Blood pressure monitoring has never been such a pregnant issue. Yet, half of the United States population suffers from hypertension which dramatically increases the risk of stroke and hypertension. According to the CDC, in 2018, hypertension-related consequences were the cause of death for 500,000 Americans.
Blood pressure monitoring equipment is getting smaller and easier to use at home—no need to go to the doctor or pharmacist to check your systolic and diastolic blood pressure accurately. By empowering the users, technology does not only save money but also saves lives.
This article will summarize how sensors dedicated to measuring arterial pressure evolved and what we can expect to reach the market in the coming years.
The past of blood pressure monitoring
In 1881, Samuel Siegfried Karl Ritter von Basch invented what can be seen as the first blood pressure apparatus. The principle was quite close to the modern sphygmomanometer we see in hospitals.
A few differences, though. The blood pressure cuff was positioned on the upper arm, filled with water, and connected to a mercury column. Nevertheless, the principles were essentially the same.
The water-filled cuff restricted the blood flow. The pulse was detected manually, of course. So the first sensor was virtually just a finger sensing the radial artery of the patient.
At that time, the concepts of diastolic and systolic were not even discovered. They only appeared in 1905 when Dr. Nikolai Korotkoff conceptualize the appearance and disappearance of sounds as pressure was applied to the blood vessel.
What is the current status of blood pressure sensors?
The use of modern electronic and optical blood pressure sensors has revolutionized blood pressure monitoring and put a clinically validated device in the home or wrist of anybody suffering from hypertension or just for health-conscious people.
The oscillometric method as the golden standard
A current blood pressure sensor has very few in common with a 19th-century rubber bulb. However, suppose you are wearing a blood pressure smartwatch right now. In that case, the chances are that it relies on an optical sensor connected to a microcontroller that detects the pulse rate and uses complex algorithms to clean the signals and diligently extract all of the components of your arterial pressure.
Automated blood pressure monitors rely on the Oscillometric method. To understand what it means, it is best to start at the beginning and describe the basic principles that contribute to blood flow within our vessels.
When the heart pumps, blood will be pushed through the arteries. The arterial walls will be pushed out due to the oscillation in the flow.
Noninvasive blood pressure (NIBP) cuffs will integrate pressure sensors that will detect the vibrations and extract what is known as the mean arterial pressure (MAP).
Contrary to the standard method relying on the Korsakoff sounds, these devices will compute the diastolic pressure and systolic pressure by relying on their internal algorithms that also rely on the analysis of the heart rate.
Most of the commercially available and FDA cleared blood pressure monitor still relies on an inflatable cuff. We recommend that you read the page maintained by the American Heart Association that gives recommendations on how to monitor blood pressure at home reliably.
The American Medical Association also maintains a list of devices (Validated Device Listing) deemed clinically reliable.
Photoplethysmography (PPG) for wearable devices
Wearable devices such as smartwatches mainly advance a new technology taking advantage of optical sensors to measure the blood flow through the wrist. However, the blood pressure reading is again extracted and computed through complex algorithms and is not a direct measurement based on the changes in sound.
To the best of our knowledge, only two smartwatches combine the advantages of an inflatable cuff with the versatility of a smartwatch: the YHE BPDoctor and the Omron Heartguide. We recently tested the YHE BPDoctor and were impressed that the device’s quality still has few rooms for improvement, especially when it comes to the integration of its software and apps.
PPG sensors are based on a technology initially used to measure heart rate and blood oxygen saturation. LED lights will shine through the superficial layers of the skin, and a photodetector will measure the amount of light transmitted or reflected from the skin.
The blood pulse wave propagation associated with each heartbeat will create periodic differences in the amount of light received by the photosensor. These periodic differences will allow measuring the speed of the blood flow, also known as Pulse Wave Velocity (PWV).
The variation of the PPG signal correlates with the Blood Pressure according to mathematical studies initiated in 1922 by Bramwell et al. The PPG signal variation correlates with the Blood Pressure.
Thus, the underlying theory between PWV and arterial stiffness is not new. Still, up until recently, the accuracy and reliability of the optical sensors and necessary computing power needed were not miniaturized enough to allow wearing it on your wrist. This has luckily changed, and for less than $100, it is now possible to benefit from the progress of technology to measure blood pressure whenever and wherever.
In a study published in 2019, Gašper Slapničar et al. analyzed the reliability and potential of the wristbands relying on PPG to measure blood pressure. Their conclusion did not leave any room for doubts. On the contrary, it was that “BP estimation with a single PPG sensor may be suitable for one-shot BP measurements, while their applicability to continuous BP monitoring is limited.”
Somehow with technology improving at a never seen before rate, yesterday’s doubts may be the reality of today. Even though, to date, no smartwatch relying on optical sensors has been FDA-cleared yet.
What will the future of BP sensors look like?
The current “holy grail” is to provide continuous bp measurement. This is the goal that a handful of innovative startups are striving to achieve.
Blumio is a silicon-valley startup founded in 2015 that has attracted a lot of grants from various prestigious institutes and has partnered with renowned companies such as the pharmaceutical giant Roche.
Blumio aims to detect arterial pulsation using a technology derived from radars and detect arterial blood flow without contact radar waveforms to determine extract blood pressure. Radar technology is not new, and Circadia Health already used it to detect respiratory patterns.
The millimeter-wave radar wearable technology is positioned near the radial artery but does not touch the skin surface. Instead, by directing a 60 Hz skin towards the skin, the device detects the microscopic movements, up to 0.05 mm, associated with the pulse propagation.
The company published several articles showcasing the developments and potential of radar-based technologies in the contactless and continuous detection of blood pressure.
Aktiia is a swiss startup relying on the research conducted at the prestigious Electronic and Microtechnic Swiss Center (CSEM). One of the founders, J. Sola, did his Ph.D. on the use of “non-invasive and continuous blood pressure monitoring.”
The technology is based on the use of PPG sensors integrated into a stylish bracelet. Aktiia’s flagship product is already available on the market and supported by five clinical studies.
The device combines optical sensors with accelerometers to start reading only when the wearer is at rest. Aktiia blood pressure wristband is undoubtedly a clever combination of what the best of technology and healthcare data science can be in the hope of continuously monitor blood flow (one measure every 9 minutes) and deliver blood pressure readings even while asleep.
Interestingly, Aktiia provides its own calibration cuff and recommends using it monthly. Regular calibration is a must for optical sensors as it ensures accuracy and clinical relevance.
NIH Funded Initiatives
In an attempt to develop new ways to monitor hypertension, the National Institute of Health (NIH) provided grants to several companies to finance their research of what the future of blood pressure monitoring will be.
Ramakrishna Mukkamala, Ph.D., is a professor of electrical and computer engineering at Michigan State University. His research team developed an app making the most of the phone fingerprint sensor and computing power.
It only takes 30 seconds for the smartphone to detect the blood flow through the index fingertip and provide what appears as an accurate bp measurement. Making the most of already available devices, such as the smartphone we all carry in our pocket, is an innovative initiative to monitor hypertension in a population unwilling to invest in a new device.
Another initiative conducted at the National Institute of Biomedical Imaging and Bioengineering (NIBIB) relates to using a smart skin patch worn at the neck. Relying on the use of ultrasound, the embedded sensor consistently detected the heart rate and allowed to extract blood pressure measures comparable to those obtained with clinically validated devices.
Finally, the SeismoWatch project, developed by researchers from the Georgia Institute of Technology, relies on detecting chest vibration. By placing the wearable device for just 10-15 against the chest, the embedded sensors detected the blood flow-induced vibration to generate an arterial blood pressure measure.
These projects are still at the research level and are still far to hist the market. Still, they reflect on the growing interest of researchers and bioengineers in developing new sensors to detect blood pressure more conveniently.
To wrap up
With obesity being one of the foremost leading factors of hypertension and heart-related diseases, the new initiative will undoubtedly arise to change the way we measure our blood pressure and take better care of the health of our heart.
They mostly rely on the detection of the pulse wave. Despite all their promises, their main drawback is that contrary to the standard methods based on sound to detect systolic and diastolic pressure, these initiatives only detect the heart rate and base their subsequent analyses on algorithms that still need to be approved by the regulatory authorities.