Deep blood pressure measurement

The patch performed as well as some clinical methods to measure blood pressure. Applications include real-time, continuous monitoring of blood pressure changes in patients with heart or lung disease, as well as patients who are critically ill or undergoing surgery. The patch uses ultrasound, so it could potentially be used to non-invasively track other vital signs and physiological signals from places deep inside the body. The work was published in Nature Biomedical Engineering ("Monitoring of the central blood pressure waveform via a conformal ultrasonic device").

“Wearable devices have so far been limited to sensing signals either on the surface of the skin or right beneath it. But this is like seeing just the tip of the iceberg,” said Prof. Sheng Xu, author of the study.  “By integrating ultrasound technology into wearables, we can start to capture a whole lot of other signals, biological events and activities going on way below the surface in a non-invasive manner.” The new ultrasound patch can continuously monitor central blood pressure in major arteries as deep as four centimeters below the skin.

The device measures central blood pressure—which differs from the blood pressure that’s measured with an inflatable cuff strapped around the upper arm, known as peripheral blood pressure. Central blood pressure is the pressure in the central blood vessels, which send blood directly from the heart to other major organs throughout the body. Medical experts consider central blood pressure more accurate than peripheral blood pressure and also say it’s better at predicting heart disease.

Measuring central blood pressure isn’t typically done in routine exams, however. The state-of-the-art clinical method is invasive, involving a catheter inserted into a blood vessel in a patient’s arm, groin or neck and guiding it to the heart.

A non-invasive method exists, but it can’t consistently produce accurate readings. It involves holding a pen-like probe, called a tonometer, on the skin directly above a major blood vessel. To get a good reading, the tonometer must be held steady, at just the right angle and with the right amount of pressure each time. But this can vary between tests and different technicians. „Even with the proper technique, if you move the tonometer tip just a millimeter off, the data get distorted. And if you push the tonometer down too hard, it’ll put too much pressure on the vessel, which also affects the data,” said co-first author Chonghe Wang, a nanoengineering graduate student. Tonometers also require the patient to sit still—which makes continuous monitoring difficult—and are not sensitive enough to get good readings through fatty tissue.

The team has developed a convenient alternative—a soft, stretchy ultrasound patch that can be worn on the skin and provide accurate, precise readings of central blood pressure each time, even while the user is moving. And it can still get a good reading through fatty tissue.

Physicians involved with the study say the technology would be useful in various patient procedures. “This has the potential to be a great addition to cardiovascular medicine,” said Dr. Brady Huang, a co-author on the paper and radiologist at UC San Diego Health. “In the operating room, especially in complex cardiopulmonary procedures, accurate real-time assessment of central blood pressure is needed—this is where this device has the potential to supplant traditional methods.”

The patch is a thin sheet of silicone elastomer patterned with what’s called an “island-bridge” structure—an array of small electronic parts (islands) that are each connected by spring-shaped wires (bridges). Each island contains electrodes and devices called piezoelectric transducers, which produce ultrasound waves when electricity passes through them. The bridges connecting them are made of thin, spring-like copper wires. The island-bridge structure allows the entire patch to conform to the skin and stretch, bend and twist without compromising electronic function.

Source: https://www.nanowerk.com/nanotechnology-news2/newsid=51065.php

Original article: https://dx.doi.org/doi:10.1038/s41551-018-0287-x