Medical Design Briefs - October 2024 - 48

problem because it means the blood
is not carrying enough oxygen to the
organs. Taking regular measurements
of blood pressure is considered one of
the best ways to monitor overall health
and to identify potential problems.
Most of us have experienced the
cuff-style measurement of blood pressure.
A nurse, doctor, or machine inflates
a cuff that fits around the upper
arm until blood can no longer flow,
and then slowly releases the air from
the cuff while listening for the sound
that blood makes as it once again begins
to flow. The pressure in the cuff
at that point corresponds to the blood
pressure in the patient's arteries. But
this technique has limitations: It can
only be performed periodically, as it
involves occluding a blood vessel, and
can only collect data from the arm.
Physicians would very much like to
have continuous readings that provide
full waveforms of a patient's blood
pressure, and not only peripheral measurements
from an arm but also central
measurements from the chest and other
parts of the body. To get the full information
they need, intensive care physicians
and surgeons sometimes resort to
inserting a catheter directly into the artery
of critical patients (a practice known
as placing an arterial line, or " a-line " ).
This is invasive and can be risky, but, until
now, it has been the only way to get a
continuous readout of true blood pressure.
In some cases, such as problems
with heart valves, full blood-pressure
waveforms can provide physicians with
diagnostic information that they cannot
get any other way.
" There's a lot of information in that
waveform that is really valuable, " says
Alaina Brinley Rajagopal, a visiting associate
in electrical engineering at Caltech,
an emergency medicine physician, and a
co-author of the paper. And other blood
pressure devices developed over the last
decade or two require a calibration step
that emergency physicians simply do not
have time for, she says. " I need to be able
to put something on a patient and have it
work immediately. "
The new device fits the bill. The current
prototype, built and tested by a
spin-off company called Esperto Medical,
is housed in a transducer case smaller
than a deck of cards and is mounted
on an armband, though the researchers
say it could eventually fit within a pack48
A
concept design for what the team envisions as final products
using the resonance sonomanometry blood pressure method. The
device on the upper arm measures from the brachial artery and
the wristwatch measures from the radial artery. The devices run
independently of one another. (Credit: Matt Fu, Esperto Medical)
age the size of a watch or adhesive patch.
The team aims for the device to first be
used in hospitals, where it would connect
via wire to existing hospital monitors. It
could mean that doctors would no longer
have to weigh the risks of placing an a-line
in order to get the continuous monitoring
of real blood pressure for any patient.
Eventually, Brinley says their device
could replace blood pressure cuffs as
well. " Blood pressure cuffs only take one
measurement as often as you run the cuff,
so if you're asking patients to monitor
their blood pressure at home, they have
to know how to use the device, they have
to put it on, and they have to be motivated
to record the information, and I would
say a majority of patients do not do that, "
says Brinley Rajagopal. " Having a device
like ours, where it is just place and forget,
you can wear it all day, and it can take
however many measurements your provider
wants, that would allow for better,
precision dosing of medication. "
n Developing a Game Changer
Rajagopal recalls the long road it has
been getting to this point with the blood
pressure device. About a decade ago, Rajagopal
returned from a global health trip
particularly frustrated by the standard of
care she could provide patients in remote
locations. Talking with Rajagopal, the two
wished they could invent something like
a medical tricorder, a handheld device
seen in Star Trek that helped the fictional
doctors of the future scan patients, gather
medical information, and diagnose.
" That got us thinking about technologies
we could adapt to get us closer to a goal
like that, " says Rajagopal. Those initial
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sci-fi-inspired discussions eventually
led them down the path to try to develop
a better blood pressure monitor.
But their first efforts did not pan
out. After years of work on a possible
solution using blood velocity to derive
blood pressure, the team decided that
they had reached a dead end. As with
many other current blood pressure
monitoring devices, that approach
could only provide the relative blood
pressure - the difference between
the high and low measurements without
the absolute number. It also required
calibration.
n Back to the Drawing Board
Rajagopal decided it was time to
reevaluate and determine if they had
any chance of solving this problem. " It
was this moment of desperation that actually
led to the key insight, " says Rajagopal.
Thinking back to his first-year physics
course at Caltech, he began scribbling on
a nearby wall. He remembered that his
physics textbook presented a canonical
problem: You have a string under tension.
How can you determine how taut the line
is? If you tweeze the string, you can relate
the velocity at which vibration waves travel
back and forth on the string to the resonance
frequency in the string, which could
give you your answer. " I thought if I could
stretch an artery in one direction and
magically tweeze it and let it go, the ringing
would give us the resonance frequency,
which would get us to blood pressure, " says
Rajagopal. After six years of failures and returning
to first principles, they finally had
their guiding insight.
And indeed, that is the underlying
idea behind the new device: Like a guitar
changing pitch as it is plucked while
being tightened, the frequency at which
an artery resonates when struck by sound
waves changes depending on the pressure
of the blood it contains.
This resonance frequency can be measured
with ultrasound, providing a measure
of blood pressure. This measurement
requires three parameters - a measurement
of the artery's radius, the thickness of
the artery's walls, and the tension or energy
in the skin of the artery.
With the physics worked out, there
were still a lot of other details to be resolved
- identifying the sound waves
that would make arteries resonate, understanding
how to measure that resonance,
and then determining how to efficiently
Medical Design Briefs, October 2024
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Medical Design Briefs - October 2024

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