(Circulation. 1997;95:1352-1354.)
© 1997 American Heart Association, Inc.
Articles |
Risk Stratification After Myocardial Infarction
Clinical Science Versus Practice Behavior
the Department of Medicine, Section of Cardiology, Baylor College of Medicine, and The Methodist Hospital, Houston, Tex.
Correspondence to Miguel A Quiñones, MD, Echocardiography Laboratory, Baylor College of Medicine, The Methodist Hospital, 6550 Fannin, SM-677, Houston, TX 77030. E-mail quelq{at}bcm.tmc.edu
Key Words: Editorials • myocardial infarction • echocardiography
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Introduction |
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 Top Introduction References |
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The survival of patients with acute myocardial infarction (MI) has improved considerably during the past 15 years with the advent of thrombolytic therapy, including better utilization of anticoagulants, aspirin, and cardioprotective drugs such as ß-blockers and ACE inhibitors. Despite this improvement, mortality rates after MI continue to demonstrate an early rise during the first 3 months, with a slower but steady increase afterward.1 The principal cardiac factors influencing survival after MI are the size of the infarct and its impact on left ventricular (LV) function, the presence of residual ischemia, recurrent infarction, and ventricular arrhythmias. These factors can coexist in the same patient and exert a negative synergistic effect on survival.
There is general consensus that reduction of the risk of recurrent ischemia improves long-term survival of post-MI patients. However, there is controversy regarding the best strategy for achieving this, particularly in low-risk patients with an uncomplicated MI. A conservative strategy uses noninvasive testing to identify important risk factors and modify therapy accordingly, including the selective use of coronary revascularization procedures, whereas a more aggressive strategy involves the routine use of coronary angiography followed by revascularization of areas supplied by significant stenotic lesions.
Ejection fraction is without doubt a strong predictor of mortality in patients with acute MI. Mortality rates increase rapidly as ejection fraction falls below 40%.2 Currently, ejection fraction is determined primarily with noninvasive techniques. In 1979, Theroux and associates3 reported on the use of submaximal exercise testing early after MI and demonstrated the negative impact of exercise-induced ischemia on long-term outcome. Since their original report, multiple studies have shown that post-MI patients with demonstrable ischemia are at high risk for cardiac events regardless of the method used for inducing or detecting ischemia.4 5 6 7 However, methods with higher sensitivity for ischemia detection, such as thallium scintigraphy or echocardiography, appear to be superior to ECG in defining subgroups of patients at low and high risk for cardiac events.4 5 Many of these studies, however, were performed in the prethrombolytic era.
The use of thrombolytic therapy has changed the clinical profile of patients receiving this unique therapy. These patients are often younger and less ill than those who do not receive thrombolysis. Furthermore, lysis of the thrombus aborts the infarct and often leaves a residual critical stenosis with viable myocardium that is potentially subject to recurrent ischemia and reinfarction.1 Thus, one could postulate that in this setting, angiographic evaluation and revascularization of suitable lesions may constitute a better option for patients than the conservative noninvasive strategy.
In this issue of Circulation, Carlos and coworkers8 provide evidence that supports the use of the noninvasive strategy even in patients treated with thrombolytic agents. The authors studied 214 patients (84% with a first MI and 57% with a Q-wave MI) and performed dobutamine stress echocardiography within 2 to 7 days after the MI. One hundred twenty-one patients received thrombolytic therapy. A major strength of the study was that the results of the test were not given to the attending physician and therefore did not influence the decision to perform coronary angiography. Nevertheless, 193 of the 214 patients had a coronary angiogram, and 39% underwent a revascularization procedure for anatomic reasons before hospital discharge. Patients were followed for a mean of 16 months, and adverse events were defined as cardiac death, nonfatal MI, sustained ventricular tachycardia or fibrillation, unstable angina, and congestive heart failure.
When the results of the dobutamine stress echocardiogram were compared with clinical and angiographic variables through the use of multiple logistic regression analysis, the only independent predictors of events came from the findings of the dobutamine stress echocardiogram and included the presence of four or more akinetic or dyskinetic segments within the infarct territory during low dose (an index of infarct size), two or more coronary artery territories demonstrating abnormal wall motion at rest or during peak dose, and lack of improvement in wall thickening (ie, lack of viability) within the infarct region during low-dose dobutamine. Importantly, the findings of the dobutamine stress echocardiogram were better at risk-stratifying patients than was angiographic demonstration of multivessel disease. Furthermore, they were capable of predicting cardiac events in patients treated with thrombolytic agents as well as in those who did not receive thrombolytic therapy. An interesting finding in this study was the association of viability within the infarct zone with reduction in risk for cardiac events. Because the dobutamine echo was performed within 2 to 7 days after MI, it is conceivable that myocardial stunning was responsible for the resting dysfunction in many of the viable segments. Therefore, the improvement in function of these segments with low-dose dobutamine indicated their likelihood for eventual recovery,9 particularly in a setting in which revascularization was commonly used.
The concept that a noninvasive assessment of the extent of ischemia and/or infarction is superior to coronary angiography for risk stratification was first established in the prethrombolytic era by Gibson and associates.4 The authors found that exercise 201Tl scintigraphy was better than coronary angiography in predicting low-risk status and more sensitive in the detection of cardiac events. The findings by Carlos and coworkers8 demonstrate that the concept holds true in the post-thrombolytic era and that it is independent of the technique used to quantify infarct size or detect ischemia. Two other studies have also shown the superiority of noninvasive testing over angiography in the post-thrombolytic era. Mahmarian et al10 used 201Tl tomography with intravenous adenosine infusion to evaluate patients during the first week after an uncomplicated MI, and Dakik et al11 used 201Tl tomography with treadmill exercise during the second week of the MI. In both studies, the strongest predictors of events were ejection fraction and the size of the ischemic perfusion defect. Ejection fraction, determined noninvasively, was combined with exercise ECG and clinical variables by Arnold et al12 in a study of 1043 patients with MI who were participating in the alteplase/placebo angioplasty trial of the European Cooperative Study Group. The authors found that coronary angiography did not perform any better in prediction of late death than did the clinical data combined with the noninvasive tests.
The evidence summarized so far well supports the use of a noninvasive strategy for the risk stratification of patients surviving an uncomplicated MI regardless of whether thrombolytic therapy is used. Despite such evidence, the trend in this country has been toward the use of invasive procedures after MI.13 14 Even in the study by Carlos and coworkers,8 90% of the patients underwent coronary angiography, and revascularization was performed for anatomic rather than clinical reasons. This practice behavior results, at least in part, from a common belief among practicing cardiologists that revascularization of an artery with residual stenosis is, in the long run, better for patients.
A recent analysis of 21 772 patients enrolled in the GUSTO-1 trial demonstrated that 71% of the patients underwent coronary angiography before discharge from the hospital and that of these patients, 58% underwent revascularization with either angioplasty (73%) or surgery.14 Analysis with the classification-and-regression tree model revealed that younger age and availability of the procedure appeared to be the two major determinants of the use of coronary angiography, whereas coronary anatomy was used to determine the use and type of revascularization. In a separate analysis of the same database, the authors noted that the use of coronary angiography and revascularization procedures varied significantly among geographical regions in the United States.15 However, no apparent relation was found between the use of these procedures and rates of recurrent infarction or death at 30 days or 1 year.
The above data suggest that routine revascularization of angiographically significant lesions may not necessarily improve the long-term outcome for patients recovering from an MI. The Should We Intervene Following Thrombolysis? (SWIFT) trial and the Thrombolysis in Myocardial Infarction Investigation (TIMI) phase II trial provided conclusive evidence that routine revascularization does not improve survival or rates of reinfarction in patients with an acute MI who have been treated with thrombolytic agents.16 17 In both trials, patients were randomized to an in-hospital interventional strategy (angiography followed by coronary angioplasty or surgical bypass of appropriate lesions) or to a conservative strategy. By 12 months, there was no difference in mortality or rates of reinfarction between the two groups. Similar results were found in the TIMI-IIIB trial at 6 weeks for patients with non–Q-wave MI.18
Given these findings, one wonders whether there is any rationale for the routine use of cardiac catheterization in patients recovering from an uncomplicated MI. One group that might benefit from coronary angiography are those with depressed LV function because they are known to be at high risk for future events. In fact, the recently published American College of Cardiology/American Heart Association guidelines for management of acute MI recommend noninvasive strategy for low-risk patients but consider the use of coronary angiography in those with an ejection fraction of 
40%.19 However, even in this group, the benefits of routine coronary angiography and revascularization were recently questioned by Rouleau and associates.20 The authors performed an analysis of patients in the United States and Canada who participated in the SAVE trial. This trial evaluated the beneficial effects of the use of captopril in 2231 patients who survived an MI with an ejection fraction of 40% (mean, 31%). Despite similar clinical characteristics for the 1573 patients from the United States and the 658 patients from Canada, coronary arteriography and revascularization procedures were more commonly performed in the United States than in Canada (68% versus 35% and 43% versus 22%, respectively). During an average follow-up of 42 months, no differences were observed between the two countries in mortality (22% in Canada and 23% in the United States) or rate of reinfarction (14% in Canada and 13% in the United States).
In summary, there is strong evidence, including the results of the study by Carlos and coworkers,8 indicating that a noninvasive strategy can be applied to most patients surviving an uncomplicated MI with at least comparable results for those receiving an invasive interventional strategy. The use of coronary angiography and revascularization should be reserved for patients with angina and/or heart failure symptoms, patients with demonstrable ischemia by noninvasive stress testing, and patients with depressed LV function in whom multivessel disease is suspected. The application of this conservative strategy on a large scale could result in substantial cost savings, primarily by reducing the use of revascularization procedures performed for anatomic reasons.
The current literature is not clear regarding which noninvasive tests are the best to use. What is clear, in my opinion, is that a measurement of ejection fraction or some comparable index of the extent of LV dysfunction should be combined with some method of detecting ischemia through the use of exercise or a pharmacological stress agent. Given the superior sensitivity for ischemia detection of the imaging modalities, my preference is the use of a radionuclide perfusion study or an echocardiographic stress test. Review of the available literature does not favor one over the other. However, an echocardiographic stress test has the distinct advantage of combining the evaluation of LV function with ischemia detection in the same setting.
The timing of these tests is also unclear from the available literature. Both exercise and pharmacological stress tests have been found to be safe during the first 2 weeks after MI.3 6 10 11 The current study by Carlos and coworkers8 suggests that maximal dobutamine stress testing is safe during the first week after MI. In my opinion, however, the sample size was not sufficiently large to establish conclusively the safety of the test when performed at such an early time, particularly between days 2 and 5 after MI. My preference is to wait until the seventh day before submitting a patient to a full dobutamine or exercise protocol. In today's climate of managed care, this means that some stable patients undergo testing as outpatients.
In summary, I believe that sufficient evidence exists to modify the current behavior of routinely performing invasive procedures and to instead use a conservative/noninvasive strategy in the management of patients with uncomplicated MIs.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editor or of the American Heart Association.
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References |
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TopIntroductionReferences |
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4.
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8.
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11.
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12.
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14.
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17.
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18.
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20.
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