Diagnostics & Imaging Week National Editor
Let's compare the measurement of blood pressure (BP) to photography.
Basically, it's usually just a single picture, says Phil Shaltis, PhD, and frequently not a very accurate one.
This measurement is most usually taken in the doctor's office, a place where, he notes, it can be especially inaccurate, since the appearance of a white coat or other caregiver in uniform isn't particularly relaxing and thus not producing a normal reading.
And Shaltis says that BP measurement even over short periods provides less than a complete picture about the heart's function and general patient health, but rather only an intermittent sort of record.
"We're trying to fill that gap," Shaltis, president of developmental firm CardioSign (Cambridge, Massachusetts), told Diagnostics & Imaging Week.
Potentially filling that gap – and producing what would essentially be more of a documentary record of heart function, and hopefully able to pick up signals of future impending problems, heart attack or stroke – is the company's BP sensing system that can be worn continuously, 24/7. The device is designed to offer accurate readings by making constant calculations that account for the wearer's movements and body positions.
Supporting the value of this continuous monitoring approach, Shaltis cites every-15-minute BP monitoring over a 24-hour period as a better predictor of heart problems, as well as response to medications.
But filling that gap means removal of the traditional cuff, its labor-intensive use – and its potential inaccuracies.
Shaltis earned his doctorate at the Massachusetts Institute of Technology (Cambridge), where he was a student working with Harry Asada, PhD, the Ford Professor of Engineering and director of MIT's d'Arbeloff Laboratory for Information Systems and Technology, that lab perhaps best known for its work in robotics.
With Asada and Andrew Reisner, MD, of Massachusetts General Hospital (Boston), Shaltis formed CardioSign last year to refine the prototype sensor and move it to commercialization.
The new monitor doesn't use the traditional BP cuff. Instead it uses elements that wrap around the wrist and attach to the finger.
It employs a method called pulse wave velocity, which allows blood pressure to be calculated, Shaltis explained, by measuring the pulse at two points along an artery.
In early models, the researchers used the heart as one of the points, with a heart monitor measuring the EKG. However, EKGs also aren't always accurate, and a heart monitor can be uncomfortable, so the researchers decided to use two points on the hand instead.
That posed a challenge.
The BP in the hand varies, depending on its position: If the arm is raised above the heart, the pressure will be higher than if it is below the heart. The researchers solved that problem by incorporating a sensor that measures acceleration in three dimensions, allowing the hand position to be calculated at any time. This compensates for the error resulting from height changes and also allows calibration of the sensor for more accurate calculation of blood pressure.
So, as the wearer raises the hand up or down, the hydrostatic pressure changes at the sensor. Correlating the change of pulse wave velocity to the hydrostatic pressure change, the system can automatically calibrate its measurement.
Shaltis told D&IW that pulse wave velocity measurement has been available since the 1960s, but without full exploitation of its value in studying the heart because of the difficulties of calibration that adjust to a person's everyday movements.
"The idea is that the speed at which a pressure wave propagates down the arterial tree is related to blood pressure – like a fire hose," he says.
Asada adds: "The human body is so complex, but the cuff gives only snapshot data. If you get signals all of the time you can see the trends and capture the physical condition quite well."
The new calibration method is a way of doing that.
This wearable approach was born from a collaboration called the Home Automation and Healthcare Consortium, which launched in 1995 at MIT and included several faculty members and about 20 companies.
The group's first project was a ring that measures pulse rate and the amount of oxygen present in the blood.
After developing the ring, Asada decided to move on to blood pressure sensing, which he saw as offering even more valuable information about a patient's health.
Shaltis told D&IW that the sensor system has been tested in healthy volunteers and that the company is now working on enhancements to the laboratory device which, he acknowledges, is rather bulky and needs to be made more "user-friendly." The enhancements, he says, will include making it smaller and developing other aesthetic improvements, and, importantly, making it cheaper to manufacture.
Another potential use he points to is in evaluation of sleep apnea and other sleep difficulties.
He says that the monitor could be available to patients within five years, though he declined specifying the internal timeline for its commercialization.
While it might be something first doctor-prescribed, an ideal configuration for the device, and the company, would be a product available on the drugstore shelf, beside the traditional blood pressure cuff system, and also enhanced to provide alerts concerning potential problems, Shaltis notes.
Once the blood pressure information is gathered, the data could be transmitted via radio signals or wireless Internet. The device runs on a tiny battery, about the same size as the ones that power watches.
The project was funded by the National Institutes of Health, National Science Foundation (Washington) and Sharp (Mahwah, New Jersey/Osaka, Japan).
The most recent prototype was developed in collaboration with Sharp, and the development of clinical applications and human testing was led by Reisner at Mass General.