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No One Knows Magic Blood Pressure Number

Gina Kolata
Doctor measuring blood pressure - studio shot on white background

Almost half a century after rigorous studies showed medicines that lower blood pressure prevent heart attacks, strokes and deaths, researchers still do not know just how low blood pressure should go.

“We all know treating hypertension is good, but we don’t know how aggressive we should be,” says Dr Michael Lauer, director of the Division of Cardiovascular Sciences at the US National Heart, Lung and Blood Institute.

The institute is seeking definitive answers as part of its mission to drive down deaths from cardiovascular disease.

The results of a large and rigorous study, called SPRINT for Systolic Blood Pressure Intervention Trial, are expected in 2017. Researchers are following 9,000 middle-age and older adults with high blood pressure. Half were assigned to get their systolic pressure — the top number that measures pressure when the heart contracts — to below 120 while the others were to get to below 140.

The study will measure not just heart attacks, strokes and kidney disease but also effects on the brain. Do people think better and avoid dementia with lower pressure?

In the meantime, doctors are making decisions in a fog of uncertainty. One school of thought says blood pressure rises with age to push more blood into the brain. Another says high blood pressure damages the brain, perhaps causing silent mini strokes.

“We don’t know which is right,” says David Reboussin, a biostatistician at Wake Forest University and a principal investigator for the new study.

The trend in geriatrics is to let pressure drift up, although not above 150, says Dr Alfred Cheung, a study investigator who is a nephrologist and professor of medicine at the University of Utah. “It’s not based on hard data,” he says.

The lack of evidence is at the heart of a dispute that is partly an artefact of the way thinking on blood pressure evolved.

When drugs to lower blood pressure came on the market in the 1950s, many doctors did not know if they should prescribe them. They thought systolic pressure should be 100 plus a person’s age. The conventional wisdom was that blood vessels stiffen with age, so higher pressure helped push blood through them.

That view was discredited in 1967 when a rigorous study comparing the drugs with a placebo ended early because those taking the medicines had so many fewer strokes and heart attacks. The drugs became mainstays in medicine, credited with saving millions of lives.

Many early clinical trials did not even address systolic pressure. Instead, they looked at diastolic pressure, the lower number, representing pressure on blood vessels when the heart relaxes between beats.

“The general thinking — incorrectly — was that as you get older, the systolic naturally goes up” to supply the brain with blood, says Dr William Cushman, the chief of preventive medicine at the VA Medical Center in Memphis, Tennessee.

It was only in 1991 that the first study on systolic pressure was published. It and subsequent research concluded the treatment goal should be a level below 150 to prevent heart attacks, heart failure and strokes. Almost no studies examined the outcomes at lower goals.

So doctors and guideline makers have a conundrum, Cushman says. “The epidemiology is consistent that having a systolic pressure of 120 or even below 120 is associated with reduced cardiovascular mortality. But that doesn’t mean that treating with medications to reach that level will give you that benefit.”

The concern is that drugs always have more effects than the one they are being used for. So a blood pressure lowered with drugs is not necessarily the same as one that is naturally lower.

Guidelines from experts are all over the map. A panel appointed by the National Heart, Lung and Blood Institute suggests a systolic pressure below 150 for those older than 60.

The American Heart Association and other groups say it should be under 140.

European guidelines call for a systolic pressure less than 150 except for older adults, but they also take into account a person’s risk of heart disease when deciding how low that number should go.

And epidemiological studies that follow large groups of people over time have found that people whose systolic pressure is naturally 120 or lower have the lowest risk of heart attacks and strokes.

The guidelines from the Heart, Lung and Blood Institute panel constituted one of the most ambitious efforts to build a consensus for blood pressure levels. The mission was to use data from rigorous studies rather than expert opinion, the older standard.

And it’s not just the question of the right goal for systolic pressure. Blood pressure and cholesterol levels are now treated differently.

Cholesterol guidelines take into account a patient’s overall risk of a heart attack. But with blood pressure, at least for US guidelines, the only thing that matters is blood pressure levels and not other factors like family history or cholesterol.

That was how the studies were designed, though, Cushman says. Cholesterol trials took other risks into account. Blood pressure trials looked at only blood pressure. But clearly some people are at lower risk than others even though they have the same blood pressure. Yet all are treated the same. Should that change?

That sort of inconsistency leaves many physicians in a quandary. Dr Michael Gaziano, a Harvard professor of medicine, considers a patient’s overall risk and is an evangelist for losing weight and exercising to boost the effects of blood pressure drugs.

Hospitals and medical practices evaluate doctors by how well patients’ pressures adhere to guidelines and often penalise them financially when patients are not adherent.

Gaziano says the grading system that targets a single value as a measure of success is flawed. “If a patient starts with a pressure of 180 and gets it down to 145, I get a bad mark…. But if a patient goes from 140 to 139, I succeeded.” A patient at his clinic, Joseph Moscillo, 65, had had a heart attack but he had reduced his pressure to 150 from 200. He would not have been seen as a success story.

But Gaziano says he believes that rather than adding more drugs to lower his pressure, it was more important for Moscillo to trim down from 100kg. “We can keep piling on meds, but it is a losing game if you don’t exercise and control your weight,” Gaziano tells Moscillo.

The results of the SPRINT study may affect doctors’ daily decisions. If it finds that a pressure of below 120 is better than below 140, then the plans for Moscillo would probably change.

Published: July 02 2015. By Gina Kolata
Copyright Business Day Live

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