Cuffless Blood Pressure Monitors: Still a
Numbers Game

Marilynn Larkin

June 13, 2023

Medscape's Editor-in-Chief Eric Topol referred to continual non-invasive, cuffless, accurate blood pressure devices as "a holy grail in sensor technology."

He personally tested a cuff-calibrated, over-the-counter device available in Europe that claims to monitor daily blood pressure changes and produce data that can help physicians titrate medications.

Topol does not believe that it is ready for prime time. Yes, cuffless devices are easy to use, and generate lots of data. But is that data accurate?

Many experts say not yet, even as the market continues to grow and more devices are introduced and highlighted at high-profile consumer events.

Burned Before

Limitations of cuffed devices are well known, including errors related to cuff size, patient positioning, patient habits or behaviors (eg, caffeine/nicotine use, acute meal digestion, full bladder, very recent physical activity) and clinicians' failure to take accurate measurements.

But are the currently available cuffless devices acceptable substitutes?

Dr Timothy Plante

Like many clinicians, Timothy B. Plante, MD, MHS, assistant professor at the University of Vermont Medical Center Thrombosis & Hemostasis Program in Burlington, is very excited about cuffless technology. However, "we've been burned by it before," he told | Medscape Cardiology.

Plante's 2016 validation study of an instant blood pressure smartphone app found that its measurements were "highly inaccurate," with such low sensitivity that more than three quarters of individuals with hypertensive blood levels would be falsely reassured that their blood pressure was in the normal range.

His team's 2023 review of the current landscape, which includes more sophisticated devices, concluded that accuracy remains an issue: "Unfortunately, the pace of regulation of these devices has failed to match the speed of innovation and direct availability to patient consumers. There is an urgent need to develop a consensus on standards by which cuffless BP devices can be tested for accuracy."

Devices, Indications Differ

Cuffless devices estimate blood pressure indirectly. Most operate based on pulse wave analysis and pulse arrival time (PWA-PAT), explained Ramakrishna Mukkamala, PhD, in a Medscape commentary. Mukkamala is a professor in the Departments of Bioengineering and Anesthesiology and Perioperative Medicine at the University of Pittsburgh.

PWA involves measuring a peripheral arterial waveform using an optical sensor such as the green light on the back of a wrist-worn device, or a 'force sensor' placed on the opposite side of the wrist over the radial artery. Certain features are extracted from the waveform and calibrated to blood pressure values using machine learning.

PAT techniques work together with PWA; they record the ECG and extract features from that signal as well as the arterial waveform to compute blood pressure values.

The algorithm used to generate the BP numbers is proprietary and may include demographics and other patient characteristics. A cuff measurement is often part of the algorithm because most cuffless devices require periodic (typically weekly or monthly) calibration using a cuffed device.

Demographics such as age and body mass index are known to correlate with BP. The cuff measurement taken for calibration can also be predictive of future BP readings, especially in stable people.

This has led some experts to question whether the waveform measurements of PWA-PAT cuffless devices actually add anything over demographics and the cuff measurement.

They don't, according to Microsoft Research in what Mukkamala and co-authors referred to (in a review published in Hypertension ) as "a complex article describing perhaps the most important and highest resource project to date (Aurora Project) on assessing the accuracy of PWA and PAT devices."

The Microsoft article was written for bioengineers. The review in Hypertension explains the project for clinicians, and concludes that, "Cuffless BP devices based on PWA and PWA-PAT, which are similar to some regulatory-cleared devices, were of no additional value in measuring auscultatory or 24-hour ambulatory cuff BP when compared with a baseline model in which BP was predicted without an actual measurement."

IEEE and FDA Validation

Despite these concerns, several cuffless devices using PWA and PAT have been cleared by the US Food and Drug Administration (FDA).

Validating cuffless devices is no simple matter. The Institute of Electrical and Electronics Engineers (IEEE) published a validation protocol for cuffless blood pressure devices in 2014 that was amended in 2019 to include a requirement to evaluate performance in different positions and in the presence of motion with varying degrees of noise artifact.

Dr Daichi Shimbo

However, Daichi Shimbo, MD, co-director of the Columbia Hypertension Center in New York City and vice chair of the American Heart Association Statement on blood pressure monitoring, and colleagues point out limitations even in the updated standard. These include not requiring evaluation for drift over time; lack of specific dynamic testing protocols for stressors such as exercise or environmental temperatures; and an unsuitable reference standard (oscillometric cuff-based devices) during movement.

Shimbo told | Medscape Cardiology that although he is excited about them, "these cuffless devices are not aligned with regulatory bodies. If a device gives someone a wrong blood pressure, they might be diagnosed with hypertension when they don't have it or might miss the fact that they're hypertensive because they get a normal blood pressure reading. If there's no yardstick by which you say these devices are good, what are we really doing — helping, or causing a problem?"

"The specifics of how a device estimates blood pressure can determine what testing is needed to ensure that it is providing accurate performance in the intended conditions of use," Jeremy Kahn, an FDA press officer, told | Medscape Cardiology. "For example, for cuffless devices that are calibrated initially with a cuff-based blood pressure device, the cuffless device needs to specify the period over which it can provide accurate readings and have testing to demonstrate that it provides accurate results over that period of use."

The FDA said its testing is different from what the Microsoft Aurora Project used in their study.

"The intent of that testing, as the agency understands it, is to evaluate whether the device is providing useful input based on the current physiology of the patient rather than relying on predetermined values based on calibration or patient attributes. We evaluate this clinically in two separate tests: an induced change in blood pressure test and tracking of natural blood pressure changes with longer term device use," Kahn explained.

Analyzing a device's performance on individuals who have had natural changes in blood pressure as compared to a calibration value or initial reading "can also help discern if the device is using physiological data from the patient to determine their blood pressure accurately," he said.

Experts interviewed by | Medscape Cardiology who remain skeptical about cuffless BP monitoring question whether the numbers that appear during the induced blood pressure change, and with the natural blood pressure changes that may occur over time, accurately reflect a patient's blood pressure.

"The FDA doesn't approve these devices; they clear them," Shimbo pointed out. "Clearing them means they can be sold to the general public in the US. It's not a strong statement that they're accurate."

Moving Toward Validation, Standards

Ultimately, cuffless BP monitors may require more than one validation protocol and standard, depending on their technology, how and where they will be used, and by whom.

And as Plante and colleagues write, "Importantly, validation should be performed in diverse and special populations, including pregnant women and individuals across a range of heart rates, skin tones, wrist sizes, common arrhythmias, and beta-blocker use."

Organizations that might be expected to help move validation and standards forward have mostly remained silent. The American Medical Association's US Blood Pressure Validated Device Listing website includes only cuffed devices, as does the website of the international scientific nonprofit STRIDE BP.

The European Society of Hypertension (ESH) 2022 consensus statement on cuffless devices concluded that until there is an internationally accepted accuracy standard and the devices have been tested in healthy people and those with suspected or diagnosed hypertension, "cuffless BP devices should not be used for the evaluation or management of hypertension in clinical practice."

This month, ESH published recommendations for "specific, clinically meaningful, and pragmatic validation procedures for different types of intermittent cuffless devices" that will be presented at their upcoming annual meeting June 26.

Updated protocols from IEEE "are coming out soon," according to Shimbo. The FDA says currently cleared devices won't need to revalidate according to new standards unless the sponsor makes significant modifications in software algorithms, device hardware, or targeted patient populations.

Device Makers Take the Initiative

In the face of conflicting reports on accuracy and lack of a robust standard, some device makers are publishing their own tests or encouraging validation by potential customers.

Biobeat cuffless blood pressure monitor

For example, institutions that are considering using the Biobeat cuffless blood pressure monitor watch "usually start with small pilots with our devices to do internal validation," Lior Ben Shettrit, the company's VP of business development, told | Medscape Cardiology. "Only after they complete the internal validation are they willing to move forward to full implementation."

Cardiologist Dean Nachman, MD, is leading validation studies of the Biobeat device at the Hadassah Ein Kerem Medical Center in Jerusalem, Israel. For the first validation, the team recruited 1057 volunteers who did a single blood pressure measurement with the cuffless device, and with a cuffed device.

Dr Dean Nachman

"We found 96.3% agreement in identifying hypertension and an interclass correlation coefficient of 0.99 and 0.97 for systolic and diastolic measurements, respectively," he told | Medscape Cardiology. "Then we took it to the next level and compared the device to ambulatory 24-hour blood pressure monitoring and found comparable measurements."

The investigators are not done yet. "We need data from thousands of patients, with subgroups, to not have any concerns," he says. "Right now, we are using the device as a general monitor — as an EKG plus heart rate plus oxygen saturation level monitor — and as a blood pressure monitor for 24-hour blood pressure monitoring."

The developers of the Aktiia device, which is the one Topol tested, take a different perspective. "When somebody introduces a new technology that is disrupting something that has been in place for over 100 years, there will always be some grumblings, ruffling of feathers, people saying it's not ready, it's not ready, it's not ready," Aktiia's chief medical officer Jay Shah, MD, told | Medscape Cardiology.

"But a lot of those comments are coming from the isolation of an ivory tower," he said.

Dr Josep Solà wearing the Aktiia bracelet on his wrist.

Aktiia co-founder and chief technology officer Josep Solà said that "no device is probably as accurate as if you have an invasive catheter," adding that "we engage patients to look at their blood pressure day by day...If each individual measurement of each of those patient is slightly less accurate than a cuff, who cares? We have 40 measurements per day on each patient. The accuracy and precision of each of those is good."

Researchers from the George Institute for Global Health recently compared the Aktiia device to conventional ambulatory monitoring in 41 patients and found that "it did not accurately track night-time BP decline and results suggested it was unable to track medication-induced BP changes."

"In the context of 24/7 monitoring of hypertensive patients," Solà said, "whatever you do, if it's better than a sham device or a baseline model and you track the blood pressure changes, it's a hundred times much better than doing nothing."

Editor’s Note: This article has been updated to correct some technical inaccuracies in explaining PWA and PAT devices.

Nachman and Plante reported no relevant financial relationships. Shimbo reported that he received funding from NIH and has consulted for Abbott Vascular, Edward Lifesciences, Medtronic, and Tryton Medical.

Marilynn Larkin is an award-winning science writer and editor. She is a frequent contributor to Medscape, was a contributing editor to The Lancet and its affiliated medical journals for more than 10 years and a regular contributor to the New York Academy of Sciences' publications and Reuters Health's professional newsfeed.

Follow Marilynn Larkin on Twitter:@MarilynnL

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