Blood pressure test still valid

Tuesday November 25 2008

“A blood pressure reading in a GP surgery may not be the most accurate way of predicting the chances of suffering a heart attack,” The Daily Telegraph reports.

The newspaper says that a study has shown that conventional blood pressure tests did not predict strokes or heart attacks, whereas multiple readings taken over a 24-hour period could.

However, all the people in this study had high blood pressure that had not responded to several drug treatments. The results are not applicable to the majority of people with high blood pressure, for whom treatment with medication is effective.

Conventional blood pressure testing by a GP remains essential as high blood pressure often has no symptoms, but can lead to serious or even fatal health problems if left untreated.

Current NICE guidelines recommend that GPs ask patients with one reading of high blood pressure to return at least twice, to confirm the diagnosis. NICE also says the value of readings over 24 hours is unclear and needs further research.

Where did the story come from?

This research was conducted by Dr Gil Salles and colleagues from the Federal University of Rio de Janeiro. The study was funded in part by the Brazilian National Research Council, Brazilian Innovation Agency and the petrol company PETROBRAS. It was published in the peer-reviewed medical journal Archives of Internal Medicine.

What kind of scientific study was this?

This was a cohort study looking at two forms of blood pressure monitoring to determine which one was a better predictor of risk of cardiovascular disease in people with medication-resistant high blood pressure (hypertension). The two forms of monitoring examined were either conventional (two blood pressure readings taken in a GP's surgery by a doctor), or multiple readings taken over a 24-hour period by a monitoring device known as ABPM (ambulatory blood pressure monitoring).

The researchers enrolled 556 people who had high blood pressure despite having been treated with full doses of three or more anti-hypertensive (blood pressure-reducing) drugs. On average, the participants were 65 years old and they had high blood pressure for 18 years. These people were enrolled at the University of Rio de Janeiro hospital outpatient clinic between 1999 and 2004.

All the participants were given a thorough assessment of their health and cardiovascular risk factors. This included a full clinical examination, an electrocardiograph (ECG), an echocardiograph (where echoes are used to develop an image of the heart) and laboratory tests.

All had their blood pressure measured twice by a doctor in the outpatient clinic while seated and had 24-hour ABPM during normal activity.

For the ABPM readings the participant wore a monitor which took blood pressure readings every 15 minutes throughout the day, and every 30 minutes at night. The researchers followed the patients three to four times a year until the end of 2007.

The researchers documented which people experienced any of a range of fatal or non-fatal cardiovascular events in this time. They also looked specifically at death from cardiovascular causes, and death from any cause. The researchers used medical records, death certificates and standard interviews with the participants’ doctors and families to identify these events.

The researchers then looked at whether there was a relationship between the risk of having a cardiovascular event and the surgery based blood pressure (BP) measurement or ABPM results. The researchers adjusted the findings for factors such as age, gender, the use of certain drugs for high blood pressure and certain health conditions, as well as lifestyle factors that affect cardiovascular risk. Analyses of the effect of ABPM were adjusted for surgery based blood pressure measurements.

The researchers also looked at whether the daytime or night-time ABPM measurements were better predictors of cardiovascular risk.

What were the results of the study?

The 556 participants were followed up for 4.8 years on average. During this time almost a fifth of participants either had a heart attack or developed angina (109 participants, 19.6%), about an eighth died (70 participants, 12.6%) and most of the deaths were from cardiovascular causes (46 participants, 8.2%).

The researchers found that surgery-measured BP was not a good indicator of which participants would have a heart attack or develop angina, die from any cause, or die from cardiovascular causes. However, ABPM measurements did predict cardiovascular events, with people with higher ABPM measurements more likely to experience an event.

For each set increase (the standard deviation) in an average 24-hour ABPM measurement, there was a 32% increase in risk of a cardiovascular event. The association between 24-hour ABPM measurement and death from any cause or death from cardiovascular causes was not statistically significant.

When looking at daytime and night-time ABPM measurements separately, they found that night-time measurements were better predictors of cardiovascular events than daytime measurements.

What interpretations did the researchers draw from these results?

The researchers concluded that higher ABPM measurements (but not office blood pressure measurements) predict increased risk of cardiovascular events in people with medication-resistant high blood pressure.

What does the NHS Knowledge Service make of this study?

There are a number of points that need to be considered when interpreting this study:

  • This study should not be taken to mean that blood pressure monitoring in a GP’s office is not useful. The study only included people who already had high blood pressure that had not responded to medication.
  • The results of this study come from participants who had medication-resistant high blood pressure, meaning they had not responded to courses of three or more drugs at maximum dose. These participants had an average age of 65 years, and had had high blood pressure for an average of 18 years. Therefore these results may not apply to other groups of people, such as those with adequately controlled blood pressure, or younger people who have not had high blood pressure for long.
  • The number of events such as cardiovascular deaths was relatively low, so this study may not have been able to detect associations between ABPM and these individual outcomes.
  • Although the authors attempted to correct for differences between participants in factors that could affect their cardiovascular event risk, these corrections may not have fully removed this effect.
  • Measurements of blood pressure and assessment of blood pressure medication were only taken at the start of the study. Changes in blood pressure and medication use over the follow-up period could be having an effect on the results.

Whether this study has any impact on clinical practice is unclear, as the aim of treatment will always be to return blood pressure to normal, measured in an office or by ABPM.

Measuring of blood pressure will continue to be part of GP consultations, and is invaluable in detecting and monitoring high blood pressure, which is otherwise difficult to detect.

Analysis by Bazian
Edited by NHS Choices