Blood pressure-are we measuring it wrong?
High blood pressure, or hypertension, is the most common risk factor for vascular disease, explaining half of the risk of stroke and heart attacks, and affecting half of adults in developed countries. In addition, hypertension is the leading cause of people being on prescribed drugs. So the way we measure and treat blood pressure is crucial.
Most of the Lancet this week is devoted to Professor Peter Rothwell, of Oxford University, who is challenging the way we measure and consider blood pressure. In a series of 3 articles in the Lancet, and one in Lancet Neurology, he argues that the idea of an absolute value of normal blood pressure has shortcomings, and in addition, we should be looking at how much the blood pressure varies.
The most common way of being diagnosed with hypertension is to have a raised reading on examination, which is then repeated after a few weeks/months. If the reading is still high, recommendations are made about lifestyle and if the readings are still raised, anti-hypertensive drugs are started. The problem is that blood pressure varies greatly between visits. This blood pressure variability between visits was measured in patients who had suffered from TIA (transient ischaemic attacks or “mini-strokes”) and found to strongly predict risk of stroke, regardless of the average blood pressure. Importantly, even in people with treated hypertension, blood pressure variability was associated with high risk of stroke. The maximum blood pressure was also a strong predictor of stroke.
In a systematic review of several randomised trials of blood pressure-lowering drugs, the drugs that caused the greatest reduction in this variability in blood pressure, were best at preventing stroke. These drugs were calcium channel blockers (e.g. amlodipine) and diuretics (e.g. furosemide), compared with beta-blockers, which were not as good at reducing variability in blood pressure, and therefore not as good at stopping strokes. When Rothwell and colleagues compared the visit-to-visit variability in blood pressure directly, they found that blood pressure variability decreased with calcium channel blockers, and increased with beta-blockers.
The take-home message is that measuring the absolute value of blood pressure should be combined with a measure of variability, and the ideal drugs to prevent strokes will both reduce the value and the variability of blood pressure. This has tremendous implications for not only how GPs diagnose and treat hypertension, it also has implications for population-level strategies such as the polypill.