The 2023 wearable photoplethysmography roadmap

Photoplethysmography is a key sensing technology which is used in wearable devices such as smartwatches and fitness trackers. Currently, photoplethysmography sensors are used to monitor physiological parameters including heart rate and heart rhythm, and to track activities like sleep and exercise. Yet, wearable photoplethysmography has potential to provide much more information on health and wellbeing, which could inform clinical decision making. This Roadmap outlines directions for research and development to realise the full potential of wearable photoplethysmography. Experts discuss key topics within the areas of sensor design, signal processing, clinical applications, and research directions. Their perspectives provide valuable guidance to researchers developing wearable photoplethysmography technology

Non-invasive venous oximetry through venous blood volume modulation

For decades, the monitoring of mixed venous oxygen saturation has been done invasively using fibre-optic catheters. This procedure is not without risk as complications may arise from catheterization. This thesis describes an alternative and novel means of monitoring venous oxygen saturation. The technique outlined involves inducing regular modulations of the venous blood volume and the associated measurement of those modulations using an optical sensor. Just as pulse oximetry utilizes the natural arterial pulse to perform spectral analysis of the peripheral blood in order to estimate the arterial blood oxygen saturation, the new venous oximetry technique uses the artificially generated pulse to perform the task of measuring peripheral venous oxygen saturation. This thesis explores and investigates the feasibility of this new venous oximetry technique. A heuristic model was first developed to predict the effects of introducing an artificially generated pulsatile signal in the venous system. The effect on the underlying natural arterial pulsation was also examined. Experiments were then conducted to justify and interpret the model developed. Other experiments were also conducted to optimize the design of the artificial pulse-based venous oximeter, to explore the effects of prolonged modulation of the venous system and to establish evidence that the measurements made were indeed related to venous oxygen saturation. It is concluded that the new venous oximetry technique is indeed feasible and with further research and development would one day replace the current invasive method

Comparison of NIRS, laser Doppler flowmetry, photoplethysmography, and pulse oximetry during vascular occlusion challenges

© 2016 Institute of Physics and Engineering in Medicine. Monitoring changes in blood volume, blood flow, and oxygenation in tissues is of vital importance in fields such as reconstructive surgery and trauma medicine. Near infrared spectroscopy (NIRS), laser Doppler (LDF) flowmetry, photoplethysmography (PPG), and pulse oximetry (PO) contribute to such fields due to their safe and noninvasive nature. However, the techniques have been rarely investigated simultaneously or altogether. The aim of this study was to investigate all the techniques simultaneously on healthy subjects during vascular occlusion challenges. Sensors were attached on the forearm (NIRS and LDF) and fingers (PPG and PO) of 19 healthy volunteers. Different degrees of vascular occlusion were induced by inflating a pressure cuff on the upper arm. The responses of tissue oxygenation index (NIRS), tissue haemoglobin index (NIRS), flux (LDF), perfusion index (PPG), and arterial oxygen saturation (PO) have been recorded and analyzed. Moreover, the optical densities were calculated from slow varying dc PPG, in order to distinguish changes in venous blood volumes. The indexes showed significant changes (p < 0.05) in almost all occlusions, either venous or over-systolic occlusions. However, differentiation between venous and arterial occlusion by LDF may be challenging and the perfusion index (PI) may not be adequate to indicate venous occlusions. Optical densities may be an additional tool to detect venous occlusions by PPG

Imaging photoplethysmography: towards effective physiological measurements

Since its conception decades ago, Photoplethysmography (PPG) the non-invasive opto-electronic technique that measures arterial pulsations in-vivo has proven its worth by achieving and maintaining its rank as a compulsory standard of patient monitoring. However successful, conventional contact monitoring mode is not suitable in certain clinical and biomedical situations, e.g., in the case of skin damage, or when unconstrained movement is required. With the advance of computer and photonics technologies, there has been a resurgence of interest in PPG and one potential route to overcome the abovementioned issues has been increasingly explored, i.e., imaging photoplethysmography (iPPG). The emerging field of iPPG offers some nascent opportunities in effective and comprehensive interpretation of the physiological phenomena, indicating a promising alternative to conventional PPG. Heart and respiration rate, perfusion mapping, and pulse rate variability have been accessed using iPPG. To effectively and remotely access physiological information through this emerging technique, a number of key issues are still to be addressed. The engineering issues of iPPG, particularly the influence of motion artefacts on signal quality, are addressed in this thesis, where an engineering model based on the revised Beer-Lambert law was developed and used to describe opto-physiological phenomena relevant to iPPG. An iPPG setup consisting of both hardware and software elements was developed to investigate its reliability and reproducibility in the context of effective remote physiological assessment. Specifically, a first study was conducted for the acquisition of vital physiological signs under various exercise conditions, i.e. resting, light and heavy cardiovascular exercise, in ten healthy subjects. The physiological parameters derived from the images captured by the iPPG system exhibited functional characteristics comparable to conventional contact PPG, i.e., maximum heart rate difference was <3 bpm and a significant (p < 0.05) correlation between both measurements were also revealed. Using a method for attenuation of motion artefacts, the heart rate and respiration rate information was successfully assessed from different anatomical locations even in high-intensity physical exercise situations. This study thereby leads to a new avenue for noncontact sensing of vital signs and remote physiological assessment, showing clear and promising applications in clinical triage and sports training. A second study was conducted to remotely assess pulse rate variability (PRV), which has been considered a valuable indicator of autonomic nervous system (ANS) status. The PRV information was obtained using the iPPG setup to appraise the ANS in ten normal subjects. The performance of the iPPG system in accessing PRV was evaluated via comparison with the readings from a contact PPG sensor. Strong correlation and good agreement between these two techniques verify the effectiveness of iPPG in the remote monitoring of PRV, thereby promoting iPPG as a potential alternative to the interpretation of physiological dynamics related to the ANS. The outcomes revealed in the thesis could present the trend of a robust non-contact technique for cardiovascular monitoring and evaluation

Reflectance photoplethysmography for non-invasive monitoring of tissue perfusion

Monitoring blood perfusion and oxygenation changes is of vital importance and for this reason many different techniques have been developed over the decades. Photoplethysmography (PPG) is an optical technique that measures blood volume variations in vascular tissue and it is well known for its utilisation in pulse oximetry for the estimation of arterial blood oxygen saturation (SpO2). In pulse oximetry, mainly the pulsatile component of the signal (AC PPG) is used while the continuous DC component is mostly excluded. Near Infrared Spectroscopy (NIRS) is another optical technique that measures changes in the concentration of oxygenated (ΔHbO2), deoxygenated (ΔHHb), and total haemoglobin (ΔtHb) from the variations in light attenuations at different wavelengths. The main motivation of this research is to explore the capability of Photoplethysmography in assessing tissue perfusion and oxygenation similarly as NIRS. The hypothesis underlining this research is that the DC component of the PPG signal contains information on the overall absorbed light and this part of the PPG signal, acquired at least two wavelengths, may be used to obtain ΔHbO2, ΔHHb, and ΔtHb as performed in NIRS. Therefore, DC PPG attenuations may be related to haemoglobin concentrations by the modified Beer-Lambert law (MBLL). In order to investigate this, novel reflectance, custom-made PPG sensors and measurement systems, including advanced signal processing algorithms, have been developed for the acquisition and analysis of raw PPG signals (AC + DC) from different anatomical locations. Three in vivo studies on healthy volunteers were carried out in order to investigate if ΔHbO2, ΔHHb, and ΔtHb estimated from PPG could indicate changes in blood perfusion and oxygenation. The studies consisted of vascular occlusions on the forearm, negative bed tilting, and whole body cold exposure. Raw PPG signals were acquired from different locations such as the forearm, fingers, and forehead, whereas simultaneous NIRS signals were used as a reference. The results showed that ΔHbO2, ΔHHb, and ΔtHb could be effectively estimated from PPG signals. These parameters indicated the changes in blood volumes and/or oxygenation, whereas comparison with NIRS signals showed good levels of correlation and trending. These promising results showed that DC PPG signals could be used to monitor changes in blood perfusion and oxygenation, extending the range of applications of Photoplethysmography

Investigation of fontanelle photoplethysmographs and oxygen saturations in intensive care neonates and infants utilising miniature photometric sensors

In children and newborn babies on intensive care, information regarding blood oxygen saturation (SpO2) is determined non-invasively by a device called a pulse oximeter. Sensors are usually placed on a hand or foot where their operation relies on the presence of pulsatile arterial blood. Light shines at two or more wavelengths (usually red and infrared) into the tissue where the pulsatile blood modulates, absorbs and scatters the different wavelengths of light in varying amounts and is detected by a photo-detector as a photoplethysmograph (PPG). The spectral information received is then processed electronically and digitally to determine the amount of haemoglobin present. In the sickest of children blood supply can become compromised to these sensor locations and the pulsatile component of the blood may diminish and pulse oximeter readings may become unreliable, especially at times when accurate blood oxygen information would be vital. Currently the alternative is to take blood from an arterial line and run a relatively lengthy analysis (pulse oximeters are near-instantaneous in their operation) that may be unnecessary if the pulse oximeter could be relied upon at these critical moments. In the smallest of babies invasive sampling of blood becomes even more of an issue as any blood loss could lead to hypovolaemia and introduce extra sites of infection plus it causes a lot of stress to the neonate. Since central blood flow may be preferentially preserved, the anterior fontanelle was investigated as an alternative monitoring site. Custom reflectance fontanelle and reference PPG sensors have been designed and built to investigate the fontanelle in those children at risk of peripheral supply compromise. Dedicated instrumentation and software has also been successfully developed for the control of the sensor electronics and the data-logging of PPG signals for retrospective analysis. Sixteen neonates were recruited for fontanelle monitoring; all were ASA 1 – 3 (ASA ranges from 1 to 5 where 1 is the least sick and 5 is the most critically ill). As part of the approved protocol the delivered oxygen to the patients was artificially altered to look for corresponding changes in PPG signal amplitudes. Amplitude results reveal strong correlations (R > 0.5) between the reference sensor (placed on the foot) and the fontanelle sensor. This suggests that the fontanelle sensor is sensitive to changes in amplitude when oxygen in the blood alters. Correlation of the health of the child, using the ASA score, and the difference in amplitudes of PPGs between the sensors reveals that the fontanelle sensor does detect increasing fontanelle PPG amplitudes when compared to the PPGs from the reference sensor the sicker the child is, confirming that pulsatile flow is being preferentially preserved at the fontanelle in those children who are the most at risk from peripheral supply compromise. SpO2 estimation at the fontanelle reveals a mean difference of 2.2 % to the SpO2 as read by the commercial device and a 1.7 % difference to the blood gas results. These results confirm that the anterior fontanelle may be used as an alternative location for SpO2 measurement in those who are at most risk of peripheral supply compromise

The 2023 wearable photoplethysmography roadmap

Photoplethysmography is a key sensing technology which is used in wearable devices such as smartwatches and fitness trackers. Currently, photoplethysmography sensors are used to monitor physiological parameters including heart rate and heart rhythm, and to track activities like sleep and exercise. Yet, wearable photoplethysmography has potential to provide much more information on health and wellbeing, which could inform clinical decision making. This Roadmap outlines directions for research and development to realise the full potential of wearable photoplethysmography. Experts discuss key topics within the areas of sensor design, signal processing, clinical applications, and research directions. Their perspectives provide valuable guidance to researchers developing wearable photoplethysmography technology

Breathing Signature as Vitality Score Index Created by Exercises of Qigong: Implications of Artificial Intelligence Tools Used in Traditional Chinese Medicine.

Rising concerns about the short- and long-term detrimental consequences of administration of conventional pharmacopeia are fueling the search for alternative, complementary, personalized, and comprehensive approaches to human healthcare. Qigong, a form of Traditional Chinese Medicine, represents a viable alternative approach. Here, we started with the practical, philosophical, and psychological background of Ki (in Japanese) or Qi (in Chinese) and their relationship to Qigong theory and clinical application. Noting the drawbacks of the current state of Qigong clinic, herein we propose that to manage the unique aspects of the Eastern 'non-linearity' and 'holistic' approach, it needs to be integrated with the Western "linearity" "one-direction" approach. This is done through developing the concepts of "Qigong breathing signatures," which can define our life breathing patterns associated with diseases using machine learning technology. We predict that this can be achieved by establishing an artificial intelligence (AI)-Medicine training camp of databases, which will integrate Qigong-like breathing patterns with different pathologies unique to individuals. Such an integrated connection will allow the AI-Medicine algorithm to identify breathing patterns and guide medical intervention. This unique view of potentially connecting Eastern Medicine and Western Technology can further add a novel insight to our current understanding of both Western and Eastern medicine, thereby establishing a vitality score index (VSI) that can predict the outcomes of lifestyle behaviors and medical conditions

Development and evaluation of venous oximetry

Photoplethysmography, a technique to measure by optical means volume changes, has been known and applied for many years. One of its most popular applications is pulse oximetry, a non-invasive method to measure oxygen content in arterial blood. It is based on the principle of arterial blood volume changes due to heart contractions, known as systoles. Systolic pulsations appear on the arterial vascular system, while blood flow in veins does not normally present pulsations, especially at remote parts of the peripheral vascular system, such as the fingers. Therefore, pulse oximetry is only applicable to arteries as their pulsations allow for separation of the pulsatile components from the rest of the absorbing components. A novel non-invasive technique permits the measurement of venous oxygen saturation by introducing a series of pulsations in the veins thus allowing the separation of venous signal components for calculation of venous oxygen saturation. This thesis presents a theoretical model describing the mechanical coupling of arteries and veins and its effects in the accuracy of oxygen saturation measurement. [Continues.

Detecting low respiratory rates using myriad, low-cost sensors

The underlying problem for two of the three most common patterns of unexpected hospital deaths (PUHD) is hypoventilation1. Current methods of post-operative respiratory monitoring give delayed signals and have a high false positive rate leading nurses to ignore alarms. We hypothesize there exists a combination of low cost sensors which are capable of providing real time feedback and alarms regarding obstructive sleep apnea and ventilatory depression. Such a monitor would be useful during space travel when monitoring personnel are limited following an injury or if astronauts were to be sedated during extended travel. Methods: Twenty-six subjects were recruited to participate in a study of the effects of Propofol and Remifentanil. Throughout the day, these patients were exposed to varying levels of both drugs simultaneously via target controlled infusions. These patients were attached to breathing and oxygen monitors including chest bands, pulse oximeters, nasal pressure sensors, CO2 capnography, breathing microphones, and thermistors. The patients were then observed for types of apnea or ventilatory depression. Results: The study is currently ongoing however preliminary analyses of the data indicate multiple low cost sensors are capable of detecting respiratory rate as well as obstructive events and apnea. Conclusion: Using only a combination of low cost sensors, we can provide real time respiratory event data to nurses and practitioners