Light sensors used to track heart rate in wearables like the Apple Watch Series 5 and Fitbit Versa 2 don’t work well for dark-skinned or obese people, The Verge reports, according to a new study modeling. That’s a problem for what’s built into current devices — but for new applications that use light sensors in wearables, such as monitoring blood pressure, said study author Jessica Ramella-Roman, an associate professor at Florida International University who studies bioimaging sensors. It’s also a bad sign. The architecture of the device has to change, she said.
The study looked at photoplethysmography (PPG) signals, a technology that uses changes in how light is reflected to measure blood flow, on three wearable devices: Apple Watch Series 5, Fitbit Versa 2 and Polar M600. The team used a model that simulates how light moves through the tissue to show how the sensors on these devices would behave under different skin properties.
Darker skin has higher melanin and absorbs more light, while obese skin tends to be thicker, has less water, and has less blood flow than non-obese skin. While previous research on the accuracy and bias of wearables has focused on skin color, Ramella-Roman said that despite these physiological differences, many studies have not included many obese people.
Ramella-Roman: That’s why we think it’s necessary to focus on there.
The researchers’ model found that the PPG signal didn’t change much with skin tone: it varied by less than 10 percent across various devices. But modeling obesity, either alone or in combination with skin color, caused up to a 60 percent change in the signal. The loss of signal appears to be due to changes in skin thickness in obese patients, the authors said in the study. There is also variation in the peak value of the PPG signal, which is used to calculate the heart rate, but its signal strength should not vary based on the heart rate value. The shape of the signal also changes — different groups are using it as a way to track blood pressure.
As we increased BMI levels and increased skin tone, the signal weakened, and then other functions started to disappear, she said. The study found that the Fitbit, which has a lower number of sensors, suffered more signal loss than the Apple Watch.
Ramella-Roman emphasizes that the study simply simulates the way these wearables detect signals in the lab. The research team still needs to examine the devices in actual people to confirm the findings. They are now conducting the study and have recruited about 100 people so far, she said.
But the problems uncovered by analyses like this complicate projects looking to use wearables to track the cardiovascular health of underserved populations, Ramella-Roman said. The new study suggests that researchers must use PPG with caution, especially for programs designed to use the device to help monitor people at high risk for cardiovascular problems — including those with obesity.
The ‘holy grail’ in this case is looking at blood pressure, she said. However, many studies are looking at blood pressure, using some combination of PPG and other modalities. The findings also suggest that other devices that use light sensors and PPGs, such as oximeters in hospitals or doctors’ offices, may not work well for people with obesity. There are very few studies evaluating these devices in these groups.
The good news, says Ramella-Roman, is that it should be possible to adjust the devices to make them more accurate for people with darker skin or obesity, without affecting the accuracy of other groups. The way these systems were originally designed may not have these people in mind, but they can certainly make changes. I don’t think there’s anything limiting that, she said.