New Device Listens for Blood Pressure

Blood pressure is one of the critical vital signs for health, but standard practice can only capture a snapshot, using a pressure cuff to squeeze arteries. Continuous readings are available, but only by inserting a transducer directly into an artery via a needle and catheter. Thanks to researchers at Caltech, however, it may soon be possible to measure blood pressure continuously at just about any part of the body.

In a paper published in July in PNAS Nexus, the researchers describe their resonance sonomanometry (RSM) approach to reading blood pressure. This new technology uses ultrasound to measure the dimensions of artery walls. It also uses sound waves to find resonant frequencies that can reveal the pressure within those walls via arterial wall tension. This information is sufficient to calculate the absolute pressure within the artery at any moment, without the need for calibration.

This last factor is important, as other non-invasive approaches only provide relative changes in blood pressure. They require periodic calibration using readings from a traditional pressure cuff. The RSM technology eliminates the need for calibration, making continuous readings more reliable.

How resonance sonomanometry works

The researchers’ RSM system uses an ultrasound transducer to measure the dimensions of the artery. It also transmits sound waves at different frequencies. The vibrations cause the arterial walls to move in and out in response, creating a distinct pattern of motion. When the resonant frequency is transmitted, the top and bottom of the artery will move in and out in unison.

This resonant frequency can be used to determine the tension of the artery walls. The tension in the walls is directly correlated with the fluid pressure of the blood within the artery. As a result, the blood pressure can be calculated at any instant based on the dimensions of the artery and its resonant frequency.

The researchers have validated this approach with both mockups and human subjects. They first tested the technology on an arterial model that used a thin-walled rubber tubing and a syringe to vary the pressure. They tested this mockup using multiple pressures and tubing of different diameters.

The researchers then took measurements with human subjects at their carotid arteries (located in the neck), using a standard pressure cuff to take intermittent measurements. The RSM technology was successful, and subsequently was also demonstrated on axillary (shoulder), brachial (arm), and femoral (leg) arteries. The readings were so clear that the researchers mention that they might even be able to detect blood pressure changes related to respiration and its impact on thoracic pressure.

Unlike traditional pressure cuff approaches, RSM provides data during the entire heartbeat cycle, and not just the systolic and diastolic extremes (In other words, the two numbers you receive during a traditional blood pressure measurement). And the fact that RSM works with different-sized arteries means that it should be applicable across different body sizes and types. Using ultrasound also eliminates possible complications such as skin coloration that can affect light-based devices.

The researchers tested their ultrasound-based blood pressure approach on subjects’ carotid arteries.Esperto Medical

“I’m a big fan of continuous monitoring; a yearly blood pressure reading in the doctor’s office is insufficient for decision making,” says Nick van Terheyden, M.D., the digital health leader with Iodine Software, a company providing machine learning technologies to improve healthcare insights. “A new approach based on good old rules of math and physics is an exciting development.”

The Caltech researchers have created a spinoff company, Esperto Medical, to develop a commercial product using RSM technology. The company has created a transducer module that is smaller than a deck of cards, making it practical to incorporate into a wearable armband. They hope to miniaturize the hardware to the point that it could be incorporated into a wrist-worn device. According to Raymond Jimenez, Esperto Medical’s chief technology officer, “this technology poses the potential to unlock accurate, calibration-free [blood pressure measurements] everywhere—in the clinic, at the gym, and even at home.”

It appears that there’s a significant market for such a product. “92 percent of consumers who intend to buy a wearable device are willing to pay extra for a health-related feature, and blood pressure ranks first among such features,” says Elizabeth Parks, the president of Internet of Things consulting firm Parks Associates.

In the future, rather than relying on arm-squeezing blood pressure cuffs, smart watches may be able to directly monitor blood pressure throughout the day, just as they already do for heart rate and other vital signs.