Correspondence
Myocardial Ischemia Detected by Ambulatory Monitoring after Myocardial Infarction
N Engl J Med 1996; 334:1545-1546June 6, 1996
- Article
To the Editor:
The article by Gill et al. (Jan. 11 issue),1 on the prognostic value of ambulatory electrocardiographic (ECG) monitoring early after myocardial infarction, concludes that the detection of ischemia during a 48-hour period was the best predictor of outcomes at 12 months.
The authors recommend the use of ambulatory ECG monitoring as the screening test of choice after myocardial infarction, over predischarge submaximal exercise testing or estimation of the left ventricular ejection fraction. Their study does not justify this recommendation for several reasons. First, a substantial proportion of patients (18 percent) did not undergo stress testing. Second, the patients who did not undergo stress testing had the highest incidence of nonfatal myocardial infarction or death (24.7 percent, P = 0.007). Since “physician's refusal” was listed as one of the reasons not to perform stress testing in patients, this group probably included patients in whom the results were expected to be positive. Third, clinicians were not blinded to the results of the stress tests, as reflected in the different rates of subsequent coronary-artery bypass surgery. The study design is therefore biased against the usefulness of exercise testing.
The outcomes in patients who underwent both ambulatory ECG monitoring and stress testing can be compared by analyzing the data Gill et al. present in Table 3 of their article. Ambulatory ECG monitoring was a better predictor only when hospitalization for angina was added as an outcome, since the positive and negative predictive values for the two tests were not statistically different for nonfatal myocardial infarction or death (Table 1Table 1
Predictive Values of Submaximal Stress Testing and Ambulatory ECG Monitoring for Death or Myocardial Infarction and for All Outcomes.). The clinical usefulness of predicting hospitalization for angina is unclear, since the majority of the patients with this condition have a good prognosis.2 Finally, in patients who underwent both ambulatory ECG monitoring and estimation of left ventricular function, a left ventricular ejection fraction of less than 35 percent was a better predictor of death (18 percent) than was the presence of ischemia (12 percent). Previous studies of patients surviving myocardial infarction have shown the important prognostic3 and therapeutic4 implications of the left ventricular ejection fraction. Thus, recommending ambulatory ECG monitoring instead of an assessment of left ventricular ejection fraction after myocardial infarction seems inappropriate.
4 ReferencesAlice A. Passer, M.D.
Dartmouth–Hitchcock Medical Center, Lebanon, NH 03756Mario J. Garcia, M.D.
Veterans Affairs Medical Center, White River Junction, VT 050091
Gill JB ,Cairns JA ,Roberts RS , et al. Prognostic importance of myocardial ischemia detected by ambulatory monitoring early after acute myocardial infarction. N Engl J Med 1996;334:65-70
Full Text | Web of Science | Medline2
van Miltenburg-van Zijl AJ ,Simoons ML ,Veerhoek RJ ,Bossuyt PM . Incidence and follow-up of Braunwald subgroups in unstable angina pectoris. J Am Coll Cardiol 1995;25:1286-1292
CrossRef | Web of Science | Medline3
The Multicenter Postinfarction Research Group
Full Text | Web of Science | Medline4
Pfeffer MA ,Braunwald E ,Moye LA , et al. Effect of captopril on mortality and morbidity in patients with left ventricular dysfunction after myocardial infarction -- results of the Survival and Ventricular Enlargement Trial. N Engl J Med 1992;327:669-677
Full Text | Web of Science | Medline
To the Editor:
Gill et al. clearly demonstrate that myocardial ischemia as detected by ambulatory ECG monitoring provides prognostic information with regard to the risk of clinically important manifestations of recurrent ischemic heart disease after myocardial infarction. Given the strategy of exercise testing used by the authors, however, it cannot be inferred that ambulatory ECG monitoring is superior.
The patients in the study underwent a single submaximal exercise test before discharge. No cardiac imaging was performed with the exercise test. Since many patients had ECG abnormalities at rest, the probability of a nondiagnostic exercise test was high, despite the exclusion of abnormalities such as left bundle-branch block. The authors do not distinguish between nondiagnostic and negative results of the stress test but only report the results as positive or negative. Negative or nondiagnostic submaximal stress testing was not followed by maximal stress testing at six weeks. Eighteen percent of the patients underwent no stress testing at all.
Exercise testing in combination with myocardial imaging after infarction is a routine procedure that provides additional prognostic information. For example, thallium-201 scintigraphy in combination with submaximal exercise testing before discharge is superior to submaximal exercise testing alone in identifying patients who are at increased risk for future cardiac events.1 The authors do not exclude the possibility that such strategies, already in clinical use, provide information similar to that obtained by ambulatory ECG monitoring.
Admittedly, serial exercise testing with myocardial imaging is expensive. It may be that ambulatory ECG monitoring is a simpler and more cost-effective approach to risk stratification after myocardial infarction. The present study, however, was not designed to address this issue.
We agree with the authors that ambulatory ECG monitoring provides important prognostic information. However, the design of the study does not permit the conclusion that “it would be logical to use ambulatory ECG monitoring as the screening test of choice after acute myocardial infarction.”
1 ReferencesPia S. Pollack, M.D.
David H. Wiener, M.D.
Temple University School of Medicine, Philadelphia, PA 191401
Gibson RS ,Watson DD ,Craddock GB , et al. Prediction of cardiac events after uncomplicated myocardial infarction: a prospective study comparing predischarge exercise thallium-201 scintigraphy and coronary angiography. Circulation 1983;68:321-336
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Author/Editor Response
The authors reply:
To the Editor: In response to Passer and Garcia: the results of our study demonstrate that a large percentage of patients who survive until the fifth day after myocardial infarction do not undergo submaximal exercise testing, and these patients are at higher risk for a subsequent cardiac event (rate of death, 16.4 percent among the patients in whom exercise testing was not performed vs. 3.3 percent among those in whom exercise testing was performed). Although many such patients will already have been identified as being at high risk by their physicians, ambulatory ECG monitoring allows further stratification of the risk within this subgroup (rate of death, 32 percent among patients with ischemia vs. 8.3 percent among those without ischemia). Physicians were not blinded to the results of exercise testing and were able to make decisions about medical management and revascularization on the basis of these results. Ambulatory ECG monitoring still provided additional prognostic information both in patients with negative exercise tests and in those with positive tests, particularly with respect to future ischemic events.
We disagree that the “clinical usefulness of predicting hospitalization for angina is unclear.” Van Miltenburg-van Zijl et al. reported that among patients admitted to the hospital with postinfarction angina, the six-month incidence of death or recurrent myocardial infarction was 20 percent, and 63 percent of the patients died, had myocardial infarction, or underwent revascularization.1 Although the left ventricular ejection fraction was a stronger univariate predictor of mortality than were the results of ambulatory ECG monitoring, no postinfarction test provided incremental prognostic value over that of a series of clinical variables. In this multivariate model, ambulatory ECG monitoring provided incremental prognostic value for the composite outcomes, whereas measurement of the left ventricular ejection fraction did not.
Pollack and Wiener point out some of the limitations of submaximal exercise testing after myocardial infarction. Of the 952 patients screened for this study, 177 (18.6 percent) were excluded because they had predefined ECG abnormalities that precluded the interpretation of ST-segment shifts. Some patients who did not undergo exercise testing might have benefited from pharmacologic-stress perfusion imaging, but the majority would probably have been excluded for the same reasons that they did not undergo exercise testing. Patients were not excluded from undergoing further stress testing after discharge from the hospital, but the number of patients who underwent such procedures is unknown. Exercise perfusion imaging is not performed routinely after acute myocardial infarction in our hospitals.
Ambulatory ECG monitoring provides incremental prognostic value beyond the prognostic value of simple clinical variables for the outcomes of death, myocardial infarction, and hospitalization for unstable angina among patients with abnormal or normal left ventricular ejection fractions and patients who do not undergo exercise testing, as well as those with positive and negative exercise tests. Accordingly, we conclude that ambulatory ECG monitoring is the screening test of choice. However, it is unlikely that a single test will provide all the prognostic information needed for risk stratification after acute myocardial infarction, and additional tests will be required as appropriate.
1 ReferencesJohn B. Gill, M.D.
John A. Cairns, M.D.
Robin S. Roberts, M.Tech.
McMaster University, Hamilton, ON L8N 3Z5 Canada1
van Miltenburg-van Zijl AJ ,Simoons ML ,Veerhoek RJ ,Bossuyt PM . Incidence and follow-up of Braunwald subgroups in unstable angina pectoris. J Am Coll Cardiol 1995;25:1286-1292
CrossRef | Web of Science | Medline
