Objective: The primary aim of the WHO-ISH Blood Pressure Treatment Trialists' Collaboration is to conduct three prospectively planned overviews (meta-analyses) of recently completed or ongoing randomized trials of blood pressure lowering drugs. These overviews should provide reliable data about the effects of several newer classes of blood pressure lowering drugs on major causes of cardiovascular mortality and morbidity in a variety of patient groups.
Methods: A registry of major ongoing or planned randomised trials (with more than 1,000 patient years of follow-up in each randomised group) of blood pressure lowering agents was established in 1995. The principal investigators of all eligible studies have been invited to collaborate in the project and to provide, upon completion of the study, a limited dataset for inclusion in the overview analyses. The principal comparisons will be of (1) newer versus older classes of blood pressure lowering drugs in patients with hypertension; (2) more versus less intensive blood pressure lowering therapy in patients with hypertension; and (3) newer blood pressure lowering drugs versus placebo in a variety of other patient groups at high risk of cardiovascular events. Separate analyses will be conducted for the main drug classes and for major patient subgroups. The principal study outcomes are stroke, major coronary heart disease events, heart failure, total cardiovascular deaths, total cardiovascular events and total mortality.
Results: A total of 37 trials of blood pressure lowering treatments were identified as potentially eligible for inclusion in this project and agreement to collaborate has been confirmed by the investigators from 36 trials. The first round of analyses was conducted in 2000 and on outcome data from a total of about 60,000 patients, involving more than 250,000 patient-years of follow-up. The second round of analyses will be conducted in 2003 on data from more than 200,000 patients, among whom it is estimated that a total of 20,000 major cardiovascular events will have occurred. This should provide good statistical power to detect even modest cause-specific differences in each of the main study outcomes.
Conclusions: The combination of trial results in these prospectively planned, systematic overviews should provide more reliable information about the effects of newer blood pressure lowering drugs than will any one study alone. The eventual use of individual patient data in the analyses will facilitate investigation of the separate effects of different drug regimens in major patient subgroups defined by characteristics such as age, gender, blood pressure, diabetes and history of renal disease, coronary heart disease or cerebrovascular disease.
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