- Email: [email protected]
Crossroads in electrocardiographic lead development: a roadmap to the future of electrocardiographic leads in clinical electrocardiography
Available online at www.sciencedirect.com
Journal of Electrocardiology 41 (2008) 183 – 186 www.jecgonline.com
Editorial
Crossroads in electrocardiographic lead development: a roadmap to the future of electrocardiographic leads in clinical electrocardiography Introduction This issue of the Journal is devoted to the important topic of electrocardiographic (ECG) leads and comprises a compendium of articles assembled from presentations given at recent international symposia and workshops addressing important issues and concerns relating to lead systems, “reduced lead systems,” their usage, and standardization. This accompanying editorial is meant to provide history, commentary, and hopefully direction for future development and resolution of some of these issues facing both the clinical and technical (vendor) electrocardiography communities. The ECG has long been recognized as one of the most successful and important tools in the rapid, noninvasive assessment of cardiac state and diagnosis of cardiac conditions. More often than not, the resting 12-lead ECG is and has been used to rule out cardiac conditions in the presence of conflicting or ambiguous clinical symptoms or signs. Does the patient have a regular heart rhythm? Are the wave intervals and morphologies normal? As clinical electrocardiography has evolved, detection and characterization of cardiac conditions have been refined to the point that ECG interpretation is used to quantify diagnoses: Where and how large is the infarct? Where is the ischemia origin? In which bundle or branch of the conduction system does conduction block occur? Where is the ectopic focus of a tachycardia? At the same time, clinical need and practice have pushed the application of the technology for use in exercise stress testing and ambulatory or ST monitoring, both of which led to the use of lead systems and reference electrode sites that differ from those of the standard 12-lead ECG for reasons of convenience, signal-to-noise considerations, or, early on, technical limitations. This clearly clouds the issue of reading and interpreting ECGs, hence the renewed interest in standardizing the technical aspects of recording ECGs. To provide a framework for discussion, it is important to acknowledge the facts and principles that must be considered in our task of improving the consistency with which the ECG can be used for diagnosing and characterizing cardiac conditions and disease. • Much of heart disease or its ramifications is localized: acute ischemia arises from local occlusion or narrowing of one or more specific vessels; infarction compromises a region of myocardium; conduction block originates at 0022-0736/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jelectrocard.2008.02.012
•
•
•
•
one or more sites in the conduction system; ectopic foci are localized. Consequently, the electrical manifestations of localized cardiac disease are also localized on the body surface. Examples of these localized patterns include the following: Occlusion of the right coronary artery resulting in inferior wall myocardial infarction (MI) produces ST segment elevation and Q waves in leads located below the rib cage. In contrast, occlusion of the left anterior descending coronary artery resulting in anterior MI produces ST segment elevation and Q waves in leads located much more superiorly on the rib cage. Conduction blocks such as left and right bundle-branch block are distinguished by their opposite sequence of QRS polarities on right and left chest regions. Although the standard 12-lead ECG may sample some aspects of the body surface electrical manifestations of underlying disease, it does not sample all regions of the chest and often undersamples these signature diagnostic features. Recently developed “reduced” or “derived” lead systems differ in their ability to capture important diagnostic ECG information. The ECG diagnostic criteria are lead specific: acute MI—presence of any Q wave in V1 through V3; presence of Q waves in any 2 contiguous leads II, III, and aVF; or V4 through V6 having widths N30 milliseconds. The ECG morphology is dependent upon the reference lead used, for example, Wilson Central Terminal (WCT) derived from distal limb leads, a WCT derived from the Mason-Likar (M-L) chest leads, or a single lead reference result in different ECG waveforms, not just in the precordial leads but also in the limb and augmented leads.
Issues and concerns in ECG lead system design and usage The following topics define the areas of ECG leads– related research, both the historical and that published within this issue, that can provide a starting point for discussion for standardizing usage and overcoming the conflicting and potentially confusing failures of ECG interpretation and disease detection. These constructs are intimately entwined and not easily separated into distinct, independent problems.
184
Editorial / Journal of Electrocardiology 41 (2008) 183–186
Signal information capture Body surface potential mapping, as the criterion standard for determining the available body surface ECG information, has provided many studies over the past 40 years that documented the extent to which a variety of lead systems captures that information. The seminal work of Barr et al first documented the fact that 24 optimally selected torso leads adequately captured “all” body surface ECG information in the sense that those leads, along with a generally applicable transformation, could reproduce or estimate ECGs at all other 104 sites on their torso recordings within a specified small error.1 Lux et al confirmed this finding using different data and methodology and showed that 30 optimally selected leads reproduced potentials at 162 additional torso sites within an error comparable with recording noise.2 The clear message from these studies was that lead systems could be devised for optimal capture of available ECG information to the extent that they could reconstruct all information. Importantly, another finding of these studies was that the standard “12” leads (really 8 different signals) captured significantly less body ECG surface information than 8 optimally selected leads. Although not directly related to the number and placement of leads for optimal signal capture, the work of Lux et al and Evans et al used principal components analysis (“eigenvector analysis”) to explore the “information content” in body surface maps. 3,4 They documented that 12 statistically independent (orthonormal) spatial distributions accounted for 98% of the trace of body surface ECG information relative to the 192 leads used in their maps. This puts an upper bound on the number of “signals” needed to capture 98% of the available information. Of course, sets of recorded ECGs are not independent and contain considerable redundancy; so more than the 12-lead ECG must be recorded to achieve the same amount of signal information. Diagnostic information capture The work of Kornreich et al, again using body surface potential mapping, clearly documented regions of the torso that were optimal for discriminating between disease dichotomies and demonstrated that optimal detection of disease (eg, localized infarction, ischemia, left ventricular hypertrophy) could be achieved by determining the body surface leads that showed the greatest statistically significant signal difference for separating data from diagnostic group pairs.5 The results showed significant improvement in ECG diagnostic performance over the standard 12-lead ECG, but clearly required the use of mapping. Implementation of such an approach is problematic in that one must know the diseases to be tested before selecting the leads to be used for optimal detection and discrimination. Kornreich et al offered a solution to this dilemma by suggesting the use of a knowledge-based classification system that selected leads from data sets representing all the diagnostic groups to be classified.6 An important “marriage” of the concepts of “signal” leads and “diagnostic” leads was proposed by Kornreich et al and Lux et al in which optimal signal leads are used to estimate
body surface maps from which the best diagnostic leads could be selected for implementing diagnosis.7,8 Diagnostic criteria Although the above studies were helpful in providing insight into and the potential for improvement in ECG diagnosis via use of optimal signal or diagnostic leads, they did not provide an obvious means for resolving the problem that virtually all clinical ECG criteria are based on the standard 12-lead ECG and its lead system. This is certainly problematic when using different lead systems for diagnosis in that there is no simple or direct translation of or means for applying criteria designed for data recorded using one lead system to data recorded using a different lead system. In other words, ECG criteria for disease detection are highly lead dependent and specific. ECG reference leads Contributing to and compounding errors in interpretation of the ECG, regardless of the lead system used, is the fact that ECG signals at specific sites are dependent upon the electrical reference signals against which they are measured. In the standard 12-lead ECG, the reference voltage against which precordial leads V1 through V6 are measured is the WCT, an algebraic summation of the distal limb leads from right and left arms and left leg. The M-L reference, an algebraic average of right and left clavicular and lower left torso leads, was developed for use in exercise treadmill testing and ambulatory or ST segment monitoring to reduce noise and minimize lead lengths. However, it results in limb, augmented, and precordial ECGs that differ from those recorded against a WCT, thus introducing errors in interpretation based on WCT reference-based criteria. Some monitoring systems rely on a single chest lead as reference. Clearly, the choice of reference and lead system has little impact on recording systems whose sole or at least primary function is detection of rhythm disturbances (arrhythmias). However, use of nonstandard lead systems and references necessitates the need for lead- and reference-specific criteria for detection and characterization of localized disease. Emerging lead systems: “alternative”, “reduced” New lead systems have arisen for use in the settings of emergency medical assessment, emergency department monitoring, and cardiac monitoring ancillary to surgical or diagnostic procedures. These new lead systems have been developed for purposes of convenience, ease and speed of application, and simplicity. Many of them include transformations that provide a means to estimate the 12-lead ECG to a known, unknown, or even unacceptable level of accuracy. A related issue is the incorrect, inconsistent (different times of electrode application), or arbitrary placement of ECG electrodes and/or ECG reference electrodes. Whether for convenience or for restrictions imposed by other medical procedures, these issues must be considered in the course of proposing guidelines and establishing standards for ECG usage.
Editorial / Journal of Electrocardiology 41 (2008) 183–186
Lead-to-lead transformations The work of Barr et al, Lux et al, and others since has introduced methodology for transforming or estimating ECGs at “unmeasured” sites from measured ECGs. These methods are based on sound mathematical and statistical techniques, such as linear, least mean-squared error estimation, linear regression, or Laplacian interpolation. Importantly, they offer reasonable and practical estimates of ECGs at unmeasured sites within the limitations of the training data used and the intrinsic limitations of the specific methods. These techniques are an important link between the measurement (signal capture) problem and the diagnosis (interpretation) problem. Thus, a system of 3, 4, or N leads can provide an estimate of the 12-lead ECG. The problem remains that no estimator is perfect and each comes with a known, verifiable estimate of error, which in general decreases with the number and placement of measurement electrodes used. In this issue The articles published within this issue of the Journal address many of the above issues and concerns and contribute to an increasing body of knowledge that should provide an important basis for developing and standardizing ECG leads for clinical use. The objectives of the research are clearly focused on improving ECG diagnosis, characterization, and monitoring of heart disease through optimizing signal capture, using prior information (statistically or regressionbased transformations), and developing diagnostic criteria that are less dependent on leads and more dependent upon diagnostic features made obvious from mapping studies that stress characterization of potential distributions. Drew and Kligfield present a comprehensive overview of the need for standardizing ECG lead systems, the focus of a recent symposium at the annual Computers in Cardiology conference held in Durham, NC, last fall. They raise concerns regarding the emerging alternative and reduced lead systems that are finding increasing use in emergency departments for rapid assessment of cardiac conditions and ischemia monitoring for event detection. They also addressed the important issue of standard vs nonstandard (M-L) limb lead electrode placement that clearly confounds and can lead to incorrect ECG-based diagnoses. Schijvenaars et al review the sources of ECG variability including the relevant issues of arbitrary lead placement, lead misplacement, and inconsistent lead placement. The ramifications on diagnosis failure are obvious in that diagnostic criteria are lead dependent and certainly do not apply for incorrect lead placement. In a related article, Pahlm and Wagner underscore this important concern and provide examples showing that ECGs recorded using the M-L reference are substantially different from those recorded with the standard distal limb leads (WCT). Importantly, such differences translate into failure of detection criteria and misinterpretation of the ECG. They offer several solutions to the problem that need to be explored. The case report by Toosi and Sochanski along with the 2 accompanying editorial comments by Drew and Hoffman provides yet further evidence of the dangers
185
arising from arbitrary and inconsistent placement of reference leads. They plead for labeling requirements to ensure that diagnostic criteria are applied only to the lead systems for which they were designed. Wagner and Pahlm review their previously published proposal for visualizing ECGs as a 24-lead system with both upright and inverted ECGs arranged in frontal and axial planes to enhance observation of the continuity of ECG changes. Clearly, the display offers no new ECG information as pointed out in the study by Kors et al but likely improves the likelihood of correct interpretation by clearly expressing waveform abnormalities along contiguous directions in the 2 planes. Wang and Horacek provide a third article on the topic of reduced or alternative leads. They used pairs of the standard precordial leads along with limb leads with simulated M-L references to estimate the standard 12-lead ECG recorded from distal limb leads. This is an important contribution in that the method estimates the standard 12-lead ECG and also includes what amounts to a correction for using the M-L reference instead of the distal limb leads, one of the suggestions of the Pahlm and Wagner article. The article by Mann et al follows along the same topic and presents the idea of an 8 to 8 lead transformation that corrects ECGs measured using the M-L reference resulting in an estimate of the 12-lead with distal limb (WCT) reference. Nelwan et al have addressed the use of reduced lead sets to estimate the 12-lead ECG and compare the performance of the “EASI” lead system with that of a subset of the standard 12-lead ECG (leads I, II, V2, and V5). In addition, they compare the performance of general transformations (estimators) with patient-specific transformations that explore the potential benefit of improved estimation provided by the latter. Clearly impractical for normal use, the improvement gained through patient-specific estimators would be useful when previous ECGs were available on the patient. Donnelly et al provide a historical perspective to the optimal signal lead selection problem and introduce some of the newer approaches under investigation. They review the important distinction between leads that are optimal in the sense of signal capture and leads that are optimal from the standpoint of diagnosis. Finlay et al contribute a variation on the optimal lead selection problem by comparing a variety of different lead systems selected on the basis of their abilities to optimally represent (estimate) unmeasured data from different diagnostic groups. They found that although the lead systems differed, they were virtually identical in the ability to capture information for other groups. The article of Kornreich and Lux confirms, once again, the advantages of supplementing the 12-lead ECG with a few leads selected for optimal signal capture and/or diagnostic information and provides evidence that transformations of measurement leads to estimate information in unsampled regions provide a means for improving diagnostic performance. In summary, this issue of the Journal provides excellent, new data and discussion of many of the important and relevant issues facing the clinical and technical electro-
186
Editorial / Journal of Electrocardiology 41 (2008) 183–186
cardiology communities. We now have many of the answers to old problems; we know that as a significant and important diagnostic and prognostic tool, the ECG can be improved; and with the enthusiastic support and technical creativity and know-how of today's clinicians and medical scientists, we have the opportunity to move forward.
Robert L. Lux, PhD Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah Salt Lake City, UT, USA E-mail address: [email protected] Fred Kornreich Unit of Cardiovascular Research and Engineering School of Medicine, Free University of Brussels (VUB) Brussels, Belgium
References 1. Barr RC, Spach MS, Herman-Giddens GS. Selection of the number and positions of measuring locations for electrocardiography. IEEE Trans Biomed Eng 1971;18:125. 2. Lux RL, et al. Limited lead selection for estimation of body surface potential maps in electrocardiography. IEEE Trans Biomed Eng 1978;25:270. 3. Evans AK, et al. Redundancy reduction for improved display and analysis of body surface potential maps. II. Temporal compression. Circ Res 1981;49:197. 4. Lux RL, et al. Redundancy reduction for improved display and analysis of body surface potential maps. I. Spatial compression. Circ Res 1981;49:186. 5. Kornreich F, et al. Identification of best electrocardiographic leads for diagnosing myocardial infarction by statistical analysis of body surface potential maps. Am J Cardiol 1985;56:852. 6. Knowledge-based system for classification of BSPM in Computers in Cardiology in 1990. 7. Kornreich F, Lux RL, MacLeod RS. Map representation and diagnostic performance of the standard 12-lead ECG. J Electrocardiol 1995;28 (Suppl):121. 8. Lux RL, et al. Estimating ECG distributions from small numbers of leads. J Electrocardiol 1995;28(Suppl):92.
Alternating right bundle-branch block
Note the significant changes in the initial forces during right bundle-branch block conduction. The Q waves in leads III and aVF become much deeper and broader, potentially leading to a misdiagnosis of inferior myocardial infarction. BEES doi:10.1016/j.jelectrocard.2008.02.032
Journal of Electrocardiology 41 (2008) 183 – 186 www.jecgonline.com
Editorial
Crossroads in electrocardiographic lead development: a roadmap to the future of electrocardiographic leads in clinical electrocardiography Introduction This issue of the Journal is devoted to the important topic of electrocardiographic (ECG) leads and comprises a compendium of articles assembled from presentations given at recent international symposia and workshops addressing important issues and concerns relating to lead systems, “reduced lead systems,” their usage, and standardization. This accompanying editorial is meant to provide history, commentary, and hopefully direction for future development and resolution of some of these issues facing both the clinical and technical (vendor) electrocardiography communities. The ECG has long been recognized as one of the most successful and important tools in the rapid, noninvasive assessment of cardiac state and diagnosis of cardiac conditions. More often than not, the resting 12-lead ECG is and has been used to rule out cardiac conditions in the presence of conflicting or ambiguous clinical symptoms or signs. Does the patient have a regular heart rhythm? Are the wave intervals and morphologies normal? As clinical electrocardiography has evolved, detection and characterization of cardiac conditions have been refined to the point that ECG interpretation is used to quantify diagnoses: Where and how large is the infarct? Where is the ischemia origin? In which bundle or branch of the conduction system does conduction block occur? Where is the ectopic focus of a tachycardia? At the same time, clinical need and practice have pushed the application of the technology for use in exercise stress testing and ambulatory or ST monitoring, both of which led to the use of lead systems and reference electrode sites that differ from those of the standard 12-lead ECG for reasons of convenience, signal-to-noise considerations, or, early on, technical limitations. This clearly clouds the issue of reading and interpreting ECGs, hence the renewed interest in standardizing the technical aspects of recording ECGs. To provide a framework for discussion, it is important to acknowledge the facts and principles that must be considered in our task of improving the consistency with which the ECG can be used for diagnosing and characterizing cardiac conditions and disease. • Much of heart disease or its ramifications is localized: acute ischemia arises from local occlusion or narrowing of one or more specific vessels; infarction compromises a region of myocardium; conduction block originates at 0022-0736/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jelectrocard.2008.02.012
•
•
•
•
one or more sites in the conduction system; ectopic foci are localized. Consequently, the electrical manifestations of localized cardiac disease are also localized on the body surface. Examples of these localized patterns include the following: Occlusion of the right coronary artery resulting in inferior wall myocardial infarction (MI) produces ST segment elevation and Q waves in leads located below the rib cage. In contrast, occlusion of the left anterior descending coronary artery resulting in anterior MI produces ST segment elevation and Q waves in leads located much more superiorly on the rib cage. Conduction blocks such as left and right bundle-branch block are distinguished by their opposite sequence of QRS polarities on right and left chest regions. Although the standard 12-lead ECG may sample some aspects of the body surface electrical manifestations of underlying disease, it does not sample all regions of the chest and often undersamples these signature diagnostic features. Recently developed “reduced” or “derived” lead systems differ in their ability to capture important diagnostic ECG information. The ECG diagnostic criteria are lead specific: acute MI—presence of any Q wave in V1 through V3; presence of Q waves in any 2 contiguous leads II, III, and aVF; or V4 through V6 having widths N30 milliseconds. The ECG morphology is dependent upon the reference lead used, for example, Wilson Central Terminal (WCT) derived from distal limb leads, a WCT derived from the Mason-Likar (M-L) chest leads, or a single lead reference result in different ECG waveforms, not just in the precordial leads but also in the limb and augmented leads.
Issues and concerns in ECG lead system design and usage The following topics define the areas of ECG leads– related research, both the historical and that published within this issue, that can provide a starting point for discussion for standardizing usage and overcoming the conflicting and potentially confusing failures of ECG interpretation and disease detection. These constructs are intimately entwined and not easily separated into distinct, independent problems.
184
Editorial / Journal of Electrocardiology 41 (2008) 183–186
Signal information capture Body surface potential mapping, as the criterion standard for determining the available body surface ECG information, has provided many studies over the past 40 years that documented the extent to which a variety of lead systems captures that information. The seminal work of Barr et al first documented the fact that 24 optimally selected torso leads adequately captured “all” body surface ECG information in the sense that those leads, along with a generally applicable transformation, could reproduce or estimate ECGs at all other 104 sites on their torso recordings within a specified small error.1 Lux et al confirmed this finding using different data and methodology and showed that 30 optimally selected leads reproduced potentials at 162 additional torso sites within an error comparable with recording noise.2 The clear message from these studies was that lead systems could be devised for optimal capture of available ECG information to the extent that they could reconstruct all information. Importantly, another finding of these studies was that the standard “12” leads (really 8 different signals) captured significantly less body ECG surface information than 8 optimally selected leads. Although not directly related to the number and placement of leads for optimal signal capture, the work of Lux et al and Evans et al used principal components analysis (“eigenvector analysis”) to explore the “information content” in body surface maps. 3,4 They documented that 12 statistically independent (orthonormal) spatial distributions accounted for 98% of the trace of body surface ECG information relative to the 192 leads used in their maps. This puts an upper bound on the number of “signals” needed to capture 98% of the available information. Of course, sets of recorded ECGs are not independent and contain considerable redundancy; so more than the 12-lead ECG must be recorded to achieve the same amount of signal information. Diagnostic information capture The work of Kornreich et al, again using body surface potential mapping, clearly documented regions of the torso that were optimal for discriminating between disease dichotomies and demonstrated that optimal detection of disease (eg, localized infarction, ischemia, left ventricular hypertrophy) could be achieved by determining the body surface leads that showed the greatest statistically significant signal difference for separating data from diagnostic group pairs.5 The results showed significant improvement in ECG diagnostic performance over the standard 12-lead ECG, but clearly required the use of mapping. Implementation of such an approach is problematic in that one must know the diseases to be tested before selecting the leads to be used for optimal detection and discrimination. Kornreich et al offered a solution to this dilemma by suggesting the use of a knowledge-based classification system that selected leads from data sets representing all the diagnostic groups to be classified.6 An important “marriage” of the concepts of “signal” leads and “diagnostic” leads was proposed by Kornreich et al and Lux et al in which optimal signal leads are used to estimate
body surface maps from which the best diagnostic leads could be selected for implementing diagnosis.7,8 Diagnostic criteria Although the above studies were helpful in providing insight into and the potential for improvement in ECG diagnosis via use of optimal signal or diagnostic leads, they did not provide an obvious means for resolving the problem that virtually all clinical ECG criteria are based on the standard 12-lead ECG and its lead system. This is certainly problematic when using different lead systems for diagnosis in that there is no simple or direct translation of or means for applying criteria designed for data recorded using one lead system to data recorded using a different lead system. In other words, ECG criteria for disease detection are highly lead dependent and specific. ECG reference leads Contributing to and compounding errors in interpretation of the ECG, regardless of the lead system used, is the fact that ECG signals at specific sites are dependent upon the electrical reference signals against which they are measured. In the standard 12-lead ECG, the reference voltage against which precordial leads V1 through V6 are measured is the WCT, an algebraic summation of the distal limb leads from right and left arms and left leg. The M-L reference, an algebraic average of right and left clavicular and lower left torso leads, was developed for use in exercise treadmill testing and ambulatory or ST segment monitoring to reduce noise and minimize lead lengths. However, it results in limb, augmented, and precordial ECGs that differ from those recorded against a WCT, thus introducing errors in interpretation based on WCT reference-based criteria. Some monitoring systems rely on a single chest lead as reference. Clearly, the choice of reference and lead system has little impact on recording systems whose sole or at least primary function is detection of rhythm disturbances (arrhythmias). However, use of nonstandard lead systems and references necessitates the need for lead- and reference-specific criteria for detection and characterization of localized disease. Emerging lead systems: “alternative”, “reduced” New lead systems have arisen for use in the settings of emergency medical assessment, emergency department monitoring, and cardiac monitoring ancillary to surgical or diagnostic procedures. These new lead systems have been developed for purposes of convenience, ease and speed of application, and simplicity. Many of them include transformations that provide a means to estimate the 12-lead ECG to a known, unknown, or even unacceptable level of accuracy. A related issue is the incorrect, inconsistent (different times of electrode application), or arbitrary placement of ECG electrodes and/or ECG reference electrodes. Whether for convenience or for restrictions imposed by other medical procedures, these issues must be considered in the course of proposing guidelines and establishing standards for ECG usage.
Editorial / Journal of Electrocardiology 41 (2008) 183–186
Lead-to-lead transformations The work of Barr et al, Lux et al, and others since has introduced methodology for transforming or estimating ECGs at “unmeasured” sites from measured ECGs. These methods are based on sound mathematical and statistical techniques, such as linear, least mean-squared error estimation, linear regression, or Laplacian interpolation. Importantly, they offer reasonable and practical estimates of ECGs at unmeasured sites within the limitations of the training data used and the intrinsic limitations of the specific methods. These techniques are an important link between the measurement (signal capture) problem and the diagnosis (interpretation) problem. Thus, a system of 3, 4, or N leads can provide an estimate of the 12-lead ECG. The problem remains that no estimator is perfect and each comes with a known, verifiable estimate of error, which in general decreases with the number and placement of measurement electrodes used. In this issue The articles published within this issue of the Journal address many of the above issues and concerns and contribute to an increasing body of knowledge that should provide an important basis for developing and standardizing ECG leads for clinical use. The objectives of the research are clearly focused on improving ECG diagnosis, characterization, and monitoring of heart disease through optimizing signal capture, using prior information (statistically or regressionbased transformations), and developing diagnostic criteria that are less dependent on leads and more dependent upon diagnostic features made obvious from mapping studies that stress characterization of potential distributions. Drew and Kligfield present a comprehensive overview of the need for standardizing ECG lead systems, the focus of a recent symposium at the annual Computers in Cardiology conference held in Durham, NC, last fall. They raise concerns regarding the emerging alternative and reduced lead systems that are finding increasing use in emergency departments for rapid assessment of cardiac conditions and ischemia monitoring for event detection. They also addressed the important issue of standard vs nonstandard (M-L) limb lead electrode placement that clearly confounds and can lead to incorrect ECG-based diagnoses. Schijvenaars et al review the sources of ECG variability including the relevant issues of arbitrary lead placement, lead misplacement, and inconsistent lead placement. The ramifications on diagnosis failure are obvious in that diagnostic criteria are lead dependent and certainly do not apply for incorrect lead placement. In a related article, Pahlm and Wagner underscore this important concern and provide examples showing that ECGs recorded using the M-L reference are substantially different from those recorded with the standard distal limb leads (WCT). Importantly, such differences translate into failure of detection criteria and misinterpretation of the ECG. They offer several solutions to the problem that need to be explored. The case report by Toosi and Sochanski along with the 2 accompanying editorial comments by Drew and Hoffman provides yet further evidence of the dangers
185
arising from arbitrary and inconsistent placement of reference leads. They plead for labeling requirements to ensure that diagnostic criteria are applied only to the lead systems for which they were designed. Wagner and Pahlm review their previously published proposal for visualizing ECGs as a 24-lead system with both upright and inverted ECGs arranged in frontal and axial planes to enhance observation of the continuity of ECG changes. Clearly, the display offers no new ECG information as pointed out in the study by Kors et al but likely improves the likelihood of correct interpretation by clearly expressing waveform abnormalities along contiguous directions in the 2 planes. Wang and Horacek provide a third article on the topic of reduced or alternative leads. They used pairs of the standard precordial leads along with limb leads with simulated M-L references to estimate the standard 12-lead ECG recorded from distal limb leads. This is an important contribution in that the method estimates the standard 12-lead ECG and also includes what amounts to a correction for using the M-L reference instead of the distal limb leads, one of the suggestions of the Pahlm and Wagner article. The article by Mann et al follows along the same topic and presents the idea of an 8 to 8 lead transformation that corrects ECGs measured using the M-L reference resulting in an estimate of the 12-lead with distal limb (WCT) reference. Nelwan et al have addressed the use of reduced lead sets to estimate the 12-lead ECG and compare the performance of the “EASI” lead system with that of a subset of the standard 12-lead ECG (leads I, II, V2, and V5). In addition, they compare the performance of general transformations (estimators) with patient-specific transformations that explore the potential benefit of improved estimation provided by the latter. Clearly impractical for normal use, the improvement gained through patient-specific estimators would be useful when previous ECGs were available on the patient. Donnelly et al provide a historical perspective to the optimal signal lead selection problem and introduce some of the newer approaches under investigation. They review the important distinction between leads that are optimal in the sense of signal capture and leads that are optimal from the standpoint of diagnosis. Finlay et al contribute a variation on the optimal lead selection problem by comparing a variety of different lead systems selected on the basis of their abilities to optimally represent (estimate) unmeasured data from different diagnostic groups. They found that although the lead systems differed, they were virtually identical in the ability to capture information for other groups. The article of Kornreich and Lux confirms, once again, the advantages of supplementing the 12-lead ECG with a few leads selected for optimal signal capture and/or diagnostic information and provides evidence that transformations of measurement leads to estimate information in unsampled regions provide a means for improving diagnostic performance. In summary, this issue of the Journal provides excellent, new data and discussion of many of the important and relevant issues facing the clinical and technical electro-
186
Editorial / Journal of Electrocardiology 41 (2008) 183–186
cardiology communities. We now have many of the answers to old problems; we know that as a significant and important diagnostic and prognostic tool, the ECG can be improved; and with the enthusiastic support and technical creativity and know-how of today's clinicians and medical scientists, we have the opportunity to move forward.
Robert L. Lux, PhD Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah Salt Lake City, UT, USA E-mail address: [email protected] Fred Kornreich Unit of Cardiovascular Research and Engineering School of Medicine, Free University of Brussels (VUB) Brussels, Belgium
References 1. Barr RC, Spach MS, Herman-Giddens GS. Selection of the number and positions of measuring locations for electrocardiography. IEEE Trans Biomed Eng 1971;18:125. 2. Lux RL, et al. Limited lead selection for estimation of body surface potential maps in electrocardiography. IEEE Trans Biomed Eng 1978;25:270. 3. Evans AK, et al. Redundancy reduction for improved display and analysis of body surface potential maps. II. Temporal compression. Circ Res 1981;49:197. 4. Lux RL, et al. Redundancy reduction for improved display and analysis of body surface potential maps. I. Spatial compression. Circ Res 1981;49:186. 5. Kornreich F, et al. Identification of best electrocardiographic leads for diagnosing myocardial infarction by statistical analysis of body surface potential maps. Am J Cardiol 1985;56:852. 6. Knowledge-based system for classification of BSPM in Computers in Cardiology in 1990. 7. Kornreich F, Lux RL, MacLeod RS. Map representation and diagnostic performance of the standard 12-lead ECG. J Electrocardiol 1995;28 (Suppl):121. 8. Lux RL, et al. Estimating ECG distributions from small numbers of leads. J Electrocardiol 1995;28(Suppl):92.
Alternating right bundle-branch block
Note the significant changes in the initial forces during right bundle-branch block conduction. The Q waves in leads III and aVF become much deeper and broader, potentially leading to a misdiagnosis of inferior myocardial infarction. BEES doi:10.1016/j.jelectrocard.2008.02.032