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19th April 2022
The cardiac data such as heart rate and rhythm obtained from a smartphone watch might not be as accurate for individuals with a darker skin tone. This was the conclusion of a systematic review by researchers from the University of Toronto, Canada, presented at the American College of Cardiology Conference 2022.
Cardiovascular disease is a leading global health issue and associated with an increasingly large economic burden. Technological innovations have become ingrained into everyday life and consumers are beginning to use consumer-grade software, such as smart wearables with numerous sophisticated sensors, to provide health insights. Today, commercial wearables can be used to collect cardiac data through electrocardiography (ECG) or photoplethysmography (PPG) and PPG has become the most popular technique for heart rate measurement. On the wrist, PPG detects blood flow rates by capturing the light intensity reflected from skin based on LEDs and photodetectors. However, questions have arisen over the accuracy of heart rate monitoring devices based on differences in skin tone. For example, because PPG detects changes in a beam of green light directed at the skin and since darker skin contains more melanin, it absorbs more green light than lighter skin. Moreover, previous research demonstrated that inaccurate PPG heart rate measurements occur up to 15% more frequently in dark skin as compared to light skin. In addition, pulse oximeter technology, which is also employed in smartwatches can be less accurate in darker skin as shown in one study, where Black patients had nearly three times the frequency of occult hypoxaemia, that was not detected by pulse oximetry as in White patients.
With smartwatches being used for health monitoring, for the present study, the team undertook a systematic review to determine the accuracy of cardiac data by wrist-worn wearable devices for participants of varying skin tones. They included studies in which heart rate and rhythm data were stratified according to the participant’s race and/or skin tone, which was measured using the Fitzpatrick score, which ranges from 1 to 6, with higher scores reflecting darker skin.
Cardiac data and skin tone
The literature search identified 10 studies with a total of 469 participants and the frequency-weighted Fitzpatrick score was reported in 6 of these studies, with 293 patients and the overall mean score was 3.5 (i.e., from the range of 1 – 6).
In 40% of studies, the researchers found a significant reduction in accuracy of heart rate measurements with a wearable device in those with darker skin compared to individuals with lighter skin tones and/or the gold standard measurements such as an ECG or a chest strap. Interestingly, one study found that wearable devices recorded significantly fewer data points for people with darker skin tones, despite no discrepancy in heart rate accuracy. A single study assessed ECG changes and noted a significant reduction in the accuracy of the R-R interval measurements in people with darker skin compared to ECG data (r = 0.98, p < 0.05).
Commenting on these findings, the lead author, Daniel Koerber, said “People need to be aware that there are some limitations for people with darker skin tones when using these devices, and the results should be taken with a grain of salt,” He added that “algorithms are often developed in homogeneous white populations, which may lead to results that are not as generalisable as we would like. Ongoing research and development of these devices should emphasise the inclusion of populations of all skin tones so that the developed algorithms can best accommodate for variations in innate skin light absorption.”
Koerber D et al. The effect of skin tone on accuracy of heart rate measurement in wearable devices: A systematic review. J Am Coll Cardiol 2022