Data from large-scale observational studies clearly show that blood pressure levels are positively and continuously related to the risk of stroke and coronary heart disease. In addition, within the usual range of both systolic and diastolic blood pressures, there is no evidence of a lower threshold below which the risk of stroke and coronary heart disease do not continue to decline.

By the mid-1990’s, it was clear from a series of randomised trials that lowering blood pressure in hypertensives had beneficial effects on cardiovascular morbidity and mortality. The combined data from a series of 17 trials, based mainly on diuretics and beta-blockers and including over 47,000 patients, demonstrated reductions in the risk of stroke and coronary heart disease of about 38% and 16% respectively (see Figure).

Collins & MacMahon; Br Med Bull 1994;50:272-98

There were few data about the effects of newer classes of antihypertensive drug such as ACE inhibitors and calcium antagonists and it was not clear whether these drugs would have similar effects on cardiovascular outcomes to those of conventional blood pressure lowering regimens.

There was no evidence about how far blood pressure should be lowered, and specifically whether there were additional benefits of more intensive blood pressure lowering strategies.

Only a few trials had directly compared the effects on cardiovascular outcomes of blood pressure lowering treatments based on different drug classes, and the data were insufficient to detect the most plausible (i.e., small to moderate) differences between the regimens.