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Originally founded in 2015, the Silicon Valley startup Blumio has emerged as a leader in wearable sensor technology. Since its start, Blumio has been dedicated to developing a cuffless blood pressure (BP) monitor.
An Overview of BP Monitoring Techniques
Blood pressure monitoring is typically achieved by an oscillatory device, which requires a cuff to be inflated over the upper arm and wrist. Once the cuff is fully inflated to a typical pressure of at least 20 mm Hg higher than what would be expected for that individual, all blood flow through the artery comes to a brief stop. The clinician will then slowly reduce the pressure of the cuff, allowing normal blood flow to return.
The value for systolic BP is identified at the first point at which the cuff pressure causes the blood to push the arterial wall open, allowing for its movement through the artery and causing a vibration to arise within the arterial walls.
The value for diastolic BP will subsequently be obtained from the second point or vibration. Taken together, these vibrations are transferred from the wall of the arteries, through the air within the BP cuff and into a transducer. The detection and later transduction of these vibrations into electrical signals can then be measured by oscillatory devices and displayed on a digital readout.
Advancements in Blood Pressure Monitoring Technologies
The ability to measure physiological parameters has a tremendous impact on the daily monitoring of public health and the detection of various conditions.
Frequent monitoring of BP is particularly crucial for the discovery and eventual maintenance of several cardiovascular diseases, including cardiac arrhythmias and hypertension, and any health risks that might be related to these conditions.
Unfortunately, conventional BP monitoring devices often interrupt the daily tasks of individuals who are required to use them. As a result, several alternative and non-invasive BP devices have been explored to operate without interruption while simultaneously improving BP measurements' accuracy.
The United States Food and Drug Administration (FDA) has recently approved the ambulatory use of a BP monitor that is based on applanation tonometry. Applanation tonometry uses a cylindrical plunger to applanate the radial artery. In doing so, a continuous external force must be applied onto the wrist's surface, which can unfortunately cause local pain, discomfort, and/or bruising to occur.
Another alternative approach to BP measurements is photoplethysmography (PPG) technology. One recent application of this technology can be found in a wristwatch system that incorporates both PPG and electrocardiogram (ECG) measurements to measure pulse transit time (PTT) and BP estimates. Whereas the ECG provides information on the timing at which blood is ejected from the aorta, the PPG measures when the pulse wave arrives at the radial artery.
Despite the potential utility of this system, there has been speculation of whether BP estimates, particularly those obtained from the ECG, could be affected by variabilities of the pre-ejection period (PEP). The susceptibility of PPG to interferences from variations in ambient light, as well as whether any tattoos and/or sweat is present at the location of the sensor, can also collectively alter the accuracy of this approach.
While these sensors have shown promising results, none of them are capable of providing direct BP measurements and, instead, predict probable BP values based on other measures. This can inevitably introduce variables into the BP monitoring process and increase the likelihood of inaccurate BP measurements.
Radar-Based Sensors for Blood Pressure Monitoring
For several years, radar-based sensors have been studied for their potential as medical sensors to measure basic vital signs such as heart and respiratory rates.
For such applications, an antenna and radar system is used to target cardiac motion from the surface of the body. The system then receives energy in microwave frequencies or millimeter-wave bands, depending upon the spatial sensitivity requirements. These electromagnetic waves are then captured and recorded to provide information on the respiratory rate and blood circulation. As this technology has progressed, these systems have successfully characterized more complex cardiovascular metrics, including PTT, pulse wave velocity (PWV), pulse arrival time (PAT), and arterial blood volume.
The Blumio Device
Many of the previous radar-based sensors that have been developed for health monitoring applications require the use of skin-contacting or near-contacting approaches.
In addition to these requirements, current radar-based sensing approaches are also associated with certain limitations involving antennas with tissues in an unpredictable reactive near-field region, while transmitted wavelengths can be much larger than the measured motion and low-contrast subsurface reflections are targeted.
Health Sensing with a Wearable Radar Sensor
Video Credit/: Blumio/YouTube.com
To resolve these issues, California-based company Blumio has developed a wearable and non-contacting millimeter-wave radar sensor that measures arterial pulses at the wrist. The basic working principle behind this innovative health monitoring device begins with a single heartbeat that causes a pulsation to travel along the artery and create microscopic motions on the skin's surface to arise.
The millimeter-wave radar device, positioned near the radial artery but does not need to be in direct contact with the skin, interrogates the pulse by transmitting a 60 Hz signal directed towards the skin surface. At this point, the signal will be reflected by the small motions that arise on the skin's surface after each heartbeat. These motions will then be translated into a high-fidelity arterial pressure waveform that will be immediately analyzed by Blumio’s proprietary algorithms to determine exact BP values, as well as other critical cardiovascular measurements.
Conclusion
The incorporation of traditional radar sensors into next-generation health monitoring devices was previously limited by the interference that motion noise can have on their sensitivity.
Since the Blumio device is measuring tiny motions on the skin’s surface, which can be as small as 0.05 millimeters in skin distension, there is a potential that larger bodily movements could interfere with the sensor’s ability to obtain accurate BP measurements. However, the research and development team at Blumio considered this potential background noise while developing its device and incorporated a motion sensor into the Blumio system to identify when a person has been still for extended periods. This sensitive detection mechanism, particularly when seated, is ideal for obtaining accurate BP measurements.
References and Further Reading
Berger A. (2001) Oscillatory Blood Pressure Monitoring Devices. BMJ 323(7318):919. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1121444/.
Johnson, J., Kim, C., & Shay, O. (2019) Arterial Pulse Measurement with Wearable Millimeter Wave Device. 2019 IEEE 16th International Conference on Wearable and Implantable Body Sensor Networks (BSN), Chicago, IL, USA; 1-4, doi:10.1109/BSN.2019.8771037.
Johnson, J., Shay, O., Kim, C., & Liao, C. (2019) Wearable Millimeter-Wave Device for Contactless Measurement of Arterial Pulses. IEEE Transactions on Biomedical Circuits and Systems 13(6); 1525-1534. doi:10.1109/TBCAS.2019.2948581.
A Scientific Approach to Cuffless Blood Pressure Monitoring [Online] Available at: https://www.blumio.com/science/.
Wearable blood pressure sensor edges closer to reality [Online] Available at: https://www.fierceelectronics.com/sensors/wearable-blood-pressure-sensor-edges-closer-to-reality.
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