ABSTRACT Wearable health monitoring is a multibillion-dollar industry. But the holy grail is probably getting it right for blood pressure monitoring without a cuff, because raised blood

pressure is very common and the leading cause of death in the world. Many have tried and failed, but industry is persisting: numerous cuffless wearable blood pressure devices are on the

market, several technologies have been developed, hundreds of patents are registered every year, and some devices already have regulatory approval. However, to convince the die-hard blood

pressure critic is a different ball game. To understand the challenges of currently accepted methods _and_ cuffless devices, I performed a 24-h blood pressure monitoring self-test, including

of these devices proved to be perfect in capturing a physiologically challenging measure, namely blood pressure, it emphasised that our current practice of a single blood pressure

measurement in clinical practice should be revisited. It further begs the question of when cuffless measurements will be good enough to incorporate in clinical decision-making. SIMILAR

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2023 METHOD-COMPARISON STUDY BETWEEN A WATCH-LIKE SENSOR AND A CUFF-BASED DEVICE FOR 24-H AMBULATORY BLOOD PRESSURE MONITORING Article Open access 15 April 2023 WEARABLE BLOOD PRESSURE

MEASUREMENT DEVICES AND NEW APPROACHES IN HYPERTENSION MANAGEMENT: THE DIGITAL ERA Article 23 March 2022 Wearable health monitoring is a multibillion-dollar industry. But the holy grail is

probably getting it right for blood pressure monitoring without a cuff, because raised blood pressure is the leading cause of death in the world [1]. Many have tried and failed, but industry

is persisting: numerous devices are on the market, several different technologies have been developed, hundreds of patents are registered every year [2], and some devices already have

regulatory approval [3]. However, to convince the die-hard blood pressure critic is a different ball game. Cuffless blood pressure devices rely on a range of different technologies to

estimate blood pressure changes, with most using pulse transit time with ECG, or pulse wave analysis combined with proprietary algorithms (and often artificial intelligence)—relying mostly

on an initial calibration with a cuff. Other potential technologies include facial video processing or ultrasound, which may not require calibration with a cuff [2]. Technical challenges

with devices may include their accuracy, stability of the measurements post calibration, and how machine learning technology is implemented—in other words, how reliant the algorithm is on

hemodynamic components and other demographic inputs, such as sex and age [2]. Although authorities such as the American Heart Association [4] and European Society of Hypertension [2]

recognise the major potential of cuffless blood pressure monitoring, both have published statements against their use in clinical practice due to accuracy issues, and uncertainty regarding

their usefulness in clinical practice. The question is not so much whether a cuffless wearable device can produce a blood pressure comparable to a cuff device—since this has been proven

relatively easy to do—but rather whether cuffless wearables can _track_ blood pressure when calm, sleeping and excited [5]. At the same time, we have to admit that blood pressure is a very

challenging physiological measure. It changes from second-to-second, day-and-night, and during winter-and-summer, when we are calm or excited [6]. Even our ‘gold standard’ method for

diagnosing hypertension, cuff-based 24-h blood pressure monitoring, has poor intra-individual reproducibility across two different days [7]. From a clinical perspective it is understandable

that for cuffless blood pressure monitoring to be accepted, there needs to be convincing evidence that the device can track pressure in a similar fashion as (imperfect) 24-h cuff-based

monitors. This is due to strong evidence that 24-h cuff blood pressure predicts death and cardiovascular events, with nighttime blood pressure being the most potent predictor of outcome

[8,9,10]. If a cuffless device does not track cuff-based blood pressure, how would a clinician feel confident to make treatment decisions? For this reason, the European Society of

Hypertension now recommends stringent criteria for cuffless device manufacturers to validate the accuracy of their products in tracking blood pressure [5]. These include procedures including

six validation tests that were developed to evaluate different aspects of so-called “intermittent” cuffless devices: (1) a static test similar to validation tests used for cuff-based

devices (absolute accuracy in blood pressure measurement); (2) a device position test (whether it captures changes in hydrostatic pressure); (3) medication treatment test (accuracy in

tracking blood pressure lowering); (4) awake/asleep test over 24-h (tracking change in blood pressure); (5) exercise test (tracking blood pressure increases); (6) and the recalibration test

(stability in cuff calibration blood pressure over time) [5]. From a patient’s perspective, it is understandable that comfort is tops [11]. A wrist-band, patch or ring that does not disturb

nighttime sleep or disrupt daily activities, or cause an anxiety response when the cuff inflates [12], would be much easier to wear for months, let alone 24 hours. Being a blood pressure

critic myself, understanding the challenges of currently accepted methods _and_ cuffless devices, I performed a 24-h blood pressure monitoring self-test. While not claiming a flawless study

by any means (with an _N_ = 1), it merely serves to demonstrate the challenges that we face. I monitored my own blood pressure while using five different devices simultaneously: cuff-based

ambulatory monitoring (left and right arm) with two validated WatchBP O3 devices (MicroLife, Switzerland) [13], and three wearable cuffless devices: the Aktiia wrist-wearable (Aktiia SA,

Switzerland) [14, 15], Biobeat chest patch (Biobeat Technologies, Israel) [16, 17], and the CAR-T ring (Skylabs, South Korea) [18, 19]. At the start of the 24-h period, the Aktiia device was

calibrated using its own supplied upper-arm cuff. The Biobeat and CAR-T ring were calibrated according to device specifications using a validated iHealth Feel (iHealthlabs, USA) upper-arm

blood pressure cuff. I then evaluated how well all devices tracked my blood pressure while I was awake, asleep and when watching the 2023 Rugby World Cup final on television (early morning

in Sydney). Being an avid Springbok rugby fan, I anticipated a substantial blood pressure response. This is likely a good emotional stressor, since in New Zealand, a strong upswing in acute

cardiac hospital admissions during Rugby World Cup tournaments were seen when the All Blacks participated [20]. When reviewing the systolic blood pressure results (Fig. 1 Panel A), it is

firstly clear that despite the two cuff-based devices inflating at precisely the same second, there were clear points of discrepancy while awake, asleep and during the rugby match. This may

have been due to movement, body position or other factors. However, there were no statistical differences between average awake (_p_ = 0.35), asleep (_p_ = 0.88) or rugby (_p_ = 0.35) blood

pressure readings between the left and right arm devices. It is also relevant to mention that systolic blood pressure dipped from an average of 113 mmHg when awake, to 91 mmHg when asleep,

rising to an average of 123 mmHg when watching the rugby. Since the blood pressure of the two cuff devices was not statistically different, they were combined and averaged for comparison

with the three cuffless devices. The Aktiia device (Fig. 1 Panel B) produced comparable systolic blood pressure when awake (_p_ = 0.73) and during the rugby match (_p_ = 0.54), but reported

higher values when asleep (_p_ < 0.001). This finding aligns with our previous independent comparison study in 41 patients [21]. The Biobeat patch device (Fig. 1 Panel C) produced lower

systolic blood pressure when awake (_p_ < 0.001) and during the rugby match (_p_ = 0.005), and higher blood pressure when sleeping (_p_ < 0.001), suggesting strong reliance on the

original calibration pressure. The CAR-T ring (Fig. 1 Panel D) showed lower systolic blood pressure when awake (_p_ = 0.002), but was comparable when sleeping (_p_ = 0.56) and when watching

the rugby (_p_ = 0.13). In Fig. 2, similar device comparisons are shown but for heart rate across the 24-h period. Overall, the cuffless devices tracked heart rate quite well compared to the

heart rate recorded by the cuff devices. The comparisons above focus only on systolic blood pressure and heart rate, but it is important to also review performance in diastolic blood

pressure. In Fig. 3 comparisons are presented in Panel A firstly showing no statistical differences between the left and right cuff-based devices for systolic, diastolic blood pressure and

heart rate. In Panel B the differences between the cuffless and cuff-based devices are shown for 24-h, awake, asleep and rugby match recordings, indicating for diastolic blood pressure,

mainly overestimation of blood pressure by cuffless devices during the nighttime, but no statistical differences during awake or rugby recordings. Heart rate recordings were comparable

between cuff-based devices and the Aktiia and BioBeat devices across the 24-h, but the CAR-T ring underestimated heart rate during the awake period (mean 78 vs 82 bpm). It is clear from the

figures that each device type has its own features that may require highlighting. * Where the cuff-based ambulatory devices were programmed to take readings every 20 min, the cuffless

devices are usually programmed to only take a reading when the person is not moving. How would this influence ‘true’ blood pressure? Since it is known that a cuff inflation causes an anxiety

response [12], would that mean that cuff-based ambulatory blood pressure should be higher overall than cuffless measurements? * Also, although the data presented here were generated from

current cuffless devices, the companies continuously strive to improve software algorithms, and therefore often launch updates of their mobile device applications. Hence, improved versions

may already be available for some. * Where a patient can usually only endure one 24-h cuff-based blood pressure measurement at a time, cuffless devices are used for weeks and months without

the user being aware that readings are taken. The benefit of weeks or months’ worth of data compared to a single 24-h period, is perhaps underestimated. * The number of readings taken by the

different devices also varies substantially. For instance, during nighttime 16 cuff blood pressure readings were taken, 10 Aktiia readings, 10 Biobeat readings, and 58 CAR-T readings. With

blood pressure changing all the time, it is likely that when more readings are taken, it may be better than fewer. In fact, when reflecting on current clinical practice mostly relying on a

single snapshot clinic blood pressure taken (often imprecisely) for decision-making [22], one cannot help but wondering whether it is time to completely overhaul how blood pressure is

measured in primary care. Current blood pressure measurement practices across thousands of busy clinics is certainly not ‘good enough’—the question is when cuffless readings will be good

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Download references FUNDING The author is supported by a National Health and Medical Research Council Investigator Grant (APP2017504). Open Access funding enabled and organized by CAUL and

its Member Institutions. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * School of Population Health, University of New South Wales, Sydney, NSW, Australia Aletta E. Schutte * The George

Institute for Global Health, Sydney, NSW, Australia Aletta E. Schutte * Hypertension in Africa Research Team, MRC Unit for Hypertension and Cardiovascular Disease, North-West University,

Potchefstroom, South Africa Aletta E. Schutte * SAMRC/Wits Developmental Pathways for Health Research Unit, Department of Paediatrics, Faculty of Health Sciences, University of the

Witwatersrand, Johannesburg, South Africa Aletta E. Schutte Authors * Aletta E. Schutte View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS All

contributions to the manuscript were made by AE Schutte. CORRESPONDING AUTHOR Correspondence to Aletta E. Schutte. ETHICS DECLARATIONS COMPETING INTERESTS The author is co-chair of STRIDE

BP (stridebp.org, an international authority evaluating blood pressure device validation status). She has received speaker honoraria from Aktiia SA and Omron Healthcare and is serving on the

Advisory Board of Skylabs (CAR-T Ring). ADDITIONAL INFORMATION PUBLISHER’S NOTE Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional

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ARTICLE Schutte, A.E. Wearable cuffless blood pressure tracking: when will they be good enough?. _J Hum Hypertens_ 38, 669–672 (2024). https://doi.org/10.1038/s41371-024-00932-3 Download

citation * Received: 02 May 2024 * Revised: 02 July 2024 * Accepted: 04 July 2024 * Published: 12 July 2024 * Issue Date: September 2024 * DOI: https://doi.org/10.1038/s41371-024-00932-3

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