J ClinEpidemiol Vol.43,No. I, pp.93-99,1990
0895-4356/90 $3.00 + 0.00 Copyright CN 1990 Pergamon Press plc
Printed in Great Britain. All rights reserved
THE EFFECT OF THE NUMBER OF ELECTROCARDIOGRAMS ANALYZED ON CARDIOVASCULAR DISEASE SURVEILLANCE: THE MINNESOTA HEART SURVEY (MHS)* STANLEY
A. EDLAVITCH,~’RICHARD CROW, GREGORY L. BURKE, JAMESHUBER, RONALD PRINEASand HENRY BLACKBURN
Divisionof Epidemiology, School of Public Health, University of Minnesota, Stadium Gate 27, 611 Beacon St SE, Minneapolis, MN 55455, U.S.A. (Received in revised form 16 January 1989)
Abstract-One method used to control costs in community cardiovascular disease surveillance is to limit the number of electrocardiograms (ECGs) used to validate acute myocardial infarction (AMI). The Minnesota Heart Survey investigated the impact of decreasing the maximum number of ECGs analyzed on classification of ECG pattern and final AMI diagnosis (definite, probable, none). A 50% sample of all 1980 acute
CHD hospital discharge records (ICD-9 code 410 or 411) from 30 of 31 Twin Cities hospitals were abstracted. Comparing results using all available ECGs in the record (maximum of 12) with those obtained using up to 4 ECGs showed little differences in the ECG classification or final AMI diagnosis. Myocardial infarction
Electrocardiogram
INTRODUCTION
In the United States, coronary heart disease mortality rates have declined 41% in the past two decades, while in many other countries rates have either declined at a lower rate or increased [l]. This observation prompted the worldwide initiation of community cardiovascular disease surveillance programs in efforts to detect, explain, and predict mortality trends. Coronary heart disease morbidity .and mortality trends, and how they relate to risk factor changes, are assessed by the World Health Organization’s multinational study (MONICA Project) [2,3], and U.S. studies such as the Minnesota Heart Survey (MHS) [4,5], the Stanford Five-City Project [6], the Pawtucket Heart Health Program [7], and the Minnesota Heart Health *Supported in part by National Heart, Lung, and Blood Institute Research Grant No. HL 23727 (Minnesota Heart Survey). tReprint requests should be addressed to Stanley A. Edlavitch, PhD.
Community surveillance
Program [8]. Each of these programs adopted standard criteria to identify acute myocardial infarction (AMI), and although criteria differ somewhat from program to program, they all use cardiac pain, serum cardiac enzymes, and electrocardiograms (ECGs), and all standardize their interpretations by reading ECGs according to the Minnesota Code [9]. Surveillance of coronary heart disease is labor intensive and expensive. Cost containment is sought by all surveillance programs. Some studies limit costs by restricting the number of ECGs used to validate hospitalized myocardial infarction (MI). The MONICA Project, conducted through 40 centers in 26 countries, copies from the patient’s chart a maximum of four ECG records for Minnesota coding [3]. In the Atherosclerosis Risk in the Community (ARIC) study, the protocol requires trained nurse abstracters to code up to three ECGs for major Q wave changes [lo, 111. At issue is whether important diagnostic in93
94
STANLEY A. EDLAVITCH et al.
formation is lost when the number of ECGs analyzed is reduced or ECG interpretation is eliminated. This study asked two questions: (1) what proportion of cases is misclassified when the number of ECGs analyzed is reduced from all available usable tracings (maximum of 12) to a maximum of 4, and (2) how often is the ECG essential to validate acute MI? If the number of ECGs needed can be significantly reduced or eliminated, substantial savings in both time commitment and costs could be realized.
METHODS
The Minnesota Heart Survey (MHS), a community surveillance study in the seven-county Minneapolis-St Paul (Twin Cities) metropolitan area, monitors trends in cardiovascular disease morbidity, mortality, and risk factors, and has been described in detail by Gillum et al. [13]. This investigation used MHS data from 1980, when a 50% sample of all medical records with discharge diagnoses of acute myocardial infarction (ICD-9, 410) or unstable angina pectoris (ICD-9, 411) in 30 of 3 1 Twin Cities hospitals were abstracted. For validation, standardized diagnostic criteria were applied to categorize events as definite, possible, or no AMI. All ECG tracings, up to a maximum of 12, were copied and sent to the Minnesota ECG Laboratory for visual coding. When more than 12 ECGs were available, the first 6 and last 6 by date and time were chosen. Analyses were conducted using all available ECGs (up to 12) and then using subsets of up to $4, 3, and 2 tracings respectively. The following criteria were used to select ECG subsets: For subset of 5 ECGs First ECG and last ECG during hospitalization, and first ECG of the day from days 2, 3, and 4 of the hospital stay. For subset of 4 ECGs First ECG and last ECG during hospitalization, and first ECG of the day from days 2 and 3 of the hospital stay. For subset of 3 ECGs First ECG and last ECG during hospitalization and the first ECG on day 2 of the hospital stay.
For subset of 2 ECGs First ECG and last ECG during hospitalization. When not available from days 2, 3 or 4 of hospitalization, ECGs were selected from alternative days. When fewer than the desired number were available (e.g. 4 desired but only 3 available), all ECGs in the record were included. Any ECG was considered unusable for coding AM1 if there were serious technical problems in the record, major intraventricular conduction block with QRS duration of greater than 0.12 seconds in any lead (includes complete left or right bundle branch block) or a cardiac pacemaker present, or no date or time was provided. The Minnesota Code classification for acute myocardial infarction, adopted for standardization in this study (Table l), uses 3 (anatomical) lead groups: lateral (I,aVL,V6), inferior (II,III,aVF), and anterior (Vl-V5). Among these classifications, the evolving diagnostic pattern is considered sufficient to validate AMI. The evolving pattern requires either the appearance of new major Minnesota Q-codes (l-l to 1-2 except l-2-6) or a change from a minor Q-code (l-3) to a major Minnesota Q-code accompanied by appearance of T wave inversion (5-1 or 5-2) or ST elevation (9-2) or ST depression (4-l or 4-2). Q-code change and ST-T change must be in the same lead group, with no major Q-code on the first ECG in the series in that lead group. We have previously demonstrated the importance of a consistent diagnostic algorithm in making comparisons over time. Fourteen AM1 rates validated using more sensitive and specific cardiac enzymes (CPK and CPK-MB) were not comparable to AM1 rates validated with only AST and LDH. Analyses were performed in 2 ways using a set of 2 enzymes, serum aspartate aminotransferase (AST) and serum lactase dehydrogenase (LDH), or a set of 4 enzymes, AST, LDH, total serum creatinase phosphokinase (CPK), and its isoenzyme CPK-MB. When only AST and LDH were available, they were considered abnormal if the maximum value of either enzyme exceeded 200% of the upper limit of normal (defined for men and women by each hospital laboratory for each batch and enzyme). If the maximum value of either was between 101 and 199% of the upper limit of normal, the enzymes were considered
Effect of the Number of ECGs Analyzed on CVD Surveillance
95
Table 1. Minnesota code classification for acute myocardial infarction (AMI) Classification 1. Major Q-code not present in a lead group in the first ECG in the series and appears in that lead group in any subsequent ECG. OY 2. (a) Change from no Q-code to minor Q-code or change from minor Z-code to major Q-code
Evolving diagnostic
ph (b) Appearance of major ST-depression or T-wave inversion or ST-elevation Diagnostic
A major Q-code is present on the first ECG of the series A majo:l’ST-elevation and major T-wave inversion appear on any ECG
Equivocal
Minnesota
Major Minor Major Major Minor Major Minor
Any ECG with a minor Q-code or Major ST-elevation or ST depression (major or minor) or T-wave inversion (major or minor) code dejinirions
Q-code Q-code ST-elevation T-wave inversion T-wave inversion ST-depression ST-depression
:
Minnesota
l-l l-3 9-2 5-1 5-3 4-1 4-3
code
to l-2-2, except l-2-6 or 1-2-S or 5-2 or 4-2
equivocal. For analyses using all 4 enzymes, CPK-MB status was a determining factor. CPK-MB was considered abnormal if it was present. When CPK-MB was absent, categorization proceeded as described above using the maximum values of CPK, AST and LDH. When CPK-MB was present, enzymes were automatically classified as abnormal.
Table 2 presents the diagnostic criteria (implemented by computer algorithm) applied in MHS. Autopsy evidence of an MI within 8 weeks of death or an evolving diagnostic ECG pattern was always considered confirmation of an AM1 diagnosis. When the evolving diagnostic ECG pattern was absent, typical cardiac chest pain accompanied by abnormal cardiac
Table 2. Minnesota Heart Survey MI diagnosis algorithm CARDIAC
PAIN PRESENT
CHEST PAIN ABSENT
MINNESOTACODE
EVOLVING DIAGNOSTIC
DIAGNOSTIC
EQUIVOCAL
OTHER
ABNORMAL ENZYMES
EQUIVOCAL ENZYMES
MISSING ENZYMES
NORMAL ENZYMES
ABNORMAL ENZYMES
EQUIVOCAL ENZYMES
MISSING ENZYMES
NORMAL ENZYMES
If an autopsy is available and shows an acute myocardial infarction within 8 weeks of death the case is coded as a definite MI.
96
STANLEY
A. EDLAWTCH et
al.
Table 3. Number of codeable ECGs in Twin Cities AMI patient records: the Minnesota Heart Survey
Table 4. Length of hospital stay and in-hospital fatality rate in Twin Cities AM1 patients by number of codeable ECGs: the Minnesota Heart Survey
Number of ECGs
N
percent
Cumulative percent
Number of ECGs*
N
Mean length of stay (days)
Fatal cases W)
12 11 10 9 8 7 6 5 4 3 2 1 0
46 20 26 49 64 96 184 256 370 288 163 142 160
2.5 1.1 1.4 2.6 3.4 5.2 9.9 13.7 19.8 15.5 8.7 7.6 8.6
2.5 3.6 5.0 1.6 11.0 16.2 26.1 39.8 59.6 75.1 83.6 91.2 100.0
None 1 to 2 3 to 4 5 to 6 7+
160 305 658 440 301
9.4 6.3 10.9 15.0 22.1
23.8 23.9 6.8 6.1 7.3
1864
100.0
100.0
Total
*Excluded unuseable ECGs (bundle branch blocks, implantable pacemakers present, no date or times provided).
enzymes was sufficient for confirmation of AMI. In the absence of chest pain, a diagnostic ECG pattern and abnormal enzymes validated an acute myocardial infarction. RESULTS
Table 3 presents the distribution of usable ECGs per patient (from the hospital record). The average number of ECGs available per patient was 4.7 (SD = 2.8), reduced to 4.2 (SD = 2.7) after eliminating unusable ECGs. Only 39.8% of the records contained more than 4 usable ECGs. Thus, limiting the number of ECGs analyzed to 4 usable ECGs reduces ECG coding costs in approximately 40% of cases. Only 16.2% of the records contained 6 or more usable ECGs and 11% 7 or more usable ECGs. As expected, the number of usable ECGs in the record is associated with length of stay (Table 4). Persons with fewer than three usable ECGs in the record were more likely to have died in the hospital. Table 5 shows that there was little impact (&3%) on the classification of ECG patterns into diagnostic, equivocal, or other when the
*Excluded unuseable ECGs (bundle branch blocks, implantable pacemakers present, no date or times provided).
maximum number of tracings was decreased from 12, 5, 4, or 3; when decreased to 2, classifications were affected slightly more. On the other hand, the evolving diagnostic ECG pattern was missed 7.7% of the time when a maximum of 5 ECGs were analyzed, 11.4% with 4 ECGs, 21.4% with 3 ECGs, and 37.9% with only 2 ECGs. Thus, the major effect of reducing the maximum number of ECGs analyzed was to lower the prevalence of the important evolving diagnostic ECG pattern. ECG classification cannot change for records with fewer total numbers of ECGs than the maximum number selected for review. For example, records with one or two ECGs are not affected by reducing the maximum number of ECGs analyzed to 5, 4, or 3 ECGs. To concentrate on records which could potentially change, the above analyses were repeated for the subgroups of records with 6 or more ECGs (485 records), 5 or more ECGs (741 records), 4 or more ECGs (1111 records), and 3 or more ECGs (1399 records). The findings for the diagnostic equivocal, normal, and other classification categories were almost identical to the overall results. The agreement in the evolving diagnostic category fell from 92.3 to 83.3% when the maximum number of ECGs analyzed was reduced to 5 for the subgroup of patients with 6 or more ECGs. No significant additional differences were noted from Table 5 when a maximum of 4, 3, 2 ECGs were analyzed. Tables 6 and 7 illustrate final MI diagnoses
Table 5. Percent agreement of Minnesota code categories using up to 12 vs fewer ECGs: the Minnesota Heart Survey Percent exact agreement with up to 12 ECGs Number of ECGs
up to 2 up to 3 up to 4 UD t0 5
Evolvina diaenostic 62.1 78.6 88.6 92.3
Diaenostic
Eauivocal
Normal
Other
92.0 97.0 91.9 99.7
92.9 97.0 98.6 99.5
100 100 100 100
100 100 100 100
Effect of the Number of ECGs Analyzed on CVD Surveillance
Table 6. Percent agreement of final AMI diagnosis comparing up to 12 vs fewer ECGs: the Minnesota Heart Survey Percent exact agreement with up to 12 ECGs Number of ECGs up up up up
to to to to
2 3 3 5
Definite
Possible
No MI
91.4 95.2 97.1 97.9
98.4 99.0 99.8 100.0
100 100 100 100
Validated using ECG, chest pain, autopsy, AST and LDH.
Table 8. The impact of omitting ECGs on identifying definite acute myocardial infarctions: MHS, 1980 No. definite AMIs Maximum No. of ECGs used 12 5 4 3 2 ~ Maximum
(definite, possible, or no) determined by ECG, pain, enzyme data, or autopsy evidence of an AM1 within 8 weeks of death, and the effect of
reducing the maximum number of ECGs considered. Table 6 presents the results of these analyses when enzymes were restricted to LDH
and AST. Table 7 presents results using more sensitive and specific cardiac enzymes (CPK and CPK-MB) in the diagnostic algorithm. A small influence on final diagnosis was found when ECGs were reduced from a maximum of 12 to 5 or 4. There was a significantly greater number of discordant diagnoses when the number was reduced to 3 or 2. These findings persisted when 2 or 4 enzymes were considered in the diagnostic algorithm. Similar findings were observed when these analyses were repeated for the subsets of records that were affected by reducing the number of analyzed ECGs. The findings were virtually identical for AM1 diagnoses or possible or no AMI. Agreement on a diagnosis of definite AM1 was decreased slightly (2.0-2.9%). Table 8 shows the results of excluding the ECG from the diagnostic algorithm. When only the cardiac enzymes AST and LDH, pain, and autopsy were considered, between 12 and 19.7% (depending on the number of ECGs used) of the AMIs identified by the full algorithm were missed. When CPK and CPK-MB findings were also included, the proportion missed by not considering ECG findings dropped to between 6.5 and 9.8%.
97
No. of EC& used 12 5 4 3 2
Percent
usingECG, autopsy, AMIs ECG pain, AST, LDH
848 830 820 807 775 No. definite AMIs using ECG, autopsy, pain, AST, LDH, CPK, CPK-MB
1157 1148 1144 1137 1116
required 19.7 18.0 17.3 16.9 12.1 Percent AMIs ECG required
9.8 9.1 8.6 8.2 6.5
DISCUSSION This study demonstrates that the ECG plays a critical role in validating AMIs in cardiovas-
cular disease surveillance. It is cost-effective to analyze a maximum of 4 ECGs rather than all ECGs in the chart. Although limiting the number of ECGs analyzed decreased the sensitivity for identifying an evolving Q-wave ECG pattern, reducing the maximum number to 4 had minima1 impact on final diagnosis of AMI. When only 2 or 3 tracings were considered (compared to a maximum of 12), a greater loss was evident in correctly classified ECG patterns and final diagnosis. When a maximum of 5 ECGS rather than 4 were interpreted, agreement on classification of ECG pattern was slightly improved, e.g. agreement for the evolving classification improved from 88.6 to 92.3%. However, including the fifth tracing had little effect on final validation of definite acute myocardial infarction, and the additional cost (U.S.$3584) did not appear warranted. As expected, the number of ECGs in the record was correlated with length of hospital stay for records with more than one usable Table 7. Percent agreement of final AM1 diagnosis comparECG. This study was not designed to determine ing up to 12 vs fewer ECGs: the Minnesota Heart Survey why certain records contain large numbers of Percent exact agreement ECGs and others only a few. However, the with up to 12 ECGs finding that the mean length of stay for individNo MI Number of ECGs Definite Possible uals with no usable ECGs in the record was 98.3 100 96.5 up to 2 9.4 days implies that a number of these patients’ 99.3 100 98.3 up to 3 AMIs occurred while they were hospitalized for 99.7 100 98.9 up to 4 other reasons. These findings were consistent 100.0 100 99.2 up to 5 whether only two enzymes, AST and LDH, Validated using ECG, chest pain, autopsy, AST, LDH, were included in the diagnostic algorithm or the CPK, and CPK-MB.
98
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A. EDLAVITCH et al.
more sensitive cardiac enzymes CPK and CPKMB were included. Whether 2 or 4 enzymes are used to identify hospitalized AMIs, the fact remains that very little information is lost by reducing the number of usable ECGs from all available in the record (maximum of 12) to a maximum of 4. In comparing 1970-1980 trends in attack and case fatality rates for AMI, we employed a computerized diagnostic algorithm which included only two enzymes, AST and LDH, because CPK determinations were not widely used and CPK-MB was not widely available in 1970 [I 51. Though this paper presents only 1980 results, all analyses were repeated and similar findings were noted using 1970 records. These data support a decision to collect and analyze a maximum of four ECGs per hospitalization. For the 1980 sample of 1864 cases, the savings in coding costs would have been 27.7% ($17,412-from $62,939 to $45,527) had the number of ECGs coded been reduced from a maximum of 12 to a maximum of 4 per case. If abstracters were trained to recognize usable tracings, additional savings would occur in abstractor time and photocopying expenses. But should ECGs be coded for cases? Another possible strategy toward cost-effectiveness would be to analyze ECGs only when the case could not be validated using other available information (history of chest pain, enzymes, autopsy findings). This strategy would have eliminated ECG coding for 36.5% (681/1864) of the 1980 cases when two enzymes were considered and for 56% (1044/1864) when 4 enzymes were used. If enzyme measures were standardized worldwide and questions on typical cardiac pain were uniform, such an approach would appear reasonable. Because no interhospital laboratory standardization is widely used, this economically attractive labor reduction would severely limit the scientific usefulness of the results and therefore cannot be recommended. For example, comparisons of morbidity statistics over time and between geographical regions would be questionable. We also considered eliminating ECGs entirely by using a completely enzyme-based diagnostic algorithm. With this algorithm, an acute myocardial infarction would be diagnosed when maximum AST, LDH, or CPK values were elevated or CPK-MB was present, or when there had been an autopsy-confirmed myocardial infarction within 8 weeks of death. Definite AMIs identified by the enzyme-autopsy algorithm
(N = 1147) were then compared with those identified using ECG, enzyme, pain, and autopsy (N = 1157). The number of ECGs analyzed had little impact on these analyses. When an enzyme-autopsy diagnostic rule was applied, 94 additional cases were classified as definite AMI, and 104 definite AMIs by our standard algorithm were not identified. We believe greater accuracy is achieved when ECGs are included. An additional reason to analyze ECGs for all cases is that they are necessary to distinguish between Q wave and non-Q wave infarctions. The Worcester, Massachusetts [ 121 and Rochester, Minnesota [ 131 community cardiovascular surveillance programs have reported differing secular trends in incidence and case fatality rates of Q wave and non-Q wave AMIs. We conclude that the ECG should be retained in the diagnostic MI algorithm for three reasons. First, the ECG plays a critical role in detecting acute myocardial infarctions. Second, the ECG allows comparability with surveys using no or different enzymes. Third, the ECG offers the only way to determine whether the infarction is Q wave or non-Q wave in type. We recommend utilizing 4 ECGs per participant in conjunction with pain, enzyme rise, and autopsy findings as a cost-effective and valid approach to identifying acute myocardial infarctions in populations. Finally, these results apply only to strategies for validating hospitalized AMIs in surveillance or in case reviews and diagnosis by expert committees. These findings are not intended as a guide for practicing clinicians who must continue to use ECGs as necessary for each patient. However, we observed a slight decrease in the number of ECGs ordered by clinicians between 1970 and 1980. As efforts to reduce the costs of medical care continue, average length of hospital stay decreases, and the more sensitive cardiac enzymes are used, the number of ECGs performed may continue to decline. Acknowledgements-The authors gratefully acknowledge the valuable scientific assitance of Drs Judith Baxter, Steve Norsted, and J. Michael Sprafka, and the editing expertise of Carolyn S. Kurtz and Carol Edlavitch.
REFERENCES 1. Stamler J. The marked decline in coronary heart disease mortality rates in the United States, 1968-l 981: Summary of findings and possible explanations. Cardiology 1985; 72: 11-22. 2. Tunstal-Pedoe H. Monitoring trends in cardiovascular
Effect of the Number of ECGs Analyzed on CVD Surveillance disease and risk factors: The MONICA Project. WHO Chronicle 1985; 39(l); 3-5. 3. The Principal Investigators of the MONICA Project. WHO MONICA Project; Geographic variation in mortality from cardiovascular diseases; baseline data on selected population characteristics and cardiovascular mortality. World Health Stat Q 1987; 40: 171-184. 4. Gillum RF, Prineas RJ, Luepker RV et al. Decline in coronary deaths: A search for explanations. Minnesota 5.
Med 1982; 65: 235-238. Gillum RF, Folsom AR, Luepker RV, Jacobs DR,
Kottke TE, Gomez-Marin 0. Prineas RJ, Taylor HL, Blackburn H. Sudden death and acute myocardial infarction in a metropolitan area, 1970-1980: The Minnesota Heart Survey. N Engl J Med 1983; 309: 1353-1358. 6. Farquhar JW, Fortman SP, Maccoby N et al. The Stanford Five-City Project: Design and methods. Am J Epidemiol 1985; 122: 323-334. 7. Lasater T, Abrams D, Artz L et al. Lay volunteer delivery of a community-based cardiovascular risk factor change program: The Pawtucket Experiment. In: Matarazzo JD, Weiss SM, Herd JA et al., Eds. Behavioral Health: A Handbook of Health Enhancement and Disease Prevention. New York: John Wiley;
8.
1984: 11661170. Blackburn H, Luepker RV, Kline FG et al. The Minnesota Heart Health Program: A research and demonstration project in cardiovascular disease prevention In: Matarazzo JD, Weiss SM, Herd JA et al., Eds. Behavioral Health: A Handbook of Health En-
99
hancement and Disease Prevention. New York: John Wiley; 1984: 1171-l 178. 9. Prineas RJ, Crow RS, Blackburn H. The Minnesota Code Manual of Electrocardiographic Findings: Standards and Procedures for Measurement and Classification. Littleton, Mass.: John Wright/P.S.G.; 1982. 10. Higgins M, Williams OD. Atherosclerosis risk in communities: Early experience. Presented for ARIC investigators at the 2nd Int MONICA Congress. Helsinki, Finland, 14-15 August 1987. 11. Steering Committee of the Atherosclerosis Risk and Communities Study. The Antherosclerosis Risk and Communities Study (ARIC) design and objective. Am J Epidemiol
1989; 129: 687-702.
12. Goldberg RJ, Gore JM, Alpert JS, Dalen JE. Non-Q wave myocardial infarction: Recent changes in occurrence and prognosis-a community-wide perspective. Am Heart J 1987; 113: 273.
13. Connolly DC, Elveback LR. Coronary heart disease in residents of Rochester, Minnesota: VI. Hospital and posthospital course of patients with transmural and subendocardial myocardial infarction. Mayo Clin Proc 1985; 60: 375-381. 14. Burke GL, Edlavitch SA, Crow RS. The effects of diagnostic criteria on trends in coronary heart disease morbidity. J Clin Epidemiol 1988. 15. Gomez-Marin 0, Folsom A, Kottke TE et al. Improved long-term survival after hospitalized acute myocardial infarction, 1970 to 1980, a populationbased study: The Minnesota Heart Survey. N Engl J Med 1987: 316: 1353-1359.
0895-4356/90 $3.00 + 0.00 Copyright CN 1990 Pergamon Press plc
Printed in Great Britain. All rights reserved
THE EFFECT OF THE NUMBER OF ELECTROCARDIOGRAMS ANALYZED ON CARDIOVASCULAR DISEASE SURVEILLANCE: THE MINNESOTA HEART SURVEY (MHS)* STANLEY
A. EDLAVITCH,~’RICHARD CROW, GREGORY L. BURKE, JAMESHUBER, RONALD PRINEASand HENRY BLACKBURN
Divisionof Epidemiology, School of Public Health, University of Minnesota, Stadium Gate 27, 611 Beacon St SE, Minneapolis, MN 55455, U.S.A. (Received in revised form 16 January 1989)
Abstract-One method used to control costs in community cardiovascular disease surveillance is to limit the number of electrocardiograms (ECGs) used to validate acute myocardial infarction (AMI). The Minnesota Heart Survey investigated the impact of decreasing the maximum number of ECGs analyzed on classification of ECG pattern and final AMI diagnosis (definite, probable, none). A 50% sample of all 1980 acute
CHD hospital discharge records (ICD-9 code 410 or 411) from 30 of 31 Twin Cities hospitals were abstracted. Comparing results using all available ECGs in the record (maximum of 12) with those obtained using up to 4 ECGs showed little differences in the ECG classification or final AMI diagnosis. Myocardial infarction
Electrocardiogram
INTRODUCTION
In the United States, coronary heart disease mortality rates have declined 41% in the past two decades, while in many other countries rates have either declined at a lower rate or increased [l]. This observation prompted the worldwide initiation of community cardiovascular disease surveillance programs in efforts to detect, explain, and predict mortality trends. Coronary heart disease morbidity .and mortality trends, and how they relate to risk factor changes, are assessed by the World Health Organization’s multinational study (MONICA Project) [2,3], and U.S. studies such as the Minnesota Heart Survey (MHS) [4,5], the Stanford Five-City Project [6], the Pawtucket Heart Health Program [7], and the Minnesota Heart Health *Supported in part by National Heart, Lung, and Blood Institute Research Grant No. HL 23727 (Minnesota Heart Survey). tReprint requests should be addressed to Stanley A. Edlavitch, PhD.
Community surveillance
Program [8]. Each of these programs adopted standard criteria to identify acute myocardial infarction (AMI), and although criteria differ somewhat from program to program, they all use cardiac pain, serum cardiac enzymes, and electrocardiograms (ECGs), and all standardize their interpretations by reading ECGs according to the Minnesota Code [9]. Surveillance of coronary heart disease is labor intensive and expensive. Cost containment is sought by all surveillance programs. Some studies limit costs by restricting the number of ECGs used to validate hospitalized myocardial infarction (MI). The MONICA Project, conducted through 40 centers in 26 countries, copies from the patient’s chart a maximum of four ECG records for Minnesota coding [3]. In the Atherosclerosis Risk in the Community (ARIC) study, the protocol requires trained nurse abstracters to code up to three ECGs for major Q wave changes [lo, 111. At issue is whether important diagnostic in93
94
STANLEY A. EDLAVITCH et al.
formation is lost when the number of ECGs analyzed is reduced or ECG interpretation is eliminated. This study asked two questions: (1) what proportion of cases is misclassified when the number of ECGs analyzed is reduced from all available usable tracings (maximum of 12) to a maximum of 4, and (2) how often is the ECG essential to validate acute MI? If the number of ECGs needed can be significantly reduced or eliminated, substantial savings in both time commitment and costs could be realized.
METHODS
The Minnesota Heart Survey (MHS), a community surveillance study in the seven-county Minneapolis-St Paul (Twin Cities) metropolitan area, monitors trends in cardiovascular disease morbidity, mortality, and risk factors, and has been described in detail by Gillum et al. [13]. This investigation used MHS data from 1980, when a 50% sample of all medical records with discharge diagnoses of acute myocardial infarction (ICD-9, 410) or unstable angina pectoris (ICD-9, 411) in 30 of 3 1 Twin Cities hospitals were abstracted. For validation, standardized diagnostic criteria were applied to categorize events as definite, possible, or no AMI. All ECG tracings, up to a maximum of 12, were copied and sent to the Minnesota ECG Laboratory for visual coding. When more than 12 ECGs were available, the first 6 and last 6 by date and time were chosen. Analyses were conducted using all available ECGs (up to 12) and then using subsets of up to $4, 3, and 2 tracings respectively. The following criteria were used to select ECG subsets: For subset of 5 ECGs First ECG and last ECG during hospitalization, and first ECG of the day from days 2, 3, and 4 of the hospital stay. For subset of 4 ECGs First ECG and last ECG during hospitalization, and first ECG of the day from days 2 and 3 of the hospital stay. For subset of 3 ECGs First ECG and last ECG during hospitalization and the first ECG on day 2 of the hospital stay.
For subset of 2 ECGs First ECG and last ECG during hospitalization. When not available from days 2, 3 or 4 of hospitalization, ECGs were selected from alternative days. When fewer than the desired number were available (e.g. 4 desired but only 3 available), all ECGs in the record were included. Any ECG was considered unusable for coding AM1 if there were serious technical problems in the record, major intraventricular conduction block with QRS duration of greater than 0.12 seconds in any lead (includes complete left or right bundle branch block) or a cardiac pacemaker present, or no date or time was provided. The Minnesota Code classification for acute myocardial infarction, adopted for standardization in this study (Table l), uses 3 (anatomical) lead groups: lateral (I,aVL,V6), inferior (II,III,aVF), and anterior (Vl-V5). Among these classifications, the evolving diagnostic pattern is considered sufficient to validate AMI. The evolving pattern requires either the appearance of new major Minnesota Q-codes (l-l to 1-2 except l-2-6) or a change from a minor Q-code (l-3) to a major Minnesota Q-code accompanied by appearance of T wave inversion (5-1 or 5-2) or ST elevation (9-2) or ST depression (4-l or 4-2). Q-code change and ST-T change must be in the same lead group, with no major Q-code on the first ECG in the series in that lead group. We have previously demonstrated the importance of a consistent diagnostic algorithm in making comparisons over time. Fourteen AM1 rates validated using more sensitive and specific cardiac enzymes (CPK and CPK-MB) were not comparable to AM1 rates validated with only AST and LDH. Analyses were performed in 2 ways using a set of 2 enzymes, serum aspartate aminotransferase (AST) and serum lactase dehydrogenase (LDH), or a set of 4 enzymes, AST, LDH, total serum creatinase phosphokinase (CPK), and its isoenzyme CPK-MB. When only AST and LDH were available, they were considered abnormal if the maximum value of either enzyme exceeded 200% of the upper limit of normal (defined for men and women by each hospital laboratory for each batch and enzyme). If the maximum value of either was between 101 and 199% of the upper limit of normal, the enzymes were considered
Effect of the Number of ECGs Analyzed on CVD Surveillance
95
Table 1. Minnesota code classification for acute myocardial infarction (AMI) Classification 1. Major Q-code not present in a lead group in the first ECG in the series and appears in that lead group in any subsequent ECG. OY 2. (a) Change from no Q-code to minor Q-code or change from minor Z-code to major Q-code
Evolving diagnostic
ph (b) Appearance of major ST-depression or T-wave inversion or ST-elevation Diagnostic
A major Q-code is present on the first ECG of the series A majo:l’ST-elevation and major T-wave inversion appear on any ECG
Equivocal
Minnesota
Major Minor Major Major Minor Major Minor
Any ECG with a minor Q-code or Major ST-elevation or ST depression (major or minor) or T-wave inversion (major or minor) code dejinirions
Q-code Q-code ST-elevation T-wave inversion T-wave inversion ST-depression ST-depression
:
Minnesota
l-l l-3 9-2 5-1 5-3 4-1 4-3
code
to l-2-2, except l-2-6 or 1-2-S or 5-2 or 4-2
equivocal. For analyses using all 4 enzymes, CPK-MB status was a determining factor. CPK-MB was considered abnormal if it was present. When CPK-MB was absent, categorization proceeded as described above using the maximum values of CPK, AST and LDH. When CPK-MB was present, enzymes were automatically classified as abnormal.
Table 2 presents the diagnostic criteria (implemented by computer algorithm) applied in MHS. Autopsy evidence of an MI within 8 weeks of death or an evolving diagnostic ECG pattern was always considered confirmation of an AM1 diagnosis. When the evolving diagnostic ECG pattern was absent, typical cardiac chest pain accompanied by abnormal cardiac
Table 2. Minnesota Heart Survey MI diagnosis algorithm CARDIAC
PAIN PRESENT
CHEST PAIN ABSENT
MINNESOTACODE
EVOLVING DIAGNOSTIC
DIAGNOSTIC
EQUIVOCAL
OTHER
ABNORMAL ENZYMES
EQUIVOCAL ENZYMES
MISSING ENZYMES
NORMAL ENZYMES
ABNORMAL ENZYMES
EQUIVOCAL ENZYMES
MISSING ENZYMES
NORMAL ENZYMES
If an autopsy is available and shows an acute myocardial infarction within 8 weeks of death the case is coded as a definite MI.
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Table 3. Number of codeable ECGs in Twin Cities AMI patient records: the Minnesota Heart Survey
Table 4. Length of hospital stay and in-hospital fatality rate in Twin Cities AM1 patients by number of codeable ECGs: the Minnesota Heart Survey
Number of ECGs
N
percent
Cumulative percent
Number of ECGs*
N
Mean length of stay (days)
Fatal cases W)
12 11 10 9 8 7 6 5 4 3 2 1 0
46 20 26 49 64 96 184 256 370 288 163 142 160
2.5 1.1 1.4 2.6 3.4 5.2 9.9 13.7 19.8 15.5 8.7 7.6 8.6
2.5 3.6 5.0 1.6 11.0 16.2 26.1 39.8 59.6 75.1 83.6 91.2 100.0
None 1 to 2 3 to 4 5 to 6 7+
160 305 658 440 301
9.4 6.3 10.9 15.0 22.1
23.8 23.9 6.8 6.1 7.3
1864
100.0
100.0
Total
*Excluded unuseable ECGs (bundle branch blocks, implantable pacemakers present, no date or times provided).
enzymes was sufficient for confirmation of AMI. In the absence of chest pain, a diagnostic ECG pattern and abnormal enzymes validated an acute myocardial infarction. RESULTS
Table 3 presents the distribution of usable ECGs per patient (from the hospital record). The average number of ECGs available per patient was 4.7 (SD = 2.8), reduced to 4.2 (SD = 2.7) after eliminating unusable ECGs. Only 39.8% of the records contained more than 4 usable ECGs. Thus, limiting the number of ECGs analyzed to 4 usable ECGs reduces ECG coding costs in approximately 40% of cases. Only 16.2% of the records contained 6 or more usable ECGs and 11% 7 or more usable ECGs. As expected, the number of usable ECGs in the record is associated with length of stay (Table 4). Persons with fewer than three usable ECGs in the record were more likely to have died in the hospital. Table 5 shows that there was little impact (&3%) on the classification of ECG patterns into diagnostic, equivocal, or other when the
*Excluded unuseable ECGs (bundle branch blocks, implantable pacemakers present, no date or times provided).
maximum number of tracings was decreased from 12, 5, 4, or 3; when decreased to 2, classifications were affected slightly more. On the other hand, the evolving diagnostic ECG pattern was missed 7.7% of the time when a maximum of 5 ECGs were analyzed, 11.4% with 4 ECGs, 21.4% with 3 ECGs, and 37.9% with only 2 ECGs. Thus, the major effect of reducing the maximum number of ECGs analyzed was to lower the prevalence of the important evolving diagnostic ECG pattern. ECG classification cannot change for records with fewer total numbers of ECGs than the maximum number selected for review. For example, records with one or two ECGs are not affected by reducing the maximum number of ECGs analyzed to 5, 4, or 3 ECGs. To concentrate on records which could potentially change, the above analyses were repeated for the subgroups of records with 6 or more ECGs (485 records), 5 or more ECGs (741 records), 4 or more ECGs (1111 records), and 3 or more ECGs (1399 records). The findings for the diagnostic equivocal, normal, and other classification categories were almost identical to the overall results. The agreement in the evolving diagnostic category fell from 92.3 to 83.3% when the maximum number of ECGs analyzed was reduced to 5 for the subgroup of patients with 6 or more ECGs. No significant additional differences were noted from Table 5 when a maximum of 4, 3, 2 ECGs were analyzed. Tables 6 and 7 illustrate final MI diagnoses
Table 5. Percent agreement of Minnesota code categories using up to 12 vs fewer ECGs: the Minnesota Heart Survey Percent exact agreement with up to 12 ECGs Number of ECGs
up to 2 up to 3 up to 4 UD t0 5
Evolvina diaenostic 62.1 78.6 88.6 92.3
Diaenostic
Eauivocal
Normal
Other
92.0 97.0 91.9 99.7
92.9 97.0 98.6 99.5
100 100 100 100
100 100 100 100
Effect of the Number of ECGs Analyzed on CVD Surveillance
Table 6. Percent agreement of final AMI diagnosis comparing up to 12 vs fewer ECGs: the Minnesota Heart Survey Percent exact agreement with up to 12 ECGs Number of ECGs up up up up
to to to to
2 3 3 5
Definite
Possible
No MI
91.4 95.2 97.1 97.9
98.4 99.0 99.8 100.0
100 100 100 100
Validated using ECG, chest pain, autopsy, AST and LDH.
Table 8. The impact of omitting ECGs on identifying definite acute myocardial infarctions: MHS, 1980 No. definite AMIs Maximum No. of ECGs used 12 5 4 3 2 ~ Maximum
(definite, possible, or no) determined by ECG, pain, enzyme data, or autopsy evidence of an AM1 within 8 weeks of death, and the effect of
reducing the maximum number of ECGs considered. Table 6 presents the results of these analyses when enzymes were restricted to LDH
and AST. Table 7 presents results using more sensitive and specific cardiac enzymes (CPK and CPK-MB) in the diagnostic algorithm. A small influence on final diagnosis was found when ECGs were reduced from a maximum of 12 to 5 or 4. There was a significantly greater number of discordant diagnoses when the number was reduced to 3 or 2. These findings persisted when 2 or 4 enzymes were considered in the diagnostic algorithm. Similar findings were observed when these analyses were repeated for the subsets of records that were affected by reducing the number of analyzed ECGs. The findings were virtually identical for AM1 diagnoses or possible or no AMI. Agreement on a diagnosis of definite AM1 was decreased slightly (2.0-2.9%). Table 8 shows the results of excluding the ECG from the diagnostic algorithm. When only the cardiac enzymes AST and LDH, pain, and autopsy were considered, between 12 and 19.7% (depending on the number of ECGs used) of the AMIs identified by the full algorithm were missed. When CPK and CPK-MB findings were also included, the proportion missed by not considering ECG findings dropped to between 6.5 and 9.8%.
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No. of EC& used 12 5 4 3 2
Percent
usingECG, autopsy, AMIs ECG pain, AST, LDH
848 830 820 807 775 No. definite AMIs using ECG, autopsy, pain, AST, LDH, CPK, CPK-MB
1157 1148 1144 1137 1116
required 19.7 18.0 17.3 16.9 12.1 Percent AMIs ECG required
9.8 9.1 8.6 8.2 6.5
DISCUSSION This study demonstrates that the ECG plays a critical role in validating AMIs in cardiovas-
cular disease surveillance. It is cost-effective to analyze a maximum of 4 ECGs rather than all ECGs in the chart. Although limiting the number of ECGs analyzed decreased the sensitivity for identifying an evolving Q-wave ECG pattern, reducing the maximum number to 4 had minima1 impact on final diagnosis of AMI. When only 2 or 3 tracings were considered (compared to a maximum of 12), a greater loss was evident in correctly classified ECG patterns and final diagnosis. When a maximum of 5 ECGS rather than 4 were interpreted, agreement on classification of ECG pattern was slightly improved, e.g. agreement for the evolving classification improved from 88.6 to 92.3%. However, including the fifth tracing had little effect on final validation of definite acute myocardial infarction, and the additional cost (U.S.$3584) did not appear warranted. As expected, the number of ECGs in the record was correlated with length of hospital stay for records with more than one usable Table 7. Percent agreement of final AM1 diagnosis comparECG. This study was not designed to determine ing up to 12 vs fewer ECGs: the Minnesota Heart Survey why certain records contain large numbers of Percent exact agreement ECGs and others only a few. However, the with up to 12 ECGs finding that the mean length of stay for individNo MI Number of ECGs Definite Possible uals with no usable ECGs in the record was 98.3 100 96.5 up to 2 9.4 days implies that a number of these patients’ 99.3 100 98.3 up to 3 AMIs occurred while they were hospitalized for 99.7 100 98.9 up to 4 other reasons. These findings were consistent 100.0 100 99.2 up to 5 whether only two enzymes, AST and LDH, Validated using ECG, chest pain, autopsy, AST, LDH, were included in the diagnostic algorithm or the CPK, and CPK-MB.
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more sensitive cardiac enzymes CPK and CPKMB were included. Whether 2 or 4 enzymes are used to identify hospitalized AMIs, the fact remains that very little information is lost by reducing the number of usable ECGs from all available in the record (maximum of 12) to a maximum of 4. In comparing 1970-1980 trends in attack and case fatality rates for AMI, we employed a computerized diagnostic algorithm which included only two enzymes, AST and LDH, because CPK determinations were not widely used and CPK-MB was not widely available in 1970 [I 51. Though this paper presents only 1980 results, all analyses were repeated and similar findings were noted using 1970 records. These data support a decision to collect and analyze a maximum of four ECGs per hospitalization. For the 1980 sample of 1864 cases, the savings in coding costs would have been 27.7% ($17,412-from $62,939 to $45,527) had the number of ECGs coded been reduced from a maximum of 12 to a maximum of 4 per case. If abstracters were trained to recognize usable tracings, additional savings would occur in abstractor time and photocopying expenses. But should ECGs be coded for cases? Another possible strategy toward cost-effectiveness would be to analyze ECGs only when the case could not be validated using other available information (history of chest pain, enzymes, autopsy findings). This strategy would have eliminated ECG coding for 36.5% (681/1864) of the 1980 cases when two enzymes were considered and for 56% (1044/1864) when 4 enzymes were used. If enzyme measures were standardized worldwide and questions on typical cardiac pain were uniform, such an approach would appear reasonable. Because no interhospital laboratory standardization is widely used, this economically attractive labor reduction would severely limit the scientific usefulness of the results and therefore cannot be recommended. For example, comparisons of morbidity statistics over time and between geographical regions would be questionable. We also considered eliminating ECGs entirely by using a completely enzyme-based diagnostic algorithm. With this algorithm, an acute myocardial infarction would be diagnosed when maximum AST, LDH, or CPK values were elevated or CPK-MB was present, or when there had been an autopsy-confirmed myocardial infarction within 8 weeks of death. Definite AMIs identified by the enzyme-autopsy algorithm
(N = 1147) were then compared with those identified using ECG, enzyme, pain, and autopsy (N = 1157). The number of ECGs analyzed had little impact on these analyses. When an enzyme-autopsy diagnostic rule was applied, 94 additional cases were classified as definite AMI, and 104 definite AMIs by our standard algorithm were not identified. We believe greater accuracy is achieved when ECGs are included. An additional reason to analyze ECGs for all cases is that they are necessary to distinguish between Q wave and non-Q wave infarctions. The Worcester, Massachusetts [ 121 and Rochester, Minnesota [ 131 community cardiovascular surveillance programs have reported differing secular trends in incidence and case fatality rates of Q wave and non-Q wave AMIs. We conclude that the ECG should be retained in the diagnostic MI algorithm for three reasons. First, the ECG plays a critical role in detecting acute myocardial infarctions. Second, the ECG allows comparability with surveys using no or different enzymes. Third, the ECG offers the only way to determine whether the infarction is Q wave or non-Q wave in type. We recommend utilizing 4 ECGs per participant in conjunction with pain, enzyme rise, and autopsy findings as a cost-effective and valid approach to identifying acute myocardial infarctions in populations. Finally, these results apply only to strategies for validating hospitalized AMIs in surveillance or in case reviews and diagnosis by expert committees. These findings are not intended as a guide for practicing clinicians who must continue to use ECGs as necessary for each patient. However, we observed a slight decrease in the number of ECGs ordered by clinicians between 1970 and 1980. As efforts to reduce the costs of medical care continue, average length of hospital stay decreases, and the more sensitive cardiac enzymes are used, the number of ECGs performed may continue to decline. Acknowledgements-The authors gratefully acknowledge the valuable scientific assitance of Drs Judith Baxter, Steve Norsted, and J. Michael Sprafka, and the editing expertise of Carolyn S. Kurtz and Carol Edlavitch.
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