Relationship Between Measurement Site and Motion Artifacts in Wearable Reflected Photoplethysmography

Abstract

Pulse rates obtained from wearable photoplethysmography (PPG) sensors are important for monitoring cardiovascular condition, especially during exercise. However, it is difficult to precisely count pulse rates during exercise because PPG is sensitive to body movement. The artifacts from body movement are caused by a change in the blood volume at the measurement site, in addition to pulsatile changes. Here, we investigated the influence of motion artifact with respect to light source and anatomical sites. In this study, we compared the signal from green-light PPG to that from infrared PPG at different anatomical sites. In these experiments, 11 subjects were asked to either assume a resting position or generate spontaneous motion artifact by jumping and swinging their arm. As a result, pulse rates obtained from green-light PPG showed a higher correlation with the ECG R-R interval as compared to those obtained with infrared. Additionally, the signal from the upper arm showed less artifact than did the peripheral one. Therefore, the green-light PPG may be useful for pulse rate monitoring.

Comparison between red, green and blue light reflection photoplethysmography for heart rate monitoring during motion

“…Thus, various techniques for the robust measurement of PPG signals during motion have been studied [1]. One approach, based on the optical characteristics of tissue, can be the choice of light wavelength [5][6][7][8]. The light sources used with PPG have been chosen at various wavelengths including the near-infrared (NIR) (e.g., 810 or 940 nm), red, green, and blue [7].…”

“…NIR and red wavelengths are generally used in PPG research and for routine clinical applications [1]. But, among the possible light colors, green light PPG has been shown to have the least influence from motion artifacts when compared with the NIR light PPG [6]. In fact, green light PPG is now often used in smartphone HR measurement, which is available not only in the laboratory but also during normal daily life.…”

“…Naturally, the influences of artifacts on the PPG signal are related to the wavelength of the light source [5,6], simply because of the wavelength dependence of the light absorption (e.g., water, melanin, oxy-and deoxyhemoglobin) and therefore the penetration depth into the tissue [9]. Absorption of the longer wavelengths such as red and NIR is relatively low giving deeper tissue penetration [9].…”

“…According to Cui et al [5], the influences of artifacts are larger in deeper site than in shallower site. Therefore, red and NIR light PPG is subject to artifacts, while the green and blue light PPG is relatively free from artifacts [5,6].…”

“…Although Maeda and colleagues have examined the influence of artifacts on green light PPG signals as compared with NIR light PPG signal [6], they did not compare with other wavelengths during motion. The purpose of our present study was to investigate the use of red, green, and blue light PPG (PPG_red, PPG_green, and PPG_blue, respectively) to discover which of these is the most suitable for measuring HR during normal daily life, where motion is likely to be a significant issue.…”