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Blockade after myocardial infarction: systematic review and
meta regression analysis
Nick Freemantle a Medicines Evaluation Group, Centre for Health
Economics, University of York, York YO10 5DD, b Department of Cardiology, Castle Hill Hospital, University of
Hull, Kingston upon Hull, U16 5JQ, c Department of Health Sciences and Clinical
Evaluation, University of York
Correspondence to: N Freemantle meg{at}york.ac.uk
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Abstract |
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Objectives:
To assess the effectiveness of
blockers in short term treatment for acute myocardial infarction and in longer term secondary prevention; to examine predictive factors that
may influence outcome and therefore choice of drug; and to examine the
clinical importance of the results in the light of current treatment.
Design:
Systematic review of randomised controlled trials.
Setting:
Randomised controlled trials.
Subjects:
Patients with acute or past myocardial infarction.
Intervention:
Blockers compared with control.
Main:
outcome measures All cause mortality and
non-fatal reinfarction.
Results:
Overall, 5477 of 54 234 patients (10.1%)
randomised to
blockers or control died. We identified a 23%
reduction in the odds of death in long term trials (95% confidence
interval 15% to 31%), but only a 4% reduction in the odds of death
in short term trials (
8% to 15%). Meta regression in long term
trials did not identify a significant reduction in effectiveness in
drugs with cardioselectivity but did identify a near significant trend towards decreased benefit in drugs with intrinsic sympathomimetic activity. Most evidence is available for propranolol, timolol, and
metoprolol. In long term trials, the number needed to treat for 2 years
to avoid a death is 42, which compares favourably with other treatments
for patients with acute or past myocardial infarction.
Conclusions:
Blockers are effective in long term
secondary prevention after myocardial infarction, but they are
underused in such cases and lead to avoidable mortality and morbidity.
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Key messages
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Introduction |
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Blockade was once heralded as a major advance in the treatment
of patients with myocardial infarction, but current evidence suggests
that less than half of eligible patients receive it.1-3 The effectiveness of
blockers was appraised by Yusuf et al in 1985,4 but since then there have been nearly 3000 deaths
among 23 000 patients randomised in new trials. Trials of
blockers now include a broader group of patients such as those at high risk or
with accompanying heart failure, enabling the benefits identified by
Yusuf et al4 in a restricted group of trials to be
extended to such patients.
Methods used in systematic reviews have also advanced. The development
of regression techniques within meta analysis enables a more robust
examination of the importance of factors that may mediate upon the
effectiveness of specific drugs.5 Two such factors,
intrinsic sympathomimetic activity and cardioselectivity, were
identified as potentially important,4 and intrinsic
sympathomimetic activity in particular seemed to be related to reduced
therapeutic action. Given the changing use of drugs after myocardial
infarction, the early promise of
blockade in these patients, and
the continuing high rates of mortality associated with myocardial
infarction, a new overview of these drugs is timely.
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Methods |
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Objective
We reappraised the effectiveness of
blockers for secondary
prevention after myocardial infarction. Our main outcome was all cause
mortality and the secondary outcomes were non-fatal reinfarction and
withdrawal from treatment. We examined the effectiveness of
blockers in the acute phase immediately after myocardial infarction;
their role in longer term secondary prevention; the importance of early
initiation after the onset of symptoms; the extent to which specific
pharmacological features of different
blockers may affect their
performance; the magnitude of benefits achieved by
blockers; and
the clinical importance of
blockers.
Inclusion criteria
We included randomised trials without crossover, with treatment
lasting more than one day, and with follow up that examined the
clinical effectiveness of
blockers versus placebo or alternative
treatment in patients who had had a myocardial infarction. Treatment
may have begun at any stage before or after myocardial infarction and
may have been commenced intravenously.
Search strategy
We conducted sensitive electronic searches of Medline (1966-97 through Ovid), Embase (1974-97 through Dialog), Biosis (1985-97 through
Edina), Healthstar (1975-97 through Ovid), Sigle (1980-97 through
Blaise-line), IHTA (1990-97 through ECRInet), conference papers index
(1984-97 through Dialog), Derwent drug file (1992-97 through Dialog),
dissertation abstracts (1992-97 through Dialog), Pascal (1992-97 through Dialog), international pharmaceutical abstracts (1992-97 through Dialog), and science citation index (1981-97 through BIDS).
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Data abstraction and appraisal of study quality
From each study we abstracted data on the total number of patients
randomised to active treatment or control,
blocker, route and dose
of drug, duration of treatment, loss to follow up, level of blinding,
concealment of allocation,6 specific study inclusion and
exclusion criteria, duration of follow up, deaths, reinfarctions, and
withdrawals. Data were checked by a second researcher.
Statistical analysis
We estimated pooled odds ratios for short and long term treatment
trials separately using the fixed effects approach of Mantel
Haenszel.
7 8
As we anticipated systematic differences
between the results of studies (heterogeneity), we also routinely
estimated random effects pooled odds ratios. Standard random effects
methods for meta-analysis (pooling the results of
studies)
9 10
may provide unduly precise estimates of
effect, as they assume that the observed distribution of effects is the true treatment distribution
an assumption that may not be valid in
sparse data.
5 11 12
Therefore, we used the full random effects approach on the basis of the numerical integration techniques using Markov chain Monte Carlo simulation, with appropriate uninformative priors and the "Bugs" software described by Smith et
al.5 This provides a more robust estimate of the precision of random effects estimates and can account for trial groups that experience no events without resorting to crude fixes such as adding a
value to each cell to estimate an individual odds ratio. A further
advantage of this approach is that the effects of predictive factors
may be examined. Our main treatment related covariates were
cardioselectivity and intrinsic sympathomimetic activity, which were
examined in the long term trials using a nested random effects logistic
regression model (see Appendix ).
blocker using the fixed effects
model.
7 8
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Results |
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We identified 82 randomised trials that examined the effects of
blockers compared with control and that had data on all cause
mortality. Overall, 5477 of 54 234 patients (10.1%) randomised died.
Fifty one trials examined acute treatment with
blockers
up to 6 weeks after onset of pain (table 1, and 31 trials examined long term
treatment with
blockers
6 to 48 months (table
2).
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Short term trials
Overall, 3062 of 29 260 patients (10.5%) randomised in short
term trials died. Although 51 trials were identified that examined the
effects of short term treatment, only 45 of these had observed deaths
in either the intervention or control groups. The major challenge to
the quality of this group of trials was that small numbers of patients
randomised to treatment or control led to many trials with either no,
or only a small number of, deaths.
0.2 to 1) as 250 patients would require treatment to
avoid one death (100 to
). Analysis of predicted benefit by drug
identified no individual drug that differed significantly in effect
from the pooled result.
Although most trials were undertaken before the second international
study of infarct survival in 198816 firmly established the
importance of thrombolysis, a large trial of thrombolysis in myocardial
infarction in 198917 randomised patients who had received
recombinant tissue plasminogen activator within 4 hours of the onset of
pain to early metoprolol or control. During 5 days of follow up, there
was no difference in mortality between the two groups. Two subsequent
myocardial infarctions were, however, avoided for every 100 patients
treated (0.2 to 4).
Long term trials
Overall effects
Overall, 2415 of 24 974 patients (9.7%) randomised in the 31 long term trials died. In general, the quality of studies was
reasonably high, with adequate follow up achieved in many trials (table
2), though the proxy quality variable, concealment of allocation, was
seldom adequately reported.
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blockers after
myocardial infarction (0.6 to 1.7); that is, about 84 patients will
require treatment for 1 year to avoid one death. A similar approach was
used to estimate the effects of treatment on reinfarction, although
only 21 of the 34 comparisons provided data on reinfarction, resulting
in wider confidence intervals and the potential for reporting bias.
This analysis suggests an annual reduction in reinfarction of 0.9 events in every 100 (0.3 to 1.6); that is, about 107 patients would
require treatment for 1 year to avoid one non-fatal reinfarction.
Predictors of benefit
Initial intravenous dose
We investigated the extent to
which initiation of treatment with an intravenous dose of
blockers
predicted mortality in the long term trials. Applying initial
intravenous dose as a covariate term in the analysis suggested no
additional benefit among patients treated in this manner (odds ratio
0.87, 0.61 to 1.22). Equally, this analysis indicates that there is no
reason to delay treatment with a
blocker and that early initiation will lead to a greater period when benefits may be accrued from treatment.
We anticipated that the presence of
cardioselectivity and intrinsic sympathomimetic activity would be
important predictors of benefit in the trials, a hypothesis examined by
Yusuf et al.4
Classification of
blockers into those with or without important
cardioselective activity or intrinsic sympathomimetic effect is not
clear cut, and there is some debate in the literature on the attributes
of acebutolol in particular.18-20 Table 3 describes the
attributes of
blockers used in the trials.
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We investigated whether
benefits were reduced in the trials with additional therapeutic options for treatment introduced, in particular the increasing use of thrombolytic treatment, and aspirin. There is no evidence that treatment in trials after 1982 (the median trial) led to differences in
benefit (odds ratio 1.04, 0.82 to 1.28).
Choice of drug
Individually, only four drugs achieved a
statistically significant reduction in the odds of death: propranolol (0.71, 0.59 to 0.85); timolol (0.59, 0.46 to 0.77); metoprolol (0.80, 0.66 to 0.96); and acebutolol (0.49, 0.25 to 0.93). The effectiveness
of acebutolol is supported by a single moderately sized study, which is
open to considerable measurement error. However, trials including
propranolol, timolol, and metoprolol include 63% of the available
evidence on the effects of long term
blockade in patients who have
had a myocardial infarction.
Withdrawal from treatment
Different definitions and reporting made comparison of withdrawal
of treatment withdrawal between trials problematic. Similar withdrawal
rates between active treatment and placebo groups concealed two
opposing effects: more patients are withdrawn from treatment groups
because of suspected adverse cardiovascular reactions (most commonly
brachycardia and hypotension), whereas in the placebo group withdrawal
is more common because of the need for
blockade for hypertension
and angina. Trials reports of dizziness, depression, cold extremities,
and fatigue were only marginally more common in the treatment than
control groups.
blockers with different
cardioselectivity or intrinsic sympathomimetic activity.
Overall, 5151 of 21 954 patients (23.5%) withdrew from treatment
(table 2). Overall, withdrawal was slightly more common in patients
taking
blockers
the difference in the annualised rate of
withdrawal compared with placebo being 1.16 in 100 patients treated
(0.56 to 1.76, random effects; fig 3). No clinically important differences in withdrawal were observed between
blockers of differing cardioselectivity and intrinsic
sympathomimeticity.
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Discussion |
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Considerable evidence supports the routine long term use of
blockers in patients who have had a myocardial infarction, with substantial benefits in terms of reduced mortality and morbidity. Short
term
blockade immediately after acute myocardial infarction seems
unlikely to be of major benefit unless treatment is continued long
term. This finding contradicts recent suggestions that
blockers
should be more commonly used intravenously in acute myocardial infarction.21 In fact, evidence strongly indicates that
unless
blockers are continued long term, the benefits suggested by Owen21 will not be observed.
The benefits of
blockers on all cause mortality are impressive when
compared with other frequently used long term treatments for the same
patient group. Table 4 shows the effects of different drugs on the
number of patients that would need to be treated for 2 years to avoid
one death
for example, after a myocardial infarction 42 patients would
need to be treated with
blockers whereas 292 patients would need to
be treated with antiplatelets.22 The number and length of
long term trials showing a consistent benefit for
blockers in
unselected patients after myocardial infarction suggest lasting
benefits in this comparatively high risk group, and suggest that
blockers should be continued indefinitely.
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Have benefits from
blockade declined with availability of new
treatments?
Our finding that
blockers benefit a broader group of patients
after myocardial infarction supports the findings of the
blocker
pooling project.23 Our finding also agrees with those of
the cooperative cardiovascular project, which examined the medical
records of 201 752 patients who had had a myocardial infarction.24 In that study, mortality was lower in every
subgroup of patients treated with
blockade than in untreated
patients. The findings of the cooperative cardiovascular project agree
with our meta regression analysis, which found no evidence of a
reduction in benefits from
blockade in more recent randomised
trials. Indeed, rather than being overtaken by newer treatments,
blockers have a comparatively large effect in reducing mortality (table 4).
Which
blocker?
Cardioselectivity was associated with a non-significant trend
towards reduced benefit. The presence of an intrinsic sympathomimetic effect predicted a near significant reduction in benefits and thus
drugs with this characteristic should be avoided. We found evidence to
support the long term use of propranolol and timolol, the only two
drugs indicated for prophylaxis after myocardial infarction in the
British National Formulary. The use of either drug led to a
substantial reduction in the odds of death, with narrow confidence
intervals (fig 2). In contrast, atenolol, which is commonly prescribed
in secondary prevention, has been inadequately evaluated in this
setting. Although similar efficacy may be achieved
we found no
evidence that all
blockers are not equal
it cannot be presumed
that the benefits from propranolol, timolol, and metoprolol will be
achieved with other drugs.
Have benefits from intravenous
blockers declined over time?
It may be hypothesised that intervention with thrombolytic drugs
and antiplatelets reduces the potential for patients to benefit from
intravenous
blockade. The first international study of infarct
survival25 was completed before the results of the second
international study16 became available, and before the use
of thrombolytic and antiplatelet treatment was established. In
contrast, the comparison of early versus delayed
blockade in a
large trial of thrombolysis in myocardial infarction was undertaken in
patients who all received thrombolytic and antiplatelet treatment.17 Although the much larger first international
study of infarct survival trial25 achieved a slightly
larger reduction in the odds of death with
blockers, measurement
error could not be excluded as an explanation for this difference, as
indicated by the test for heterogeneity between the trials (Q=0.025,
df=1, P=0.87). The thrombolysis in myocardial infarction trial did
suggest that early use of intravenous
blockers could reduce the
early risk of serious arrhythmias.
Are
blockers underused?
Concern has been voiced that
blockers are used in less than
half of eligible patients after myocardial infarction,1-3 despite substantial benefits and generally low treatment costs. Concern
that side effects affect the usefulness of
blockers must be
tempered by the low yearly withdrawal from
blockers in the long
term trials we reviewed. The clinical implications of our results are
clear. New is not necessarily better, especially if the aim is to
reduce mortality in patients after myocardial infarction. Furthermore,
the underuse of
blockers in this group leads to a rate of avoidable
death that should not be considered acceptable among those keen to
practice evidence based medicine.
blockers, particularly in patients with heart
failure,26-28 may lead to substantial benefits for a
broad range of patients.
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Acknowledgments |
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We thank Andrew Herxheimer, who assisted in the categorisation of included compounds, and Anne Burton for her diligent help in locating studies and in the preparation of the manuscript.
Contributors: NF developed the protocol for the review, abstracted data, and undertook the majority of statistical analyses. JC conceptualised the review, developed the protocol, and provided clinical interpretation of the included trials and the results. PY developed the meta regression approach and provided methodological support in the review. JM contributed to the development of the protocol, data abstraction, and some of the statistical analyses. JH designed and implemented the electronic search strategies. NF and JC will act as guarantors for the paper.
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Footnotes |
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Funding: SmithKline Beecham Pharmaceuticals UK. The views expressed are those of the authors and not necessarily those of the sponsor.
Competing interests: This study was funded through an unrestricted educational grant by SmithKline Beecham, who supply carvedilol in the United States. JGFC has spoken at many meetings and educational programmes on drugs in heart failure, organised by pharmaceutical and device companies, and received fees and expenses. He has also received research funding from industry as well as the NHS, British Heart Foundation, and US Veterans Administration.
website extra: References for the trials appear on the BMJ's website www.bmj.com
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Appendix |
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Statistical model for random effects regression analysis
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is
a constant,
describes the overall treatment effect,
describes
the effect of intrinsic sympathomimetic activity, and
describes the
effect of cardioselectivity.
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(Accepted 6 April 1999)
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