Introducing OBPM – The Optical Revolution for Blood Pressure Monitoring

Measuring Pressure

Introducing OBPM – The Optical Revolution for Blood Pressure Monitoring

Above: Visualization of the aktiia bracelet that will deploy the OBPM technology for the prevention, diagnosis and management of hypertension.

According to the World Health Organization, every third adult suffers from hypertension—that is 1.5 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 worldwide. 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 performed with a cuff placed around the arm. This 110-year-old technology is cumbersome and leads to low compliance for patients prescribed to self-monitor. Together with patients’ associations and global-health managers, professionals agree on the fact 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 (OHRM)

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 monitoring of heart rate 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 technology (OHRM) that is integrated into today’s smartwatches and connected bracelets.

The technology behind OHRM was inherited from clinical pulse oximetry and relied on the so-called Photo-Plethysmography 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 to collect the light that had been scattered within the tissues by means of a photo-diode 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 the analysis of those PPG time series.

The major impact that the OHRM technology has had on worldwide spread of wearable devices is partly to be granted to that Swiss pioneering team. However, by that time the CSEM team realized that the full potential of the PPG principle at the wrist was not being fully exploited.

The Invention of Optical Blood Pressure Monitoring (OBPM)

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

Let’s revisit some 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 of less 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 it as your own arterial experience of a water hammer. For instance, if you place any of your fingertips of the right hand on top of the left radial artery (the small artery on the back side of the left wrist) the radial pulse that you will feel is nothing else than a peripheral version of such wall-distending wave. Indeed, this pressure wave originates by the mechanical opening of the aortic valve, but it will very fast propagate along the arterial tree, and as any wave transmission phenomenon, it will be subject to transmission line principles.

In particular two principles are of interest here: the first one is 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. On the other side, reflection phenomena will also appear. Every time that the forward pressure pulse propagating downstream along the aorta will encounter a change of impedance (either because of an arterial branching or by a structural arterial tapering) part of its pulsatile energy will be reflected, creating a backwards pressure pulse that will propagate upstream towards the heart. The complex superposition of the forward and the subsequent backwards pressure pulses will create your own pressure pulse mixture that will depend on your arterial tree topology, your cardiovascular status and… your blood pressure.

For example, realizing that you forgot to address the most important request from a reviewer of one of your manuscripts will trigger on 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 modifying thus 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 morphology of the arterial pressure pulse) contains relevant information on your underlying blood pressure.

Several research groups working on cardiovascular physiology and arterial biomechanics had studied the arterial propagation phenomena since the sixties. While these works came up with very interesting techniques for the 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 undergoing activity in OHRM, CSEM had already gathered large amounts of datasets of optical signals acquired at the wrist of volunteers while they were undergoing 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 into its proprietary datasets of wrist optical signals. The outcome of this pioneer R&D activity was a first 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 toolbox of firmware, middleware, and backend software that enables cuffless and continuous measurement of blood pressure for, virtually, any PPG sensor. These algorithms are tailored towards the 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, connected armbands, and connected bracelets and smartwatches. Figure 1 (below) provides an overview on how the oBPM™ algorithms can be applied to such different sensor types.

Figure 1: The use of the OBPM technology to calculate blood pressure values from optical signals.

Figure 1: The use of the OBPM technology to calculate blood pressure values from optical signals.

During the past years, close collaboration with university hospitals around Switzerland have provided very promising results on the accuracy of the oBPM™ algorithms in anesthetized patients (including invasive reference means and involving very challenging hemodynamic variations) and in healthy volunteers underdoing daily life activities (proving the ability of oBPM™ to cope with any type of artefacts occurring in realistic use cases). Even more, the oBPM™ latest results indicate that devices relying on the technology may not require any re-calibration for, at least, two to three weeks.

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

While the oBPM™ technology has been able to prove its accuracy 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 will be coupled with clinical-proven algorithms: ranging from global prevention campaigns to the titration of hypertensive drugs in patients. The wrist declination of the oBPM™ technology will disrupt the prevention of high blood pressure, will save lives and will help to reduce healthcare costs worldwide. In order to enable the full exploitation of such possibilities, CSEM has recently announced the creation of a new startup company, aktiia.

In the following months aktiia will industrialize its proprietary version of the oBPM™ technology that will be released as the first medical CE-certified and FDA-approved 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 (pictured at the top of this article) will integrate the 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 depth. Embedded oBPM™ algorithms will process arterial pulsation information that will be displayed directly on the wrist wearable. The smartphone/cloud connection will aggregate physiological information from the bracelet empowered by continuously improving machine learning algorithms from the cloud. A health application will serve as the day-to-day interface for users and patients. The cloud infrastructure will also allow on-demand blood pressure records for users, patients as well as healthcare professionals, providing detailed feedback on one’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 time has come to change. We envision that while automated cuff-based devices will keep their position in acute and sub-acute clinical settings, optical-based blood pressure monitors will invade the ambulatory space for prevention, diagnosis and hypertension management purposes. The new data generated by these cuffless monitors is expected to disrupt the existent clinical practices. As stated by Prof. George Stergiou from the European Society of Hypertension, “when this will become available, we will have to re-write the chapter of diagnosis and management of hypertension and all the methods for blood pressure evaluation (auscultatory, oscillometric office, home, ambulatory) will eventually become redundant.”

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

For author queries, contact Josep Sola, aktiia S.A., Neuchâtel (Switzerland), josep@aktiia.com.