09/03/2013 04:16 EDT | Updated 11/03/2013 05:12 EST

International study shows hypertension poorly controlled; Canadian data questioned

TORONTO - Despite the fact the risks posed by high blood pressure are well known, the condition remains under-diagnosed and poorly controlled around the world, a large Canadian-led international study suggests.

The study, published Tuesday in the Journal of the American Medical Association, showed that 41 per cent of adults from 17 countries had high blood pressure.

When that figure was adjusted to compensate for the fact that the study participants were mainly older adults — a group more likely to have high blood pressure — the findings suggested about 28 per cent of the world's population has high blood pressure. It suggested only a fraction of those people were successfully controlling their blood pressure using medication.

"Lots of people don't know they have hypertension. And even those who know about their hypertension, a proportion did not receive treatment. And of those who received treatment, blood pressure is not well controlled," said Dr. Koon Teo, one of the co-lead authors of the paper and a cardiologist at McMaster University in Hamilton.

But the director of the hypertension unit of the University of Ottawa Heart Institute raised questions about the study, noting that the numbers from Canada paint a picture that is far more dismal than other recent studies have shown.

Dr. Frans Leenen, who was not involved in the McMaster-led study, said two large Canadian surveys that looked at hypertension in Ontario and across Canada showed that about 65 per cent of Canadians with the condition have their blood pressure under control.

In the McMaster study, the control rate for high income countries was between 16 and 18 per cent. Canada was one of three countries in the high income classification, but Canadians made up about two-thirds of the people from those three countries who were enrolled in the study.

Likewise, Leenen said, the two Canadian studies showed that only about 15 per cent of people who had high blood pressure were unaware of the fact. In the McMaster study, about 48 per cent of the participants from high income countries were unaware they had hypertension.

Leenen, who was a co-author of a scientific article that explored the findings of the two Canadian studies, was at a loss to explain why the McMaster study's numbers were so different, and why the authors didn't cite the Canadian surveys. The article Leenen co-authored was published in the Canadian Medical Association Journal in June 2011.

"Both the Canadian surveys showed much higher treatment and control rates than this study shows," Leenen said.

"That really is a major concern. And they don't discuss it at all.... That (leads to) questions then to what extent the other numbers — control rates — are accurate, yes or no."

The findings of the McMaster study combine to tell a story that has been recounted frequently: high blood pressure is too often undetected and even when detected, is often not effectively treated.

But Leenen called that "an outdated myth" — at least as far as Canada and the United States are concerned. "It doesn't mesh with the recent survey data."

The most up-to-date U.S. data shows that hypertension is controlled in about half of detected cases, he said. (The U.S. was not among the countries in the study. High income countries included were Canada, Sweden and the United Arab Emirates.)

Leenen said whereas the Canadian data were based on surveys where adults were randomly selected for testing — random selection is considered critical to ensure results aren't biased or skewed — the way the international study's participants were enrolled was not clear.

In fact, the authors note in the paper that the non-random selection of subjects means the results cannot be deemed to be representative of the individual countries. They go on to say that as the overall rates they found are in keeping with global estimates, there appears to be no major biases in their study.

The findings come from the PURE study — the acronym stands for Prospective Urban Rural Epidemiology — which is designed to track how urbanization is affecting risk factors for and rates of cardiovascular disease in various parts of the world.

The 17 countries in the study represent a mixture of upper income, developing and low income nations.

The study found people with hypertension in higher income countries were more likely to be aware of their condition than people in lower income countries. The same was true for the likelihood that a person with high blood pressure would be treated.

The new president of the World Hypertension League called the findings of the McMaster study evidence of a widespread failure to address hypertension prevention and control.

"There's a recognition that this needs to be dealt with," said Dr. Norm Campbell, a professor of medicine at the University of Calgary who has been a leader of Canadian efforts to combat hypertension.

Campbell suggested the tragedy of hypertension — a leading cause of death and disability worldwide — is that it is largely preventable.

"If we look back to hunter-gatherer societies, which are almost non-existent now, they might have had five per cent of the population hypertensive, with very little increase of blood pressure with age," said Campbell, who began his term as head of the World Hypertension League on Tuesday.

"So this is really something that we've done to our society by becoming physically inactive, obese and eating highly processed foods with lots of salt, low potassium, low fibre, etc."