The reflected light variation due to heart rate is typically no more than about 2% – a change that is imperceptible to the human eye. Right: a higher blood pressure pulse causes wider arteries and more light absorption (lower reflectivity). Left: lower pressure preceding the pulse wave means narrower arteries and less absorption (higher reflectivity) of the green light source. PPG devices detect this variation in reflected light and use it to estimate heart rate. The amount of reflected light depends on several factors, one of which is the volume of arteries near the skin’s surface.īlood in the arteries absorbs light better than the surrounding body tissues so, as arteries contract and swell in response to the pulsating blood pressure, the intensity of the reflected light rises and falls. When light emitted by the monitor enters the skin, most of it is absorbed by body tissues, but some is reflected. The science behind optical heart-rate monitors is something called photoplethysmography (PPG): the measurement of artery volume using light. So how do optical heart-rate monitors work, and why aren’t they always accurate? How they work Disappointment with device performance has led to a class action lawsuit in California against Fitbit, alleging that its heart-rate monitors are “grossly inaccurate and frequently fail to record any heart rate at all”. Similarly, Garmin makes it clear that its Vivosmart device is for “recreational purposes and not for medical purposes” and that “inherent limitations” may “cause some heart rate readings to be inaccurate”.ĭespite these disclaimers, monitors that include a heart-rate reading can elicit consumer expectations. For example, Fitbit declares that their product is “not a medical device” and “accuracy of Fitbit devices is not intended to match medical devices or scientific measurement devices”. Makers of consumer heart-rate monitors, however, clearly state that they are not intended for medical purposes. The problem is, they’re not always very accurate.Ī search of, the world’s largest clinical trials database, reveals nearly 200 trials involving Fitbit devices. Tim Collins, Manchester Metropolitan University Ivan Miguel Pires, University of Beira Interior Salome Oniani, Georgian Technical University, and Sandra Woolley, Keele Universityįitbit, Garmin and other consumer heart-rate monitors are increasingly being used in clinical trials.
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