Introducing OBPM – The Optical Revolution for Blood Pressure Monitoring

Measuring Pressure

Introducing oBPM, the optical revolution for blood pressure monitoring.

Above: A possible visualization of the aktiia bracelet that will deploy oBPM technology for the prevention, diagnosis, and management of hypertension. The final form factor of the bracelet and its functionalities will be defined during the coming months in consultation with hypertension experts worldwide.

According to the World Health Organization, every third adult suffers from hypertension—which amounts to 1 billion adults worldwide. Hypertension can lead to severe complications, such as stroke and heart failure. Each year, this illness results in 7.5 million premature deaths. The paradox of hypertension is that most people suffering from this condition are unaware of it. As such, hypertension is known as the “silent killer.”

The current gold standard for measurement is perform – ed with a cuff placed around the arm. This 110-year-old technology is cumbersome and leads to low compliance for patients prescribed to selfmonitor. Together with pa- tients associations and global health managers, professionals agree that only a blood pressure measurement that is comfortable (cuffless) and continuous (beat to beat, including during the night) will empower the real fight against the silent killer.

The Revolution of Optical Heart Rate Monitoring

The Swiss Research and Technology Organization (CSEM) initiated the development of cuffless blood pressure monitoring back in 2004. At that time, CSEM was optimizing algorithms for the 24/7 heart rate monitoring based on the analysis of optical signals at the wrist. The publications and patents from those years led to the pioneering works of optical heart rate monitoring (OHRM) technology integrated into today’s smartwatches and connected bracelets.

The technology behind OHRM was inherited from clinical pulse oximetry and relied on the so-called photoplethysmography principle (PPG). The simplicity of the approach was a revolution at that time: one simply needed to illuminate the skin of the wrist via a light source (only red and infrared LEDs were available then) and collect the light that had been scattered within the tissues by means of a photodiode placed on the skin. Because the collected light had been amplitude-modulated by the pulsation of skin arterioles, one could then extract information on heart rate from analysis of those PPG time series.

The singular impact OHRM has had on the worldwide spread of wearable devices can be attributed, in part, to that pioneering Swiss team. However, CSEM representatives realized in 2004 that the full potential of the PPG principle at the wrist was not being fully exploited.

The Invention of Optical Blood Pressure Monitoring

The analysis of PPG to extract heart periodicity information in real life was, of course, a great invention. But those signals contained even more valuable insights. The question was how to identify this additional information and fully exploit its potential.

Let’s revisit the basic physiological background on the genesis of PPG wrist signals. The primary source of any arterial pulsatile activity is the heart. At each new cardiac cycle, the opening of the aortic valve generates a pressure pulse that propagates along the walls of the entire arterial tree. We are not talking about the transport of oxygenated blood within the arteries here (which occurs at low velocities, fewer than 50 cm/s) but about a wall-distending wave that propagates independently of the blood flow and at higher velocities (ranging from 4 m/s in large, elastic arteries up to 30 m/s in small, muscular arteries). Think of your own arterial experience as a water hammer. For instance, if you place any of your right-hand fingertips on top of the left radial artery (the small artery on the back side of the left wrist), the radial pulse you will feel is nothing but a peripheral version of such a wall-distending wave. Indeed, this pressure wave originates via the mechanical opening of the aortic valve, and it then propagates very fast along the arterial tree, and, as with any wave transmission phenomenon, it is subject to transmission-line principles.

Two principles are of particular interest here. The first is the wave propagation velocity. Because of the elasticity of the arterial walls, pressure waves will not propagate instantaneously but will be transmitted according to their mechanical distensibility: an old, stiff aorta will transport pulses faster than a young, elastic aorta. The second principle involves reflection phenomena. Every time the forward pressure pulse propagating downstream along the aorta encounters a change of impedance (either because of an arterial branching or a structural arterial tapering), part of its pulsatile energy will be reflected, creating a backward pressure pulse that will propagate upstream toward the heart. The complex superposition of the forward and the subsequent backward pressure pulses will create one’s own pressure pulse mixture, which will depend on one’s arterial tree topology, cardiovascular status, and … blood pressure.

Imagine, for example, realizing that you forgot about an important meeting. This will trigger in you a fight-or-flight response, accelerating your heart rate, constricting most muscular arteries, and increasing your blood pressure. The increase in blood pressure will distend your elastic arteries, such as the aorta, increasing its stiffness and thus modifying the way it transports pressure pulses. And your own pressure pulse mixture will be completely altered. The principle is thus well identified: your pressure pulse mixture (or the morphology of your arterial pressure pulse) contains relevant information on your underlying blood pressure.

Several research groups working on cardiovascular physiology and arterial biomechanics had studied arterial propagation phenomena beginning in the 1960s. While this research came up with very interesting techniques for pulse wave analysis of the morphology of arterial pressure pulses, their implementation relied on the use of either invasive sensors inserted into the arterial tree or on tonometric sensors placed on top of palpable arteries.

In 2010 (and thanks to the ongoing activity in OHRM), the CSEM had already gathered large numbers of data sets for optical signals acquired at the wrists of volunteers while they were pursuing daily life activities. At that point, the Swiss research group started investigating how to apply the large volume of available know-how in arterial pulse wave analysis to its proprietary data sets of wrist optical signals. The outcome of this pioneer research and development activity was an initial implementation of the Optical Blood Pressure Monitoring (oBPM) algorithms.

The Potential of OBPM

It’s been a long road since the release of the first versions of the oBPM algorithms. Today, oBPM is a full tool box of firmware, middleware, and backend software that enables cuffless and continuous measurement of blood pressure for virtually any PPG sensor. These algorithms are tailored for analysis of PPG signals acquired at different body locations, and they support signals from different sensor topologies. Some implemented solutions include the use of oBPM in pulse oximeters, smartphones, and connected armbands, bracelets, and smartwatches. The below figure provides an overview of how the oBPM algorithms can be applied to such different sensor types.

The use of the OBPM technology to calculate blood pressure values from optical signals.

The use of the OBPM technology to calculate blood pressure values from optical signals.

In recent years, close collaboration with university hospitals around Switzerland has provided very promising results for the accuracy of the oBPM algorithms both in anesthetized patients (including the use of invasive arterial lines to generate reliable reference measurements and involving very challenging hemodynamic variations) and in healthy volunteers engaging in daily life activities (proving the ability of oBPM to cope with any type of artifact occurring in realistic use cases). Even more important, the latest oBPM results indicate that devices relying on the technology may not require any recalibration for at least two to three weeks.

The First Accurate Medical Blood Pressure Monitor Solely Based on Optical Measurements

While oBPM technology has had its accuracy proven in clinical trials and reduced field tests, the time has come to deploy its full potential as a healthcare and commercial breakthrough—in particular, at the wrist. Think about the possibilities that arise when a consumer wrist wearable is coupled with clinically proven algorithms—possibilities that range from global prevention campaigns to the titration of hypertension drugs in patients. The use of oBPM technology at the wrist will contribute to the proliferation of high blood pressure prevention campaigns, save lives, and help reduce healthcare costs worldwide. To enable the full realization of such possibilities, the CSEM recently announced the creation of a new startup company, aktiia S.A.

In upcoming months, aktiia plans to industrialize its proprietary version of the oBPM technology, which will be released as the first medical European Union CE-certified and U.S. Food and Drug Administration-cleared optical-only blood pressure monitor at the wrist. The aktiia ecosystem will consist of three elements: a bracelet, the oBPM algorithms, and a smartphone/cloud connection. The bracelet (see Figure 2) will integrate the optical module, enabling the measurement of reflection PPG signals. Based on proprietary know-how, the sensor will allow the assessment of the pulsation of arterioles at different skin depths. The embedded oBPM algorithms will process arterial pulsation information. The smartphone/cloud connection will aggregate physiological information from the bracelet. A health application will serve as the day-to-day interface for users and patients. The cloud infrastructure will also allow on-demand access to blood pressure records for users, patients, and healthcare professionals, providing detailed feedback on a person’s cardiovascular health trends.

Rewriting the Chapter of Prevention, Diagnosis, and Management of Hypertension

After more than a century of limited access to the real-time, 24/7 blood pressure profiles of hypertensive patients, the moment has come for change. We envision that, while automated cuff-based devices will still be used in acute and subacute clinical settings, optical-based blood pressure monitors will overtake the ambulatory space for prevention, diagnosis, and hypertension management purposes. The new data generated by these cuffless monitors are expected to disrupt existing clinical practice. As summarized by Prof. George Stergiou of the European Society of Hypertension and aktiia’s medical consultant, “When [cuffless monitors] become available, we will have to rewrite the chapter of diagnosis and management of hypertension, and all the methods for blood pressure evaluation will eventually become redundant.”

The oBPM technology originally developed at the CSEM will be a catalyst for that change. Today, we are proud to be further pioneering and revolutionizing the field of nonocclusive blood pressure monitoring with aktiia. For more information, please visit www.aktiia.com.