Photoplethysmography and its application in clinical physiological measurement

Local pulse wave velocity (PWV) is evolving as one of the important determinants of arterial hemodynamics, localized vessel stiffening associated with several pathologies, and a host of other cardiovascular events. Although PWV was introduced over a century ago, only in recent decades, due to various technological advancements, has emphasis been directed toward its measurement from a single arterial section or from piecewise segments of a target arterial section. This emerging worldwide trend in the exploration of instrumental solutions for local PWV measurement has produced several invasive and noninvasive methods. As of yet, however, a univocal opinion on the ideal measurement method has not emerged. Neither have there been extensive comparative studies on the accuracy of the available methods. Recognizing this reality, makes apparent the need to establish guideline-recommended standards for the measurement methods and reference values, without which clinical application cannot be pursued. This paper enumerates all major local PWV measurement methods while pinpointing their salient methodological considerations and emphasizing the necessity of global standardization. Further, a summary of the advancements in measuring modalities and clinical applications is provided. Additionally, a detailed discussion on the minimally explored concept of incremental local PWV is presented along with suggestions of future research questions.

Local Pulse Wave Velocity: Theory, Methods, Advancements, and Clinical Applications

Younger aortas can expand 5 times more than older ones as fluid pumps through them, a finding that could help to design more successful aortic prostheses.

https://link.aps.org/pdf/10.1103/PhysRevX.10.011015

Nonlinear Dynamics of Human Aortas for Material Characterization

Blood pressure (BP) is a crucial indicator of cardiac health and vascular status. This study explores the relationship between radial artery BP and wrist skin strain. A BP estimation method based on the physical model of wrist skin tissues and pulse wave velocity (PWV) is proposed. A photoplethysmography (PPG) sensor and strain gauge are used in this method. The developed strain-based pulse wave sensor consists of a pressing force sensor, which ensures consistent pressing force, and a strain gauge, which measures the cardiac pulsation on the wrist skin. These features enable long-term BP monitoring without incurring the limb compression caused by a cuff. Thus, this method is useful for individuals requiring continuous BP monitoring. In this study, the BP of each participant was measured in three modes (before, during, and after exercise), and the data were compared using a clinically validated sphygmomanometer. The percentage errors of diastolic and systolic BP readings were, respectively, 4.74% and 4.49% before exercise, 6.38% and 6.10% during exercise, and 5.98% and 4.81% after a rest. The errors were compared with a clinically validated sphygmomanometer.

https://www.mdpi.com/2072-666X/9/11/556/pdf

Estimation of Blood Pressure in the Radial Artery Using Strain-Based Pulse Wave and Photoplethysmography Sensors

Humans are an important component of human-machine systems. A better understanding of the role and the status of humans can facilitate and improve the overall human-machine system performance, as well as ensure the well-being of humans. In human factors and ergonomics (HF/E), conventional methods of human performance evaluation usually require the efforts of trained personnel for data collection, data analysis, and the explanation of results. There is an emerging need for a novel and cost-efficient method of assessing human work and status in various systems. The development of wearable technologies has improved the potential for developing a smarter and automatic solution to performing relevant evaluations. In this paper, the authors conduct a scoping review of the studies published from 2001 to 2017 that focused on wearable sensor technology and propose a framework to summarize the research topics. The main steps in the framework include data collection, data processing, and system feedback. Specifically, considering the gaps between current HF/E studies of wearables and the available resources, the authors conducted a detailed review of the application of wearables in HF/E human work evaluation. The opportunities and challenges of introducing wearable sensors into HF/E evaluation are discussed.

Human Work and Status Evaluation Based on Wearable Sensors in Human Factors and Ergonomics: A Review

In light of the need of health monitoring, the paper presents a flexible polyethylene terephthalate (PET)-based wearable sensor for arterial pulse waveform measurement. The sensor encompasses a polydimethylsiloxane (PDMS) microstructure embedded with an electrolyte-enabled 5×1 transducer array, which spans 6mm and has a spatial resolution of 1.5mm. A pulse signal exerts a deflection on the microstructure and is recorded as a resistance change by a transducer at the site of the pulse. An untrained individual can easily align the sensor on a targeted artery with a negligible margin and then acquire the arterial pulse waveform continuously and non-invasively. This sensor is fabricated using microfluidics technology and thus features low cost for mass production. The sensor is hand-held on an artery and records its pulse signal for a 10s period, which bears baseline drift, due to the respiration and the motion artifact. Discrete Meyer Wavelet Transform (DMWT) and Cubic Spline Estimation (CSE) are employed to remove baseline drift in a pulse signal. The pulse waveform is expressed in terms of the sensor deflection as a function of time. Carotid arterial pulse waveforms are measured by the sensor on three subjects at rest and on two subjects post-exercise. Additionally, radial arterial waveforms are measured on one subject at rest. The measured pulse pattern change of the two subjects between at rest and post-exercise is consistent with the literature. As the pulse transmits from central (carotid) to peripheral (radial) for one subject, the ratio of amplitude of main peak to amplitude of dicrotic wave goes up and the up-stroke time becomes shorter. This is consistent with the related observations in the literature. Thus, the limited amount of data collected here demonstrates the feasibility of using the sensor as a wearable health monitoring device.

A Flexible PET-based Wearable Sensor for Arterial Pulse Waveform Measurement

Arterial stiffness and endothelial function are two established surrogate markers of subclinical atherosclerosis and are quantified by three arterial parameters: elasticity, viscosity and radius of the arterial wall. Yet, the current methods for their assessment are unsuitable for routine use. Post-exercise response of the cardiovascular (CV) system serves as a more sensitive detection of subclinical arterial abnormalities that are not apparent at-rest. The objective of this study is to propose a novel method that can measure post-exercise response of arterial parameters and is also suitable for routine use. A microfluidic tactile sensor with a location-insensitive configuration was used for arterial pulse signal measurements on six asymptomatic male subjects, offering measurement reliability, ease use by a layperson, and affordability. By treating the arterial pulse signal as a vibration signal of the arterial wall, vibration-model-based analysis of only one measured pulse signal with no calibration was conducted for simultaneous estimation of three arterial parameters. Exercise-intensity-normalized percent changes in arterial parameters were utilized to remove the influence of variation in exercise intensity on post-exercise response, and then their measured values were compared for difference in post-exercise response between the subjects. One subject who was obese, on subject who had insomnia, and the oldest subject in the study demonstrated differences in post-exercise response at the radial artery (RA), as compared with the three subjects free of those three factors. Despite a lack of statistical significance, the observed difference at the RA between subjects was supported by (i) their consistency with the related findings in the literature, and (ii) their consistency with the measured values at the carotid artery (CA) and superficial temporal artery (STA) and the anatomical difference between the three arteries. The proposed method has the potential of offering an affordable and convenient diagnosis tool for routine arterial health assessment.

Post-exercise Response of Arterial Parameters for Arterial Health Assessment Using a Microfluidic Tactile Sensor and Vibration-Model-Based Analysis: A Proof-of-Concept Study.

The pulsatile pressure in an artery is accompanied by the radial motion of the arterial wall. In this paper, the governing equation of the radial wall motion is first derived, indicating that longitudinal stretching of the arterial wall plays a critical role in the radial wall motion. Based on the derived equation and time-harmonic nature of its radial motion, the arterial wall is modeled as a second-order dynamic system. A microfluidic-based tactile sensor is used to acquire the arterial pulsatile pressure waveform, which is used to represent the radial displacement of the arterial wall. Consequently, the 1st-order and 2nd-order derivatives of the radial displacement are the wall velocity and wall acceleration. In the context of time-harmonic vibration, the key features in the radial displacement and its two derivatives are interpreted to obtain the elasticity and viscosity of the arterial wall: spring stiffness and damping coefficient in the circumferential direction. Pulse signals at radial artery (RA) and superficial temporal artery (STA) on seven healthy human subjects are measured and processed to estimate the two physical properties of the arterial wall. The measured difference in the physical properties between the two arteries and among the seven subjects validates the feasibility of the proposed dynamic modeling of the radial wall motion to capture the physical properties of the arterial wall, and at the same time demonstrates the necessity of subject-specific and artery-site-specific measurements.

Arterial Wall Motion and its Dynamic Modeling for Arterial Stiffness and Damping

A Flexible Microfluidic-Based Sensor for Monitoring the Bending and Tilting of a Metal Bar for Pectus Excavatum (PE) Patients

For the purpose of cardiovascular system monitoring, this paper presents a flexible microfluidic sensor for measuring arterial pulse waveforms. Sitting on a flexible substrate, the core of the sensor is a polymer microstructure embedded with an electrolyte-enabled 5 × 1 resistive transducer array. As a time-varying load, a pulse signal deflects the microstructure and registers as a resistance change by the transducer at the site of the pulse. Radial, carotid and temporal pulse signals are all originally recorded as an absolute resistance signal by the sensor. Moreover, carotid pulse signals are measured on two subjects at-rest and post-exercise. A wavelet-based cascaded adaptive algorithm is written in Matlab to remove baseline drift (or motion artifacts) in a recorded pulse signal. The obtained pulse waveforms are consistent with the related findings in the literature. The sensor features low cost for mass production and easy use by an untrained individual.

Dan Wang

Papers

A Flexible PET-based Wearable Sensor for Arterial Pulse Waveform Measurement

Post-exercise Response of Arterial Parameters for Arterial Health Assessment Using a Microfluidic Tactile Sensor and Vibration-Model-Based Analysis: A Proof-of-Concept Study.

Arterial Wall Motion and its Dynamic Modeling for Arterial Stiffness and Damping

A Flexible Microfluidic-Based Sensor for Monitoring the Bending and Tilting of a Metal Bar for Pectus Excavatum (PE) Patients

Arterial Pulse Waveform Monitoring via a Flexible PET-Based Microfluidic Sensor