteen lead system, including four extra electrodes, can be used to form Frank X, Y and Z leads. 
         [0016]    Referring to  FIG. 2 , an electrocardiogram 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  16  and  18  is defined as the QRS complex, and represents the electrical activation of the ventricles. The portion of the PQRST complex defined between reference points  18  and  20  is defined as the T-wave, and represents the electrical recovery or recharge phase of the ventricles. As is known in the art, the distinct portions or segments of the PQRST complex can be broken up and individually evaluated through a process known as segment extraction. It has been observed that an evaluation of the shape of an extracted T-wave can be used to evaluate the electrical stability of a patient&#39;s heart. For example, it is known that overly flat T-waves, certain shapes of asymmetric T-waves, and T-waves that include one or more “notches” are linked to unstable cardiac electrical activity that can lead, for example, to cardiac arrhythmia. T-wave shape evaluation may be performed manually or may be automated such as with a computer algorithm configured to identify T-wave data falling outside a predefined range. 
         [0017]    The twelve ECG leads (e.g., leads I, II, III, V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , aVR, aVL and aVF) obtained by the cardiac diagnostic/monitoring system  10  (shown in  FIG. 1 ) produce twelve different electrocardiograms that represent the electrical activity of a patient&#39;s heart with varying degrees of precision. The degree of precision with which a particular lead represents the heart&#39;s electrical activity is based in part on the placement of the corresponding electrode or electrodes. The electrodes V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , RA, LA, RL, LL that are in closest proximity to and/or are most optimally aligned with the electrical activity of the heart at any given time receive the strongest signal and are therefore best adapted to monitor such electrical activity. Accordingly, the ECG lead based on an electrode or electrodes that are in closest proximity to and/or are most optimally aligned with the electrical activity of the heart generally provide the most accurate reflection of this electrical activity. 
         [0018]    The optimal location on a given patient for electrode placement varies during the course of the heart&#39;s electrical cycle, and varies from patient to patient. Therefore, it is often the case that variation of the placement of the electrodes V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , RA, LA, RL, LL can affect the diagnostic accuracy. As will be described in detail hereinafter, an embodiment of this invention estimates the data that would be provided from a robust representative lead even if none of the electrodes are accurately placed on the patient  12 . Advantageously, this provides the most accurate and reflective data for purposes of generating and evaluating an electrocardiogram. 
         [0019]    Having described the cardiac diagnostic/monitoring system  10  in accordance with an embodiment, a method for electrocardiogram evaluation will now be described. The method will hereinafter be described as being applied to the T-wave segment of an electrocardiogram for purposes of evaluating T-wave shape. It should, however, be appreciated that alternative methods may be applied to an entire electrocardiogram, or to any other electrocardiogram segment. 
         [0020]    Referring to  FIG. 3 , a block diagram illustrates a method  100 . The individual blocks  102 - 112  represent steps that may be performed in accordance with the method  100 . Unless otherwise specified, the steps  102 - 112  need not be performed in the order shown. 
         [0021]    At step  102 , a patient&#39;s electrical activity is measured such as, for example, with the cardiac diagnostic/monitoring system  10  (shown in  FIG. 1 ). According to one example, the electrical activity measured at step  102  is produced by the patient&#39;s heart. The patient may be a human or an animal. 
         [0022]    At step  104 , the electrical activity measured at step  102  is processed to form a multi-lead ECG signal such as, for example, with the cardiac diagnostic/monitoring system  10 . According to the exemplary embodiment illustrated in  FIG. 1 , the multi-lead ECG signal is a twelve lead ECG signal including the leads I, II, III, V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , aVR, aVL and aVF. 
         [0023]    Step  106  is an optional step wherein the multi-lead ECG signal of step  104  is converted to a multi-dimensional ECG representation. According to one embodiment, the multi-lead ECG signal is a twelve lead ECG signal, and the multi-dimensional ECG representation is a three-dimensional ECG representation having orthogonal X, Y and Z leads. Step  106  can be skipped if the cardiac diagnostic/monitoring system  10  (shown in  FIG. 1 ) is replaced with a system adapted to provide orthogonal X, Y and Z lead data such as, for example, the Frank lead system. The following equations may be implemented to convert twelve lead data (eight independent leads) into orthogonal X, Y and Z lead data: 
         [0000]        X=− 0.1106* v 1+0.045* v 2−0.04* v 3+0.2146* v 4−0.067* v 5+0.6868* v 6+0.3872* I− 0.1993* II    
         [0000]        Y= 0.1855* v 1−0.0728* v 2+0.0186* v 3+0.0154* v 4−0.1148* v 5+0.0682* v 6−0.0695* I+ 1.145* II    
         [0000]        Z= 0.3665* v 1−0.0363* v 2+0.165* v 3+0.2041* v 4+0.1395* v 5−0.4688* v 6+0.0587* I+ 0.0815* II    
         [0000]    This conversion matrix was developed from 5,000 fifteen lead ECG systems (i.e., standard twelve lead plus Frank X, Y and Z lead systems), and a regression analysis run between 5,000 paired twelve lead and Frank X, Y and Z lead systems. Any other known methods for converting from a multi-lead ECG signal to a multi-dimensional ECG representation, such as the inverse Dower method, may alternatively be implemented at step  106 . According to the exemplary embodiment wherein the multi-dimensional ECG representation is a three-dimensional representation, the orthogonal X, Y and Z lead data models the heart as a three-dimensional dipole source producing an electrical signal that varies based on the distance from the heart in the X, Y and Z directions. This is known as a dipole vector model, and the X, Y and Z lead data respectively defines the X, Y and Z components of the dipole source. 
         [0024]    Step  108  is an optional step in which one or more segments of the PQRST complex (shown in  FIG. 2 ) are extracted. According to one embodiment, the T-wave segment of the PQRST complex is extracted at step  108 . According to another embodiment, both the T-wave and the QRS complex segments of the PQRST complex are extracted. 
         [0025]    At step  110 , the multi-dimensional ECG signals of step  106  are transformed into at least one synthesized ECG signal representing an optimally oriented projection vector. The synthesized ECG signal defines the highest energy vector at any given time. The “highest energy vector” conveys both magnitude and direction of the cardiac electrical activity as measured by an optimally placed electrode. Even if none of the electrodes V 1 , V 2 , V 3 , V 4 , V 5 , V 6 , RA, LA, RL, LL (shown in  FIG. 1 ) are actually placed in the optimal location, the synthesized ECG signal still provides an estimate of the data that would represent the major heart re-polarization activity. The synthesized ECG signal of step  110  yields a more robust representation of heart&#39;s actual electrical activity than would otherwise be obtainable by selecting a single physically obtained or derived lead. According to one embodiment, the transform of step  110  is obtained by performing principal component analysis (PCA) on the multi-dimensional ECG signals of step  106 . Principal component analysis is known to those skilled in the art and therefore will not be described in detail. According to alternate embodiments, the transform of step  110  may be performed by other processes such as independent component analysis (ICA), nonlinear component analysis (NLCA), factor analysis (FA), projection pursuit (PP), singular value decomposition (SVD), and similar techniques. 
         [0026]    At step  112 , the output of the transform performed at step  110  is analyzed. It should be appreciated that many different types of analyses can benefit from the method  100  as the data on which the analysis is based is most reflective of the patient&#39;s actual electrical cardiac activity. In a non-limiting manner, the following will describe several exemplary analyses that may be performed at step  112 . 
         [0027]    According to an embodiment wherein the T-wave segment of the PQRST complex is extracted at step  108 , the analysis of step  112  may include evaluating the shape of the T-wave defined by a principal PCA vector. The T-wave shape evaluation may look for features such as flatness, asymmetry, and the presence of a notch which are linked to unstable cardiac electrical activity. 
         [0028]    According to an embodiment wherein the T-wave segment of the PQRST complex is extracted at step  108 , the analysis of step  1   12  may include evaluating the shape of the T-wave defined by a second PCA vector. The T-wave shape evaluation may look for features such as flatness, asymmetry, and the presence of a notch which are linked to unstable cardiac electrical activity. 
         [0029]    According to an embodiment wherein the T-wave segment of the PQRST complex is extracted at step  108 , the analysis of step  112  may include evaluating the orientation of the principal PCA vector relative to the second PCA vector. 
         [0030]    According to an embodiment wherein the T-wave segment of the PQRST complex is extracted at step  108 , the analysis of step  112  may include evaluating the orientation of the projection vector represented by the synthesized ECG signal relative to the X, Y and Z lead vectors. These angles can be calculated using basic geometry principles as will be appreciated by those skilled in the art. The orientation of the projection vector represented by the synthesized ECG signal relative to the X, Y and Z lead vectors is one of the features that is indicative of cardiac electrical stability such that the method  100  can be used to predict heart problems including arrhythmia. 
         [0031]    According to an embodiment wherein both the T-wave and the QRS complex segments of the PQRST complex are extracted at step  108 , the analysis of step  112  may include evaluating the orientation of the projection vector representing the T-wave relative to the projection vector representing the QRS complex. The orientation of the projection vector representing the T-wave relative to the projection vector representing the QRS complex is indicative of cardiac electrical stability such that the method  1   00  can be used to predict heart problems including arrhythmia. 
         [0032]    According to an embodiment wherein both the T-wave and the QRS complex segments of the PQRST complex are extracted at step  108 , the analysis of step  112  may include evaluating the length of the QT interval as represented by the synthesized ECG signal. The length of the QT interval is indicative of cardiac electrical stability and therefore can be used to predict heart problems including arrhythmia. 
         [0033]    While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made to the embodiments without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only, and should not limit the scope of the invention as set forth in the following claims.