J. ELECTROCARDIOLOGY, 8 (1 53-59, 1975
Special Article: ECG Data Acquisition. A Discussion BY G. E. DOWER, J. A. OSBORNE, O. SURANYI, D. E. STEWART, AND K. BONHAM
The use of c o m p u t e r s in electrocardiography is spreading rapidly and substantial funds are being committed. But do we need to use a c o m p u t e r to do s o m e t h i n g which a c a r d i o l o g i s t can do v e r y e f f i c i e n t l y ? An affirmative answer must be on the grounds of economy or better diagnosis. Economy is more often cited t h a n i m p r o v e m e n t - - in fact, automated interpretation may compare unfavorably with that of the cardiologist. 1 But economic grounds m a y be hard to justify b e c a u s e r e c o r d i n g a n d i n t e r p r e t i n g an electrocardiogram (ECG) is a relatively simple p r o c e d u r e r e q u i r i n g r a t h e r m o d e s t equipment and very little of a cardiologist's time. It is not easy to save technician's time and the capital cost of a computing system is about two orders of magnitude greater than that of a conventional ECG service. So let us look again at the justification that use of the computer may lead to a better diagnosis. Early efforts directed at merely automating the cardiologist did not take into account the very good possibility that the cardiologisti n t e r p r e t e r can be aided by the computer rather than replaced by it. Examples of the way in which the computer can help are as follows: making various measurements from the ECG signals, comparing them with norm a l and m o r b i d v a l u e s , g i v i n g r e l e v a n t probabilities, making various computations such as spatial angles and magnitudes, providing various displays of information, and keeping an extensive file of previous data. Another way in which the computer can help is via polarcardiography. Polarcardiog r a m s (PCGs) can lead to c o n s i d e r a b l e e n h a n c e m e n t of the accuracy of the E C G diagnosis. 2 Furthermore, the PCG criteria for infarction and various other conditions are relatively simple to program; this can assist the cardiologist in learning a new technique. -
The intelligence of computers can be improved indefinitely so that, ultimately, the prospects for computer analysis and interpretation are good. Unfortunately, some centers have optimistically adopted computer techniques on grounds that others had done so, without there being a clear, impartial assessment of performance. Much depends upon how the computer is to be used. In this connection, m a n y fundamental decisions have been made which deserve a second look in the light of recent technological d e v e l o p m e n t s . T h e s e decisions, right or wrong, could affect the future value of c o m p u t e r s to e l e c t r o c a r d i o g r a p h y . T h i s editorial will examine some of them.
Choosing the Types of Data There are essentially two types of data: the 12-lead ECG signals and the xyz signals. Which are better for computer analysis? This depends on how the computer is to be used. If the intent is merely to automate, matching the cardiologist's interpretation as closely as possible, the decision of Caceres 3 and others to transmit the 12-lead signals, in sequence, to the computer would seem to be the most reasonable. On the other hand, if the intent is to employ the computer as a diagnostic aid in its own right, t h e r e b y offering something more than can be provided from a singlechannel, direct-writing electrocardiograph, a good case can be made for the decision of Pipberger and others in favor of the xyz signals. Pipberger showed experimentally that these signals contain virtually all the information utilized by a cardiologist reading the 12-lead ECG. 4 The reduction of the array of body surface potentials to three, made possible by the heart vector concept, has attractions for comp u t e r program development unfettered by traditional methods. There are attractions, too, in employing xyz leads, which have a rational, physical basis, rather than ECG leads, which have a more traditional background. The experience of m a n y cardiologists with s o m e t y p e s of v e c t o r c a r d i o g r a p h s o f t e n prompts the expression of doubt as to the practicability of employing an xyz lead system routinely. The following comments of a reviewer are representative of this view: "The proper recording of a vectorcardiogram with the Frank lead system is a long and laborious
-
From the Departments of Cardiology, Vancouver General Hospital, and Polarcardiography. Shaughnessy Hospital, Vancouver, B.C., Canada. Reprint requests to: Dr. J. A. Osborne, Cardiology, Vancouver General Hospital, Vancouver, Canada. 53
54
ECG DATA ACQUISITION
process and in my opinion is the primary reason to doubt the applicability of this technique to routine electrocardiography." But with suitable equipment, recording xyz sign a l s can be simpler than recording a convent i o n a l 12-lead ECG. 5 In a recent field study of Cretan villagers, nearly 600 xyz magnetictape recordings were made in a dozen villages in ten days; on some days over 100 recordings were made. The Frank system was employed. The tracings were recorded by one of the authors (GED) and his wife with occasional help from nurses at village clinics. Of course, this experience says much for the magnificent cooperation of the villagers, but it also clearly demonstrates the practicability of the Frank lead system. At the Mayo Clinic the decision was made to acquire both the 12-lead ECG signals and the xyz signals. 6 This meant some extra work for t h e t e c h n i c i a n s , and some i n g e n i o u s m e a n s for m a k i n g the operation practical were worked out. One of these involved combining the F r a n k and 12-lead ECG electrodes in the left mid-axillary line a n d i n the region of the left nipple. This entailed modifying the F r a n k electrode positions slightly. Although there have been no reported studies of the effect of this modification of Frank's system, s e v e r a l m a n u f a c t u r e r s have adopted this arrangement as standard. Actually, there is a more attractive alternative: to record only the F r a n k xyz s i g n a l s and to s y n t h e s i z e t h e 12-lead ECG from them. ECG Lead Synthesis The xyz signals from the F r a n k network m a y be combined in accordance with Frank's image space data to yield simulations of the 12-lead ECG which are often surprisingly close. 7,s This has been subjected to extensive clinical study and the agreement is satisfactory for q u a l i t a t i v e comparisons. Table I shows a comparison of 121 typical, unselected, 12-lead ECGs, conventional and simul a t e d , from p a t i e n t s u n d e r g o i n g c a r d i a c catheterization. There were 182 abnormalities identified. The tracings were read independently by various interpreters who were u n a w a r e of the test, the tracings being introduced into the daily pile of ECGs to be interpreted. The concordance of the diagnoses is r e a s s u r i n g . The differences observed are largely due to two features of the simulated 12-lead ECG: the P waves are often inverted in V1, occasionally in lead I, and the potentials in some of the precordial leads tend to be somewhat less. Consequently, there was a tendency to over-read atrial hypertrophy and to under-read left ventricular hypertrophy. These shortcomings disappear if data concerning the spatial magnitudes of the maxim u m P and QRS vectors (P and R) are avail-
able, because simple cut-off points can be used for them. Obviously, these data will be available, if the xyz signals are sent to the computer. But, in any case, once the reader becomes familiar with the simulated ECG he is not apt to be misled, and when these two minor points (concerning P and R) are taken into consideration, the clinical differences between the conventional and simulated ECG become negligible. The cardiologist can read the simulated ECG as if it were obtained in the conventional manner. He may even come to prefer the simulated ECG because it has positive points: reproducibility is better and individual variation is less, at least theoretically. These are because there is less positional variation (a protractor is employed to determine the position of the electrode near the left nipple), and because the Frank lead system is designed to compensate for variations in the location of the electrical center of the heart. Synthesis of the 12-lead ECG req u i r e s r e l a t i v e l y simple i n s t r u m e n t a t i o n which can be taken to the bedside whenever the need arises, although it would normally be done later from the xyz signals recorded on magnetic tape. The decision to synthesize results not only in a saving of technician's time 9 but also in a substantial reduction of the amount of data sent to the computer and stored for future comparisons. An interesting corollary to the fact that the cardiologist can use the simulated 12-lead ECG is that the computer can, too. This means that the decision to record only the xyz signals does not restrict computer analysis to programs based on them; 12-lead E C G a n a l y s i s p r o g r a m s can be used also. It is sometimes asked: Since the xyz signals are being recorded, why not learn to read the orthogonal (xyz) ECG rather than synthesize the 12-lead ECG? The a n s w e r is that, although the xyz signals contain all the necessary information, the orthogonal ECGs are not easy to read. To give an example, the x and z signals m a y look normal yet the xz, or transverse, loop m a y indicate infarction, s The trouble is that the xyz signals may fail to display the information they contain. Obviously, a distinction must be drawn between information and display. Data Display Significant information m a y be contained in the xyz signals without its being apparent to an observer. This will not matter if the computer displaces the observer altogether, but computer programming has not yet progressed to this stage. Let the computer assist the cardiologist by presenting the data in diverse ways. Among these m a y be cited the simulated 12-lead ECG (attractive because of J. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
DOWER
ET A L
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TABLE 1 Comparison of ECG Diagnoses Based on Standard vs. Synthesized 12-lead ECGs
Standard
N
Synthesized N (normal)
17
AMI (anterior infarction)
AMI
IMI
LAE
P RVH incr.
Dil. R.V.
LVH
N. LBBB RBBB AV NSP Volt. arrhy.
121 PATIENTS
12
182 FEATURES
IMI
(inferior infarction)
11
LAE (left atrial
enlargement)
24
2
P incr. (P wave increased) RVH (right ventricular hypertrophy) Dil. R.V.
(dilated right ventricle)
4
3
LVH (left ventricular hypertrophy)
20
N. Volt. (normal voltage)
12
LBBB (left bundle branch block) RBBB
(right bundle branch block) AV arrhy. (atrio ventricular arrhythmia) NSP (non specific pattern)
26
1
1
32
Comment: The voltage criteria for the synthesized 12-lead ECG are somewhat different. However, much simplier criteria are based on the spatial magnitudes derived from the XYZ signals. These are normally available with the simulated 12-lead ECG and have a diagnostic performance at least as good as that of the 12-lead ECG.
the familiarity of its format), the vectorcardiogram (VCG), the PCG, various numerical d a t a such as time m e a s u r e m e n t s and t h e magnitudes and directions of selected heart vectors, such as P and ~ mentioned above, and a computer assessment of disease probabilities. All these displays can be written out automatically, without the need for mounting J. ELECTROCARDIOLOGY, V O L 8, NO. 1, 1975
of tracings or loops, and any or all of them can be provided for any beat of recorded xyz data - - a considerable aid in the determination of the point of origin of ectopic beats in, for example, the exercising patient. 1~ Of course, all forms of display need not be studied in every case, but they can be produced without recalling the patient.
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ECG DATA ACQUISITION
Data Editing Nowadays, in an effort to save the technician's time, t h e r e is a tendency to feed unedited data into the computer. Actually, the saving m a y be more apparent than real and the computer system may be encumbered with unnecessary data, the transmission system may be used inefficiently, and the quality of the data m a y be substandard. What is editing? Whenever a technician cuts and mounts an ECG she edits data. This editing is retrospective, being done after the data are acquired. When a technician records an ECG she is editing because she has to decide when the flow of d a t a will start and whether or not the quality will be satisfact o r y . This t y p e of e d i t i n g is prospective because it is carried out before the acquisition of data. With a direct link to the computer - an on-line system - - this is the only type of editing possible. If the data are first recorded on magnetic tape, retrospective editing m a y be carried out as well, considerably enhancing the efficiency of the editing program. Unfortunately, this option is not available if the tape-recording stage is merely an ancillary to a normal on-line operation. In V a n c o u v e r , r e t r o e d i t i n g t h e t a p e recorded xyz signals is done by playing them b a c k and v i e w i n g t h e m on a screen. The operator watches until about six successive beats of good quality have been seen and then presses a switch that results in those very b e a t s being saved for t r a n s m i s s i o n , computer analysis and future comparison. Normally, the operator has a train of 20 or 30 pre-recorded beats to select from. If desired, for rhythm analysis, for example, they can all be run off.
Data Compression The ECG data m u s t be digitized before they can be analyzed by the computer. Analog-todigital conversion requires repeated sampling of the analog signals at rates ranging, at different centers, from 250 to 1000 per second. L o w s a m p l i n g r a t e s m a y o b s c u r e highfrequency detail in the xyz signals, but high rates may result in an excessive volume of digital information. In order to surmount this problem a data compressor has been developed 11 which takes advantage of the fact that high sampling rates are required only when analog signals are changing rapidly - a phenomenon limited to the QRS complex. An example of data compression is shown in Fig. 1. Data compression can lead to a tenfold reduction in the volume of information without any actual loss. The six beats, or so, selected by the editing technician, when compressed form a data block which requires minimal storage space. Compared with the prac-
Z SEC.
I
0.0
0.3
i
0.6
I
0.9
I
1.2
I
1.5
Yc
zc// CDMPRESSED ]]ME SCRLE
CARDIAC DATA COMPRESSION. FRANK LEADS
Fig. 1. Computer plots of digitized ECG signals before compression (X, Y, Z) and after compression (Xc, Yc, Zc). The compression ratio in this example is approximately 10:1. Note that there is relatively little compression of the QRS complex compared to that of slower parts of the tracings. (Resolution: vertical, 8 bits; horizontal, 1000 samples/sec.) This form of compression discards only data which are redundant, i.e. samplings which show no numerical change. The effective sampling rate for the transmitted samplings varies with the rate of change of the ECG signals. By transmitting counts of dropped samples, exact reconstitution of uncompressed digitized signals is possible.
tice of sending trains of 12-lead and xyz signals to the computer, this combination ofretroediting and compression gives an approximately 50-fold reduction. A reduction of this magnitude invites a re-evaluation of methods for the transmission of data.
Data Transmission There are good reasons for minimizing the amount of data to be transmitted to the comp u t e r . The t r a n s m i s s i o n of t h r e e simultaneous signals in analog form over standard telephone lines tends to produce deterioration. Furthermore, the band-width is limited to about 100 Hz, the m i n i m u m required for direct-writing electrocardiographs. Unless the intent is merely to automate rather than to improve, this m a y not be good enough. R e q u i r e m e n t s for v e c t o r c a r d i o g r a p h y and polarcardiography are considerably higher. The transmission of data in digital form is attractive because it is virtually noise-free and does not impose rigid bandwidth restricJ. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
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tions. However, a substantially longer time is required to send digitized xyz signals over t e l e p h o n e lines t h a n to send t h e p a r e n t analog signals. But editing and compression m a k e it possible to reduce the transmission time to be comparable with that required for the transmission of raw ECG data in analog form. Consequently, digital transmission becomes feasible over long distances. In a system proposed for British Columbia and currently under development, there will be preprocessing stations, each with a telecommunications typewriter that allows twoway communication with the computer. The preparation of a data package proceeds as follows: the analog tape is replayed and the patient's name and other data verbally recorded on the tape are typed out and stored in a digital cassette. The operator reviews the previously recorded analog xyz signals, monitoring t h e m as frontal and sagittal loops on a cathode ray screen. These analog signals are continuously digitized, compressed, and held in t e m p o r a r y storage. The capacity of the temporary storage facility is approximately six heart beats. As more beats come, those in longest are pushed out and discarded. However, when the operator presses a footswitch, the stored beats are inserted into the digital cassette, following the typed data. This completes the data package. The data package is transmitted from the digital cassette via telephone line to the computer by the same route. The computer returns its report, which is typed out automatically. Transmission of the data package can be immediate or it m a y be delayed and sent upon command from the computer - - if lower rates apply. The decision to adopt data pre-processing affects the decision whether to employ an online or an off-line system. On-line vs Off-line In an on-line s y s t e m t h e r e is a d i r e c t connection between the patient and the computer, usually a telephone line. The advantage of this is that there is no need to record the signals on magnetic tape. When analog tape recorders tended to introduce appreciable noise, the on-line system seemed the more attractive and probably less expensive. However, modern tape recorders g e n e r a t e insignificant electrical noise and the cost of the on-line system has been pushed up by the complex nature of the data acquisition carts. Such carts contain a three-channel oscillograph coupled to some form of automatic lead switching; they employ a numerical code to identify the patient and convey other information. A number of twelve or more digits m a y be used. Although elaborate means for re-checking the code are offered, the chance of J. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
57
error is not insignificant, and errors suspected later cannot be checked. The data acquisition equipment for the offline system is a good deal simpler. It consists merely of a portable tape recorder with associated electronics that permit monitoring during recording. Verbally recording the patient's n a m e and other data with the xyz signals is virtually error-free. (If the wrong name is used the patient is apt to correct it!) The off-line data acquisition unit is more versatile, compact, portable, and economical than current equivalent on-line units. If an immediate 12-lead ECG is required this can be synthesized either from the tape or directly from the patient. But the equipment to do this need not be taken routinely to the bedside. Extra equipment is needed to transmit the data to the computer (unless the tape itself is sent). This may involve retroediting and compression carried out at a preprocessing station, which can handle the data from several data acquisition units. Traditionally, interpretation of the ECG has been "off-line" in that the record is cut, mounted, and interpreted later. Even in an on-line system, records that m a y receive '~offline" interpretation are still produced. In the off-line system the procedure is hardly different in that records and computer outputs are generated to be interpreted or checked later. Although some delay exists between the recording and transmission of data, this can be reduced to m i n u t e s when the need arises. Consequently, the on-line system m a y not be significantly faster in practice. In any case, when an ECG is ordered, minutes or hours elapse before the technician prepares and connects the patient, so that the advantages of being on-line m a y seldom be utilized. If the merits of being on-line m a y be questionable, there is no doubt that a premium must be paid. The on-line system lacks the f l e x i b i l i t y of t h e off-line s y s t e m . Transmission and data storage are less satisfactory, more expensive, or both. The on-line system is vulnerable to interruptions of the communicating and computing system, whereas, with the off-line system, data may continue to be collected and held until the rest of the system is operational. The on-line system is less adaptable to field use and it does not provide such positive identification, in that there are no means for rechecking later if an error is suspected. In all of these respects the off-line system is superior; in addition, it permits the pre-packaging of data, which can be transmitted at a convenient or economical time upon a command generated by the computer. A common feature of on-line data acquisition carts, designed to record the 12-lead ECG and to feed the xyz signals to the computer, is
58
ECG DATA ACQUISITION
the automatic switching of sets of leads, recorded three at a time, to give a write-out of the 12-lead ECG with strips of equal length for each lead, in a format that does not require cutting and mounting. It is interesting to compare autosequencing with the operat i o n of a s i n g l e - c h a n n e l , d i r e c t - w r i t i n g electrocardiograph. The only time saved is that required for the tracing to be written out and mounted; instead of 35 seconds, or so, the write-out takes 10 seconds - - unless the cycle has to be repeated, which is common. The mounting time for a single-channel 12-lead ECG is between 31/2 and 4'/2 minutes. Let us say the total time saved is five minutes. If the technician is paid $3.50 per hour, the saving is 30 cents per patient. Now let us look at the bad points. It is a feature of ECGs that large deflections tend to be either upward or downward, rather t h a n both. In o p e r a t i n g the conventional electrocardiograph the baselines are moved up or down to suit the lead being recorded. In autosequencing there is no opportunity for this; the baselines must be kept centered and t h e l i m i t s of t h e u p w a r d and d o w n w a r d deflections are 25 mm. Deflections of this magnitude are at the upper bounds of normal a t n o r m a l s e n s i t i v i t y . As a r e s u l t , t h e amplification used with autosequencing is often half what it would be in an ECG recorded on a conventional electrocardiograph, in which, by m o v i n g t h e b a s e l i n e appropriately, deflections up to 5 mm, upward or downward, may be recorded. After setting the amplification, the technician may run off the complete sequence before she knows if t h e setting is correct; she m a y have to repeat the s e q u e n c e at a n e w amplification, t h e r e b y wasting time and paper. She may even decide that 12-lead ECGs at two amplifications will be needed: one to show small P waves in the limb leads and one to show large R or S waves in the precordial leads. Near the end of an otherwise satisfactory tracing, the baseline m a y wander off in one of the leads, especially if the patient is restless. This will normally r e q u i r e repetition of t h e whole sequence. Autosequencing requires that six chest electrodes be used to record leads V1-6 rather than a single electrode, which is moved. If the F r a n k system is added, the total number may be 14 or 16 electrodes. From the interpreter's point of view there are no benefits to autosequencing, only difficulties. The overall quality of the tracings tends to be poorer because the technician's task, in diffcult patients, is actually harder. The n u m b e r of tracings at half sensitivity turns out to be considerably greater, and serial tracings on the same patient m a y easily be
recorded at different sensitivities, rendering comparison more difficult. The interpreter must be alert to the fact that increasing the n u m b e r of electrodes greatly increases the possibility for error in electrode placement. These factors m a y appreciably increase the time it takes to read an ECG. Before leaving the on-line vs off-line issue, a word should be said about cardiac monitoring. This is a special case for the on-line system in which initial diagnoses are not required - - only changes are important. This monitoring function is probably better handled by a local dedicated computer in continuous contact with the monitored subjects and capable of responding to a variety of inputs, the ECG being only one. Data Storage and Retrieval The clinical importance of comparing present with past data leaves no doubt that computer analysis of the future must include this process. It is not rash to predict that such data will exceed one entry per capita. The magnitude of this problem is not immediately obvious because the capacity of computer systems to store vast amounts of data is revealed in m a n y familiar instances, and it m a y not be appreciated how voluminous are the results of digitizing ECG signals. Typically, the volume of d a t a flowing to the computer would be e q u i v a l e n t to 3000 words of print, or five typewritten pages. Whenever a repeat study is made it will be necessary to retrieve these data quickly and automatically. Methods for storing large amounts of data for rapid retrieval are being actively studied and no doubt solutions will soon be available which can be applied to ECG data. The possibility will exist for storing ECGs from every individual in a central repository for ready access from anywhere in the country. It is in this perspective that the 50-fold reduction aff o r d e d by r e t r o - e d i t i n g 'and c o m p r e s s i o n should be viewed. The Decisions T h e s e a r e t h e f u n d a m e n t a l decisions: 12-lead vs xyz signals, or both; retroediting and compression, or not. ECG lead simulation and on-line or off-line, a n a l o g or digital transmission are influencing factors which ultimately depend upon those basic decisions. It appears that nothing of significance is lost by omitting t h e conventional 12-lead ECG signals. There is a considerable benefit from recording the xyz signals as a first step in d a t a reduction. The efficiency of retroediting dictates that it should be employed, as J. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
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well as c o m p r e s s i o n . T h e a d v a n t a g e s of online t r a n s m i s s i o n c a n be questioned. D i g i t a l t r a n s m i s s i o n is v a l u a b l e , a t l e a s t for long distances. Always relevant are diagnostic accuracy, c o n v e n i e n c e , cost, a n d flexibility. All t h e s e factors a r e to be c o n s i d e r e d a n d related. REFERENCES 1. BRUCE, R A, AND YARNALL, S R: Reliability and normal variations of computer analysis of Frank electrocardiogram by Smith-Hyde program (1968 version). Am J Cardiol 29:389, 1972 2. DOWER, G E, HORN, H E AND ZIEGLER, W G: The polarcardiograph: diagnosis of myocardial infarction. Am Heart J 69:369, 1965 3. CACERES, E A: Computer analysis in the determination of standard and normal ranges for cardiograph values. In Vectorcardiography 2: Proceedings of the XI Symposium on Vectorcardiography, I HOFFMAN, ed. North-Holland, Amsterdam, 1971, p 192 4. PIPBERGER, H V, BIALEK, S M, PERLOFF, J K AND SCHNAPER, H E: Correlation of clinical information in the standard 12-lead ECG and in a corrected orthogonal 3-lead ECG. Am Heart J 61:31, 1961
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5. OSBORNE, J A: Diagnosis by means of X, Y, Z s i g n a l s in a l a r g e g e n e r a l h o s p i t a l . In Vectorcardiography, 2: Proceedings of the XI S y m p o s i u m on V e c t o r c a r d i o g r a p h y , I HOFFMAN, ed. North-Holland, A m s t e r d a m , 1971, p 170 6. SMITH, R A AND HYDE, C M: C o m p u t e r analysis of the electrocardiogram in clinical practice. In Electrical activity of the heart, G W MANNINGand S P AHUJA, eds. Charles C. Thomas, Springfield, Illinois, 1969, p 305 7. DOWER, G E: A lead synthesizer for the Frank s y s t e m to s i m u l a t e the s t a n d a r d 12-lead electrocardiogram. J Electrocardiology 1:101, 1968 8. DOWER, G E: Polarcardiography. Charles C . Thomas, Springfield, Illinois, 1971, p 247 9. DOWER, G E: Extending vectorcardiography, In Vectorcardiography 2: Proceedings of the XI S y m p o s i u m on V e c t o r c a r d i o g r a p h y , I HOFFMAN, ed. North-Holland, Amsterdam, 1971, p 194 10. BRUCE, R A, DETRY, J-M, EARLY, K AND EARLY, R: Polarcardiographic responses to maximal exercise in healthy young adults. Am Heart J 83:206, 1972 11. STEWART, D E, DOWER, G E AND SURANYI, O: Letter to the editor - - an ECG compression code. J Electrocardiology 6:175, 1973
Special Article: ECG Data Acquisition. A Discussion BY G. E. DOWER, J. A. OSBORNE, O. SURANYI, D. E. STEWART, AND K. BONHAM
The use of c o m p u t e r s in electrocardiography is spreading rapidly and substantial funds are being committed. But do we need to use a c o m p u t e r to do s o m e t h i n g which a c a r d i o l o g i s t can do v e r y e f f i c i e n t l y ? An affirmative answer must be on the grounds of economy or better diagnosis. Economy is more often cited t h a n i m p r o v e m e n t - - in fact, automated interpretation may compare unfavorably with that of the cardiologist. 1 But economic grounds m a y be hard to justify b e c a u s e r e c o r d i n g a n d i n t e r p r e t i n g an electrocardiogram (ECG) is a relatively simple p r o c e d u r e r e q u i r i n g r a t h e r m o d e s t equipment and very little of a cardiologist's time. It is not easy to save technician's time and the capital cost of a computing system is about two orders of magnitude greater than that of a conventional ECG service. So let us look again at the justification that use of the computer may lead to a better diagnosis. Early efforts directed at merely automating the cardiologist did not take into account the very good possibility that the cardiologisti n t e r p r e t e r can be aided by the computer rather than replaced by it. Examples of the way in which the computer can help are as follows: making various measurements from the ECG signals, comparing them with norm a l and m o r b i d v a l u e s , g i v i n g r e l e v a n t probabilities, making various computations such as spatial angles and magnitudes, providing various displays of information, and keeping an extensive file of previous data. Another way in which the computer can help is via polarcardiography. Polarcardiog r a m s (PCGs) can lead to c o n s i d e r a b l e e n h a n c e m e n t of the accuracy of the E C G diagnosis. 2 Furthermore, the PCG criteria for infarction and various other conditions are relatively simple to program; this can assist the cardiologist in learning a new technique. -
The intelligence of computers can be improved indefinitely so that, ultimately, the prospects for computer analysis and interpretation are good. Unfortunately, some centers have optimistically adopted computer techniques on grounds that others had done so, without there being a clear, impartial assessment of performance. Much depends upon how the computer is to be used. In this connection, m a n y fundamental decisions have been made which deserve a second look in the light of recent technological d e v e l o p m e n t s . T h e s e decisions, right or wrong, could affect the future value of c o m p u t e r s to e l e c t r o c a r d i o g r a p h y . T h i s editorial will examine some of them.
Choosing the Types of Data There are essentially two types of data: the 12-lead ECG signals and the xyz signals. Which are better for computer analysis? This depends on how the computer is to be used. If the intent is merely to automate, matching the cardiologist's interpretation as closely as possible, the decision of Caceres 3 and others to transmit the 12-lead signals, in sequence, to the computer would seem to be the most reasonable. On the other hand, if the intent is to employ the computer as a diagnostic aid in its own right, t h e r e b y offering something more than can be provided from a singlechannel, direct-writing electrocardiograph, a good case can be made for the decision of Pipberger and others in favor of the xyz signals. Pipberger showed experimentally that these signals contain virtually all the information utilized by a cardiologist reading the 12-lead ECG. 4 The reduction of the array of body surface potentials to three, made possible by the heart vector concept, has attractions for comp u t e r program development unfettered by traditional methods. There are attractions, too, in employing xyz leads, which have a rational, physical basis, rather than ECG leads, which have a more traditional background. The experience of m a n y cardiologists with s o m e t y p e s of v e c t o r c a r d i o g r a p h s o f t e n prompts the expression of doubt as to the practicability of employing an xyz lead system routinely. The following comments of a reviewer are representative of this view: "The proper recording of a vectorcardiogram with the Frank lead system is a long and laborious
-
From the Departments of Cardiology, Vancouver General Hospital, and Polarcardiography. Shaughnessy Hospital, Vancouver, B.C., Canada. Reprint requests to: Dr. J. A. Osborne, Cardiology, Vancouver General Hospital, Vancouver, Canada. 53
54
ECG DATA ACQUISITION
process and in my opinion is the primary reason to doubt the applicability of this technique to routine electrocardiography." But with suitable equipment, recording xyz sign a l s can be simpler than recording a convent i o n a l 12-lead ECG. 5 In a recent field study of Cretan villagers, nearly 600 xyz magnetictape recordings were made in a dozen villages in ten days; on some days over 100 recordings were made. The Frank system was employed. The tracings were recorded by one of the authors (GED) and his wife with occasional help from nurses at village clinics. Of course, this experience says much for the magnificent cooperation of the villagers, but it also clearly demonstrates the practicability of the Frank lead system. At the Mayo Clinic the decision was made to acquire both the 12-lead ECG signals and the xyz signals. 6 This meant some extra work for t h e t e c h n i c i a n s , and some i n g e n i o u s m e a n s for m a k i n g the operation practical were worked out. One of these involved combining the F r a n k and 12-lead ECG electrodes in the left mid-axillary line a n d i n the region of the left nipple. This entailed modifying the F r a n k electrode positions slightly. Although there have been no reported studies of the effect of this modification of Frank's system, s e v e r a l m a n u f a c t u r e r s have adopted this arrangement as standard. Actually, there is a more attractive alternative: to record only the F r a n k xyz s i g n a l s and to s y n t h e s i z e t h e 12-lead ECG from them. ECG Lead Synthesis The xyz signals from the F r a n k network m a y be combined in accordance with Frank's image space data to yield simulations of the 12-lead ECG which are often surprisingly close. 7,s This has been subjected to extensive clinical study and the agreement is satisfactory for q u a l i t a t i v e comparisons. Table I shows a comparison of 121 typical, unselected, 12-lead ECGs, conventional and simul a t e d , from p a t i e n t s u n d e r g o i n g c a r d i a c catheterization. There were 182 abnormalities identified. The tracings were read independently by various interpreters who were u n a w a r e of the test, the tracings being introduced into the daily pile of ECGs to be interpreted. The concordance of the diagnoses is r e a s s u r i n g . The differences observed are largely due to two features of the simulated 12-lead ECG: the P waves are often inverted in V1, occasionally in lead I, and the potentials in some of the precordial leads tend to be somewhat less. Consequently, there was a tendency to over-read atrial hypertrophy and to under-read left ventricular hypertrophy. These shortcomings disappear if data concerning the spatial magnitudes of the maxim u m P and QRS vectors (P and R) are avail-
able, because simple cut-off points can be used for them. Obviously, these data will be available, if the xyz signals are sent to the computer. But, in any case, once the reader becomes familiar with the simulated ECG he is not apt to be misled, and when these two minor points (concerning P and R) are taken into consideration, the clinical differences between the conventional and simulated ECG become negligible. The cardiologist can read the simulated ECG as if it were obtained in the conventional manner. He may even come to prefer the simulated ECG because it has positive points: reproducibility is better and individual variation is less, at least theoretically. These are because there is less positional variation (a protractor is employed to determine the position of the electrode near the left nipple), and because the Frank lead system is designed to compensate for variations in the location of the electrical center of the heart. Synthesis of the 12-lead ECG req u i r e s r e l a t i v e l y simple i n s t r u m e n t a t i o n which can be taken to the bedside whenever the need arises, although it would normally be done later from the xyz signals recorded on magnetic tape. The decision to synthesize results not only in a saving of technician's time 9 but also in a substantial reduction of the amount of data sent to the computer and stored for future comparisons. An interesting corollary to the fact that the cardiologist can use the simulated 12-lead ECG is that the computer can, too. This means that the decision to record only the xyz signals does not restrict computer analysis to programs based on them; 12-lead E C G a n a l y s i s p r o g r a m s can be used also. It is sometimes asked: Since the xyz signals are being recorded, why not learn to read the orthogonal (xyz) ECG rather than synthesize the 12-lead ECG? The a n s w e r is that, although the xyz signals contain all the necessary information, the orthogonal ECGs are not easy to read. To give an example, the x and z signals m a y look normal yet the xz, or transverse, loop m a y indicate infarction, s The trouble is that the xyz signals may fail to display the information they contain. Obviously, a distinction must be drawn between information and display. Data Display Significant information m a y be contained in the xyz signals without its being apparent to an observer. This will not matter if the computer displaces the observer altogether, but computer programming has not yet progressed to this stage. Let the computer assist the cardiologist by presenting the data in diverse ways. Among these m a y be cited the simulated 12-lead ECG (attractive because of J. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
DOWER
ET A L
55
TABLE 1 Comparison of ECG Diagnoses Based on Standard vs. Synthesized 12-lead ECGs
Standard
N
Synthesized N (normal)
17
AMI (anterior infarction)
AMI
IMI
LAE
P RVH incr.
Dil. R.V.
LVH
N. LBBB RBBB AV NSP Volt. arrhy.
121 PATIENTS
12
182 FEATURES
IMI
(inferior infarction)
11
LAE (left atrial
enlargement)
24
2
P incr. (P wave increased) RVH (right ventricular hypertrophy) Dil. R.V.
(dilated right ventricle)
4
3
LVH (left ventricular hypertrophy)
20
N. Volt. (normal voltage)
12
LBBB (left bundle branch block) RBBB
(right bundle branch block) AV arrhy. (atrio ventricular arrhythmia) NSP (non specific pattern)
26
1
1
32
Comment: The voltage criteria for the synthesized 12-lead ECG are somewhat different. However, much simplier criteria are based on the spatial magnitudes derived from the XYZ signals. These are normally available with the simulated 12-lead ECG and have a diagnostic performance at least as good as that of the 12-lead ECG.
the familiarity of its format), the vectorcardiogram (VCG), the PCG, various numerical d a t a such as time m e a s u r e m e n t s and t h e magnitudes and directions of selected heart vectors, such as P and ~ mentioned above, and a computer assessment of disease probabilities. All these displays can be written out automatically, without the need for mounting J. ELECTROCARDIOLOGY, V O L 8, NO. 1, 1975
of tracings or loops, and any or all of them can be provided for any beat of recorded xyz data - - a considerable aid in the determination of the point of origin of ectopic beats in, for example, the exercising patient. 1~ Of course, all forms of display need not be studied in every case, but they can be produced without recalling the patient.
56
ECG DATA ACQUISITION
Data Editing Nowadays, in an effort to save the technician's time, t h e r e is a tendency to feed unedited data into the computer. Actually, the saving m a y be more apparent than real and the computer system may be encumbered with unnecessary data, the transmission system may be used inefficiently, and the quality of the data m a y be substandard. What is editing? Whenever a technician cuts and mounts an ECG she edits data. This editing is retrospective, being done after the data are acquired. When a technician records an ECG she is editing because she has to decide when the flow of d a t a will start and whether or not the quality will be satisfact o r y . This t y p e of e d i t i n g is prospective because it is carried out before the acquisition of data. With a direct link to the computer - an on-line system - - this is the only type of editing possible. If the data are first recorded on magnetic tape, retrospective editing m a y be carried out as well, considerably enhancing the efficiency of the editing program. Unfortunately, this option is not available if the tape-recording stage is merely an ancillary to a normal on-line operation. In V a n c o u v e r , r e t r o e d i t i n g t h e t a p e recorded xyz signals is done by playing them b a c k and v i e w i n g t h e m on a screen. The operator watches until about six successive beats of good quality have been seen and then presses a switch that results in those very b e a t s being saved for t r a n s m i s s i o n , computer analysis and future comparison. Normally, the operator has a train of 20 or 30 pre-recorded beats to select from. If desired, for rhythm analysis, for example, they can all be run off.
Data Compression The ECG data m u s t be digitized before they can be analyzed by the computer. Analog-todigital conversion requires repeated sampling of the analog signals at rates ranging, at different centers, from 250 to 1000 per second. L o w s a m p l i n g r a t e s m a y o b s c u r e highfrequency detail in the xyz signals, but high rates may result in an excessive volume of digital information. In order to surmount this problem a data compressor has been developed 11 which takes advantage of the fact that high sampling rates are required only when analog signals are changing rapidly - a phenomenon limited to the QRS complex. An example of data compression is shown in Fig. 1. Data compression can lead to a tenfold reduction in the volume of information without any actual loss. The six beats, or so, selected by the editing technician, when compressed form a data block which requires minimal storage space. Compared with the prac-
Z SEC.
I
0.0
0.3
i
0.6
I
0.9
I
1.2
I
1.5
Yc
zc// CDMPRESSED ]]ME SCRLE
CARDIAC DATA COMPRESSION. FRANK LEADS
Fig. 1. Computer plots of digitized ECG signals before compression (X, Y, Z) and after compression (Xc, Yc, Zc). The compression ratio in this example is approximately 10:1. Note that there is relatively little compression of the QRS complex compared to that of slower parts of the tracings. (Resolution: vertical, 8 bits; horizontal, 1000 samples/sec.) This form of compression discards only data which are redundant, i.e. samplings which show no numerical change. The effective sampling rate for the transmitted samplings varies with the rate of change of the ECG signals. By transmitting counts of dropped samples, exact reconstitution of uncompressed digitized signals is possible.
tice of sending trains of 12-lead and xyz signals to the computer, this combination ofretroediting and compression gives an approximately 50-fold reduction. A reduction of this magnitude invites a re-evaluation of methods for the transmission of data.
Data Transmission There are good reasons for minimizing the amount of data to be transmitted to the comp u t e r . The t r a n s m i s s i o n of t h r e e simultaneous signals in analog form over standard telephone lines tends to produce deterioration. Furthermore, the band-width is limited to about 100 Hz, the m i n i m u m required for direct-writing electrocardiographs. Unless the intent is merely to automate rather than to improve, this m a y not be good enough. R e q u i r e m e n t s for v e c t o r c a r d i o g r a p h y and polarcardiography are considerably higher. The transmission of data in digital form is attractive because it is virtually noise-free and does not impose rigid bandwidth restricJ. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
DOWER ET AL
tions. However, a substantially longer time is required to send digitized xyz signals over t e l e p h o n e lines t h a n to send t h e p a r e n t analog signals. But editing and compression m a k e it possible to reduce the transmission time to be comparable with that required for the transmission of raw ECG data in analog form. Consequently, digital transmission becomes feasible over long distances. In a system proposed for British Columbia and currently under development, there will be preprocessing stations, each with a telecommunications typewriter that allows twoway communication with the computer. The preparation of a data package proceeds as follows: the analog tape is replayed and the patient's name and other data verbally recorded on the tape are typed out and stored in a digital cassette. The operator reviews the previously recorded analog xyz signals, monitoring t h e m as frontal and sagittal loops on a cathode ray screen. These analog signals are continuously digitized, compressed, and held in t e m p o r a r y storage. The capacity of the temporary storage facility is approximately six heart beats. As more beats come, those in longest are pushed out and discarded. However, when the operator presses a footswitch, the stored beats are inserted into the digital cassette, following the typed data. This completes the data package. The data package is transmitted from the digital cassette via telephone line to the computer by the same route. The computer returns its report, which is typed out automatically. Transmission of the data package can be immediate or it m a y be delayed and sent upon command from the computer - - if lower rates apply. The decision to adopt data pre-processing affects the decision whether to employ an online or an off-line system. On-line vs Off-line In an on-line s y s t e m t h e r e is a d i r e c t connection between the patient and the computer, usually a telephone line. The advantage of this is that there is no need to record the signals on magnetic tape. When analog tape recorders tended to introduce appreciable noise, the on-line system seemed the more attractive and probably less expensive. However, modern tape recorders g e n e r a t e insignificant electrical noise and the cost of the on-line system has been pushed up by the complex nature of the data acquisition carts. Such carts contain a three-channel oscillograph coupled to some form of automatic lead switching; they employ a numerical code to identify the patient and convey other information. A number of twelve or more digits m a y be used. Although elaborate means for re-checking the code are offered, the chance of J. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
57
error is not insignificant, and errors suspected later cannot be checked. The data acquisition equipment for the offline system is a good deal simpler. It consists merely of a portable tape recorder with associated electronics that permit monitoring during recording. Verbally recording the patient's n a m e and other data with the xyz signals is virtually error-free. (If the wrong name is used the patient is apt to correct it!) The off-line data acquisition unit is more versatile, compact, portable, and economical than current equivalent on-line units. If an immediate 12-lead ECG is required this can be synthesized either from the tape or directly from the patient. But the equipment to do this need not be taken routinely to the bedside. Extra equipment is needed to transmit the data to the computer (unless the tape itself is sent). This may involve retroediting and compression carried out at a preprocessing station, which can handle the data from several data acquisition units. Traditionally, interpretation of the ECG has been "off-line" in that the record is cut, mounted, and interpreted later. Even in an on-line system, records that m a y receive '~offline" interpretation are still produced. In the off-line system the procedure is hardly different in that records and computer outputs are generated to be interpreted or checked later. Although some delay exists between the recording and transmission of data, this can be reduced to m i n u t e s when the need arises. Consequently, the on-line system m a y not be significantly faster in practice. In any case, when an ECG is ordered, minutes or hours elapse before the technician prepares and connects the patient, so that the advantages of being on-line m a y seldom be utilized. If the merits of being on-line m a y be questionable, there is no doubt that a premium must be paid. The on-line system lacks the f l e x i b i l i t y of t h e off-line s y s t e m . Transmission and data storage are less satisfactory, more expensive, or both. The on-line system is vulnerable to interruptions of the communicating and computing system, whereas, with the off-line system, data may continue to be collected and held until the rest of the system is operational. The on-line system is less adaptable to field use and it does not provide such positive identification, in that there are no means for rechecking later if an error is suspected. In all of these respects the off-line system is superior; in addition, it permits the pre-packaging of data, which can be transmitted at a convenient or economical time upon a command generated by the computer. A common feature of on-line data acquisition carts, designed to record the 12-lead ECG and to feed the xyz signals to the computer, is
58
ECG DATA ACQUISITION
the automatic switching of sets of leads, recorded three at a time, to give a write-out of the 12-lead ECG with strips of equal length for each lead, in a format that does not require cutting and mounting. It is interesting to compare autosequencing with the operat i o n of a s i n g l e - c h a n n e l , d i r e c t - w r i t i n g electrocardiograph. The only time saved is that required for the tracing to be written out and mounted; instead of 35 seconds, or so, the write-out takes 10 seconds - - unless the cycle has to be repeated, which is common. The mounting time for a single-channel 12-lead ECG is between 31/2 and 4'/2 minutes. Let us say the total time saved is five minutes. If the technician is paid $3.50 per hour, the saving is 30 cents per patient. Now let us look at the bad points. It is a feature of ECGs that large deflections tend to be either upward or downward, rather t h a n both. In o p e r a t i n g the conventional electrocardiograph the baselines are moved up or down to suit the lead being recorded. In autosequencing there is no opportunity for this; the baselines must be kept centered and t h e l i m i t s of t h e u p w a r d and d o w n w a r d deflections are 25 mm. Deflections of this magnitude are at the upper bounds of normal a t n o r m a l s e n s i t i v i t y . As a r e s u l t , t h e amplification used with autosequencing is often half what it would be in an ECG recorded on a conventional electrocardiograph, in which, by m o v i n g t h e b a s e l i n e appropriately, deflections up to 5 mm, upward or downward, may be recorded. After setting the amplification, the technician may run off the complete sequence before she knows if t h e setting is correct; she m a y have to repeat the s e q u e n c e at a n e w amplification, t h e r e b y wasting time and paper. She may even decide that 12-lead ECGs at two amplifications will be needed: one to show small P waves in the limb leads and one to show large R or S waves in the precordial leads. Near the end of an otherwise satisfactory tracing, the baseline m a y wander off in one of the leads, especially if the patient is restless. This will normally r e q u i r e repetition of t h e whole sequence. Autosequencing requires that six chest electrodes be used to record leads V1-6 rather than a single electrode, which is moved. If the F r a n k system is added, the total number may be 14 or 16 electrodes. From the interpreter's point of view there are no benefits to autosequencing, only difficulties. The overall quality of the tracings tends to be poorer because the technician's task, in diffcult patients, is actually harder. The n u m b e r of tracings at half sensitivity turns out to be considerably greater, and serial tracings on the same patient m a y easily be
recorded at different sensitivities, rendering comparison more difficult. The interpreter must be alert to the fact that increasing the n u m b e r of electrodes greatly increases the possibility for error in electrode placement. These factors m a y appreciably increase the time it takes to read an ECG. Before leaving the on-line vs off-line issue, a word should be said about cardiac monitoring. This is a special case for the on-line system in which initial diagnoses are not required - - only changes are important. This monitoring function is probably better handled by a local dedicated computer in continuous contact with the monitored subjects and capable of responding to a variety of inputs, the ECG being only one. Data Storage and Retrieval The clinical importance of comparing present with past data leaves no doubt that computer analysis of the future must include this process. It is not rash to predict that such data will exceed one entry per capita. The magnitude of this problem is not immediately obvious because the capacity of computer systems to store vast amounts of data is revealed in m a n y familiar instances, and it m a y not be appreciated how voluminous are the results of digitizing ECG signals. Typically, the volume of d a t a flowing to the computer would be e q u i v a l e n t to 3000 words of print, or five typewritten pages. Whenever a repeat study is made it will be necessary to retrieve these data quickly and automatically. Methods for storing large amounts of data for rapid retrieval are being actively studied and no doubt solutions will soon be available which can be applied to ECG data. The possibility will exist for storing ECGs from every individual in a central repository for ready access from anywhere in the country. It is in this perspective that the 50-fold reduction aff o r d e d by r e t r o - e d i t i n g 'and c o m p r e s s i o n should be viewed. The Decisions T h e s e a r e t h e f u n d a m e n t a l decisions: 12-lead vs xyz signals, or both; retroediting and compression, or not. ECG lead simulation and on-line or off-line, a n a l o g or digital transmission are influencing factors which ultimately depend upon those basic decisions. It appears that nothing of significance is lost by omitting t h e conventional 12-lead ECG signals. There is a considerable benefit from recording the xyz signals as a first step in d a t a reduction. The efficiency of retroediting dictates that it should be employed, as J. ELECTROCARDIOLOGY, VOL. 8, NO. 1, 1975
DOWER ET AL
well as c o m p r e s s i o n . T h e a d v a n t a g e s of online t r a n s m i s s i o n c a n be questioned. D i g i t a l t r a n s m i s s i o n is v a l u a b l e , a t l e a s t for long distances. Always relevant are diagnostic accuracy, c o n v e n i e n c e , cost, a n d flexibility. All t h e s e factors a r e to be c o n s i d e r e d a n d related. REFERENCES 1. BRUCE, R A, AND YARNALL, S R: Reliability and normal variations of computer analysis of Frank electrocardiogram by Smith-Hyde program (1968 version). Am J Cardiol 29:389, 1972 2. DOWER, G E, HORN, H E AND ZIEGLER, W G: The polarcardiograph: diagnosis of myocardial infarction. Am Heart J 69:369, 1965 3. CACERES, E A: Computer analysis in the determination of standard and normal ranges for cardiograph values. In Vectorcardiography 2: Proceedings of the XI Symposium on Vectorcardiography, I HOFFMAN, ed. North-Holland, Amsterdam, 1971, p 192 4. PIPBERGER, H V, BIALEK, S M, PERLOFF, J K AND SCHNAPER, H E: Correlation of clinical information in the standard 12-lead ECG and in a corrected orthogonal 3-lead ECG. Am Heart J 61:31, 1961
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5. OSBORNE, J A: Diagnosis by means of X, Y, Z s i g n a l s in a l a r g e g e n e r a l h o s p i t a l . In Vectorcardiography, 2: Proceedings of the XI S y m p o s i u m on V e c t o r c a r d i o g r a p h y , I HOFFMAN, ed. North-Holland, A m s t e r d a m , 1971, p 170 6. SMITH, R A AND HYDE, C M: C o m p u t e r analysis of the electrocardiogram in clinical practice. In Electrical activity of the heart, G W MANNINGand S P AHUJA, eds. Charles C. Thomas, Springfield, Illinois, 1969, p 305 7. DOWER, G E: A lead synthesizer for the Frank s y s t e m to s i m u l a t e the s t a n d a r d 12-lead electrocardiogram. J Electrocardiology 1:101, 1968 8. DOWER, G E: Polarcardiography. Charles C . Thomas, Springfield, Illinois, 1971, p 247 9. DOWER, G E: Extending vectorcardiography, In Vectorcardiography 2: Proceedings of the XI S y m p o s i u m on V e c t o r c a r d i o g r a p h y , I HOFFMAN, ed. North-Holland, Amsterdam, 1971, p 194 10. BRUCE, R A, DETRY, J-M, EARLY, K AND EARLY, R: Polarcardiographic responses to maximal exercise in healthy young adults. Am Heart J 83:206, 1972 11. STEWART, D E, DOWER, G E AND SURANYI, O: Letter to the editor - - an ECG compression code. J Electrocardiology 6:175, 1973
