''We have shown that it is variations in people's blood pressure rather than the average level that predicts stroke most powerfully," says study lead author Peter Rothwell, MD, professor of clinical neurology at the Stroke Prevention Research Unit, John Radcliffe Hospital in Oxford, England.
The study, published in The Lancet, followed more than 2,000 patients who had a transient ischemic attack or TIA, a ''mini-stroke'' predictive of a larger stroke, and validated the results with results from three other studies, in finding the link between occasional high blood pressure and stroke risk.
The researchers focused on the systolic blood pressure reading, the top number in the measurement, reflecting the pressure when the heart contracts while pumping blood. Normal blood pressure readings are below 120/80 millimeters of mercury, or mmHg.
How much variability predicts stroke risk? ''One certainly sees an increased risk of stroke when the systolic blood pressure fluctuates 40 mmHg or more (say between 120 mmHg and 160 mmHg) even when mean [or average] blood pressure is very well controlled," Rothwell tells WebMD in an email interview.
Occasional High Blood Pressure and Stroke Risk: Study Details
Rothwell and his colleagues evaluated data from 2,435 patients who had been enrolled in the UK-TIA aspirin trial, which assigned patients with a recent TIA or ischemic stroke to take aspirin or a placebo. Rothwell's team evaluated only the 2,006 patients from this study who had TIAs but no strokes, to avoid compromising the results due to the effect of a recent stroke on blood pressure.
Blood pressure in these patients was measured once at every four-month follow-up visit during the study, which ran from 1979 to 1985.
The results from this study were validated by Rothwell's team with results from three other large studies, each involving more than 2,000 patients.
Occasional High Blood Pressure Predicts Stroke
Patients with the most variation in their systolic blood pressure over seven clinic visits were found six times more likely to have a stroke, Rothwell found.
The highest blood pressure readings were also associated with higher stroke risk. Those with the highest readings over the seven visits were 15 times more likely to have a stroke during the follow-up period.
Not all the patients were being treated for hypertension, Rothwell tells WebMD. "Variability was predictive of stroke in both groups," he says, treated and untreated. "Some had stable hypertension, some had episodic hypertension and some had stable normal blood pressure. The episodic hypertension group had the highest risk of stroke.''
In one of the studies, variability in blood pressure also predicted the risk of heart attacks.
Blood Pressure Study Findings: Implications
The new research isn't the first to evaluate the risks of episodic high blood pressure, Rothwell notes. Although some variability in blood pressure is normal, the new research, he says, should inspire a change in thinking.
"I think that the risk associations, and other evidence, are sufficiently strong for us to stop reassuring patients with variable blood pressure that they don't have hypertension and don't need treatment, which is what current guidelines argue if their [average] blood pressure is OK,'' Rothwell says. " We should be concerned about episodic hypertension in patients who are not on treatment and about residual variability in patients who are already on treatment."
Some variability is normal, he tells WebMD. How much depends on age (typically increasing with age) and gender, with women more likely to have more variability. African-American people, too, tend to have more variability, he says. And other factors, such as the stiffness of the arteries, can affect the amount of variability, he says.
For patients who have high blood pressure and take their pressure at home to monitor it, Rothwell offers this advice based on his findings: "I think that they should consult their doctor if the systolic blood pressure is variable, particularly if they find that it is sometimes 150 mmHg or higher, even if it is well controlled at other times.''
Blood Pressure Study: Second Opinion
The suspected link between episodes of high blood pressure and stroke risk is not new, says Patrick Lyden, MD, chair of neurology at Cedars-Sinai Medical Center in Los Angeles and a 30-year stroke researcher who reviewed the new research for WebMD.
But the new findings, he says, are ''the most important demonstration ... that the attacks of high blood pressure really are bad for you."
Lyden says he is aggressive about treating high blood pressure in his patients, whether the pressure is up some time or all the time. But not all physicians are as aggressive, he suspects. "I would say there is a widespread practice on both continents (UK and here) that occasional high blood pressure is ignored."
"This data teaches us how foolish that is," he says.
"This is still a hypothesis," Lyden tells WebMD. But "I think he's showing us it's critically important to pay attention to this."