There is no evidence that average blood pressure in the whole of the general population has risen over the seventy or so years over which measurements are available. There is some indirect evidence that it may be lowered and may still be falling quite independently of medical treatment. Whatever the definition of high blood pressure used, the proportion of people with high blood pressure is directly related to average pressure throughout the population.
Standardized and accurate measurements of blood pressure for large representative populations have only been available since the 1950s in the United Kingdom, Scandinavia and the United States and more recently for other countries. There is some evidence from countries whose general diet has shifted from very high to much lower salt intakes – specifically Japan, Belgium and Portugal – that average blood pressure in the general population has fallen, probably for this reason. These reductions in salt intake reflect shifts in methods of food preservation from the traditional methods of salting, pickling and smoking to the modern methods of refrigeration and rapid transport of fresh food. As these changes have occurred in most economically developed countries, average blood pressure has probably fallen everywhere, compared with average levels in the 19th century.
Research in the USA has always shown higher average blood pressures in poorer people. Although research in this country in the 1950s and 1960s failed to show any systematic difference between social groups, this seemed to have changed by the 1980s, with higher average blood pressure in poorer populations throughout the UK. These differences in average blood pressures are not too large, but differences in other risk factors for heart disease show the same trend. Although there is some evidence that coronary heart disease became common after World War I and that it started among rich people, ever since the 1950s it has more and more become a disease for poorer people. This is partly because of the difference in smoking habits, but some of the changes to the pattern may now be caused by these variations in average blood pressure and perhaps by access to accurate diagnosis and effective treatment.
It should be noted that blood pressure also varies between races and different sorts of societies, but why it does is not an easy question to answer. A number of different factors have been considered. High blood pressure is a serious problem throughout black Africa and black people in North America have much higher blood pressure and stroke problems than Caucasians. However, the differences between black and white North Americans disappear if differences in income are taken into consideration. In the 1960s and 1970s, comparisons between British citizens of Afro-Caribbean and European descent showed no differences, but more recent studies have begun to show the same differences an in the USA.
These differences are evident in the social aspect, rather than racial and social differences in average blood pressure seem to depend mainly on income differences. When North American incomes were much more polarized than British incomes, social differences in the average blood pressure were much higher in the USA than in the UK, but there is evidence that as these societies converge, social differences in average blood pressure are becoming similar in the two countries.
Recent changes in the national health care system contract for general practitioners should have led to most people with high blood pressure being identified, although it is much more doubtful whether most of these are followed up with effective treatment. If all general practice teams developed cardiovascular follow-up clinics for high blood pressure on the same lines as the best have already done and all their patients were treated to the level of quality attained in large clinical trials, an additional 15% of fatal strokes would be prevented and probably 5% of coronary heart disease.
Michael Russell
Your Independent guide to Blood Pressure
