Non-contact device detects patient blood flow at a distance

For nearly 90 years, doctors have used a simple combination of light and a photo sensor to monitor vitals like blood oxygen saturation and heart rate. Measuring fluctuations in local blood volume over time, this technique, called Photoplethysmography (PPG), is used in devices ranging from clip-on finger sensors to fitness trackers. But while PPG devices are simple and accurate, they’re also limited to one point of contact on a single patient and therefore can miss problems in other parts of the body.

To overcome these limitations, researchers at the University of Waterloo have developed a portable system, called Coded Hemodynamic Imaging, that can monitor the blood flow of patients at multiple arterial points simultaneously and without direct contact with the skin (i.e. at a distance).

“Traditional systems in wide use now take one blood-pulse reading at one spot on the body. This device acts like many virtual sensors that measure blood-flow behaviour on various parts of the body. The device relays measurements from all of these pulse points to a computer for continuous monitoring,” said Robert Amelard, a PhD candidate in systems design engineering at Waterloo and recipient of the prestigious Alexander Graham Bell Canada Graduate Scholarship from the Natural Sciences and Engineering Research Council of Canada. “By way of comparison, think of measuring the traffic flow across an entire city rather than through one intersection.”

A paper recently published in Nature’s Scientific Reports describes how the system works. Light from a high-powered LED is reflected off the patient’s skin and captured by 100 fps camera. A microcontroller coordinates frame captures between the camera and light source to introduce temporally coded illumination (TCI) data, which is then used to remove ambient lighting artifacts from the frames. The system then uses signal processing software to remove camera sensor and process noise. The result is a stable PPGI signal. Tests run on the researchers’ prototype accurately measured heart rate with the camera set up to 1.5 meters from the test subjects.

With further refinement, the researchers say the system could be used to monitor multiple patients for whom skin contact pose challenges, such as in an infant nursery or burn victim ward.
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