What new sensing approaches measure blood pressure?

Today’s cuff technique for measuring blood pressure (BP) goes back 100 years or so. While the sensors used to measure pressure have changed considerably, the inflate, deflate, and then-measure approach is quite similar. It has even been automated for home use. However, it is not a continuous measurement.

In contrast, researchers are exploring several continuous, noninvasive BP (cNIBP) measurement techniques based on different physical signals and phenomena. These techniques include: volume clamping (measurement of fingertip blood perfusion), tonometry (pressure signals at the surface of the skin), photoplethysmography (light reflectance from blood), pulse transit time (pressure wave velocity), ballistocardiography (reactive forces from cardiac ejection), Doppler ultrasound (blood velocity), and bioimpedance (electrical conductivity of blood). How these measurements are achieved provides an interesting comparison. The last approach and a new technique will be discussed here.

Graphene electronic tattoos

Bioimpedance techniques that measure the electrical conductivity of blood to determine the user’s blood pressure have been used extensively in wearables such as watches and wristband fitness trackers for many years. However, the relationship between the measured bioimpedance and blood pressure requires precise stability. If the electrodes in the user’s wearable device shift by just a few millimeters during the measurement process, the data is not reliable.

More recently, researchers have developed a wearable, continuous BP monitoring platform based on electrical bioimpedance that uses atomically thin, self-adhesive, and lightweight graphene electronic tattoos (GETs) as human bioelectronic interfaces. The GETs can monitor arterial BP for more than 300 minutes. This length is ten times longer than previously reported in other studies.

As shown in Figure 1, blood pressure is recorded continuously and non-invasively, with an accuracy of 0.2 ± 4.5 mm Hg for diastolic (the lower) pressures and 0.2 ± 5.8 mm Hg for systolic (the higher) pressures. This corresponds to a performance equivalent to the Grade A classification for measuring blood pressure referring to the American Association for the Advancement of Medical Instrumentation (AAMI) or British Hypertension Society (BHS) protocols. With a machine learning model trained to analyze the correlation between bioimpedance readings and blood pressure, measurements from graphene electronic tattoos can noninvasively continuously monitor blood pressure.

Figure 1. Two red (injecting) electrodes send a 50 µA current through the arm, where the current moves through the underlying artery (since blood conducts electricity better than tissue). With the four blue (sensing) electrodes measuring the bioimpedance (the body’s resistance to electric current), the bioimpedance changes with every heartbeat. (Image: IEEE Spectrum)

Resonance sonomanometry

Researchers claim resonance sonomanometry to be the first noninvasive method to continually measure true blood pressure. In the well-established ultrasound procedure, a sonogram is the image produced by that procedure. In contrast, manometry is an approach used to measure pressure.

As shown in Figure 2, the new patented technique uses sound waves to obtain blood pressure data from blood vessels while monitoring the response with ultrasound. The sound waves gently stimulate resonance in an artery, and then ultrasound imaging measures the artery’s resonance frequency to achieve a true measurement of blood pressure. In practice, the procedure gives patients a gentle buzzing sensation on the skin and produces results similar to those obtained using a standard-of-care blood pressure cuff. Researchers expect the new technique will enable better blood pressure monitoring at home, in hospitals, and possibly even in remote locations.