Unobtrusive Photoplethysmographic Monitoring Under the Foot Sole while in a Standing Posture.

Seunghyeok Hong,Kwang Park

doi:10.3390/s18103239

Abstract

Photoplethysmography (PPG) of the foot sole could provide additional health-related information compared with traditional PPG of the finger or wrist. Previously, foot PPG required the procedural binding of a light-emitting diode (LED)-photodetector (PD) pair. We achieved PPG of the foot sole without binding any sensors to the foot while the participant stood in a natural standing position on the testing device. Foot PPG was performed using multiple LED-PD pairs to overcome motion artefacts caused by stabilization. We identified regions of the sole suitable for reliable sensor positioning with optimal LED-PD pairs on the basis of the estimated heart rate (HR) and signal quality index derived by dynamic time warping (wSQI). The first experiment included four participants with direct skin-to-sensor contact, and the results showed a mean HR estimation error of 0.01 beats/min and a wSQI of 0.909. The extended experiment with 53 participants, which involved including a gap between the skin and sensors to consider real-life applications, yielded a mean HR estimation error of 0.638 beats/min and a wSQI of 0.751. Based on the selection ratio of optimal LED-PD pairs, the best region of the sole for PPG was the midfoot, except the medial longitudinal arch. In conclusion, we confirmed that foot PPG using multiple LED-PD pairs is appropriate for HR evaluation and further applications.

Highlights

As a human stands naturally on a surface, it is possible to obtain information about the heart, blood vessels, and foot tissue without the person wearing any devices: this is the aim of unobtrusive photoplethysmography (PPG) of the sole of the foot
97.67% and a low wSQI of 0.773, indicates the high accuracy of the valley detection. It shows that the correlation-based signal quality indices (SQIs) do not always vary dependably with each other because the correlation coefficient is sensitive to not just the valley points and the overall morphology of the PPG signal based on amplitude variations
The brightness of each square of coordinates indicates the quality of foot PPG (SIHR and wSQI) using a PD (F17) as a function of the probe location and time window

Summary

Introduction

As a human stands naturally on a surface, it is possible to obtain information about the heart, blood vessels, and foot tissue without the person wearing any devices: this is the aim of unobtrusive photoplethysmography (PPG) of the sole of the foot. Foot photoplethysmograms include physiological information from the heart towards the lower extremities because PPG detects light variations originating from changes in the blood volume being transmitted from the left ventricle. Previous studies have reported various medical applications of foot PPG, including the monitoring of the vascular status, the prevention of diabetic foot ulcers [1,2], and the prediction of sympathetically mediated pain in the lower limbs to evaluate the success of pain-relieving treatments [3]. Compared with conventional finger PPG, foot PPG has been reported to be more accurate for estimating

Methods

Results

Conclusion