Publish date: 12 May 2023

In the largest ever study of its kind, blood pressure measured repeatedly over 24 hours is five times more predictive of the risk of death from all causes when compared to the way blood pressure is usually measured in the clinic, according to research led by UCLH Director of Research Professor Bryan Williams alongside a research consortium from Spain and the University of Oxford.

The study, published in The Lancet, highlights the very strong association between blood pressure and risk of death in almost 60,000 people followed for an average of almost 10 years. Importantly, the study showed that the average of multiple readings taken over a 24hr period, through the day and night, was much more strongly associated with the risk of death over the next 10 years than conventional measurement of blood pressure in the clinic.

The strongest association was with the night-time systolic pressure. The results reinforce the importance of 24hr blood pressure monitoring as a diagnostic tool in the assessment of high blood pressure and the importance, wherever possible, of not relying solely on blood pressure measurements taken in the clinic for the diagnosis and management of hypertension.

24hr ambulatory blood pressure measurement is done using a conventional blood pressure cuff around the upper arm, connected to a small blood pressure monitor, usually worn attached to a belt around the waist. The monitor is programmed to measure blood pressure every 20 minutes during the day and every thirty minutes at night. The average of these readings is then used to define the daytime, night-time and whole 24hr blood pressure value.

It was already known that ambulatory readings are a better predictor of patient outcomes and have been recommended by NICE in the UK since 2012 to confirm the diagnosis of high blood pressure. However, prior studies have not had the same statistical power or duration of follow up as this new study, to enable quantification of just how much stronger the association was between different methods of blood pressure measurement and risk of death. This very large study puts a robust figure on the size of the increased predictive power of ambulatory measurements.

For the study, 59,124 patients from 223 primary care centres in Spain were followed for almost ten years. The research team compared ambulatory and clinic blood pressure measurements at baseline with data on subsequent mortality in the same patients, using data from the Spanish National Registry of deaths.

They found ambulatory systolic blood pressure was 4.79 times more informative than clinic systolic blood pressure for predicting death from all causes, and 4.65 times more informative for predicting cardiovascular deaths (for example heart attack or stroke). Systolic pressure is the higher of the two numbers which measure blood pressure.

The research team also found that night-time ambulatory blood pressure was the most predictive of death and six times more informative for death than clinic blood pressure, and nearly twice as informative for predicting mortality as daytime ambulatory pressure.

Both masked hypertension (where clinic blood pressure is normal ambulatory blood pressure is elevated) and sustained hypertension (raised blood pressure in both clinic and with ambulatory readings) were linked with a greater risk of death from all causes compared to those whose blood pressure was normal in both the clinic and with ambulatory readings.

But in an important finding that differs from some previous studies, this study found that ‘white coat hypertension’, where blood pressure measured in the clinic is elevated, but the ambulatory blood pressure away from the clinic setting is normal, was not linked to a greater risk of death.

Professor Bryan Williams, who is a specialist in hypertension at UCLH, Chair of Medicine at UCL, and President of the International Society of Hypertension, said: “We have known for some time that ambulatory measurement of blood pressure over 24hrs is a better measurement to confirm the diagnosis and guide treatment of hypertension, but for the first time we have been able to provide robust quantification of how big this effect is, and show that the difference in predictive power for death is very much larger with ambulatory blood pressure, especially night-time pressure, when compared to the blood pressure measurements we make in the clinic.”

“Wherever possible and available, ambulatory readings should be taken to confirm a diagnosis of hypertension. It helps avoid treating those with white coat hypertension, which we have shown is not associated with increased risk of death and provides a more accurate diagnosis. One would hope that this would ultimately lead to better treatment decisions and better outcomes for our patients.”

Read the paper published in The Lancet: Relationship between clinic and ambulatory blood pressure and mortality: an observational cohort study in 59,124 patients