Advances in basic and applied research in photoplethysmography

Welcome to this Frontiers in Physiology Research Topic Collection on the Advances in Basic and Applied Research in Photoplethysmography. Photoplethysmography (PPG) is a vascular optics technique that can be used to detect blood volume changes in the microvascular bed of tissue with each heartbeat (Allen 2007, Kyriacou and Allen 2021). In recent decades there has been a significant increase in the number of published articles describing basic and applied research in PPG, hailing it as a non-invasive, low cost, and simple optical measurement technique applied at the surface of the skin to measure a whole host of physiological parameters. The popularity of this topic can be attributed to the realization that PPG has important implications for a wide range of applications including cardiovascular system assessment, vital signs monitoring including non-invasive blood pressure estimation and heart rate, and the study of pain. In addition, the recent significant contributions of PPG to wearable devices have had a major impact on the popularity and usability of PPG. There is currently a large body of literature contributing new knowledge on the relation of the PPG pulse morphology, pulse wave analysis and pulse features extraction with the physiological status of peripheral blood vessels, such as vascular ageing and stiffness, blood pressure and compliance, microvascular disease, autonomic function, amongst others. There are also significant efforts in the utilization of the PPG for the detection of heart arrhythmias such as Atrial Fibrillation (AF). Researchers are continuing to strive in combining the PPG sensory capabilities of wearables, such as smartwatches, with Artificial Intelligence (AI) machine learning approaches in delivering ubiquitous health monitoring solutions that go beyond the current available consumer devices. The motivation and aim of this Research Topic is to bring together the latest cutting edge basic and applied research in the field of Photoplethysmography. The Research Topic for Frontiers in Physiology: Computational Physiology and Medicine showcases 16 original research papers covering a diverse range of contributions in the broad field of PPG measurement and analysis

Spectral analysis for pulse rate variability assessment from simulated photoplethysmographic signals

Introduction: Pulse rate variability (PRV) refers to the changes in pulse rate through time and is extracted from pulsatile signals such as the photoplethysmogram (PPG). Although PRV has been used as a surrogate of heart rate variability (HRV), which is measured from the electrocardiogram (ECG), these variables have been shown to have differences, and it has been hypothesised that these differences may arise from technical aspects that may affect the reliable extraction of PRV from PPG signals. Moreover, there are no guidelines for the extraction of PRV information from pulsatile signals.Aim: In this study, the extraction of frequency-domain information from PRV was studied, in order to establish the best performing combination of parameters and algorithms to obtain the spectral representation of PRV.Methods: PPG signals with varying and known PRV content were simulated, and PRV information was extracted from these signals. Several spectral analysis techniques with different parameters were applied, and absolute, relative and centroid-related frequency-domain indices extracted from each combination. Indices from extracted and known PRV were compared using factorial analyses and Kruskal-Wallis tests to determine which spectral analysis technique gave the best performing results.Results: It was found that using fast Fourier transform and the multiple signal classification (PMUSIC) algorithms gave the best results, combined with cubic spline interpolation and a frequency resolution of 0.0078 Hz for the former; and a linear interpolation with a frequency resolution as low as 1.22 × 10−4, as well as applying a fifth order model, for the latter.Discussion: Considering the lower complexity of FFT over PMUSIC, FFT should be considered as the appropriate technique to extract frequency-domain information from PRV signals

Advances in basic and applied research in photoplethysmography

Welcome to this Research Topic in Frontiers in Physiology, focusing on Advances in Basic and Applied Research in Photoplethysmography.Photoplethysmography (PPG) is a low-cost and simple vascular optics technique that can be used to detect blood volume changes in the microvascular bed of tissue with each heartbeat (Allen, 2007; Kyriacou and Allen, 2021). The popularity of this Research Topic area can be attributed to the realization that PPG has important implications for a wide range of applications including assessment of the cardiovascular system, monitoring of vital signs including non-invasive estimation of blood pressure and heart rate, and the study of pain. There is currently a large body of literature contributing new knowledge on the relation of PPG pulse morphology, pulse wave analysis and pulse feature extraction with the physiological status of peripheral blood vessels, such as vascular aging and stiffness, blood pressure and compliance, microvascular disease, autonomic function, and others. There are also significant efforts in the utilization of the PPG for the detection of heart arrhythmias such as Atrial Fibrillation (AF). In addition, the recent significant contributions of PPG to wearable devices have had a major impact on the popularity and usability of PPG. Researchers continue to strive to combine the PPG sensory capabilities of wearables, such as smartwatches, with Artificial Intelligence (AI) machine learning approaches to deliver ubiquitous health monitoring solutions that go beyond currently available consumer devices. PPG and AI have a bright future together for the benefit of patients.The aim of this Research Topic for Frontiers in Physiology is to bring together the latest cutting-edge basic and applied research in the field of Photoplethysmography. Our Research Topic comes from world-leading authors in the field and showcases 16 original research papers covering a diverse range of contributions in PPG measurement and analysis

Establishing best practices in photoplethysmography signal acquisition and processing.

Photoplethysmography is now widely utilised by clinical devices such as pulse oximeters, and wearable devices such as smartwatches. It holds great promise for health monitoring in daily life. This editorial considers whether it would be possible and beneficial to establish best practices for photoplethysmography signal acquisition and processing. It reports progress made towards this, balanced with the challenges of working with a diverse range of photoplethysmography device designs and intended applications, each of which could benefit from different approaches to signal acquisition and processing. It concludes that there are several potential benefits to establishing best practices. However, it is not yet clear whether it is possible to establish best practices which hold across the range of photoplethysmography device designs and applications.This paper is based upon work from COST ACTION CA18216 ‘Network for Research in Vascular Ageing’, supported by COST (European Cooperation in Science and Technology): www.cost.eu . The work was supported by British Heart Foundation (BHF) grant FS/20/20/34626, and by the Estonian Ministry of Education and Research under personal post-doctoral research funding PUTJD815

Customisable Silicone Vessels and Tissue Phantoms for In Vitro Photoplethysmography Investigations into Cardiovascular Disease

Age-related vessel deterioration leads to changes in the structure and function of the heart and blood vessels, notably stiffening of vessel walls, increasing the risk of developing cardiovascular disease (CVD), which accounts for 17.9 million global deaths annually. This study describes the fabrication of custom-made silicon vessels with varying mechanical properties (arterial stiffness). The primary objective of this study was to explore how changes in silicone formulations influenced vessel properties and their correlation with features extracted from signals obtained from photoplethysmography (PPG) reflectance sensors in an in vitro setting. Through alterations in the silicone formulations, it was found that it is possible to create elastomers exhibiting an elasticity range of 0.2 MPa to 1.22 MPa. It was observed that altering vessel elasticity significantly impacted PPG signal morphology, particularly reducing amplitude with increasing vessel stiffness (p p-value of 5.176 × 10−15 and 1.831 × 10−14 was reported in the red and infrared signals, respectively. It has been concluded in this study that a femoral artery can be recreated using the silicone material, with the addition of a softener to achieve the required mechanical properties. This research lays the foundation for future studies to replicate healthy and unhealthy vascular systems. Additional pathologies can be introduced by carefully adjusting the elastomer materials or incorporating geometrical features consistent with various CVDs

Inaccuracy of pulse oximetry with dark skin pigmentation: clinical implications and need for improvement.

Recent reports highlight potential inaccuracies of pulse oximetry in patients with various degrees of skin pigmentation. We summarise the literature, provide an overview of potential clinical implications, and provide insights into how pulse oximetry could be improved to mitigate against such potential shortcomings

Multi-Modal Spectroscopic Assessment of Skin Hydration

Human skin acts as a protective barrier, preserving bodily functions and regulating water loss. Disruption to the skin barrier can lead to skin conditions and diseases, emphasizing the need for skin hydration monitoring. The gold-standard sensing method for assessing skin hydration is the Corneometer, monitoring the skin’s electrical properties. It relies on measuring capacitance and has the advantage of precisely detecting a wide range of hydration levels within the skin’s superficial layer. However, measurement errors due to its front end requiring contact with the skin, combined with the bipolar configuration of the electrodes used and discrepancies due to variations in various interfering analytes, often result in significant inaccuracy and a need to perform measurements under controlled conditions. To overcome these issues, we explore the merits of a different approach to sensing electrical properties, namely, a tetrapolar bioimpedance sensing approach, with the merits of a novel optical sensing modality. Tetrapolar bioimpedance allows for the elimination of bipolar measurement errors, and optical spectroscopy allows for the identification of skin water absorption peaks at wavelengths of 970 nm and 1450 nm. Employing both electrical and optical sensing modalities through a multimodal approach enhances skin hydration measurement sensitivity and validity. This layered approach may be particularly beneficial for minimising errors, providing a more robust and comprehensive tool for skin hydration assessment. An ex vivo desorption experiment was carried out on fresh porcine skin, and an in vivo indicative case study was conducted utilising the developed optical and bioimpedance sensing devices. Expected outcomes were expressed from both techniques, with an increase in the output of the optical sensor voltage and a decrease in bioimpedance as skin hydration decreased. MLR models were employed, and the results presented strong correlations (R-squared = 0.996 and p-value = 6.45 × 10−21), with an enhanced outcome for hydration parameters when both modalities were combined as opposed to independently, highlighting the advantage of the multimodal sensing approach for skin hydration assessment