A new wearable ultrasound patch that non-invasively monitors blood pressure in arteries deep beneath
the skin could help people detect cardiovascular problems earlier on and with greater precision.
In tests, 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.
A team of researchers led by the University of California San Diego describe their work
in a paper published in Nature Biomedical Engineering.
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
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 (more than one inch) below the skin.
Physicians involved with the study say the technology would be useful in various inpatient
procedures.
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.
It’s highly operator-dependent.
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.
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 UC San Diego-led 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.
The patch was tested on a male subject, who wore it on the forearm, wrist, neck and foot.
Tests were performed both while the subject was stationary and during exercise.
Recordings collected with the patch were more consistent and precise than recordings from
a commercial tonometer.
The patch recordings were also comparable to those collected with a traditional ultrasound
probe.
The patch is a thin sheet of silicone elastomer patterned with what’s called an “island-bridge”
structure—an array of small electronic parts that are each connected by spring-shaped wires
. 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.
The patch uses ultrasound waves to continuously record the diameter of a pulsing blood vessel
located as deep as four centimeters below the skin.
This information then gets translated into a waveform using customized software.
Each peak, valley and notch in the waveform, as well as the overall shape of the waveform,
represents a specific activity or event in the heart.
These signals provide a lot of detailed information to doctors assessing a patient’s cardiovascular
health.
They can be used to predict heart failure, determine if blood supply is fine, etc.
Researchers note that the patch still has a long way to go before it reaches the clinic.
Improvements include integrating a power source, data processing units and wireless communication
capability into the patch.
The team is looking to collaborate with experts in data processing and wireless technologies
for the next phase of the project.