Friday March 12 2010
Fluctuation in blood pressure may be able to predict risk of stroke
“Swings in blood pressure ‘could better predict stroke than average high readings’,” reported The Daily Telegraph. The paper reported that “variations in people’s blood pressure rather than the average level predicts stroke most powerfully”.
The news story was based on a collection of studies published in The Lancet. The authors have presented a compelling argument that fluctuations in blood pressure may help to predict the risk of vascular events such as stroke.
As mentioned in an accompanying editorial published in the same journal, it is important to point out that the authors are not questioning the validity of using average blood pressure to predict risk, but are suggesting that variable blood pressure could also be used as a supplementary indicator of increased risk.
At this stage, further evidence needs to be presented that fluctuating blood pressure can be used in this way before guidelines on identifying cardiac risk would be updated. Patients should not stop taking their blood pressure medication, but should consult their GP if they have any queries regarding it.
Where did the story come from?
The story is based on a collection of papers published in The Lancet and The Lancet Neurology, both peer-reviewed medical journals. The papers were written by Professor Peter Rothwell from the Stroke Prevention Research Unit at the John Radcliffe Hospital, Oxford, and colleagues from institutions in England, Ireland and Sweden. Funding was provided by several institutions and organisations, including the UK Medical Research Council, the National Institute for Health Research and Pfizer.
What kind of research was this?
The collection of papers includes a cohort study, a separate systematic review and meta analysis, and a narrative review in The Lancet, and an article in The Lancet Neurology. All papers looked at the relationship between blood pressure and the risk of vascular disease such as stroke.
Professor Rothwell says that high blood pressure is the most prevalent treatable risk factor for vascular events such as stroke, but how blood pressure causes the damage that leads to such vascular events is poorly understood. Most clinical guidelines base their advised courses of action on the risks of vascular events according to a person’s stable (usual) blood pressure. The stable blood pressure reading would be calculated as the average of measurements taken at the doctor’s surgery over a number of visits.
In this review, the professor puts forward the theory that fluctuations in blood pressure, rather than maintained high blood pressure readings, may be a more accurate prediction of the risk of vascular events.
What did the research involve?
The review covered the following areas:
- Whether there is likely to be variability in blood pressure measurements between an individual’s visits to the doctors. If there is high variability, an average measurement may not give a complete picture of the patient’s blood pressure status throughout time, and risk statistics for stroke calculated using average values may be affected.
- If drugs used to treat hypertension (high blood pressure) and the risk of vascular events also have an effect of reducing fluctuations in blood pressure.
- Particular attention was paid to the risk of stroke and its relationship to blood pressure. The author looked at studies where patients had their blood pressure monitored for 24 hours, and that had assessed the risk of stroke.
Professor Rothwell provides some background to these issues and discusses them in some detail. He mentions one study showing that 69% of people who had previously experienced a stroke had episodic (every now and again) hypertension, whereas 12% had stable hypertension as consistently demonstrated over a 24-hour period.
He reviews several epidemiological studies looking at how estimated stable blood pressure can predict the risk of vascular events. The author discusses how fluctuations in blood pressure may play a role in this. He notes that some epidemiological evidence appears to support this, including the fact that there appears to be an increase in strokes mid-morning, which matches the daily pattern of blood pressure variation, and that other reasons for increases in transient blood pressure are also risk factors for stroke.
The author says that risk calculations of stroke are based on the usual blood pressure measurement based on the average of measurements taken on multiple visits to the doctor. He argues that as there can be large variations in readings between visits, any risk predictions based on average readings alone may not reflect the whole picture.
In his review, Professor Rothwell also looks at trials examining the effect of calcium channel blockers (for reducing blood pressure variability) compared with other blood pressure lowering drugs such as angiotensin-converting enzyme inhibitors or beta-blockers, which have differing modes of action. He notes that all the drugs lowered patients’ blood pressure to the same extent, but the calcium channel blockers lowered the risk of stroke compared with the other drugs.
The cohort study by Professor Rothwell and his colleagues reappraised data from previous cohort studies to assess whether variations in blood pressure were a better predictor of stroke outcome than an averaged measurement of blood pressure. The first part of this review assessed the risk of stroke in relation to visit-to-visit variability in blood pressure in people who had experienced a previous stroke. For this, they used data from the UK-TIA aspirin trial and three similar cohort studies. The second part of the review used data from the Anglo-Scandinavian Cardiac Outcomes Trial Blood Pressure Lowering Arm (which involved 24-hour blood pressure monitoring) to assess the effect of blood pressure variability in people treated for hypertension. The researchers found that visit-to-visit variation in blood pressure was a strong predictor of subsequent stroke, and this was independent of the average of all of the patients’ measurements. They also found that the maximum blood pressure measure recorded was also a strong predictor of stroke. They found that in studies where patients’ blood pressure was measured continually over 24 hours, the variation measured in this short period was also a weak predictor of stroke, and was most predictive in younger patients.
A separate systematic review and meta analysis of trials looked at the effects of different classes of blood pressure lowering drugs in preventing stroke. Those trials included took multiple measures of blood pressure at baseline and during follow-up, rather than just quoting a single average measure. The meta analysis found that, compared with other drugs - such as angiotensin-converting enzyme (ACE) inhibitors - there was 19% lower variation in participants’ blood pressure measurements when patients were taking calcium channel blockers and 13% less variation in patients receiving non-loop diuretic drugs.
How did the researchers interpret the results?
Professor Rothwell concludes that increased mean blood pressure is an important cause of arterial disease, but variability and instability in blood pressure also have important roles in the progression of organ damage and the likelihood of vascular events such as stroke. He suggests that variability in blood pressure measurements should be routinely reported in trials looking at the role of high blood pressure on stroke, and that more research is needed to quantify variability and instability of blood pressure in routine practice.
Professor Rothwell has presented a compelling argument in support of his theory that fluctuations in blood pressure may help to predict the risk of vascular events such as stroke.
As mentioned in an accompanying editorial, it is important to point out that Professor Rothwell is not questioning the validity of using average blood pressure to predict risk, but is advocating also using variable blood pressure as a supplementary indicator of increased risk.
As with all narrative reviews the research methods that the author used to identify evidence to support his theories are not definitively laid out. It is therefore not possible to carry out a full appraisal of this evidence. However, the systematic appraisal of data on blood pressure and stroke is a robust and standardised way of appraising all of the available data in a research area.
At this stage, further evidence needs to be presented that fluctuating blood pressure can be used in this way before guidelines on identifying cardiac risk would be updated. This research does not affect patients who are currently taking blood pressure lowering medications. Patients should not stop taking their blood pressure medication, but should consult their GP if they have any queries regarding it.
Currently NICE recommendations on the treatment of hypertension should be followed and drug therapy offered to those who:
- have persistent (measurement on >2 occasions) high blood pressure of 160/100 mmHg or more
- are at raised cardiovascular risk (10-year risk of cardiovascular disease (CVD) of 20% or more, or existing CVD or target organ damage) with persistent blood pressure of more than 140/90 mmHg