Wearable Multichannel Photoplethysmography Framework for Heart Rate Monitoring During Intensive Exercise

We propose a multichannel-PPG sensor that uses truncated singular value decomposition (SVD) for accurate estimation of heart rate (HR) during high-intensity exercise. We implemented a sensor to acquire nine channel signals simultaneously, and to enable real-time monitoring of PPG signals and HR. For denoising, we used SVD programmed in the microcontroller unit and showed that the pure pulse is concentrated mostly within the first largest singular value with the related two eigenvectors by extracting the significant feature components of the pulse signals. Our proposed sensor was tested in 32 subjects during exercise of various intensities: walking and medium- and high-intensity jogging. We compared our proposed method with the single-channel method, the multichannel weighting method, and the multichannel best signal selection. Furthermore, we incorporated acceleration signals and a HR tracking algorithm, into our proposed multichannel framework and evaluated its performance using the 12 data sets from the 2015 IEEE Signal Processing Cup Database. Our method showed the lowest absolute errors in HR estimation when compared to the single-channel, multichannel weighting, and multichannel best signal selection methods, irrespective of exercise intensity. We also found that our proposed multichannel framework incorporating acceleration signals and a HR tracking algorithm provided the lowest average absolute error of 0.94 beats/m. The results show that our proposed multichannel framework can be used for accurate HR estimation during high-intensity exercise.

I. Introduction

A photoplethysmogram (PPG) signal consists of direct current (DC) and alternating current (AC) components. The AC component represents the changes in arterial blood volume between the systolic and diastolic phases of a cardiac cycle. The DC component corresponds to the detected light intensity from tissues, venous blood, and non-pulsatile components of arterial blood. A PPG sensor is categorized into transmission and reflectance types. In the transmission type, a light emitting diode (LED) and a photodetector face each other. The light from the LED travels through absorbing substances such as skin pigmentation, bone, and arterial and venous blood, and is received by the photodetector and quantized through filters and an analog-to-digital converter (ADC) [1]–[3]. An example of the transmission type is a fingertip pulse oximeter, which is clinically accepted and widely used [4], [5]. In the reflectance type, an LED and a photodetector are placed on the same side. The light from the LED is reflected from the surface, received by the photodetector, and similarly quantized through filters and an ADC [6]–[8]. The reflectance type is mainly used on parts of the body that are too thick for light transmission (e.g., wrist and forehead); thus, these devices take diverse forms, for example, a band, watch, or patch. Many wearable PPG sensors that monitor heart rate in real time are commercially available.