Article • Hypertension and CVD assessment and prediction

Blood pressure measurement: deceivingly simple, yet highly complex

Blood pressure (BP) is one of the most basic parameters in any healthcare status assessment and holds considerable diagnostic significance. However, clinicians should never underestimate the complexity behind the assessment, cautions Dr Rhian M. Touyz, MBBCh, PhD. At the ESC congress in Barcelona, the expert from the Research Institute of the McGill University Health Centre (RI-MUHC) in Montréal, Canada, went into detail on the implications of time and variability in BP measurement, challenging the conception of longer measurements always yielding superior results.

Report: Wolfgang Behrends

A major challenge in blood pressure measurement is achieving accurate and reproducible results, while finding practical solutions for the clinical setting at the same time. ‘One can’t emphasize enough the importance of accurate blood pressure measurement,’ she stressed. Not only is it essential to reliably diagnose hypertension, but also to predict and manage cardiovascular risk. The value affects when and how a patient is treated, and it reflects on how well a patient responds to therapy.

However, blood pressure is far from static, but is influenced by a patient’s physiology, circadian rhythm and posture, measurement environment and method, as well as many additional factors. For example, a patient who has just had a meal will present with a slightly lowered systolic BP reading due to ingestion, while caffeine intake, exposure to cold or the mere presence of an observer is known to raise blood pressure.¹ It is vital to take this huge variability into account, Touyz pointed out.

Measurement inaccuracy: more than just a cosmetic error

The current gold standard to assess a patient’s BP is through intra-arterial measurement – an impractical method due to its highly invasive nature. Therefore, practically all clinical institutions rely on indirect alternatives, and office BP measurements are mostly performed using automatic oscillometric devices with a brachial cuff.

Historical development of blood pressure measurement methods

Image source: Şentürk et al., Turkish Journal of Electrical Engineering and Computer Sciences 2019 (CC BY 4.0)

While this is in accordance with the International Society for Hypertension’s best practice guidelines², medical professionals should be aware of the limitations, Touyz cautioned: ‘This is a snapshot measurement, so there is a failure to capture the variability in blood pressure.’ Another factor to keep in mind is white coat hypertension, which has 15-25% of patients present with higher BP simply because the reading is performed in an unfamiliar setting, such as the GP’s office. On the opposite side, these measurements may underdiagnose so-called “masked hypertension”, where blood pressure is uncharacteristically low during the patient’s office visit.

The expert made it clear that these fluctuations are not to be dismissed easily: ‘Inaccuracies of just 5 mmHg correspond to an incorrect classification of hypertension in 84 million people worldwide.³ The economic, societal and individual impact is huge, so we do have to measure blood pressure accurately and we do need to consider all these variables.’

Home-based measurement: device validation is essential

A possible way out of the problem comes in the form of automated and out-of-office blood pressure measurement, Touyz suggested. According to current recommendations, most automated devices measure BP several times with 1-minute intervals. Recent research even suggests that repeated measurements achieve comparable accuracy with only 30-second intervals.⁴ On the other hand, studies with extended intervals beyond one minute did not observe improved results.⁵

elderly woman measuring blood pressure at home

Various types of devices for home-based BP measurement have seen widespread use, not least due to the Covid-19 pandemic. Not only is this method practical and convenient, but also predictive of cardiovascular events, the experts said. ‘It is actually a very good method for long term follow up of treated patients.’ She also welcomed that self-measuring contributes to a sense of involvement and ownership in a patient’s care. ‘But it is absolutely essential that these home blood pressure devices are accurately validated.’ The expert emphasized this aspect in view of a recent Australian study that found the majority of commercially available devices to not be proven accurate according to international validation standards.⁶ She therefore appealed to her colleagues to only recommend devices to their patients that have been validated by institutions such as the British and Irish Hypertension Society⁷ or the German Hochdruckliga⁸.

Upcoming technologies

In the coming years, the field might benefit from the introduction of promising new technologies, Touyz predicted. Case in point: researchers currently explore the potential of graphene tattoos, which noninvasively measure the bioimpedance between the radial and ulnar arteries and use an algorithm to assess blood pressure.⁹ This system, which can measure BP continuously for more than 300 minutes, shows how new technology could address current challenges in terms of BP variability, the expert concluded.

‘The most important message is to be aware of the impact of variability,’ she pointed out, urging clinicians not to think of blood pressure as just one number that defines the risk of hypertension. Sharing this knowledge with patients is another critical factor, as patients tend to get confused when their BP values are different across several measurements. ‘It is important to make patients aware that variability is normal, but that blood pressure should be measured at the same time of day and under the same conditions.’

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