The dynamics of blood pressure in populations and hypertensive cohorts.

Two sets of data, derived from the WHO Cooperative Hypertension Community Control Project and concerned with spontaneous changes of blood pressure over a period of five years, are described. The first deals with the community as a whole, studied through the examination of two independent random samples of the same population made five years apart. The second pertains to cohorts of hypertensive subjects included in the hypertension registers from various centres participating in the WHO programme and followed up for four years.The population blood pressure distribution showed a clear shift towards lower levels for both systolic and diastolic values. The mean changes, however, were smaller than 5 mmHg. In the subjects initially labelled as "hypertensive", the effects of "regression to the mean" were apparent both in the short-term evaluation (five months) and, more importantly, after four years of follow-up. In the latter case, the decreases were more remarkable in the first year but continued to show until the third year.These findings suggest that a "controlled" design is necessary not only in prospective clinical trials but also in community projects where the effects of an intervention on blood pressure are to be evaluated.