Method to identify electrode placement

An ECG signal is acquired in multiple channels, and sources of interference are filtered from the signals. A covariance matrix is then formed with the channels of data. The invention then employs matrix mathematics to discover a set of eigenvectors that organize the variability of data in a multi-dimensional space along new directions, orthogonal to each other and ranked in order of significance. For each eigenvector, a corresponding eigenvalue is calculated. In addition, coefficients are calculated which correspond to the portion of each eigenvector that is necessary to reconstruct each original vector. From the eigenvector solution of the covariance matrix, the angles between the eigenvectors and the original vectors are determined. The eigenvector coefficients and the angles between the eigenvectors and the original vectors are related by a cosine relationship. The angles calculated for each particular ECG test are compared to a reference set of angles to determine whether the electrodes are placed in the standard ECG electrode placement, an alternative electrode placement, or an incorrect electrode placement.

BACKGROUND OF THE INVENTION
 The invention relates to measuring a physiological characteristic of a
 patient, and particularly, to an electrocardiograph including a method and
 apparatus for identifying the relative position of the electrodes
 connected to the patient.
 It is commonly known that ten electrodes and ten leadwires are needed to
 record and present what is commonly referred to as a twelve lead
 electrocardiogram (ECG), i.e., a group of twelve signals representing
 twelve different "views" of the electrical activity in the patient's
 heart. For standard or resting ECG electrode placement, one electrode is
 attached to each of the four body limbs at the right wrist, left wrist,
 right ankle, and left ankle. Additionally, six electrodes are attached to
 the chest over the heart. The ten electrodes connect via several resistor
 networks to enough amplifiers to record twelve channels of ECG. The twelve
 leads (i.e., signals) are generally split into two groups comprising the
 frontal plane and the horizontal plane. The frontal plane leads (I, II,
 III, aVr, aVl, aVf) are variously referred to as limb leads, Einthoven
 leads, or bipolar leads. The horizontal plane leads (v1, v2, v3, v4, v5,
 v6) are likewise variously referred to as precordial leads, chest leads,
 or unipolar leads.
 Accurate placement of the electrodes on the patient's body surface is
 required to record a useful ECG using an electrocardiograph or patient
 monitor. The ideal placement of electrodes for a standard ECG is well
 defined and accepted within the medical industry. However, routine correct
 placement of the electrodes in the clinical environment is difficult to
 achieve for several reasons. First, nurses and ECG technicians are
 frequently not adequately trained or are too inexperienced to accurately
 locate the attachment points. Moreover, individual physical
 characteristics vary widely from patient-to-patient. These variations lead
 to misinterpretation of the "anatomical guideposts" used to locate the
 proper attachment points. Additionally, patients sometimes have wounds or
 bandages that preclude access to the patient's body surface at the proper
 attachment points. Also, attachment of the electrodes to an ECG machine is
 often accomplished using long individual ECG leadwires. Even if the
 electrodes are accurately placed on the patient, the leadwires connecting
 them to the electrocardiograph may be crossed such that signals are
 switched at the instrument.
 Many inventors have attempted to solve the problem of electrode connection
 to the chest. Numerous belts, pads, vests, harnesses and strip electrodes
 have been developed that place a multitude of electrodes into an ordered
 arrangement to facilitate the attachment of the leads to the patient and
 eliminate the possibility of some types of attachment errors. In general,
 these inventions attempt to fix the six horizontal electrodes in relation
 to each other while adapting to different patient sizes. None of these
 teachings address the issue of placement of the limb electrodes. Moreover,
 the location of the horizontal lead electrodes may still not be at the
 proper anatomical positions.
 In some ECG applications the patient must be free to move. Thus, it becomes
 inconvenient or impossible to place the electrodes on the wrists and
 ankles. Applications where the patient must be free to move include long
 term recordings, known as holter; ambulatory patient monitoring, such as
 telemetry monitoring; and exercise testing on treadmills or bicycles,
 known as stress testing. In these tests, the wrist and ankle electrode
 positions are unacceptable for electrode placement due to inconvenience,
 increased danger of tangling of the lead wires, and increased noise from
 limbs in motion. Generally, in each of these ECG applications the limb
 electrodes are moved onto the torso and placed near the shoulders and
 hips. The Mason-Likar system is one variation of electrode placement on
 the torso. Twelve-lead bedside monitoring also requires placement of the
 electrodes on the torso. In each of the systems for alternative electrode
 placement, useful ECG data is obtained, but the data differs significantly
 from standard EGC data. Important differences in amplitudes and waveforms
 occur between standard ECGs and alternative electrode placement ECGs.
 Due to tile differences between data obtained from standard ECGs and
 alternative electrode placement ECGs, a complication in ECG analysis
 arises when all ECG test results, regardless of the type of electrode
 placement, are stored in the same hospital storage system. The same
 patient may have ECG data stored on the hospital system for a standard ECG
 and an ECG obtained during a stress test. If no explanation is given for
 the differences in the data, cardiologists and hospital technicians may be
 confused when both sets of ECG data are viewed together.
 SUMMARY OF THE INVENTION
 Accordingly, the invention provides a method and apparatus for analyzing
 twelve-lead electrocardiograms (ECGs) and for identifying the angles
 between all the lead vectors. This information allows recognition of the
 placement of electrodes (either the unintended misplacement or the
 intentional choice of alternative placements), without the requirement for
 additional placement of other devices on the patient such as belts, pads,
 vests, harnesses, electrode strips, or non-standard additional electrodes,
 and without the need for additional electronics such as impedance current
 injectors, impedance measurement circuits, sonic or magnetic digitizers,
 and/or digital cameras.
 For the method of the invention, ten seconds of ECG data from eight leads
 is gathered. Data from two of the frontal leads and all six of the
 horizontal leads is gathered. A representative heartbeat is located in
 each channel of data, and sources of interference are filtered from the
 data. A covariance matrix is then formed with the eight channels of
 remaining data.
 The invention then employs matrix mathematics, referred to as the
 Karhunen-Loeve transform (KLT), singular value decomposition, principal
 components analysis, or principal forces analysis, to discover a set of
 basis vectors or eigenvectors that organize the variability of data in a
 multi-dimensional space along new directions, orthogonal to each other and
 ranked in order of significance. For each eigenvector, a corresponding
 eigenvalue is calculated. In addition, eigenvalue coefficients are
 calculated which correspond to the portion of each eigenvector that is
 necessary to reconstruct each original lead vector. This technique has
 been used in the prior art to reduce the redundancy of multi-dimensional
 data, to compress and transmit ECG data, to organize features for ECG
 waveform classification, and to reduce noise sources in ECG. However, none
 of the disclosed prior uses of KLT, SVD, PCA, PFA, or like methods allow
 the identification of electrode placement.
 From the eigenvector solution of the covariance matrix, the angles between
 the eigenvectors and the original vectors are determined. The eigenvalue
 coefficients and the angles between the eigenvectors and the original
 vectors are related by a cosine relationship. The angles calculated for
 each particular ECG test can be compared to a reference set of angles to
 determine whether the electrodes are placed in the standard or resting ECG
 electrode placement, an alternative electrode placement, or an incorrect
 electrode placement.
 The invention further includes an ECG machine capable of alerting an ECG
 technician of non-standard or incorrect electrode placement. The ECG
 machine is capable of instructing the ECG technician as to how far and in
 what direction the electrodes are out of place. The ECG machine is also
 capable of labeling the ECG test data with information regarding the
 particular type of electrode placement used during the ECG test, including
 standard electrode placement and various alternative electrode placements.
 The invention still further includes a software program capable of
 analyzing ECG test data. The software program is capable of analyzing ECG
 data to determine what type of electrode placement was used during the
 test. The software program can then label the ECG test data to inform
 cardiologists that standard electrode placement was used or an alternative
 electrode placement was used.
 It is an advantage of the invention to provide a method of evaluating lead
 placement in an ECG.
 It is another advantage of the invention to eliminate the need to manually
 measure the positions of the electrodes on the patient.
 It is still another advantage of the invention to provide a method of
 evaluating lead placement for stored ECGs, i.e., ECGs that have previously
 been acquired and copied into patient information storage and retrieval
 systems.
 It is still another advantage of the invention to provide a method of
 evaluating lead placement in an ECG, which method does not require prior
 knowledge about how the electrodes were originally placed on the patient.
 Various other features and advantages of the invention are set forth in the
 following drawings, detailed description and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 The method will be described in the context of twelve-lead
 electrocardiograms. It is understood that the essence of the invention is
 applicable to any signal-processing environment where a redundancy of data
 can be used to infer relationships between the sources of the data
 regarding directions and/or angles. The scope of the invention is not
 limited to one heartbeat or ten seconds of ECG. The scope of the invention
 is not limited to eight or twelve leads or channels, but the example of a
 ten second, twelve-lead ECG is very useful and common and will be used
 herein for the purpose of describing the invention.
 FIG. 2 illustrates an ECG machine 10 embodying the invention. While the
 invention is described in the context of an ECG machine, other devices
 embodying the invention include data storage and retrieval devices used
 for long term storage of patient data. In such devices, the ECG data is
 originally acquired or input from an ECG machine or other patient
 monitoring device (e.g. stress testing machine, Holter monitor, bedside
 monitor, etc.), and is stored in memory (not shown) for later (even long
 term) retrieval and analysis. The ECG machine 10 includes lead wires LA,
 RA, LL, RL and v1-v6 (only a few of which are shown in FIG. 2). For a
 standard ECG, ten electrodes are attached to a patient's body. One
 electrode is attached to each of the patient's four limbs at the wrists
 and ankles. These electrodes are referred to as left arm (LA), right arm
 (RA), left leg (LL), and right leg (RL). As shown in FIG. 1, six
 electrodes are attached in standard positions on the chest around the
 heart. As is commonly known in the art, the ten electrodes are connected
 via the respective lead wires and several resistor networks (not shown) to
 enough amplifiers 15 (only a few of which are shown in FIG. 2) to record
 twelve separate ECG signals or twelve leads.
 The leads are split into two groups: the frontal plane and the horizontal
 plane. If a straight line were drawn from the heart to each wrist and each
 ankle, the four lines would lie in the frontal plane. Similarly, if a
 straight line were drawn from the heart to each of the six electrodes
 placed on the patient's chest, the six lines would generally lie in the
 horizontal plane. The leads in the frontal plane are referred to as the
 frontal leads, the limb leads, the Einthoven leads, or the bipolar leads,
 and include leads I, II, III, aVr, aVl, and aVf. The leads in the
 horizontal plane are referred to as the horizontal leads, the precordial
 leads, the chest leads, or the unipolar leads, and include leads v1, v2,
 v3, v4, v5, and v6.
 The frontal leads are obtained with various permutations of the LA, RA, and
 LL electrodes, with the RL electrode serving as an electrical ground. As
 shown in FIG. 2, bipolar leads are comprised of the potential between two
 electrodes: lead I corresponds to the potential between LA and RA, lead II
 corresponds to the potential between LL and RA, and lead III corresponds
 to the potential between LL and LA. As shown in FIG. 2, augmented leads
 are comprised of the potential between one electrode and a reference
 input, the reference input being the average of two electrodes. For
 example, lead aVf is the signal between LL and a reference input, where
 the reference input is the average of the potentials at electrodes RA and
 LA.
 As shown in FIG. 2, the horizontal leads are obtained with various
 permutations of the six electrodes attached to the patient's chest, in
 addition to the four limb electrodes. Each of the six horizontal leads is
 comprised of the signal between the potential at the particular electrode
 placed on the patient's chest and the potential at Wilson's central
 terminal. Wilson's central terminal refers to the average potential
 between the RA, LA, and LL electrodes. For example, lead v1 is the signal
 between electrode v1 and Wilson's central terminal.
 The ECG machine 10 also includes an ECG control 20 connected to the
 amplifiers 15. The ECG control 20 receives the ECG data from the
 amplifiers 15, analyzes the ECG data and stores the ECG for later
 retrieval. The ECG control 20 includes an analysis module 25 for analyzing
 the ECG data. As will be appreciated by those of skill in the art, the
 analysis module 25 can effect analysis of the ECG data using electronic
 hardware, or a combination of electronic hardware and software.
 The ECG control includes a summary storage device 30, and a display device
 35 connected to the analysis module 25. The display device 35 may be a
 printer or monitor, and any number of display devices may be connected to
 or controlled by the analysis module 25. Moreover, other external
 connections (not shown) or other internal devices (also not shown) may be
 included in the ECG machine to effect long term storage and retrieval of
 ECG data and other patient data.
 Generally, twelve channels of ECG data are acquired by the ECG machine 10
 and stored in the memory storage device 30. Most of the data acquired from
 the six frontal leads is repetitive data. It is well known in the art that
 the six frontal plane leads are easily related to each other by
 Einthoven's triangle. Kirchhoff's voltage law can be used with lead
 vectors I, II, and III, forming a triangle in the frontal plane, known as
 Einthoven's triangle. If any two vectors are known, the third vector can
 be calculated, because the sum of the three vectors must equal zero. Only
 two leads of the three leads contain independent data. Moreover, leads
 aVr, aVl, and aVf are just permutations of leads I, II, and III, so if any
 two frontal leads are known, the other four frontal leads can be
 calculated.
 As shown in the block diagram of FIG. 13, the method of the invention
 begins with the acquisition of the lead data. Due to the repetition of the
 data, only the data from two frontal leads is gathered. In the preferred
 embodiment, only the data from leads I and II is gathered. If only the
 data from leads I and II is gathered, the data for the remaining four
 frontal leads is calculated using the following equations:
EQU III=II-I
EQU aVr=-(I+II)/2
EQU aVl=(I-III)/2
EQU aVf=-(II+III)/2
 In addition to the two frontal leads, data is gathered from each of the six
 horizontal leads. Only ten seconds of data from each of the eight leads is
 necessary to practice the method of the invention. Ten seconds of eight
 channels of ECG is gathered at 500 samples per second. The data from each
 of the original eight leads is shown in FIG. 3. From this data, a
 representative heartbeat is located or derived in each channel by methods
 known in the art. Sources of interference, such as power line frequencies,
 respiration, muscle tremor, or baseline drift, are then removed. The
 remaining signal represents only the electrical activity of the heart
 according to the dipole model. This signal is dominated by an equivalent
 instantaneous electrical vector in three dimensions located at a point
 source within the heart. It is assumed that the recorded voltages on the
 body surface are principally projections in different directions of that
 instantaneous electrical vector.
 Once the data is acquired and filtered, the analysis module 25 forms a
 covariance matrix of the ECG data. The method includes removal of the mean
 value from each of eight lead vectors consisting of, for example, 600
 samples each. Each vector is multiplied by each other vector, sample by
 sample, and the products summed to obtain the dot product solution. The
 dot product solution is referred to as the covariance. ECG leads that are
 very similar in shape will have high covariances.
 The analysis module 25 then calculates the eigenvector solution to the
 covariance matrix using one of the following generally equivalent
 mathematical methods: Karhunen-Loeve transform (KLT), singular value
 decomposition, principal forces analysis, or principal components
 analysis. The method referred to for purposes of this description is the
 Karhunen-Loeve transform or KLT.
 The KLT is most easily understood in the context of three-dimensional data,
 such as data acquired using the Frank lead system. FIG. 1 illustrates the
 Frank lead system, which in addition to the standard ten ECG electrodes,
 includes four additional electrodes (H,I,E,M). The Frank lead system is
 used to acquire data that represents the heart activity in only three
 vectors. Basically, the KLT is applied to the data to analyze the
 variability of the data in the x, y, z coordinate system and determine a
 new u, v, w coordinate system. More specifically, KLT is implemented by
 the analysis module 25 to determine the direction in which there is the
 most variation in the data. The direction of most variability in the data
 becomes the u coordinate. The direction with the second most variability
 becomes the v coordinate, with the v coordinate being perpendicular to the
 u coordinate. The direction with the third most variability becomes the w
 coordinate, with the iv coordinate being perpendicular to the plane
 defined by the u and v coordinates. From the first three directions of
 variation, a new three-dimensional coordinate system is established that
 corresponds to each particular set of data. The first three eigenvectors
 correspond to the first three directions of most variability represented
 by the u, v, and w coordinates. Although difficult to visualize, each
 additional direction of variability is also determined by the KLT. These
 additional dimensions may represent variations in the patient's tissue,
 such as whether the electrode is placed over rib bones or over intercostal
 space.
 Since most data can be represented with three dimensions, the first three
 directions of variability, and thus, the first three eigenvectors are the
 most significant. The eigenvector solution for the original data from FIG.
 3 is shown in FIG. 4. For clarification purposes, the first eigenvector
 (e1) does not correspond to the data from lead I, rather the first
 eigenvector (e1) corresponds to the direction of most variability of the
 data from all eight leads. As shown in FIG. 4, the first three
 eigenvectors (e1, e2 and e3) contain most of the relevant data, since the
 fourth through eighth eigenvectors (e4, e5, e6, e7 and e8) have limited
 signal content. Moreover, it is clear from FIG. 4 that most of the heart's
 activity is along a single direction represented by the first eigenvector.
 Eigenvalues for each eigenvector are then determined by the analysis module
 25 from the position of each data point along the eigenvector. Each data
 point is given a value corresponding to the variation of the data point
 from the eigenvector. The mean of the values corresponding to each data
 point is found. The standard deviation from the mean is considered the
 eigenvalue for that particular eigenvector. The eigenvalues for the first
 three eigenvectors will be the largest, since the first three eigenvectors
 represent the directions along which the data varies the most.
 For the method of the invention, all but the three largest eigenvalues and
 their corresponding eigenvectors are discarded. As shown in FIG. 5, tables
 for all the eigenvalues and the most important three eigenvectors are
 constructed. The first table of eigenvalues in FIG. 5 corresponds to the
 total eigenvalues for each of the eight eigenvectors. The second table in
 FIG. 5 consists of the coefficients of each eigenvector necessary to
 reconstruct the original lead data. For example, original lead I is
 approximately represented by about 20% of eigenvector e1, 15% of
 eigenvector e2, and 12% of eigenvector e3. As shown in FIG. 6, the
 eigenvalues and their corresponding eigenvectors can be used to accurately
 reconstruct the original data.
 More importantly than reconstructing the original data, the coefficients in
 the second table in FIG. 5 can be used to determine the angles between
 each of the original lead vectors and the eigenvectors. The eigenvalue
 coefficients in the second table can be interpreted as cosines of the
 angles between each original lead vector and the eigenvector. For example,
 the eigenvalue coefficient corresponding to eigenvector e1 and lead v4 is
 -0.522. The cosine of 60 degrees is 0.5. Thus, the angle between
 eigenvector e1 and lead v4 is about 60 degrees. Similarly, the eigenvalue
 coefficient corresponding to eigenvector e2 and lead v1 is -0.570, so the
 angle between eigenvector e2 and lead v1 is also about 60 degrees. Since
 the cosine of 90 degrees is zero, as the coefficients approach zero, the
 angle between the eigenvector and the original lead approaches 90 degrees.
 For example, the eigenvector coefficient corresponding to eigenvector e2
 and v4 is 0.001, thus the angle between eigenvector e2 and original lead
 v4 is almost 90 degrees.
 A reference eigenvector solution including a set of reference angles
 representative of a typical standard or resting ECG is stored in the
 memory 30 of the ECG machine 10. The angles between the eigenvectors and
 the original lead vectors for a particular ECG test are determined and
 then compared to the reference angles. If the angles for the ECG test do
 not match with the reference angles, the electrode placement is determined
 by the analysis module 25 to be either non-standard or incorrect. For
 example, the reference angle between eigenvector e1 and lead v2 may be
 about 90 degrees. If an ECG test is conducted and the angle between
 eigenvector e1 and lead v2 is only 30 degrees, electrode v2 is not in the
 correct position on the patient's chest for standard electrode placement.
 FIG. 7 illustrates the lead angle presentation for standard electrode
 placement. The frontal plane leads are only shown from the front, since
 these leads are defined to lie exactly in the frontal plane. The lead
 angle presentation in FIG. 7 serves as the reference for standard
 electrode placement for comparison with the lead angle presentations in
 FIGS. 8-12. A comparison between the reference angles in FIG. 7 and the
 lead angles in FIG. 8 shows that leads v1 and v2 have been reversed. A
 comparison between FIG. 7 and FIG. 9 shows that lead v5 is placed 2 cm too
 low.
 FIGS. 10, 11, and 12 depict lead angle presentations for alternative
 electrode placement schemes. FIG. 10 shows the lead angle presentation for
 Mason-Likar electrode placement. For Mason-Likar electrode placement, the
 limb electrodes are placed at the outside tips of the shoulders and at the
 hips. The lead angle presentation for Mason-Likar electrode placement
 compares favorably to the standard lead angle presentation in FIG. 7. FIG.
 11 shows an alternative to Mason-Likar electrode placement, in which the
 limb electrodes are placed at the middle of the collarbones and at the
 hips. The leads are presented in a more vertical alignment for this
 alternative electrode placement. Since the arm electrodes are placed
 closer together than for standard electrode placement, lead I becomes much
 shorter. FIG. 12 shows another alternative to Mason-Likar electrode
 placement, in which the limb electrodes are placed just to the left and
 right of the sternum and at the bottom of the rib cage. This electrode
 placement presents an extreme distortion to the standard lead angle
 presentation in FIG. 7.
 The results obtained from the above-described mathematical process can be
 used during an ECG test in two ways: (1) to alert an ECG technician to
 reversed or incorrectly positioned electrodes, and (2) to alert an ECG
 technician to non-standard electrode placement. In a situation in such as
 LA/RA reversal, the analysis module 25 alerts the ECG technician via the
 display device 35 that the leads are reversed. In a situation in which one
 of the horizontal leads is placed incorrectly on the chest, the anlaysis
 module 25 alerts the ECG technician to the incorrect lead placement and
 instructs the technician how far and in what direction to reposition the
 electrode. In the case of non-standard electrode placement, the ECG
 machine 10 simply defines the non-standard placement, e.g. Holter or
 stress and provides an indication on the display, and in the memory of the
 non-standard electrode placement.
 FIG. 13 illustrates a flowchart detailing the method of the invention. The
 method includes the acts of acquiring ten seconds of ECG information 110;
 isolating a single heartbeat in each channel of the ECG data 120; forming
 a covariance matrix of the ECG data 130; solving for the eigenvectors and
 eigenvalues of the covariance matrix 140; discarding the eigenvectors
 associated with the smallest eigenvalues 150; keeping the three
 eigenvectors associated with the largest three eigenvalues 160;
 interpreting the angles of the original lead vectors in terms of the three
 most important eigenvectors 170; and comparing the derived angles to
 reference angles 180. After the comparison, if the angles are the same
 within a tolerance, then the ECG data is stored in a memory with an
 indication that the data is from a standard ECG and that the correct
 electrode placement was used 190. If the angles are not within a
 tolerance, then the method determines whether the ECG data is from a
 standard ECG 200. If the ECG data is from a standard ECG, then the ECG
 data is stored in the memory with an indication that the electrodes were
 placed incorrectly during acquisition of the ECG data 210. If the ECG data
 is determined not to be standard ECG data, then the ECG data is further
 analyzed to determine the type of ECG electrode configuration 220. The
 information relating to the type of ECG configuration or the incorrect
 electrode placement may either be displayed on the display device
 immediately upon acquiring the ECG data, or may be stored as a message to
 be generated later upon production of the ECG report.
 The results obtained from the above-described mathematical process are
 useful in an ECG machine or in a software package for conducting ECG
 analysis to determine and record with the ECG data what type of electrode
 placement scheme is being used or was used with the ECG test, or whether
 the electrodes were placed in the correct position on the body, or whether
 leads were accidently switched during the test.
 It should be understood that the invention is not limited in its
 application to the details of the construction and the arrangements of the
 components set forth in the description or illustration in the drawings.
 The invention is capable of other embodiments and of being practiced or
 being carried out in various ways. Also, it is to be understood that the
 phraseology and terminology used herein is for the purpose of description
 and should not be regarded as limiting.