A light-based wearable to detect sleep apnea

Sleep apnea, the disorder where normal breathing is disturbed during sleep, can negatively affect someone’s mental and physical health. In an Eindhoven MedTech Innovation Center (e/MTIC) research collaboration involving TU/e and the Kempenhaeghe Epilepsy and Sleep Center, Gabriele Papini has developed a way to monitor sleep apnea using a compact wrist-worn device that is comfortable to wear and can be used for long-term home monitoring. Papini defended his PhD thesis on February 18th at the department of Electrical Engineering, so the Eindhoven University of Technology, TU/e in a press release.

“Up to 80 percent of those with moderate or severe obstructive sleep apnea (OSA) might go undiagnosed, which can have a significant health impact,” says Gabriele Papini, PhD researcher in the Biomedical Diagnostics Lab at the department of Electrical Engineering.

OSA blocks the regular flow of air, which can disrupt sleep and deprives the body of an adequate supply of oxygen. “If left untreated, the effects of OSA include daytime sleepiness, fatigue, and cardiovascular issues,” notes Papini. “So, well-timed diagnosis is essential.”

Wearables

In clinical tests, the gold standards for OSA diagnosis are polysomnography and polygraphic sleep apnea tests, but both require a person to wear sensors overnight to monitor functions like brain activity, heart rate, and muscle activity.

“Signals from the different sensors are combined, and the number of unusual respiratory events per hour of sleep is counted in the combined signal,” says Papini. “This is known as the apnea-hypopnea index.”

However, the gold standard OSA tools are not suited for screening and for monitoring over multiple nights as the sensors can be uncomfortable to wear, can affect sleep, and would be impossible for a person to use at home. “My research looked at a way to take measurements in comfortable way that could be used by a patient at home, and that also solves the screening and monitoring issues.”

Papini’s solution is a method inspired by popular wearables such as smartwatches and fitness trackers.

“We wanted to design a system that was compact, that didn’t need to be operated by a sleep clinician, and that someone could easily use themselves,” says Papini.

So, Papini and his colleagues developed a method using a wrist-worn device that looks like a smartwatch or fitness tracker and involves illuminating the skin with green light from LEDs. Part of the light is reflected by the blood in the body, and then detected by a light sensor.

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And for the research 500 people, both healthy individuals and those with sleeping disorders such as OSA or insomnia, wore the device while sleeping. More on these experiments in a bit, but what information is contained in the reflected light signal?

“First, the signal contains heartbeat pulses, which can be checked for changes that might be related to OSA,” says Papini.  “The same signal also contains information on the respiratory system, and this can also be used to diagnose OSA, but before properly analyzing the signal, we needed to improve signal quality.”

“The AHI values calculated using our wrist-worn device were in good agreement with the AHI values calculated with the gold standard approaches. So, it is conceivable for a person to use this device to screen and monitor OSA at home,” says Papini. 

Future sleep

The promising results have Sebastiaan Overeem, somnologist at Kempenhaeghe and Papini’s main supervisor, very optimistic about the future: “Hopefully, this research will lead to new techniques that, in addition to a better diagnosis, can also check on the efficiency of treatments for patients with sleep disorders. And importantly, the device could be used at home and for prolonged periods of time.”

Papini is quick though to point out an important aspect of the wrist-worn device. “Unfortunately, it’s not as accurate as the gold standard approaches, particularly those that measure brainwaves, and will never replace the gold standard. But clinicians could ask patients to use the device when they want to monitor OSA for a long time and without the need for lots of cables and sensors.”