The Pierre Rijlant lecture 2007: the future of electrocardiography.
Predicting the future with any certainty is essentially impossible but on the other hand, it is feasible to make an informed guess as to the likely trend of developments in any specific area. To some extent, electrocardiography is no exception. On the other hand, the rapid growth of other diagnostic techniques which, to some extent, are complementary to electrocardiography but nonetheless compete in some senses, makes prediction a little more difficult. This brief paper attempts to look a short distance into the future.
Value of the 12-lead ECG
It is now over 100 years since the limb leads of the electrocardiogram (ECG) were introduced (1), 75 years since the unipolar chest leads were designed (2) though only 65 years since the precordial electrode positions were standardised (3) and approximately the same time since augmented unipolar leads were introduced (4). While the technology has undoubtedly advanced significantly since then, these 12 leads have in many ways stood the test of time, been the subject of much research and proved an invaluable aid to the physician and cardiologist among many other health professionals.
There is much basic diagnostic information that can be obtained only from the 12-lead ECG with respect to rhythm, conduction defects, acute and subsequent serial changes as well as secondary ST-T changes of ventricular hypertrophy that it hardly needs to be repeated here.
The basic 12-lead ECG, in some form, will continue to be of significant diagnostic value for the foreseeable future.
12-lead ECG electrode placement
Historically, the limb leads were measured with ankles and wrists in conducting solution, which formed the "electrode". This type of electrode was replaced by a metal plate held in to the ankles and wrists. These electrode positions were involved in recording of thousands of ECGs from which the normal limits of the ECG evolved , e.g. (5).
With respect to exercise electrocardiography, an alternative lead system placement, the "Mason Likar" system (6), came into general use. In this system, the four limb electrodes were placed on the torso. The right arm electrode was shifted to a point in the infraclavicular fossa medial to the border of the deltoid muscle and 2 cm below the lower border of the clavicle. The left arm electrode was repositioned in a mirror image position in the left infraclavicular fossa. The left leg electrode was positioned in the anterior axillary line halfway between the costal margin and the iliac crest. This position is not critical. The right leg electrode can be placed anywhere but is usually positioned in the right iliac fossa. A number of studies showed that this resulted in differences of the ECG pattern and hence interpretation compared to electrodes placed on wrists and ankles (7).
On the other hand, for 12-lead Holter ECG recording, the Mason Likar lead positions are used in view of the convenience of avoiding limb electrodes being linked by lengthy wires to a central recording unit while an individual went about the normal activities of daily living.
In other situations such as emergencies, where it might not be possible to have easy access for electrode connection, the Mason Likar electrode positions are also used. Also, in the monitoring situation it is clearly much more convenient for the patient to have extremity electrodes placed on the torso at the risk of distorting Q waves and ST-T appearances in the limb leads.
At the recent International Society of Computerised Electrocardiology annual meeting in Cancun in Mexico (April 2007), the President of the Society, Professor Barbara Drew, supported by Dr. Paul Kligfield, suggested that the Mason Likar positions be used for routine resting 12 lead electrocardiography. If this were to happen from the point of view of convenience, then some form of mapping of limb lead measurements from the conventional to the Mason Likar approach would need to be undertaken or else completely revised limits of normality, particularly for limb leads, would need to be developed. This in turn relates to revised limits of the P, QRS and T axes.
Sad to say, many technicians and nurses nowadays are, out of ignorance, recording the resting ECG using the Mason Likar lead positioning. Automated ECG analysis programs, however, not to mention whoever reviews the ECG, have no idea that this is being done in any specific ECG and are therefore likely to make a wrong interpretation in certain situations.
There will be increasing pressure to move to the use of torso positions for the extremity ECG electrodes, i.e adopt the Mason-Likar system for routine 12- lead ECG analysis.
Derived 12-lead ECG systems
A different approach to recording a 12-lead ECG is to make use of the EASI lead system (8) where 4 electrodes are placed on the torso at A (mid axillary line at level of V6), E (sternum at level of 5th intercostal space), I (mid axillary line at level of V6R) and S (sternum below the sternal notch). From these four electrodes, three essentially orthogonal leads can be recorded and from them, a full 12-lead ECG can be derived. There have been many studies now where the method of deriving the transformation has been examined and a variety of transformations is available for converting the EASI derived XYZ leads into the full 12-lead ECG (9). This 4- electrode system (plus 1 electrode added for technical reasons) is very simple to apply and can be very useful for rhythm monitoring etc, but in general terms, transformation to a 12-lead ECG is not possible with 100% accuracy though proponents of the system argue that differences are of small clinical consequence.
The EASI Lead system will be used basically for patient monitoring only.
Reduced lead systems
Some authors have suggested that by using 2 chest leads together with the 6 limb leads, the remaining 4 chest leads can be derived with very high accuracy (10) using a general transformation. However, by recording initially a standard 12-lead ECG and developing a patient specific transformation, it is possible thereafter to use only 2 chest leads from which the other 4 can be derived with even higher accuracy (10).
This approach will have limited use for patient monitoring.
Rudy et al (11) have, for many years, worked intensively on the development of an inverse model, which allows calculation of electrical activity on the epicardium from body surface electrode recordings made using over 200 electrodes. The technique is sometime called ECG imaging or ECG-I. This work is gaining increasing acceptance, among other things, for localisation of arrhythmogenic foci, which can then be ablated (12). The difficulty with this approach is that a CT scan is required prior to the mapping in order to obtain the geometry of the heart for each patient. This enhances the accuracy of the technique but at the same time limits its application to small numbers of individuals and, even more so, to one specific research centre at the present time.
In due course, ECG-I will be replicated in a number of centres worldwide possibly through commercial development.
Body surface mapping
Body surface mapping has been in existence for over 60 years. Recently, proponents of the approach claim that body surface maps derived from say 80 or more electrodes allow a much more sensitive detection of changes accompanying acute myocardial infarction (13). This has led to a very significant recent investment of over $25m into marketing a system for detecting such abnormalities.
Body surface mapping in acute myocardial infarction will have limited, though greater, acceptability in major centres than at present.
A major review of the technology and criteria for diagnostic electrocardiography has been taking place over the past couple of years under the aegis of the American Heart Association and a series of six guideline papers is appearing during 2007 (e.g.14,15). These will help to redefine criteria for all aspects of diagnostic electrocardiography and attempts at standardising diagnostic terminology will also be involved. For example, the guideline on acute ischaemic changes makes a recommendation that computer programs should attempt to specify the coronary artery that is responsible for an acute ischaemic change.
There will be significant enhancements to computer based ECG reports over the next few years on account of the new guidelines.
The increasing availability of cardiac magnetic resonance imaging (MRI) is currently leading to a number of studies where the location of myocardial infarction can be defined using delayed contrast enhanced magnetic resonance images against which ECGs can be assessed. Already this has led to one report (16) suggesting that the term "posterior" should be omitted from electrocardiographic diagnostic terminology while other correlations between Q waves on the ECG and the exact location of the corresponding infarct have been outlined (17).
MRI ECG correlations will greatly enhance the accuracy of the ECG diagnosis of myocardial infarction in the not too distant future.
There is no question that miniaturisation of electronic equipment has led to a reduction in the size of an electrocardiograph. On the other hand, for hospital use, where an A4 size ECG report is desirable, the size of the equipment is limited by the size of the paper drive and perhaps a monitor screen provided for the convenience of the technician. Completely miniaturised systems with a single channel printer can be produced but for hospital use, the larger electrocardiographs are much to be preferred.
Miniaturisation of equipment for 12-lead electrocardiography has reached its nadir.
Other enhancements to equipment are likely to be enhanced firmware for detecting implanted cardiac pacemaker activity, including that of biventricular pacemakers. This results from increased sampling rates of the ECG waveforms in excess of 64.000 samples per second per channel. A report of biventricular pacing may be possible (18).
Electrocardiographs will continue to be enhanced in respect of their signal processing and ability to detect implanted cardiac pacemaker stimuli.
A final point to make in relation to equipment is the likely increase in wireless transmission of ECGs from the bedside to a central ECG management system. Of course, this necessitates the availability of suitable facilities spread throughout a large hospital for example and the purchase of a more expensive electrocardiograph.
The uptake of wireless technology for routine ECG management will be limited until costs are acceptable.
It is often said that there will be an explosion in home monitoring given the increasing size of population aged over 60 years. Set against this has to be the difficulty in monitoring signals, which from a technological point of view, can readily be transmitted from the home to a central monitoring facility. Various approaches to recording the 12-lead ECG in the home situation have been proposed, from vests with electrodes stitched into the fabric through to a glove, which can then be held against the chest to allow the precordial leads to be derived. A major problem of course is the accuracy of electrode positioning and the quality of the recording, which in many cases may be so unsatisfactory as to be useless.
From a technological point of view, many exciting possibilities exist as exemplified by the Epimedics project being developed by Rubel and colleagues in the University of Lyon (19). This allows a patient to have his or her ECG monitored by a pocket sized device and should any abnormality be detected, a warning signal including the ECG can be sent immediately using conventional telephone technology to a monitoring centre. At the same time, a physician can be sent to assist the patient if need be and again appropriate GPS technology for example can be used to guide the physician to the patient.
There will be an increase in the number of devices available for home ECG monitoring but there could be an inability for health services worldwide to handle such an increased demand for home monitoring, leading to a growth of private companies involved in this area.
Electrocardiography is here to stay despite the competition from other technologies. There is still the prospect of a lengthy career for any young engineer or doctor interested in furthering the science of electrocardiography!!
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(10.) Nelwan SP, Kors JA, Meij SH, van Bemmel JH, Simoons ML. Reconstruction of the 12-lead electrocardiogram from reduced lead sets. J Electrocardiol 2004; 37: 11-8.
(11.) Ramanathan C, Ghanem RN Jia P Ryu K, Rudy Y. Noninvasive electrocardiographic imaging for cardiac electrophysiology and arrhythmia. Nat Med 2004; 10: 422-8.
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(13.) Owens CG, McClelland AJ, Walsh SJ, Smith BA, Tomlin A, Riddell JW, et al. Prehospital 80-lead mapping: does it add significantly to the diagnosis of acute coronary syndromes? J Electrocardiol 2004; 37 Suppl: 223-32.
(14.) Kligfield P, Gettes LS, Bailey JJ, Childers R, Deal BJ, Hancock EW, et al; American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; American College of Cardiology Foundation; Heart Rhythm Society. Recommendations for the standardization and interpretation of the electrocardiogram: part I: the electrocardiogram and its technology a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society endorsed by the International Society for Computerized Electrocardiology. Circulation 2007; 115: 1306-24
(15.) Mason JW, Hancock EW, Gettes LS, Bailey JJ, Childers R, Deal BJ, et al; American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; American College of Cardiology Foundation; Heart Rhythm Society Recommendations for the standardization and interpretation of the electrocardiogram: part II: Electrocardiography diagnostic statement list: a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society: endorsed by the International Society for Computerized Electrocardiology. Circulation 2007; 115: 1325-32.
(16.) Bayes de Luna A, Wagner G, Birnbaum Y, Nikus K, Fiol M, Gorgels A, et al; International Society for Holter and Noninvasive Electrocardiography. A new terminology for left ventricular walls and location of myocardial infarcts that present Q wave based on the standard of cardiac magnetic resonance imaging: a statement for healthcare professionals from a committee appointed by the International Society for Holter and Noninvasive Electrocardiography. Circulation 2006; 114: 1755-60.
(17.) Bayes de Luna A, Cino JM, Pujadas S, Cygankiewicz I, Carreras F, Garcia-Moll X, et al. Concordance of electrocardiographic patterns and healed myocardial infarction location detected by cardiovascular magnetic resonance. Am J Cardiol 2006; 97: 443-51.
(18.) Fairweather J, Johnston P, Luo S, Macfarlane PW. Computer analysis of implanted cardiac pacemaker rhythm. Computers in Cardiology. In press 2007.
(19.) EPI-MEDICS. Enhanced Personal, Intelligent and Mobile System for Early Detection and Interpretation of Coronary Syndromes. [cited 2007 May 5]; Available from: http://epi-medics.univlyon1. fr/flash/epimedics.html
Peter W. Macfarlane
Division of Medical Sciences, University of Glasgow, Glasgow, UK
Address for Correspondence: Professor Peter W. Macfarlane, Cardiology, Level 4, QEB, Royal Infirmary, Glasgow G31 2ER Scotland, UK E-mail: firstname.lastname@example.org
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|Author:||Macfarlane, Peter W.|
|Publication:||The Anatolian Journal of Cardiology (Anadolu Kardiyoloji Dergisi)|
|Date:||Jul 1, 2007|
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