Smart watches and other wearables can measure data like the number of steps, calories burned, heart rate—and also blood pressure—in real time. The potential benefits of wearable blood pressure monitoring devices for the general public are real. But studies on today’s products point to accuracy issues, and cardiological societies remain cautious about the use of these devices, which are not yet recommended for the detection and medical management of hypertension. In addition, the systems deemed most reliable are also the bulkiest and most expensive. So, technology research is still needed1,2,3.
CEA-List worked with CEA-Leti and BioMaps to make wearable blood pressure monitoring systems more robust, reliable, compact, and affordable.
Each time the heart beats, arterial blood volume changes. Continuous blood pressure monitoring depends on being able to detect these changes. “Pulse wave velocity, measured optically, and changes in arterial diameter, measured acoustically, are both necessary,” said Sylvain Chatillon, a research engineer at CEA-List.
CEA-Leti is currently developing a wearable optical system for the measurement of physiological parameters based on photoplethysmography, a technique that uses light to measure variations in arterial blood volume with each heartbeat. Ultrasound will be integrated into this system to provide additional information and make physiological measurements—especially blood pressure—more robust. CEA-List drew on its expertise in ultrasound to develop an innovative technology that not only improves signal quality, but that is also more compact. BioMaps, a laboratory at functional MRI center Service Hospitalier Frédéric Joliot (SHFJ), completed initial medical validation testing of the measurement protocol. The protocol is able to deliver the level of accuracy expected for blood pressure monitoring.
CEA-List’s technology is based on ultrasound. Probes placed on the body emit ultrasound waves which, when propagated through an environment, bounce off the different obstacles encountered, creating echoes. These echoes can be analyzed to reconstruct an image. In obstetrics, for instance, a fetus can be “seen” in utero using ultrasound. For blood pressure, ultrasound waves are propagated through the artery in the arm.
CEA-List used its CIVA Healthcare software to calculate the optimal dimensions for the ultrasound probe in order to achieve the target performance for a device worn on the wrist. In validation testing done at BioMaps, multi-element probes made up of several small, independently functioning sensors were used to construct an inter-element matrix of all of the combinations of emitted and received waves. CEA-List also designed algorithms capable of reconstructing an image of the artery using coherent addition of the signals obtained. Measurements of the lapsed time between the echoes coming from the proximal and distal walls located on either side of the arm artery can be used to continuously measure artery diameter and variations in that diameter.
In vitro testing of CEA-List’s algorithms was conducted at BioMaps, first on several different sizes of silicone artery models in water, and then on humans. The algorithms were enhanced for even more precise and dynamic detection and location of echo peaks at the inner and outer artery walls. The result? A real-world in vivo demonstration of reliable, robust, rapid arterial diameter measurement. A total of four patents have been filed to protect the ultrasound technology. A scientific paper is also forthcoming.
CEA-List and CEA-Leti will continue to work toward integrating this new ultrasound capability into the multi-mode wearable system. They will also assess the potential of CEA-Leti’s micromachined ultrasonic transducers (MUTs) for these types of use cases. These advances could soon make smart watches even smarter and our ability to monitor our health even better.
We got great results, which means we now have new capabilities ready to transfer to a manufacturer looking for ways to better respond to the general population’s health monitoring needs. This kind of project is both stimulating and rewarding.
