Glabella: Continuously Sensing Blood Pressure Behavior using an Unobtrusive Wearable Device

By continuously observing the body’s blood pressure response to various stimuli, we are equipped with an opportunity to uncover new insights as to how an individual may respond to certain foods and drugs, observe patterns of blood pressure variability throughout the day, and potentially provide new understanding of a multitude of pathologies.

In order to gain such insights, it is necessary to develop more continuous blood pressure monitoring technologies. One promising method is measuring the pulse transit time (PTT), which is the time that it takes a pulse wave generated by a heartbeat to travel down the arterial tree between two or more sites of the body. Unlike a traditional oscillometric cuff, PTT measurement does not require the use of a cuff and can be measured continuously during each beat. However, typical methods for measuring PTT either require the use of ECG and pulse sensing, which necessitate cumbersome electrodes, or active interaction with a device that potentially obstructs regular activities to accomplish a measurement.

Our device measures the wearer’s pulse using photoplethysmography on three locations of the user’s head, which produces three slightly offset pulse signals. From the offset in these waves, Glabella computes the pulse transit times between the locations, thereby continuously monitoring the behavior of the wearer’s blood pressure on a beat-by-beat basis unobtrusively, conveniently, and throughout their regular day.

We believe that the form factor of our prototype strikes a good balance between wearability and sensing a rich signal; while glasses are exceeded only by wrist watches in the convenience of wear and social acceptability, glasses are constantly worn on the body. Coincidentally, they reach across multiple arteries at different distances from the wearer’s heart, making them optimal for sensing pulse transit times in a usable and ordinary form factor. This is a key feature that current wrist-worn devices do not afford.

In order for PTT to actually become useful for continuous blood pressure monitoring outside the hospital, natural living environments of daily life, the measurement techniques of PTT need to be more unobtrusive and socially acceptable. Our work expands on the use of multi-site PPG measurements to derive PTT, but avoids measuring in unobtrusive locations, such as the finger and the toes, which restricts movement and is typically covered by clothing, respectively. Instead, we opt for measurements on the facial arteries and integrate our system into a pair of glasses, making the use of the system possible in normal everyday life.

Winokur et al. developed a hearing aid-like device that measures single-point ECG, BCG, and PPG through a connection to a PC3. The proposed device requires the user to wear an ear plug and an ECG electrode below the nape and has the potential to continuously measure PAT and PTT an ECG electrode, though further miniaturization would be required to make it comfortable for long-term use and visually unobtrusive. Our system is similar to Winokur et al.’s approach in terms of the head-mounted mechanism, but uses multi-site PPG to avoid the use of ECG and is incorporated into a single device.