Recent studies have shown that it is variability in patients' blood pressure that predicts the risk of a stroke most powerfully and not a high average or usual blood pressure level.

The results of the studies, led by Peter Rothwell, a professor, and colleagues at the University of Oxford, have major implications for the diagnosis and treatment of high blood pressure in prevention of stroke and heart disease.

'Raised blood pressure, or hypertension, accounts for over 50 percent of the risk of stroke and other vascular events in the population,' says Rothwell of the Department of Clinical Neurology.

'It has long been believed that it is the underlying average blood pressure that determines most of the risk of complications from hypertension and all of the benefit from the drugs that are used to lower blood pressure. The work that we have done shows that this hypothesis is only partly true - at least when it comes to stroke, the most common complication of hypertension,' he adds.

In the four papers, Rothwell and colleagues show that variability in blood pressure is important as well as a raised average blood pressure.

In one study, Rothwell's team found that variability of blood pressure between visits to clinics was a strong predictor of stroke, heart failure, angina, and myocardial infarction, independent of the patient's mean blood pressure. In contrast to assumptions in current guidelines that patients with only occasional high readings do not require treatment, they show that such patients have a high risk of stroke and other complications.

'We have shown that it is variations in people's blood pressure rather than the average level that predicts stroke most powerfully. Occasional high values, and what might be called episodic hypertension, carry a high risk of stroke. Previously, such fluctuations were considered to be benign and uninformative,' says Rothwell.

The research was published in The Lancet.