Precordial electrocardiographic mapping after exercise in the diagnosis of coronary artery disease

METHODS

Precordial Electrocardiographic Mapping After Exercise In the Diagnosis of Coronary Artery Disease

KIM FOX, MRCP ANDREW SELWYN, MRCP JOHN SHILLINGFORD.FRCP, FACC London, England

A technique is described for recording the precordial electrocardiographic body surface map before and after exercise. The technique provides an extra dimension to the conventional exercise electrocardiogram because a measurement can be made of the area and severity of S-T segment changes that are projected onto the front of the chest. Sixteen lead isopotential surface maps were recorded before and after exercise in 109 patients with angina who subsequently underwent coronary arteriography. In addition, exercise electrocardiograms were obtained in 53 of these patients using three orthogonal leads and in all patients using a single chest unipolar chest lead. Precordial surface mapping after exercise was found to have a greater sensitivity (95 percent) than electrocardiography using either the orthogonal leads (66 percent) or a single chest lead (64 percent) (P 0.05). The technique of precordial surface mapping after exercise improves the ability to diagnose coronary artery disease and can easily be applied to clinical practice.

The aim of exercise or stress testing is to increase myocardial oxygen demand in order to elicit the electrocardiographic manifestations of ischemia (S-T segment changes) that are absent at rest. The ability to elicit these alterations in the S-T segment depends on the degree of narrowing of the coronary arteries, the amount of work performed and the accuracy with which areas of myocardial ischemia can be detected. It is usually accepted that 70 percent luminal narrowing of a major coronary vessel is necessary before the blood supply to the myocardium can be compromised by exertion, l-3 and the modern multistage stress tests aiming at maximal heart rates ensure that an adequate work load is nearly always achieved.4 The principal limiting factor of these tests is the detection of myocardial ischemia. Although multiple lead and orthogonal systems are adequate in most cases,5,6 they are not sufficiently sensitive. This paper describes a technique for recording serial precordial surface maps after exercise that has proved superior to other methods in the diagnosis of coronary artery disesse. Methods From the Division of Cardiovascular Disease, Royal Postgraduate Medical School, Hammersmlth Hospital, London, England. This work was supported by a grant from the t3ritish Heart Foundation, London, England. Manuscript received July 24, 1978; revised manuscript received September 29, 1978, accepted October 5. 1978. Address for reprints: Kirr Fox, MRCP, Cardiovascular Research Unit, Royal Postgraduate Medical School, Hammersmith Hospital, London, W12 OHS, England.

Electrocardiographic mapping: One hundred nine consecutive patients (93 men and 16 women with a mean age of 48 years) were studied before undergoing diagnostic coronary arteriography. They were referred from the outpatient department and all were sufficiently fit to undergo an exercise test. In all patients a 16 point precordial electrocardiogram was recorded using a direct writing ink jet Mingograf (Elema Schonander) recording on four channels simultaneously. The gain employed was 10 mm for 1 mv, and the paper speed was 25 mm/set. The 16 points were distributed evenly over the left hemithorax, and the boundaries were the angle of Louis, the right sternoclavicular joint, the posterior axillary line and 6 cm below the xiphisternum (Fig. 1). Disposable press stud electrodes7 were used and then secured into position using adhesive tape

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FIGURE 1. Patient with 16 press stud electrodes evenly distributed over the left hemithorax. The electrodes are secured in position with adhesive tape and connected to the four channel electrocardiograph by means of a four-way switching system.

sufficiently tight to keep the electrodes stable without interfering with respiration. Screened electrode cable was used and the 16 electrodes were connected to the electrocardiograph using a four-way switching system (Fig. 1). The 16 precordial leads were unipolar V leads connected to a central Wilson terminal. One of the positions (C3, Fig. 2) corresponded exactly to lead Vs and was selected to act as the single chest lead. Exercise testing: Graded maximal exercise tests were performed using a bicycle ergometer.8 Patients cycled at a constant speed (50 revolutions/min), and the work load was increased by 25 watts each minute. Exercise was performed for 6.04 f 2.23 minutes (mean f 1 standard deviation). The exercise tests were limited by the appearance of chest pain, dyspnea, fatigue or multiple ventricular ectopic complexes.9 All recordings were made with the patient sitting comfortably

-+--v---+-+---

with the arms extended resting on the handlebars. A recording was taken from the 16 positions before exercise, immediately after and then 1,3,5,8 and 10 minutes later. Recordings were then continued at 3 minute intervals, as necessary, until the electrocardiogram had returned to the pre-exercise state. Connecting the electrodes to the patient in preparation for the test took 10 to 15 minutes, and usually 15 to 20 minutes was required for the exercise test and postexercise recordings. Each 16 lead precordial map took about 10 seconds to record, and about 10 minutes was required to analyze a series of maps from a patient. Fifty-three of these patients were randomly selected to undergo, in addition, a graded maximal exercise test using a treadmill.‘0 All these patients had orthogonal three lead electrocardiograms using the Frank lead system6 recorded before, during and at regular intervals after exercise using a

-+--+---k-b

4

Before Exercise

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FIGURE 2. Example of recording of the 16 positions before and after exercise. The precordial area of S-T segment depression drawn as a contour map is shown on the left.

After Exercise

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Hewlett-Packard 1513EI three channel electrocardiograph. X and Z leads were placed in the fourth intercostal space. All exercise tests were recorded before the coronary arteriograms were obtained. The criteria of the Scandinavian committee on electracardiographic classification were used to interpret the exer-

cise tests.11 S-T segment changes were measured from the P-Q segment, and the segment was considered depressed if there was a change of 1 mm or more lasting 0.08 second or longer. S-T segment elevation was measured 0.06 second from the end of the QRS complex and was considered significant if the value was 1 mm or more. Contour maps of the area of S-T segment change were drawn for each precordial surface map.12 Coronary arteriography: All patients had left ventricular angiography and coronary arteriogaphy performed with the and cardiologists who Judkins technique. l3 The radiologists performed the investigation also interpreted the coronary arteriograms without knowledge of the exercise tests. Statistical analysis was performed using a chi square test

TABLE

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I

Results of Coronarv

Arteriosraphv

in 109 Patients Patients (no.)

Single vessel disease Left anterior descending artery (LAD) Circumflex artery (LCx) Right coronary artery (RCA) Total

14 1 2:

Two vessel disease Left main stem LAD + LCx LAD + RCA RCA + LCx Total

:: 4 29

Three vessel disease Left main stem + RCA LAD + LCx + RCA Total

:: 36

4

and paired t tests.

Results It was possible to obtain satisfactory recordings permitting measurement of the S-T segment changes in all patients. Figure 2 shows a typical example of the quality of recording obtained before and after exercise in a patient with a significant narrowing of the left anterior descending coronary artery. Also shown is the contour map drawn of the area of S-T segment depression. It can be seen that the baseline is stable even immediately after severe exertion when a heart rate of 150 beatslmin has been achieved. It was not necessary to shave the chests of the men to obtain a satisfactory tracing, and no difficulties were experienced obtaining good tracings in the 116women.

immediately

we-exercise

FIGURE 3. Sequence of contour maps (with a sample electrocardiographic complex taken from position C2 below) recorded before and after exercise in a patient with significant narrowing of the left anterior descending coronary artery. S-T segment depression is present immediately after the end of exercise and increases in both area and severity during the first minute after the end of exercise.

3 mins after

The maximal heart rate achieved with use of the bicycle ergometer ranged from 111 to 167 beats/min (mean 148). In the 53 patients who also had an orthogonal lead stress test, the maximal heart rate was 123 to 167 beats/min (mean 152) on the bicycle ergometer and 130 to 167 beats/min (mean 152) on the treadmill (P >0.05). Sequence of S-T changes in surface maps: Eighty-five of the 109 patients were found to have significant (70 percent or greater) luminal narrowing of at least one major coronary artery (Table I). Figure 3 shows the sequence of contonr maps obtained before and after exercise in a patient with significant narrowing of the left anterior descending coronary artery. S-T segment

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after

cl

exercise

lmm

1 min after exercise

10 mins

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depression is present immediately after the end of exercise, and the area continues to increase over the first 1 minute after exercise with a large central area of maximal S-T segment depression present. Thereafter, both the area and severity gradually decrease, returning to the resting state at 8 minutes. Sequential changes in the S-T segment similar to those seen in this patient were present in 75 patients. In a further eight patients S-T segment depression was present only in the recording made immediately after exercise. In 63 patients

differences were present in the amount of S-T depression recorded in adjacent electrode sites. Correlation with coronary arteriographic findings: In 95 percent (81 of 85) of the patients with sig-

nificant narrowing of a major coronary artery, a correct diagnosis was made using exercise precordial mapping (S-T depression in 61 patients, S-T elevation in 8 patients and both S-T elevation and depression in 12 patients). By comparison, the diagnosis was correct in only 64 percent (54 patients) when the single chest lead (Vs)

TABLE II

Details of the 11 Patients With a Positive Precordial Exercise Map and a Negative Orthogonal Lead Stress Test Heart Rate Achieved (beatsimin) Precordial Orthogonal Test Test

Age (yr) & Sex

Case no. 1

55F 49F 50M SOM

150 140 160 135

150 150 167 125

;

45M 35M 45M 52M

130 150 145 160

130 125 145 167

9

36M

130

111

62M 57M

160 130

150 130

z 4

:

:‘:

Site of Precordial lschemia High High Inf High Inf High High High Low Inf Low Inf High Inf

ant = anterior; CAD = vessel narrowing on coronary arteriography; LCx = left circumflex coronary artery; RCA = right coronary artery.

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m

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3 4

ant,

;

ant lat ant lat,

4 2 5

LAD LCX LAD RCA, LCx

lat,

4

RCA, LCx

ant

2 1

LAD LAD, RCA

LAD LCX RCA RCA, LAD, LCx

inf = inferior; LAD = left anterior descending coronary artery; lat = lateral;

1 min 5 mins after after exercise exercise exercise IRE 4. Patient with severe narrowing of the left anterior descending coronary artery. Exercise on both the bicycle ergometer (precordial mapping, left) and the treadmill (orthogonal leads, rlghi) was stopped because severe chest pain developed. An area of S-T segment depression in the high anterior part of the surface map was recorded after exercise. A sample complex recorded from position 82 is shown below each map. No S-T segment changes were seen using the Frank system of three orthogonal leads. X and Z leads were placed in the fourth intercostal space.

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ant lat

preexercise

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Discussion Since the introduction of exercise electrocardiography as a routine investigation in the diagnosis of ischemit heart disease, several reportsr4J5 have correlated this technique with the findings at coronary arteriography. An improvement in the ability to identify patients with the coronary artery disease was found after the introduction of rnultistage stress tests using a treadmill or bicycle ergometer.16 Further modifications of the lead systems used has been accompanied by an increased diagnostic sensitivity, with 60 to 80 percent of patients with significant narrowing of one or more major coronary vessels having a positive response to the stress test.lsJ7 However, a significant number of patients with advanced coronary arterial lesions will not be identified with these techniques. Because some of these patients have les#ions amenable to surgery, widespread use of coronary arteriography has become necessary. It is clear therefore that the development of a noninvasive test that will more accurately identify pa-

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III

Sensitivity

and Specificity

Precordial surface mapping Orthogonal lead system S;;gy chest lead 5

of Three

Lead

Systems

Sensitivity

Specificity

(%I

(%I

95

92

68

85

64

92

tients who require coronary arteriography will be a considerable asset in the routine management of patients presenting with chest pain. Advantages of exercise precordial surface mapping: Precordial surface mapping has been used to observe the changes in the S-T segment and R and Q waves that occur after the development of an acute myocardial infarction.12J8Jg Theoretically, application of this technique to stress testing would have many advantages over use of the conventional lead systems. However, this application has seldom been attempted because of technical difficulties including the time required to record a surface electrocardiogram and develop a stable electrode system. Furthermore, the belief that it would be necessary to have multichannel electrocardiographic recorders with computer analysis precluded any widespread application. In this study we described a method for recording a 16 lead isopotential precordial surface map with electrocardiographic equipment that is routinely used in clinical practice. The equipment necessary differs little from that already used in the standard exercise laboratories. Although care must be taken when connecting the electrodes from 16 precordial positions, it is possible to outline areas of ischemia that are inaccessible to the standard electrocardiograph. However, it is important to evaluate the false negative and false positive results. Four patients with coronary artery disease but negative precordial stress tests had obvious angiographic evidence of collateral blood supply from one of the major coronary arteries. We speculate that the collateral circulation was able to provide sufficient blood supply to meet these patients’ metabolic requirements of exercise. These patients did not have severe intractable angina and it is unlikely that they would have been helped much by insertion of a coronary bypass graft into the diseased vessel. Likewise, the false positive results can be understood; mitral valve prolapse20 is known to be associated with S-T segment changes and the development of ventricular tachycardia in a patient is likely to compromise coronary blood flow with ensuing S-T segment changes. The limitations of the orthogonal lead system were previously shown by Kornreich.21 He identified nine precordial electrodes that provided wave form information not available using the orthogonal leads. Surface mapping does, in addition, add an extra dimension to exercise electrocardiography. It provides a measurement of the area of S-T segment changes that are pro-

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jetted onto the front of the chest. It is therefore possible to evaluate objectively by noninvasive means the effects of medical and surgical interventions. Each patient can be used as his own control and a semiquantitative measurement can be made of the area and severity of the S-T segment changes projected onto the front of the chest before and after medical treatment and coronary bypass surgery, Diagnostic sensitivity of method: Our study has shown that, as assessed from the findings at coronary arteriography, the sensitivity of precordial surface mapping was 95 percent. This is an improvement on the results reported for the currently favored systems using 12 leads5 and 3 orthogonal leads even when computeraveraging techniques and multivariate analysis of S-T segment data are performed.22 A comparison in this study among precordial surface mapping, orthogonal system and a single chest lead showed surface mapping to have a significantly higher sensitivity than the other two techniques although there was little difference in

specificity. Clearly by recording from a large number of unipolar precordial positions small areas of ischemia were identified that might otherwise have been missed. This is exemplified by the relatively large number of patients (25 percent) who manifested S-T segment elevation after exercise in the precordial map. This rate is higher than that previously reported using standard exercise tests.2” It is likely that recording over the entire left hemithorax will reveal a greater frequency of S-T segment elevation after exercise. Implications: This is a simple technique for recording the precordial surface map after exercise. Surface mapping is more accurate than recording with the orthogonal leads and single chest lead in identifying patients with coronary artery disease and has a greater sensitivity than that reported when the 12 lead electrocardiogram or vectorcardiogram is recorded after exercise. The technique is not unduly time-consuming and should be of value in patients presenting with chest pain, reducing the need for angiocardiography.

References 1. Wegria I?, Legers M, Ksating RP, Ward HP: Relationship between the reduction in coronary flow and the appearance of electrocardiographic changes. Am Heart J 38:90-96, 1949 2. Diethrich EB, Liddicoat JE, Kinard SA, Garrett HE, Lewis JM, DeBakey ME: Surgical significance of angiographic patterns in coronary arterial disease. Circulation 35,36:Suppl 1:1-155-l-162, 1967 3. Sewell WH: Coronary tine arteriography for recognition of “demand” for collateral arteries. JAMA 186:224-228, 1963 4. Sheffield LT, Roitman D: Stress testing methodology. Prog Cardiovasc pis 19:33-49, 1976 5. Mason RE, Likar I, Biern RO, Ross RS: Multiple lead exercise electrocardiography. Experience in 107 normal subjects and in 67 patients with angina pectoris and comparison with coronary arteriography in 84 patients. Circulation 36:517-525, 1967 Frank E: An accurate clinically practical system for spatial vector cardiography. Circulation 13:737-749, 1956 Fluck D, Burgess PA: A press-stud electrode for continuous monitoring of the electrocardiogram. Lancet 1:1405, 1966 Lanooy C, Bonier FH: A hyperbolic ergometer cycling and cranking. J Appl Physiol 9:499-500, 1956 Redwood DR, Rosing DR, Goldstein RE, Beiser CD, Epstein SE: Importance of the design of an exercise protocol in the evaluation of patients with angina pectoris. Circulation 43:617-628, 1971 10. Bruce RA: Exercise testing of patients with coronary heart disease. Principles and normal standards for evaluation. Ann Clin Res 3: 323-332, 197 1 11. Scandinavian Committee on ECG classification. The “Minnesota Code”‘for ECG classification. Adaption to CR leads and modification of the code for ECGs recorded during and after exercise. Acta Med Stand Suppl481:1-26, 1967 12. Reid DS, Pellides LJ, Shillingford JP: Surface mapping of the RS-T segment in acute myocardial infarction. Br Heart J 33:370-374, 1971

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13. Judkins MP: Selective coronary arteriography. 1. A percutaneous transfemoral technic. Radiology 89:815-824, 1967 14. Roitman D, Jones EB, Sheffield LT: Comparison of submaximal exercise ECG test with coronary cineangiocardiogram. Ann Intern Med 72:641-647, 1970 15: Kassebaum DG, Sutherland KI, Judkins MP: A comparison of hypoxaemia and exercise electrocardiography in coronary artery disease: diagnostic precision of the methods correlated with coronary arteriography. Am Heart J 75:759-776, 1968 16. Bruce RI\: Methods of exercise testing. Step test, bicycle, treadmill, isometrics. Am J Cardiol 33:715-720, 1974 17. Martin CM, Major MC, McConahay DR: Maximal treadmill exercise electrocardiography. Correlations with coronary arteriography and cardiac hemodynamics. Circulation 46:956-962, 1972 ia. Muller JE, Maroko PR, Braunwald E: Evaluation of precordial electrocardiographic mapping as a means of assessing changes in myocardial ischaemic injury. Circulation 52:16-27, 1975 19. Selwyn A, Shillingford JP: Precordial mapping of Cl waves and RS ratio changes in acute myocardial infarction. Cardiovasc Res 11:167-171, 1977 20. Devereux RB, Perloff JK, Reichek N, Josephson ME: Mitral valve proplase. Circulation 54:3-14, 1976 21. Kornreich F: The missing waveform information in the otthogbnal electrocardiogram (Frank leads). Where and how can this missing waveform be retrieved? Circulation 48:984-995, 1973 22. Simoons ML, Hugenholtz PC& Estimation of the probability of exercise-induced ischemia by quantitative EGG analysis. Circulation 56:552-559, 1977 23. Fortuin NJ, Friesinger GC: Exercise-induced S-T segment elevation. Clinical electrocardiographic and arteriographic studies in twelve patients. Am J Med 49:459-464, 1970 24. Kilpatrick D: Exercise vectorcardiography in diagnosis of ischemic heart disease. Lancet 21332-334. 1976

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