(Circulation. 2000;102:2031.)
© 2000 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Department of Medicine (D.A.M., E.M.A., J.A.d.L., R.P.G., C.H.M., E.B.), Brigham and Womens Hospital, Boston, Mass; Nottingham Clinical Research (A.C., R.C.), Nottingham, UK; and the Department of Medicine (S.A.M.), University of California, San Francisco.
Correspondence to David A. Morrow, MD, Cardiovascular Division, Brigham and Womens Hospital, 75 Francis St, Boston, MA 02115. E-mail damorrow{at}bics.bwh.harvard.edu
| Abstract |
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Methods and ResultsWe developed and evaluated a convenient bedside clinical risk score for predicting 30-day mortality at presentation of fibrinolytic-eligible patients with STEMI. The Thrombolysis in Myocardial Infarction (TIMI) risk score for STEMI was created as the simple arithmetic sum of independent predictors of mortality weighted according to the adjusted odds ratios from logistic regression analysis in the Intravenous nPA for Treatment of Infarcting Myocardium Early II trial (n=14 114). Mean 30-day mortality was 6.7%. Ten baseline variables, accounting for 97% of the predictive capacity of the multivariate model, constituted the TIMI risk score. The risk score showed a >40-fold graded increase in mortality, with scores ranging from 0 to >8 (P<0.0001); mortality was <1% among patients with a score of 0. The prognostic discriminatory capacity of the TIMI risk score was comparable to the full multivariable model (c statistic 0.779 versus 0.784). The prognostic performance of the risk score was stable over multiple time points (1 to 365 days). External validation in the TIMI 9 trial showed similar prognostic capacity (c statistic 0.746).
ConclusionsThe TIMI risk score for STEMI captures the majority of prognostic information offered by a full logistic regression model but is more readily used at the bedside. This risk assessment tool is likely to be clinically useful in the triage and management of fibrinolytic-eligible patients with STEMI.
Key Words: coronary disease prognosis myocardial infarction mortality risk factors
| Introduction |
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Sophisticated multivariable models developed for the prediction of mortality among patients with STEMI identify independent clinical predictors and quantify their relative contribution to mortality risk.6 Although such models offer important insight into the relationships between clinical data and prognosis, they are not readily applied in routine clinical practice. Therefore, we developed a clinical risk score that can be calculated easily at the bedside but is derived from a comprehensive multivariable analysis in a well-characterized population of nearly 15 000 patients with STEMI from the Intravenous nPA for Treatment of Infarcting Myocardium Early II (InTIME II) trial. The prognostic performance of the Thrombolysis in Myocardial Infarction (TIMI) risk score for STEMI was then compared with that of other risk models and validated in an external data set composed of nearly 3700 patients with STEMI.
| Methods |
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18 years
and exhibited chest pain and ST elevation or left bundle branch block
on the qualifying ECG. Exclusion criteria included any history of
cerebrovascular disease, a systolic blood pressure of
>180 mm Hg, a diastolic blood pressure of >110,
cardiogenic shock, or increased risk of severe bleeding. The InTIME II
protocol was approved by the institutional committee on human research
at each of the participating centers.
Clinical End Points
Vital status was assessed through 30 days and every 6 months
until trial completion. The primary end point of the trial was death
from any cause within 30 days of randomization. Mortality data after
discharge were obtained through telephone follow-up or outpatient
visitation.
Statistical Analysis
Performance of the multivariate
analysis and derivation of the risk score were based on
patients with complete baseline data (93.7%), with subsequent
reevaluation in the full population. Univariate
relationships between baseline characteristics and 30-day mortality
were assessed by logistic regression analysis. Thresholds for
categorization of continuous variables were determined graphically
and were based on prevalence in the population. Independent predictors
of 30-day mortality were identified by stepwise logistic regression.
All baseline variables entered the initial model and were
maintained if P<0.05.
Selection of independent predictors for inclusion in the TIMI risk
score for STEMI was based on their relative prognostic contribution in
the full logistic regression model. Variables were ranked by
z score, and those with the least contribution were
sequentially removed from the model until reaching 10 variables
that captured 97% of the overall prognostic information from the full
multivariate model (evaluated as a ratio of the global
2 statistic from the reduced compared with
full model). For each patient, the TIMI risk score for STEMI was
calculated as the simple arithmetic sum of point values assigned to
each risk factor based on the multivariate-adjusted
risk relationship: 1 point for odds ratio (OR) 1.0 to <2, 2 points for
OR 2.0 to 2.5, and 3 points for OR >2.5. Age was weighted in 2 strata,
with 2 points for an age range of 65 to 74 years and 3 points for ages
75 years. The 3 historical variables that remained in the model
(diabetes, history of angina, and history of hypertension) had risk
relationships of similar magnitude and were combined to form a single
composite variable.
The discriminatory capacity of the risk score was assessed by using the area under the receiver operating characteristic curve (c statistic) as an index of model performance.7 The c statistic reflects the concordance of predictions with actual outcomes in rank order, with a c statistic of 1.0 indicating perfect discrimination.7 The prognostic performance of the TIMI risk score was compared with the full multivariable model as well as 2 previously described risk models.6 8 The reliability of risk score prediction was also evaluated by comparing the observed mortality rates with those predicted by the risk score across deciles of risk established by dividing patients according to predicted mortality from the multivariate model and then determining the actual mortality for each group.6 Risk score categories were collapsed (eg, >8) when the prevalence of a given score was <1%. For evaluation of the risk score in the full population, missing variables contributed a point value of 0 to the total score. A value of P<0.05 was considered significant. Analyses were performed by use of S-PLUS (version 3.4, MathSoft) and SAS (version 6.12, SAS Institute).
Validation Set
The TIMI Risk Score for STEMI was assessed in an external data
set from the TIMI 9 trial. TIMI 9A and 9B were multicenter randomized
trials evaluating the safety and efficacy of hirudin as an adjunct to
fibrinolytic therapy (tissue plasminogen
activator or streptokinase at the physicians
discretion).9 10 The combined database included 3687
patients with vital status established at 30 days.
| Results |
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Predictors of Mortality
Each of the baseline clinical characteristics was evaluated as a
univariate predictor of mortality (Table 1
). When
all of the candidate variables were assessed
simultaneously in multivariate
analysis, 16 remained significant predictors of mortality
(Figure 1
). Assessed by the area under
the receiver operating characteristic curve (concordance of the
predictions with actual outcomes), the full 16-variable regression
model demonstrated a strong discriminatory capacity (c statistic
0.784). Ten characteristics accounted for 97% of the predictive
capacity of the multivariate model and were selected
for inclusion in the TIMI risk score for STEMI (Figure 1
), with
the 3 historical characteristics (diabetes, history of hypertension,
and prior angina) subsequently grouped as a composite variable
(adjusted OR 1.6, 95% CI 1.4 to 1.9).
|
TIMI Risk Score for STEMI
The TIMI risk score for STEMI (Figure 2
) showed a strong association with
mortality at 30 days, with a >40-fold graded increase in mortality
between those with a risk score of 0 and those with a score >8
(P(trend)<0.0001). At the high end, a
score >5 identified 12% of patients with a mortality risk >2-fold
higher than the mean for the population. In contrast, the 12% of
patients with a risk score of 0 had a mortality rate <1%.
Discriminating among the lower risk groups, nearly two thirds of the
population had risk scores of 0 to 3, with a 5.3-fold gradient in
mortality over this range (P<0.0001, Figure 2
).
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The TIMI risk score demonstrated a strong predictive capacity, comparable to the full multivariable model (c statistic 0.779 versus 0.784). The reliability of the TIMI risk score predictions were assessed by comparison with the observed mortality rates across the population divided into deciles of risk. Excellent concordance of the risk score predictions with observed mortality rates was evident (correlation coefficient 0.994).
Comparison With Other Models
To evaluate the TIMI risk score in the context of previously
developed models, we tested the performance of the
logistic regression equation developed in the Global Utilization of
Streptokinase and t-PA for Occluded Arteries (GUSTO)-I
trial6 as well as an unweighted risk score derived in the
TIMI 2 trial8 in the InTIME II data set. The TIMI risk
score offered prognostic capacity comparable to both the
multivariable model from GUSTO-I (c statistic 0.803) and the risk
score from TIMI 2 (c statistic 0.753).
Predictive Consistency and Validation
The prognostic capacity of the TIMI risk score was stable over
multiple time points, ranging from 24 hours to 365 days after
presentation (Table 2
).
Furthermore, the discriminatory capacity of the model remained good for
prediction of 1-year mortality among 30-day survivors (c statistic
0.725, Figure 3
). Notably, the proportion
of deaths occurring by 30 days increased with ascending TIMI risk
score, ranging from 44% among those with a score of 0 to 77% for
those with risk scores >8
(P(trend)<0.0001).
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The risk score was predictive of 30-day mortality among important
subgroups, such as men and women and smokers and nonsmokers (Table 3
), with a similar graded relationship
between the risk score and mortality across each of these subgroups.
The model was also evaluated in the full 15 060 patient population
(including patients with missing risk score variables) without
substantial change from the derivation set (c statistic 0.776, Table 3
).
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The risk score was also strongly associated with 30-day mortality in an
external population of patients treated with fibrinolytics for STEMI
(Figure 4
). Application of the TIMI risk
score for STEMI in the TIMI 9A/B population revealed a similar nearly
40-fold gradient in mortality risk. Mortality was again <1% among
patients with a risk score of 0. In addition, a high discriminatory
capacity of the TIMI risk score was evident in this external validation
set (c statistic 0.746).
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Application as an Epidemiological Tool
To illustrate the utility of the TIMI risk score for STEMI
in adjusting for baseline risk profile, we performed an
analysis of regional revascularization
practice patterns among patients treated with fibrinolytics in the
InTIME II study. For the purpose of this example, we used the risk
score as a framework to stratify revascularization
rates (coronary artery bypass grafting or
percutaneous intervention) in the US and non-US sites
participating in the InTIME II study. A consistent pattern of
increased utilization of revascularization
procedures in the United States was evident across all groups as
stratified by risk score (Figure 5
).
Furthermore, a pattern of decreasing frequency of
revascularization among the highest risk patients
was apparent among US and non-US centers.
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| Discussion |
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Effective risk stratification is integral to the management of patients with acute coronary syndromes.11 Even among patients with STEMI, for whom initial therapeutic options are well-defined, patient risk characteristics have an impact on early therapeutic decision making.1 12 13 In addition, increasing economic pressures have intensified the need for appropriate triage and clinical resource utilization, including decisions regarding transfer to tertiary centers.14 In particular, the capacity to reliably identify patients at very low risk for fatal recurrent events may offer the opportunity to select low-risk patients for early hospital discharge.15 16 Tools that enhance the clinicians ability to rapidly and accurately assess risk are thus of substantial interest.
Risk Modeling in STEMI
Carefully performed multivariable analysis for
mortality prediction in the GUSTO-I trial provided significant
information regarding demographic, clinical, and historical factors
that offer independent prognostic information among
fibrinolytic-eligible patients with STEMI.6 The complex
(>20-term) model produced in the GUSTO-I analysis allowed for
the interplay of risk markers, including nonlinear relationships and
interaction between variables. The risk estimates offered by the
GUSTO-I and other multivariable models for mortality in STEMI were
highly accurate in their derivation data sets but required a computer
for calculation.6 17
Investigators have developed a number of simplified risk stratification schemes, which may be calculated at the bedside without the aid of a computer.4 8 18 Several of these models were developed before the widespread use of throm-bolysis.18 19 20 Of those derived in the era of reperfusion, several were formed by using general measures of severity of illness, such as the Acute Physiology and Chronic Health Evaluation II scoring system,21 whereas others were based on expert opinion and prior investigation.8 Models that have integrated weighting information from multivariate regression in a fashion similar to the TIMI risk score are few and have not been derived for prediction of short-term outcomes in STEMI.4
TIMI Risk Score for STEMI
The clinical data included in the TIMI risk score for STEMI
are all routinely collected at hospital presentation.
Consistent with prior observations,2 6 22 all of
the variables included in this model were independent predictors of
30-day mortality in the InTIME II population. Notably, the finding of
an association between low body weight and increased mortality risk
reported by others3 6 was again observed to be
significant. When used in combination with a simple integer-weighting
system, these basic risk factors constitute a robust risk scoring
scheme that can be calculated at the bedside by any care provider with
the aid of a simple score card (Figure 6
). The TIMI risk score for STEMI
reliably identifies patients at very high risk while maintaining good
discriminatory capacity in the low-risk range, where smaller absolute
differences are more likely to impact clinical decisions.
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Used as an epidemiologic tool, the TIMI risk score provides a convenient mechanism to identify baseline differences in risk profile and offers a framework for analyses stratified by risk group at presentation. In our illustrative example, stratification of revascularization rates by risk score effectively demonstrated differences in regional practice among similar risk patients. In addition, this approach highlighted the disproportionate number of lower-risk patients undergoing revascularization and the need to evaluate whether interventions that might improve outcomes are being performed less frequently for the highest-risk patients, who may derive the greatest benefit.13 23
The TIMI risk score for STEMI may also be used in designing clinical trials. By eliminating patients with low risk scores, a population with higher event rates can be identified. This strategy permits testing for a relative treatment effect with a smaller sample size for the trial.
Study Limitations
Development of any useful prediction model must balance
accuracy and complexity. Higher discriminatory capacity in the
derivation data set may come at the cost of both reduced
generalizability and increased complexity, which hinder practical
application. Although a full regression equation using ß coefficients
from multivariable analysis offers the highest index of
predictive discrimination, it does not meet our objective for easy
bedside application. In contrast, a highly simplified model may be more
easily generalized but yields less discriminatory power. Thus, we
proceeded with the extensive evaluation of an intermediate model.
Although we recognize the loss of some information in the reduction of
the number of variables and categorization of continuous
variables, the impact on the predictive performance of the
TIMI risk score was shown to be small.
The TIMI risk score was derived and validated among fibrinolytic-eligible patients enrolled in clinical trials. It is recognized that patients ineligible for thrombolysis or excluded from clinical trials may be at higher risk for adverse outcomes.24 25 The absolute quantitative observations made in the present report may not apply to other populations. Nevertheless, the strong consistency between the major risk markers that emerged in our present analysis and those identified in registries outside of clinical trials2 3 5 26 suggest that the risk relationships are likely to be similar.
Finally, the TIMI risk score for STEMI is designed for risk assessment early after patient presentation and thus does not incorporate noninvasive and invasive data, including provocative testing for ischemia, evaluation of left ventricular function, and coronary angiography. Furthermore, other important early prognostic indicators, such as cardiac biomarkers and ST-segment resolution, were not included in this analysis. The interaction of the TIMI risk score with these prognostic measures may be an area of interest for future investigation.
Conclusions
Building from clinical variables identified as
independent risk markers in InTIME II, we have developed a convenient
clinical risk score for predicting mortality among patients with STEMI.
The TIMI risk score for STEMI may be readily applied at the bedside at
the time of hospital presentation and captures the majority
of prognostic information offered by a full logistic regression model.
This risk assessment tool is likely to be clinically useful in the
triage and management of patients eligible for fibrinolytic therapy and
may also serve as a valuable aid in clinical research.
| Acknowledgments |
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Received April 27, 2000; revision received June 6, 2000; accepted June 7, 2000.
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