Blood Pressure Estimation Using Custom Photoplethysmography Sensors Located on Radial Artery at Wrist
Authors: Nguyen Long , Kim Jong-Jin , Chung Wan-Young 
Published: July 2021

Introduction

Cardiovascular diseases (CVD), diabetes, and chronic kidney disease are among the most society concerns with a high correlation to blood pressure (BP) factor. As a result, developing a wearable device with a BP monitoring function is highly demanded health caring and monitoring. In order to measure BP, traditional methods are based on a sphygmomanometer as a gold standard, but it is not suitable for wearable applications. A solution, therefore, employed some sensors placed along artery paths of a human body and estimated BP from time transfer delay or Pulse Transit Time (PTT) of blood volume through these paths [1]. In practice, it is comfortable if all sensors are located centrally in a specific region and the wrist hand is considered the most convenient. For detecting the pulse wave of blood flow, photoplethysmography (PPG) is the king reigning the wearable device market for healthcare because of its small form factor, none-electrode-contact requirement, and multi-wavelength applications in extracting health indexes such as heart rate, SPO2, Glucose, Hydration levels, etc. Hence, a custom PPG sensor aiming to estimate BP in this paper is designed to optimize signal strength collecting and then applied to measure PTT, Reflective PTT (R-PTT) on the radial artery at three wrist’s location as Fig. 1.

Method

First, a three-sensor array with four LEDs at different wavelengths (525nm, 630nm, 850nm, and 940nm) is designed. The distances between LEDs and a single photodetector (PD) are considered to find the optimized places to detect blood pulse and the intensity of light by controlling the electric current on LEDs. The PD is a broadband sensitivity sensor permitting it absorbs all reflecting lights from LEDs on blood flow.

Next, because light penetrates the skin depends on wavelength and intensity, we inherited the research work by Paul C.-P. Chao et al. [2] to select the optimal distances for placing LEDs. Next, a wearable device utilizing an ultra-low-energy analog-front-end, high-resolution 24bit-ADC, and small form factor is designed for collecting PPG data. After that, three well-known locations on a radial artery in Traditional Chinese Medicine at wrist, namely Cun, Guan, and Chi are considered to measure PTT for inter-sites and reflective PTT (R-PTT) for each site and compare each other’s to evaluate how different in each method, which one is better for estimating BP. The position of these points and the depth of the radial artery had been widely studied in [3].
Finally, BP estimation based on R-PTT with a validation using a 24-hour ambulatory BP meter namely Oscar 2TM from SunTech Medical® permits us to extract heart rate, systolic BP and diastolic BP. Five healthy subjects with an average age of 30 ± 6, height 170 ± 8 cm are recruited for seven days (9am-9pm) experiments.

Results and Conclusions

From our experiment, we discover the in-phase and invert-phase effect which affect the ratio AC/DC for each wavelength. In invert-phase status, the longer wavelength gives the better AC/DC ratio while this rule will change in in-phase condition. In other words, depending on the locations and status of the PPG pulse, the AC/DC ratio of these signals have a different outcome. Next, for measuring PTT between each location pair Cun-Guan, Chi-Guan and Cun-Chi are ambiguous by phase effect and resolution of data sampling while R-PTT takes advantage because of using just a single site. The correlation values of the R-PTT to SBP and DBP are r = 0.8 and 0.65 respectively.

References

[1] Mukkamala, Ramakrishna, Jin-Oh Hahn, Omer T. Inan, Lalit K. Mestha, Chang-Sei Kim, Hakan Töreyin, and Survi Kyal, Toward ubiquitous blood pressure monitoring via pulse transit time: theory and practice, IEEE Trans. on Biomedical Engineering 62, no. 8 (2015): 1879-1901.
[2] Kao, Y.H., Chao, P.C.P. and Wey, C.L., 2018. Design and validation of a new ppg module to acquire high-quality physiological signals for high-accuracy biomedical sensing. IEEE Journal of Selected Topics in Quantum Electronics, 25(1), pp.1-10.
[3] Kim, J.U., Lee, Y.J., Lee, J. and Kim, J.Y., 2015. Differences in the properties of the radial artery between cun, guan, chi, and nearby segments using ultrasonographic imaging: a pilot study on arterial depth, diameter, and blood flow. Evidence-Based Complementary and Alternative Medicine, 2015.

Figure 1