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Early Clinical Outcomes and Routine Management of Patients With Non–ST-Segment Elevation Myocardial Infarction
A Nationwide Perspective
Richard C. Becker, MD;
Maureen Burns, MD;
Nathan Every, MD, MPH;
Charles Maynard, PhD;
Paul Frederick, MPH, MBA;
Frederick A. Spencer, MD;
Joel M. Gore, MD;
Costas Lambrew, MD;
for the National Registry of Myocardial Infarction Participants
Arch Intern Med. 2001;161:601-607.
ABSTRACT
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Background Myocardial infarction (MI) in the absence of electrocardiographic ST-segment
elevation or new bundle branch block is the cause of hospitalization for a
large and steadily increasing proportion of patients with acute ischemic chest
pain. Despite its prevalence, the common demographic features, current hospital-based
management, and short-term clinical outcome among patients with non–ST-segment
elevation MI remain poorly defined.
Methods A total of 183 113 patients with non–ST-segment elevation
MI were identified in the National Registry of Myocardial Infarction database.
Using a validated model, 43 928 patients (24.0%) were retrospectively
placed in major, 34 917 (19.1%) in intermediate, and 104 268 (56.9%)
in minor severity clinical event categories that included hospital death,
recurrent myocardial ischemia, and nonfatal recurrent MI.
Results The administration of widely available and universally recommended pharmacologic
therapies, including aspirin and ß-adrenergic blocking agents, was suboptimal,
particularly among patients with major severity clinical events. In contrast,
coronary angiography and mechanical revascularization procedures were commonplace
(>60% of all patients) and most frequently performed in patients within the
minor (compared with the major) severity clinical event category (58.2% and
42.7%, respectively).
Conclusions Patients with non–ST-segment elevation MI are a heterogeneous
population, with readily identifiable demographic characteristics and clinical
features associated with important early outcomes, including death. Nationwide
efforts directed toward maximizing pharmacologic therapy utilization and the
performance of invasive procedures according to established guidelines must
continue.
INTRODUCTION
MYOCARDIAL ischemia and subsequent injury in the absence of diagnostic
electrocardiographic (ECG) changes are recognized as increasingly common causes
for hospital admission, occurring in more than 750 000 individuals yearly
in the United States. In most cases, acute myocardial infarction (MI) without
ST-segment elevation or new bundle branch block is caused by atheromatous
plaque disruption, intraluminal thrombosis, and distal microembolization.
Although variable in its pathobiologic predominance of platelets and fibrin
and rarely of sufficient mass to fully or permanently occlude an epicardial
coronary vessel, the derangement of coronary artery blood flow is capable
of compromising myocardial perfusion for a critical period.1, 2, 3, 4
Early descriptions suggested that patients with non–ST-segment
elevation MI, compared with those with ST-segment elevation or bundle branch
block MI, represent a homogeneous population whose risk for adverse clinical
outcomes, including recurrent myocardial ischemia, infarction, and cardiac
death, was uniform and greatest in the weeks to months following hospital
discharge5, 6, 7, 8, 9, 10;
however, more recent observations have challenged the belief that patients
are generally at low risk during the early postinfarction period.11 An ability to risk characterize patients represents
an important step toward developing targeted and cost-effective management
strategies for wide-scale implementation.
The National Registry of Myocardial Infarction (NRMI-2), a database
that includes more than 350 000 patients, represents a powerful resource
for determining population heterogeneity, demographic characteristics, and
clinical features that identify individuals who are either likely or unlikely
to experience in-hospital adverse outcomes and can also be used to define
nationwide trends in pharmacologic, diagnostic, and intervention-based management
of patients with non–ST-segment elevation MI.
METHODS
The NRMI was initiated in 1989 as a pilot project of patients with acute
MI. The first large-scale registry (NRMI-1), at its conclusion in September
1994, included more than 350 000 patients and was designed to determine
national practice patterns and facilitate continuous quality improvement at
individual participating hospitals.12
The NRMI-2 represents an expanded data collection instrument that has
been used to determine cardiovascular risk factors, resource utilization,
and the safety of commonly used therapies and processes of care. Demographic,
procedural, and outcome data on patients with suspected acute MI were collected
by an appointed coordinator at each site. Participation in the registry was
voluntary, and hospitals were encouraged to enter consecutive patients irrespective
of treatment strategy and outcome. To be enrolled, patients were required
to have experienced an acute MI according to on-site criteria that included
elevated cardiac enzyme levels (creatine kinase or its MB fraction), an abnormal
ECG, and/or an abnormal coronary angiogram. A nondiagnostic ECG was defined
by the presence of nonspecific ST or T wave abnormalities (no injury pattern
or Q waves).
All collected data were sent to a central data collection center (ClinTrials
Research Inc, Lexington, Ky) for processing and subsequent analysis. Double
key entry was used to add each case report form to the NRMI database.
The original data derived from the NRMI-2 database, inclusive from June
1994 through January 1997, included a total of 446 970 observations.
Patients were eliminated from the study for the following reasons: transfer
out of an NRMI-2 site, ECG ST-segment elevation or left bundle branch block
on presentation, missing discharge data, age older than 110 years (or missing
age), and missing region assignment. The final analysis was based on a total
of 183 113 observations.
CLINICAL EVENT SEVERITY CATEGORIZATION
Patients were retrospectively divided into major, intermediate, and
minor severity clinical event categories. The diagnostic criteria for each
predefined category was based on a consensus among the investigators.
Major severity clinical events included the following: hospital death
due to cardiac rupture, recurrent MI, cardiogenic shock, ventricular tachycardia
or ventricular fibrillation, or intracranial hemorrhage; nonfatal but disabling
stroke; nonfatal recurrent MI; and sustained ventricular tachycardia.
Intermediate severity clinical events included recurrent ischemia, congestive
heart failure, sustained atrioventricular block, and serious (but non–life-threatening)
hemorrhage.
Low-severity clinical events were those not included in either the intermediate
or major severity categories.
STATISTICAL ANALYSIS
Patient characteristics across clinical event outcomes were compared
with the  2 statistic for categorical variables and 1-way analysis
of variance for continuous variables such as age and weight. Patients who
experienced in-hospital death (after the first 24 hours) were identified by
multivariable logistic regression. First, stepwise logistic regression was
used to identify predictors of mortality in a 50% random sample of the patient
population. Using the regression coefficients and model constant, we tested
the model developed in stage 1 on the remaining half of the population. When
the model was applied to the test set, the goodness-of-fit as assessed by
the area under the receiver operating characteristic curve was 79.2%.
A model predicting the probability of cardiac catheterization and/or
percutaneous coronary interventions (PCIs) (propensity score) was calculated
for each data set observation. The score represents the relation between multiple
characteristics and the dependent variable as a single characteristic. Based
on the median value, the score is then divided into low or high and placed
in the original model. The resulting model references the high-propensity
category with the low-propensity category. The area under the receiver operating
characteristic curve was 76.3%.13
RESULTS
A total of 183 113 patients diagnosed as having acute non–ST-segment
elevation MI were identified. The baseline demographic characteristics and
clinical features for the overall population are outlined in Table 1.
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Table 1. Baseline Demographic and Clinical Characteristics for the
Overall Study Population*
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Patient characteristics for the clinical event severity category groups
are summarized in Table 2. From
the total cohort of patients, 43 928 (24.0%) were placed in the major
severity clinical event category, 34 917 (19.1%) in the intermediate
severity clinical event category, and 104 268 (56.9%) in the minor severity
clinical event category. Compared with patients in the minor severity clinical
event group, patients experiencing major severity events were older, more
often female, more likely to have a history of prior MI and diabetes and less
likely to abuse tobacco.
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Table 2. Baseline Demographics for the Clinical Event Severity Groups*
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The ECG findings and clinical features according to clinical event severity
groups are presented in Table 3.
Patients with major severity clinical events were more likely to have ST-segment
depression and less likely to have either nonspecific ST-T wave changes or
a normal ECG at the time of hospital admission than patients in lower-severity
categories. In addition, they more often had ECG evidence of anterior site
of infarction, a creatine kinase–MB fraction more than 2 times the upper
limit of normal, and an ejection fraction less than 0.40.
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Table 3. Electrocardiographic and Enzymatic Features According to Clinical
Event Severity*
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Overall, 11.7% of study patients enrolled in NRMI-2 did not survive
their event. The mortality among patients with major severity clinical events
was 48.8%. Recurrent ischemia and reinfarction occurred in 12.4% and 2.6%
of patients, respectively (Figure 1).
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Death, recurrent angina or myocardial ischemia, and myocardial infarction
(MI) among 183 113 patients who participated in the National Registry
of Myocardial Infarction according to clinical event category designation.
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The early (within 24 hours of hospital admission) and predischarge administration
of adjunctive pharmacologic therapy is summarized in Table 4. Although the use of intravenous heparin sodium, aspirin,
and ß-adrenergic blocking agents was relatively low in all patients,
those within the major severity clinical event group were least likely to
be treated. In these individuals, intravenous heparin, aspirin, and ß-adrenergic
blocking agents were administered 57.4%, 56.0%, and 21.4% of the time, respectively.
A similarly low administration rate for aspirin, ß-adrenergic blocking
agents, and angiotensin-converting enzyme inhibitors was observed at the time
of hospital discharge.
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Table 4. Early (<24 Hours) Adjunctive Pharmacologic Therapy According
to Clinical Event Severity*
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Patients with non–ST-segment elevation MI underwent coronary angiography,
PCIs, and bypass grafting at rates of 53.7%, 20.5%, and 13.4%, respectively.
Invasive diagnostic testing and percutaneous revascularization were more common
in patients with minor rather than major severity clinical events, although
the latter group did undergo surgical revascularization at a slightly higher
rate (12.3% vs 14.8%). By definition, patients in the minor severity clinical
event group did not experience recurrent myocardial ischemia during their
hospitalization; however, a relatively small proportion of these patients
underwent either standard exercise stress testing or an alternative functional
study before hospital discharge to better delineate their risk for future
cardiac events (Table 5).
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Table 5. Procedures Performed Before Hospital Discharge According to
Clinical Event Severity*
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A comparison between clinical event severity groups and within groups
according to the presence of absence of interventional procedures is shown
in Table 6. Patients undergoing
percutaneous or surgical revascularization, considered collectively, were
younger and less likely to have diabetes, hypertension, or prior MI than those
treated more conservatively.
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Table 6. Patient Characteristics and Clinical Features Among Patients
Who Did (Yes) and Did Not (No) Undergo Interventional Procedures*
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The predicted probability of cardiac catheterization and/or PCIs by
multivariable logistic regression analysis was as follows: history of congestive
heart failure (odds ratio [OR], 0.47), systolic blood pressure less than 100
mm Hg (OR, 0.56), pulse greater than 100/min (OR, 0.62), prior MI (OR, 0.69),
age (increments of 10 years) (OR, 0.94), male sex (OR, 1.23), Killip class
I (OR, 1.54), prior PCIs (OR, 1.93), chest pain on presentation (OR, 2.19),
and transfer to NRMI-2 participating hospital (OR, 6.46). All P values were less than .001.
A multivariable logistic regression analysis including propensity score
of cardiac catheterization and PCIs was performed to determine demographic,
medical history, and clinical predictors of in-hospital death after the first
24 hours (Table 7). The observed
and predicted outcomes correlated with forward and forward stepwise regression
models with P values less than .001.
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Table 7. Multivariate Logistic Regression Analysis (Including Propensity
Score) for Predictors of In-Hospital Death (After 24 Hours)
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COMMENT
The approach to patients with acute coronary syndromes in general and
non–ST-segment elevation MI in particular is based on a clear understanding
of pathobiologic principles, results of randomized clinical trials, and decades
of clinical experience. Our findings, derived from a large, nationwide registry
that reflects routine care, underscore the great diversity of patients who
experience non–ST-segment elevation MI and the related spectrum of clinical
outcomes, ranging from early death to an uncomplicated hospital course. Yet,
despite the observed differences among patients with a common diagnosis, processes
of care and initial management strategies were similar, irrespective of in-hospital
clinical event severity and overall inherent risk. Of added concern, the administration
of time-tested and widely recommended pharmacologic therapies, including ß-adrenergic
blocking agents and aspirin, was relatively low in all clinical event severity
groups, particularly patients at greatest risk for in-hospital mortality.
CLINICAL EVENT OUTCOMES IN NON–ST-SEGMENT ELEVATION MI
The management of patients with ST-segment elevation or bundle branch
block MI has been a national health priority for nearly 2 decades. Early recognition
of symptoms, prompt transport to a medical facility, and rapid diagnosis and
treatment are the goals set forth by the National Heart Attack Alert Program,14, 15, 16, 17, 18, 19, 20
the American Heart Association, and the American College of Cardiology.21 Information derived from randomized clinical trials,
national registries, and multicenter databases has been formulated to develop
clinical scales that reliably identify patients at high22, 23
and low24, 25, 26 risk
for adverse outcomes. Risk scales and clinical event severity scores serve
as the basis for management pathways, with the ultimate goal of providing
optimal patient care.
The NRMI-2 data and validated model show convincingly that patients
with non–ST-segment elevation MI represent a heterogeneous population
and, as in patients with ST-segment elevation MI,27
increasing age, female sex, anterior site of infarction, and hemodynamic instability
are associated with a poor outcome. Although several respected groups have
reported that the risk of recurrent MI and death among patients with non–ST-segment
elevation MI is greatest between 30 days and 1 year after the initial event,7, 8, 11 the NRMI-2 experience
suggests that many patients are, in fact, at equal or even greater risk for
early events than patients who present to the hospital with ST-segment elevation.
It is clear that the recognition and aggressive management of patients with
high-risk features, regardless of ECG findings, is of paramount clinical importance.
PHARMACOLOGIC THERAPY
The findings derived from large-scale clinical trials do not support
the use of fibrinolytic therapy among patients with non–ST-segment elevation
MI.28, 29, 30 In fact,
an overview of randomized placebo-controlled trials31
leads one to conclude that fibrinolytic therapy is not beneficial and, in
fact, may be harmful in this particular clinical setting. Our analysis of
routine clinical practice in the United States revealed that nearly 10% of
patients with non–ST-segment elevation MI received fibrinolytic therapy
within 24 hours of hospitalization. Although it is tempting to speculate that
treatment was driven by concomitant high-risk features, more than 50% of patients
were in the minor severity clinical event category.
Because the pathobiology of non–ST-segment elevation MI shares
common features with other acute coronary syndromes,1, 2
the initial pharmacologic approach (with the exception of reperfusion modalities)
is nearly identical across the spectrum of potential clinical presentations.
The American Hospital Association/American College of Cardiology guidelines21 recommend that all patients receive heparin (unfractionated
or low molecular weight), aspirin, and ß-adrenergic blocking agents.
Nitrates are suggested in the care of recurrent angina, and calcium channel
blockers are reserved for those who either have a contraindication to ß-adrenergic
blocking agent use or experience persistent symptoms despite ß-blockade.
The Unstable Coronary Artery Disease Council32
has proposed a similar pharmacologic strategy.
The NRMI-2 data raise concerns regarding compliance with recommended
therapies for patients with non–ST-segment elevation MI. Just two thirds
of patients received aspirin and heparin during the initial 24 hours, and
only one third were treated with ß-adrenergic blocking agents. The suboptimal
use of standard anti-ischemic and antithrombotic therapies was particularly
evident in patients at risk for hospital death and those within the major
severity clinical event category. Regrettably, these observations are similar
to those reported previously for patients with ST-segment elevation and bundle
branch block MI.33 Although a somewhat better
performance was reported in the TIMI III (Thrombolysis in Myocardial Infarction
Phase III) registry,34 it remains clear that
patients with acute coronary syndromes all too often do not receive a full
complement of pharmacologic therapy. Although it could be argued that there
are sound, clinically based reasons for withholding certain therapies, the
dramatically different treatment rates that emerge when patients with similar
diagnoses and demographic characteristics who are enrolled in multicenter
registries are compared with those entered into clinical trials suggest that
the mere reminder served by an existing protocol has a significant impact
on patient management. This observation supports the development of on-site
clinical pathways as a mechanism to improve guideline compliance and overall
performance.
Most patients enrolled in NRMI-2 were treated at a time before the approval
of platelet glycoprotein IIb/IIIa receptor antagonists. Although the management
of acute coronary syndromes will progressively evolve, the addition of a new
therapy should not detract from the very clear message provided by the current
analyses. Because the trials of platelet glycoprotein IIb/IIIa receptor antagonists
were designed and conducted on a background of standard anti-ischemic and
antithrombotic treatment regimens, their introduction to the armamentarium
of management strategies serves to supplement not replace existing treatment.
CORONARY ANGIOGRAPHY AND INTERVENTIONAL THERAPY
There is considerable variation in the use of diagnostic coronary angiography
and interventional procedures among patients with non–ST-segment elevation
MI.
The ever-increasing rates of PCIs and surgical procedures are undoubtedly
multifactorial in origin; however, there is little question that the guidelines
for coronary angiography published in 1987 strongly influenced management.35 At that time it was recommended that all patients
(with non–Q wave MI) be considered for coronary angiography. More recently,
the guidelines have been modified and recommend that high-risk patients, defined
as those with recurring episodes of spontaneous or exercise-induced myocardial
ischemia, shock, pulmonary congestion, malignant ventricular arrhythmias,
or left ventricular dysfunction, undergo invasive testing.20, 35, 36, 37, 38, 39
Coronary angiography was performed in nearly 40% of study patients enrolled
in the NRMI-2 and resulted in revascularization procedures, irrespective of
clinical event severity category, in approximately one third of individuals.
These rates are comparable with those found within the conservative arm of
VANQUISH (Veterans Affairs Non–Q-Wave Infarction Strategies in Hospital)37 but lower than those reported in TIMI IIIB30 and GUSTO IIb (Global Utilization of Streptokinase
and Tissue Plasminogen Activator for Occluded Coronary Arteries IIb),40 3 separate studies that address the potential benefit
of an early aggressive management strategy (with or without adjunctive pharmacologic
therapy). Although it is difficult to compare studies and draw conclusions,
the findings from NRMI-2 suggest that patients with non–ST-segment elevation
MI frequently undergo invasive procedures with modest attention to anticipated
risk for subsequent clinical events.
The emergence of data that support an aggressive medical regimen for
patients with postinfarction myocardial ischemia38
coupled with our observations suggest that clinicians should consider pharmacologic
therapy with much greater resolve than is currently practiced. Two large-scale
clinical trials that are currently in the development phase, SOCRATES (Study
Of Coronary Revascularization And Therapeutic EvaluationS) and COURAGE (Clinical
Outcomes Utilization Revascularization and Aggressive druG Evaluation), will
provide important information for patient management.
STUDY LIMITATIONS
The NRMI-2 is a nationwide registry, not a randomized clinical trial.
As a result, patient outcome measures determined according to hospital-based
procedures, management strategies, and overall processes of care should be
interpreted cautiously because of the potential for inherent selection bias.
In addition, the definitions for recurrent myocardial ischemia and reinfarction
were not standardized across all participating sites, nor were they reviewed
by a separate clinical events committee. The prospective data collection design
does, however, permit a detailed overview of current hospital-based clinical
practice. Although the criteria chosen to divide patients into major, intermediate,
and minor severity clinical event categories represented investigator consensus,
previously published risk assessment tools, severity scores, and guidelines11, 21, 23, 26, 27
were used to develop the classification scheme.
CONCLUSIONS
Patients with non–ST-segment elevation MI are a heterogeneous
population who exhibit readily identifiable characteristics and clinical features
that define their potential risk for adverse outcomes, including early death.
Based on the NRMI-2 cohort of nearly 200 000 individuals, it is apparent
that a considerable proportion of patients currently do not receive even the
most fundamental (and recommended) anti-ischemic and antithrombotic medical
therapies and that the use of invasive diagnostic testing, followed often
by percutaneous and surgical revascularization procedures, is not routinely
tailored according to proposed guidelines and evidence-based risk assessment
criteria. These practices likely have an impact on clinical outcome, resource
utilization, and nationwide health care costs.
AUTHOR INFORMATION
Accepted for publication August 3, 2000.
From the Cardiovascular Thrombosis Research Center, Division of Cardiovascular
Medicine, University of Massachusetts Medical School, Worcester (Drs Becker,
Burns, Spencer, and Gore); Myocardial Infarction Triage and Intervention Coordinating
Center, Seattle, Wash (Drs Every and Maynard and Mr Frederick); and Maine
Medical Center, Portland (Dr Lambrew).
Corresponding author: Richard C. Becker, MD, Division of Cardiovascular
Medicine, University of Massachusetts Medical School, 55 Lake Ave N, Worcester,
MA 01655 (e-mail: beckerr{at}ummhc.org).
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