Continuous-Wave Radar Systems May Replace Cuffed Blood Pressure Armbands
Uncomfortable cuffed armbands for blood pressure monitoring may be a technology of the past with continuous-wave radar systems.
Most of us have experienced the discomfort of having our blood pressure measured by a healthcare professional with a cuffed armband, ranging from moderate pressure to bruising. For some critically ill patients who require constant monitoring, the only available methods involve invasive arterial measurements that carry inherent risks, such as secondary infection.
Researchers, engineers, and physicians have been seeking solutions that involve continuous monitoring of blood pressure without the need for an armband or direct arterial pressure sensing.
Recently, Infineon Technologies AG partnered with the startup Blumio through Infineon’s Silicon Valley Innovation Center (SVIC) to bring a new non-invasive continuous blood pressure monitoring device to the medical industry.
This new partnership will merge Infineon’s monolithic microwave integrated circuits (MMIC) XENSIV 60 GHz radar technology with Blumio cardiovascular health expertise and proprietary DSP algorithms to develop a new concept for wearable technology, tentatively available by 2021.
A block diagram of Infineon’s existing 77 GHz MMIC radar transceiver IC (RASIC). Image used courtesy of Infineon
This technology will monitor cardiovascular signals like pulse rate and systolic/diastolic blood pressure. The goal is to construct a wearable electronic system that provides arterial measurements that are “contactless, continuous, and precise,” according to Blumio.
Continuous Wave Radar and the Beat of Your Heart
Infineon offers frequency-modulated continuous-wave radar (FMCWR) systems. These systems operate based on a known transmission frequency impinged upon a surface at the range and subsequent reception of the echo reflection. The received signal is measured with respect to a delta in time and frequency of a reference—usually the transmitted signal. This allows the system to determine the range and relative velocity.
Recent research published in November 2019 in Nature compared CWR to two other technologies, photoplethysmogram (PPG) and electrocardiogram (ECG), in the measurement of systolic blood pressure. The research indicated that there is a significant improvement in accuracy when measuring systolic blood pressure by using CWR over ECG and PPG.
Researchers’ test measurement setup for comparing CWR to ECG & PPG. Image used courtesy of Malikeh Pour Ebrahim et. al
Blumio is using a variation of the CWR principal to measure the motion of human skin using the heartbeat to determine arterial pressure at the wrist. As the blood travels across the section of the body (1st frame below) being measured, the system is able to translate that motion into coherent signals using CWR (2nd frame below), and extract the following parameters: 1) diastolic pressure, 2) systolic pressure, 3) augmentation pressure, and 4) pulse pressure (3rd frame below).
Contactless analysis of blood pressure circulation using 60 GHz CW radar. Image used courtesy of Blumio
Design Challenges for DSP Engineers
Obtaining meaningful data from electronic systems in the presence of noise and motion artifacts (in the case of radar) remains one of the greatest challenges of radio designers. Filters (primarily digital in MMIC devices) can be used to remove significant portions of noise from the system, but filtering can also cause significant loss of required signal data. Designers will have to cross educational domains, working with bio-medical professionals, to determine the validity of test data.
Additionally, the radar antennas must be able to provide replicable, valid data while allowing patients to remain mobile and perform their daily activities. As researchers in the Nature note, the antenna must be flexible and small so it is not electrically disturbed by motion.
Finally, a third challenge (which may actually benefit the wearable's power efficiency) is the mandatory requirements for specific absorption rates of RF energy in living tissues. The general limits on human exposure in uncontrolled environments is 1.6 W/kg for 1 g of tissue (or 4 W/kg averaged over any 10 g of tissue), according to Health Canada.
The ability to reliably transmit and receive resolvable signals over varying distances is limited principally by the transmission power from the source. In the research paper published by Nature, the continuous-wave radar transceiver was limited to 2 mW (or 2 dBm).
A transceiver operating at 2 dBm could integrate well into low-power IoT medical or commercial devices. The race to be the first to market with this new technology is on, and Blumio and Infineon are looking to be first past the goal post.
Featured image (modified) used courtesy of Blumio
Have you ever helped design a medical device that in some way affected your healthcare experience down the line? If so, share your experience in the comments below.