ACE inhibition after myocardial infarction: can megatrials provide answers?

International trials and national practice: a questionnaire survey of current physician practice in the treatment of acute myocardial infarction

To establish current national clinical practice in the care of patients with acute myocardial infarction (AMI), a questionnaire survey of 50 consultant physicians currently working in the Republic of Ireland was carried out. There were 45 (90%) respondents. 32/45 (71%) give thrombolysis in CCU only; 13/45 (29%) give thrombolysis in casualty also. Streptokinase (Stk) is the first choice thrombolytic agent for the majority. 14/45 (31%) use tPA for anterior AMI in patients under 60 years. Angiotensin converting enzyme (ACE) inhibitors are given by 34/45 (76%) to patients with evidence of left ventricular dysfunction. ACE inhibitors are neither used routinely nor are they prescribed in the first three days after the AMI by the majority of the physicians surveyed. Serum magnesium is checked routinely by 5/45 (11%) and intravenous magnesium is given routinely by 5/45 (11%). The percentage of AMI patients considered for angiography varied from 10-50%. Despite reports from randomised, controlled trials showing reduced mortality in patients given tPA (versus Stk), routine early ACE inhibition and intravenous magnesium post-AMI, most clinicians in Ireland use streptokinase, selective late ACE inhibition and no magnesium. The reasons for the dichotomy between the favourable results of randomised clinical trials and routine practice are speculative.

Mega-trials and management of acute myocardial infarction

Clinical management of acute myocardial infarction has been strongly influenced by large, simple trials (mega-trials) with unrestrictive protocols and limited data collection. The design has been adopted to increase statistical power to a maximum. Its validity rests on an effective randomisation procedure and intention-to-treat analysis of deaths. Experience has shown that mega-trials tend to generate effect-estimates nearer the null than those from conventional trials or meta-analyses. When a small or absent observed treatment effect (or subgroup effect) in a mega-trial contrasts with the results of conventionally designed trials, it is necessary to assess both null bias and failure to increase the true treatment effect to a maximum in the mega-trial. Null bias will arise when the contrast between treatment and no-treatment, or between subgroups, is blunted either by non-protocol therapy or by inaccuracy of data, including misclassification between subgroups. Each is more likely with an unrestrictive design. To increase the true treatment effect to a maximum, trial conditions must be specified with insight into mechanism, dose-dependence, and time-dependence. The mega-trial design is therefore unsuited to an exploratory role. These issues are illustrated by the examples of nitrates, angiotensin-converting-enzyme inhibitors, and magnesium in acute myocardial infarction but have general relevance to the validity and generalisability of simple trials.