N Engl J Med 2001;344:1879-1887
Background: Acute coronary syndrome is broadly categorized into unstable angina, non-ST-elevation myocardial infarction (NSTEMI) and ST-elevation myocardial infarction (STEMI). In unstable angina, there is no rise in cardiac biomarkers, although some challenge this clinical entity in the current era of high sensitivity troponins. In NSTEMI, there is elevation of cardiac biomarkers but no ST segment elevation on the electrocardiogram. In STEMI, there is an ST segment elevation on the electrocardiogram as well as a rise in cardiac biomarkers.
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In patients with STEMI, percutaneous coronary intervention (PCI) significantly improves outcomes. However, its role in acute coronary syndrome without ST-segment elevation is less clear for several reasons. Patients with NSTEMI tend to be older and have more comorbidities, increasing procedural risks. This also means that they have competing risks for mortality, potentially reducing the benefit of PCI. Another key challenge is that NSTEMI patients frequently have multivessel disease, making it more difficult to identify the culprit lesion; since there is usually only partial occlusion of the culprit coronary artery. In contrast, there is usually complete occlusion of a coronary artery in STEMI and ST-segment elevation on the electrocardiogram helps localize the infarcted area, making it relatively easy to identify the culprit artery.
The findings from previous randomized trials of revascularization in unstable angina and NSTEMI, have been inconsistent. The TACTICS–Thrombolysis in Myocardial Infarction 18 trial sought to compare early invasive vs conservative strategy in patients with unstable angina or NSTEMI.
Patients: Eligible patients had angina within 24 hours that was: >20 minutes in duration, accelerating angina, or recurrent episodes at rest or with minimal effort. Patients also had to have one of the following: ST-segment depression of at least 0.05 mV, transient (<20 minutes) ST-segment elevation of at least 0.1 mV, or T-wave inversion of at least 0.3 mV in at least two leads; elevated cardiac biomarkers; or history of coronary artery disease.
Patients were excluded if they have persistent ST elevation, PCI or CABG within 6 months, LBBB or paced rhythm, high bleeding risk, severe congestive heart failure or cardiogenic shock, serious systemic illness, or serum creatinine > 2.5 mg/dL.
Baseline characteristics: The trial randomized 2,220 patients – 1,114 randomized to early invasive strategy and 1,106 randomized to conservative strategy.
The average age of patients was 62 years and 66% were men. Approximately 28% had diabetes and 39% had prior myocardial infarction.
Troponin T levels were elevated (>0.01 ng/ml) in 54% of the patients.
Procedures: Patients were randomly assigned in a 1:1 ratio to undergo early invasive vs conservative strategy.
Patients received aspirin 325 mg daily, intravenous unfractionated heparin (5000U bolus, followed by an infusion at 1000U/ hour for 48 hours), and intravenous tirofiban (0.4 μg/kg/minute for 30 minutes followed by an infusion of 0.1 μg/kg/minute for 48 hours or until revascularization with tirofiban administered for at least 12 hours after PCI).
Patients in the early invasive arm underwent coronary angiogram between 4 and 48 hours after randomization and underwent PCI as appropriate. Patients in the conservative arm were treated medically. If stable, they underwent an exercise-tolerance test before discharged (83% of these tests were with nuclear perfusion or echocardiography imaging). Patients in the conservative arm underwent coronary angiography with PCI if they had angina at rest associated with ischemic EKG changes or elevation in cardiac biomarkers, had clinical instability or had ischemia on their stress test.
Endpoints: The primary outcome was a composite of death from any cause, nonfatal myocardial infarction, and rehospitalization for an acute coronary syndrome, at six months.
The estimated sample size to provide 80% power was 1,720 patients. This assumed that 22% of the patients in the conservative arm would experience the primary outcome and that the early invasive strategy would result in 25% relative risk reduction in the primary outcome. The sample size was later increased to 2,220 patients.
Results: In the early invasive strategy, 97% of the patients underwent coronary angiogram after a medium of 22 hours after randomization, and 60% underwent PCI or CABG. In the conservative arm, 51% underwent coronary angiogram and 36% underwent revascularization during the index hospitalization.
The primary composite endpoint was lower with the early invasive strategy (15.9% vs 19.4%, odds ratio: 0.78, 95% CI: 0.62 - 0.97; p= 0.025). The Kaplan-Meier curves started to separate at approximately one week. This benefit was driven by lower myocardial infarction and lower rehospitalization for an acute coronary syndrome with the early invasive strategy; (4.8% vs 6.9%) and (11.0% vs 13.7%), respectively. There was no difference in all-cause death (3.3% vs 3.5%).
There were 3 important subgroup interactions. First is based on ST changes where patients with ST changes at presentation had all the benefit with an early invasive strategy (16.4% vs 26.3% [for patients with ST changes] and 15.6% vs 15.3% [for patients without ST changes]). Second is based on Troponin T levels where patients with troponin T> 0.1 ng/mL had significantly more benefit with an early invasive strategy (16.4% vs 24.5% and 15.1% vs 16.6%). The third is based on TIMI score where patients with higher TIMI score had more benefit with an early invasive approach. For a high TIMI score of 5-7, the event rate was 19.5% with early invasive vs 30.6% with conservative approach. Patients with TIMI score of 0-2 had no benefit with an early invasive strategy (12.8% with early invasive vs 11.8% with conservative strategy).
Note to readers: TIMI score is a risk stratification tool used to predict 14-day adverse outcomes in patients with unstable angina or NSTEMI. The score ranges from 0 to 7 with higher scores indicating worse prognosis.
Conclusion: In patients with unstable angina or NSTEMI, an early invasive strategy reduced the composite endpoint of death from any cause, nonfatal myocardial infarction, and rehospitalization for an acute coronary syndrome at six months with a number needed to treat of approximately 29 patients.
The subgroup analysis of this trial is particularly important and biologically plausible, as the presence of ST changes and level of cardiac biomarkers elevation indicate more significant myocardial ischemia or necrosis. Patients without ST changes comprised 62% of the study participants, while those with negative cardiac biomarkers made up 59%, and the study results should not be generalized to these subgroups.
Another key consideration is the lack of detailed criteria for what was deemed ‘appropriate’ revascularization. Only 60% of patients in the early invasive strategy group underwent revascularization, underscoring that not all patients with unstable angina or NSTEMI benefit from coronary angiography and that further risk stratification is necessary.
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