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INFLUENCE OF SKIN TONE ON ACCURACY OF WRIST-WORN PHOTOPLETHYSMOGRAPHIC HEART RATE DURING EXERCISE: A PILOT STUDY

Abstract

BACKGROUND: Wearable technology that uses photoplethysmography (PPG) to estimate heart rate (HR) by measuring light reflectance at the epidermis is attractive because it is both inexpensive and noninvasive. However, higher melanin content may affect the accuracy of HR measurement from PPG by reducing the amount of light passing through the epidermis. Previous studies investigating whether subjective ratings of skin color­—as a surrogate for melanin content—are related to PPG HR measurement accuracy are mixed; importantly, none of these studies used an objective assessment of epidermal melanin. The purpose of this pilot study was to investigate the association between melanin content when measured objectively and accuracy of PPG HR measurement. METHODS: Skin color was measured noninvasively in 7 healthy adults (mean±SD; age = 24±5 y; White, n=4; Black, n=2; Middle Eastern, n=1) using a handheld colorimeter. Individual Typology Angle (ITA°), which is highly correlated with epidermal melanin, was calculated from colorimeter measurements and used to classify participants as having dark (ITA° ≤10°), intermediate (10° < ITA° < 41°), or light (ITA° ≥41°) skin. Participants then cycled continuously in temperate conditions (22 °C, 40% relative humidity) for 10 min each at 50%, 60%, 70%, and 80% of age-predicted maximal HR, respectively, followed by a 10-min seated recovery. Chest-worn HR (criterion) and wrist-worn PPG HR were recorded continuously and summarized into 30-s epochs for analysis. An a priori threshold for acceptable mean absolute error (MAE) between criterion and PPG HR was set at ≤5 bpm. RESULTS: MAE was not different (P=0.52) between light (0.9±1.2 bpm, n=1), intermediate (1.3±2.5 bpm; n=4), and dark (0.8±1.6 bpm; n=2) skin tones. In this preliminary analysis, there was also no interaction between skin tone and exercise intensity (P=0.66 for 2-way mixed model ANOVA). Average MAE was <5 bpm for all intensities and during recovery. CONCLUSIONS: These preliminary data demonstrate a high level of agreement between PPG HR and criterion HR across a limited sample of skin tones. A larger sample is needed to further elucidate whether epidermal melanin, represented as ITA°, is associated with accuracy of PPG HR.

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