Taking your blood pressure with your cell phone or watch: there are already devices on sale, but they do not yet have medical validity

If by just looking at our watch or mobile phone we can know in real time our heart rate, the number of steps we take, the calories we burn or the hours of sleep achieved the night before, why can't we do the same to know our blood pressure?

Blood pressure is the force that blood exerts against the walls of the arteries.

It is defined by two values: the pressure that results from the push of blood pumped to the rest of the body by the contraction of the heart, called systolic (or maximum) pressure, and that which occurs when the organ relaxes, called diastolic (or minimum). .

The American Heart Association considers blood pressure normal when these indicators do not exceed, respectively, 120 and 80 millimeters of mercury.

When the values ​​are below 90/60 millimeters of mercury, the person has hypotension.

In athletes it can be asymptomatic and carries no risks.

Other times it causes symptoms such as dizziness, nausea, paleness, blurred vision, confusion, or fainting, because the brain does not receive enough blood flow.

Very low blood pressure can be life-threatening because it can cause

shock

, in which organs suffer damage due to the absence of blood flow.

It is more common in the elderly and can be precipitated by sudden changes in position, dehydration, infections, bleeding, certain medications or also by diseases such as Parkinson's or diabetes.

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Above 140/90 the person is said to have high blood pressure.

Researchers have calculated that, in people between 40 and 69 years old, for every increase of 20 millimeters in systolic blood pressure and 10 millimeters in diastolic blood pressure, the risk of coronary heart disease and cerebrovascular accidents or stroke doubles.

Worldwide, some 1.28 billion people between 30 and 78 years of age have hypertension.

Most live in low- or middle-income countries and more than half do not receive treatment to control it, notes a 2021 study published in

The Lancet

.

This is despite the fact that hypertension can be easily detected by measuring blood pressure (at home or in a health center) and can often be treated effectively with low-cost medications.

Now, a new generation of blood pressure measuring devices seeks to facilitate the diagnosis, and control, of hypertension.

Unlike traditional equipment, they dispense with the inflatable cuff or bracelet that is placed around the arm and allow values ​​to be obtained on demand—for example, by pressing a sensor with the finger—;

or continuously, if wearing a watch, ring or bracelet.

“Regular blood pressure monitoring in all adults would improve awareness of hypertension and, for those who have already developed it, may improve their control,” says Ramakrishna Mukkamala, a bioelectrical engineer at the University of Pittsburgh, United States, who is a co-author. from a review article in the 2022

Annual Review of Biomedical Engineering

on blood pressure measurement using cuffless devices.

“For example, if patients continue to see that their blood pressure is high, they will eventually be able to comply with taking their medications,” explains Mukkamala.

The measurement of blood pressure has a history of almost three centuries until it leads us to the mechanism that we all know and that our family doctor performs every time we go to his office, by putting on a cuff that surrounds the arm, inflating it and then deflating it controlled, until determining the maximum and minimum blood pressure.

Leave the sleeve behind

But the use of blood pressure monitors with an inflatable cuff has some drawbacks.

One is that unless a person has a blood pressure monitor for home use—a survey of adults ages 50 to 80 in the United States found that only 55 percent of hypertension patients surveyed owned one—then they'll need to go Go to a pharmacy, doctor's office, or health center to find out your blood pressure.

Another barrier is that repeated inflations and deflations of the cuff are disruptive and even difficult to apply when frequent monitoring of a patient in a hospital is necessary.

And a third drawback is that they do not allow continuous measurement of blood pressure;

It only offers us the specific measurement at an exact moment.

The new cuffless devices promise to reveal a more complete picture of physiological blood pressure changes that cannot be detected with spot measurements, instead offering a more realistic blood pressure profile, says Alberto P. Avolio, a biomedical engineer at the Macquarie University, in Sydney, Australia, and co-author of the review published in the

Annual Review of Biomedical Engineering.

The challenge is to ensure that users can take their blood pressure as easily as they measure their heart rate. deimagine (Getty Images)

The different cuffless measurement devices are based on methods that, instead of directly determining blood pressure, use sensors to capture certain indirect signals that they process using different algorithms (or sets of mathematical procedures) that allow the value to be obtained.

It would be like inferring fever by measuring the increase in palpitations and sweating instead of using a thermometer, or the result of a soccer game from outside the stadium by listening only to the screams of the spectators.

One of the detection methods is optical sensors.

They are based on the principle of photoplethysmography (FPG): it consists of illuminating a segment of the skin and analyzing the difference between the light emitted and that detected by a photoreceptor, which will depend on the diameter of the artery, the volume of blood and the the concentration of hemoglobin (oxygen-carrying protein) at the measurement site.

During the systolic phase, when the heart pumps blood, the difference between the emitted and reflected light will be maximum, because there will be more blood flow and, therefore, more hemoglobin and other proteins that absorb light.

During the diastolic or relaxation phase, this difference will be minimal.

The algorithm relates these measurements to blood pressure.

It is the same method used by the Apple Watch and other devices to measure heart rate, and also by pulse oximeters that became popular during the Covid-19 pandemic to record the level of saturation or oxygenation of the blood.

It is also the method used by the bracelet from the Swiss company Aktiia, available for now only in Europe, which automatically records blood pressure values ​​for 24 hours (even when sleeping, averaging the results every two hours) and which can then be view through an application on mobile phone.

There are also electrical sensors, a modified version of an electrocardiogram, which measures the electrical activity of the heart;

mechanical sensors, such as ballistocardiography and seismocardiography, that adhere to the surface of the skin to capture small variations in pressure;

and bioimpedance sensors, similar to instruments that analyze body composition by measuring the body's resistance to the flow of an electrical current.

Calibrated and uncalibrated

There are two broad categories into which these new cuffless blood pressure measurement devices can be grouped: those that require calibration—the records obtained with a manual or digital cuff sphygmomanometer need to be periodically compared—and those that require calibration. no.

Both types of devices capture body signals non-invasively, whether from the fingertip, ear or wrist, among others.

The estimated blood pressure is then displayed or transmitted to nearby devices, such as smartphones or tablets.

Among the measurement methods that require calibration is pulse transit time or PTT, which represents the time in milliseconds it takes for the pulse to travel between two arterial points: the stiffer an artery, the higher the blood pressure and the lower the TTP, because the pulse travels faster.

This method is the one that, to date, has the most scientific evidence.

Another is based on the analysis of the shape and amplitude of the pulse wave, which is the pressure wave that traces the propagation of the blood pumped by the heart through the entire arterial tree, and whose characteristics depend, in part, of the rigidity of the arterial walls.

More recently, equipment has appeared that, based on images captured with a camera (as if they were selfies), detect changes in the TTP or subtle changes in the color of the face, imperceptible to the eye, that accompany each heartbeat.

In this way, the blood flow under the skin and the shape of the pulse waves are reconstructed.

Cuffless blood pressure measuring devices capture signals from the body non-invasively and then display them on devices such as watches, smartphones or tablets. KNOWABLE MAGAZINE

In contrast, uncalibrated cuffless blood pressure measurement methods aim to eliminate the step of comparing device measurements with those captured by a sphygmomanometer.

They use only machine learning and artificial intelligence to establish, from the signals captured by the sensors, the person's blood pressure values.

In the same way that a stream of water can exert more or less force on the walls of a hose if the height or opening of a faucet is changed, the analysis of oscillations or fluctuations in blood volume can be measured when a ring is worn and The arm is lowered, because the internal blood pressure of the finger increases as it receives more blood flow due to gravity.

Alternatively, a ring can also obtain measurements of blood volume oscillations by periodically applying gentle pressure to the finger.

A sensor in a smartphone can also do this analysis, when pressed following the instructions given by the device.

Other calibration-free methods use ultrasound waves to visualize variables such as artery dimensions and blood flow rate, which are also related to blood pressure.

The path to clinical application

The development of these cuffless blood pressure devices is advancing at high speed, but that does not mean they are ready for use in the medical world.

“Unfortunately, the pace of evidence, regulation and validation testing of these devices so far lags behind the pace of innovation and direct-to-consumer marketing,” said Stephen P. Juraschek, a medical researcher at Beth Israel Medical Center. Deaconess, in Boston, and colleagues in a review published in September in

Current Cardiology Reports

.

Currently, there is no standardized validation protocol to evaluate the accuracy of cuffless devices, as required by the US Food and Drug Administration, although several of these developments have already received authorization for marketing in the US market.

For its part, the European Society of Hypertension has issued its guidelines, where they emphasize that, for now, cuffless devices should not be used to make diagnosis and treatment decisions.

“The potential clinical value of cuffless pressure measurement is enormous.

However, the limitations or precautions are just as great,” says James Sharman, an expert in blood pressure measurement methods and exercise physiologist at the University of Tasmania (Australia).

Before advocating for broader use, it will be necessary to see whether cuffless devices perform accurate measurements, have clinical superiority over the current standard of blood pressure measurement, and determine how they would be integrated into current medical practice, Sharman adds.

Furthermore, since each device has its own algorithm and method for estimating blood pressure, each of them should demonstrate its performance individually.

The work is already in process.

According to the ClinicalTrials.gov database, as of October 2023, more than ten studies were seeking participants to evaluate cuffless devices intended to measure blood pressure.

The Samsung Health Monitor, a photoplethysmographic technology that allows blood pressure to be measured through the Samsung Galaxy Watch, was approved as a medical device by the South Korean Ministry of Food and Drug Safety in April 2020.

There are also several studies already completed.

In Switzerland, a team evaluated its use for ambulatory blood pressure monitoring (ABPM), which measures this parameter continuously for 24 hours and is a better predictor of cardiovascular health than non-continuous measurements.

The study involved 67 patients who performed a traditional ABPM, with a cuff device, but also had a watch-like optical sensor placed on the upper arm or wrist, on the opposite extremity to which They wore the muff.

Although differences were recorded between the measurements of both devices, the difference was small and within the limits recommended by the international standard.

“These results are encouraging and suggest that cuffless 24-hour ABPM may soon become a clinical possibility,” the authors noted in their conclusions.

Additionally, those participating in the study reported that the optical sensor was more comfortable and overwhelmingly preferred it to its cuff alternative.

And in South Korea, a recent observational study followed 760 people who used a Samsung Galaxy watch approved in that country to monitor blood pressure for a month.

The device requires calibration once a month, but curiously 75% of the participants did not trust a single monthly calibration as had been suggested and performed more calibrations.

This allowed the researchers to identify that poor calibration can affect the measurements of the device and it is necessary to standardize the calibration processes to guarantee the proper functioning of the device.

But the study also found that “smartwatch-based blood pressure measurement is feasible for out-of-office blood pressure monitoring in the real world,” as, on average, participants measured this parameter 1.5 times per day.

Will the day come when we can accurately know our blood pressure just by looking at our smart watch or mobile phone?

It's likely, “but not in the near future,” Avolio says.

Even more research is needed so that cuffless devices can provide reliable quantitative information to track physiological changes with acceptable precision, the expert concludes.

Origins of instruments to measure blood pressure

In 1733, Stephen Hales, an English clergyman, was the first to measure and observe the dynamic variation of blood pressure by inserting a glass tube into the artery of a mare.

In 1881, Samuel Siegfried Karl Ritter von Basch, a Czech-Austrian pathologist and microscopist, unveiled his invention of the sphygmomanometer (a water column tensiometer), which was the first non-invasive device to measure blood pressure indirectly. .

The tool was improved over the next quarter century: in 1896, Italian physician Scipione Riva-Rocci presented a variant that used a column of mercury and an inflatable elastic cuff that was placed on the arm.

Later, in 1905, the Russian doctor Nikolai Sergeevich Korotkof made key observations during the repair of blood vessels of soldiers wounded in the Russo-Japanese War of 1904-1905;

He discovered that certain blood flow noises heard during physical examination after inflating and deflating the cuff coincided with the maximum, or systolic, and minimum, or diastolic, values ​​of blood pressure.

This is the origin of classic aneroid or auscultatory sphygmomanometers, which have a bulb to inflate the cuff and also require a stethoscope, and which are still used today in many health centers.

However, since the seventies, automatic oscillometric devices were developed, which contain a calibrated electronic pressure sensor that analyzes the oscillations of the brachial artery of the arm when it is collapsed with the cuff, and which are today the most used and recommended by medical societies and the World Health Organization.

Article translated by

Debbie Ponchner

.

This article originally appeared on

Knowable en español

, a nonprofit publication dedicated to making scientific knowledge available to everyone.

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