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 Multimodal Monitoring in Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and disability, often resulting in increased intracranial pressure (ICP) and cerebral ischemia. Current ICP measurement methods involve invasive, non-therapeutic procedures. This research aims to develop a non-invasive, continuous optical system for monitoring ICP and cerebral oxygenation. Using backscattered brain optical signals, it leverages cerebral pulsatile photoplethysmograms (PPGs) and non-pulsatile near-infrared spectroscopy (NIRS) signals to assess ICP and oxygenation. The innovation lies in using cerebral NIRS-PPGs to measure ICP, based on the hypothesis that changes in ICP affect cerebral PPG signal morphology. These changes in morphological features, with the support of advanced algorithms including Machine Learning (ML) models, could be utilised in translating the changes in the pulsatile signals in absolute measurements of ICP. The research firstly implemented Monte Carlo simulations to fully understand the effect of multi-source detector separations on brain light tissue interaction. Secondly, a novel reflectance, custom-made TBI multiwavelength and multisource-detector optical sensor and instrumentation, including advanced signal processing algorithms, was designed to acquire, pre-process, and analyse raw PPG signals (AC + DC) from the brain. Thirdly, a novel head phantom and an in vitro brain haemodynamic system were developed for evaluating the sensor. The phantom was the ideal tool for simulating different clinical scenarios that cannot be implemented in real in vivo studies. Fourthly, this research carried out three in vitro studies to investigate the sensor's capability to non-invasively monitor intracranial pressure and oxygenation. The first study evaluated the quality of the optical signals acquired from the developed probe at different source-detector (S-D) separations and multiple wavelengths. It was concluded that the optimal S-D separation to reach the cerebral tissue, and acquire good quality PPG signals, was within 3 cm and 4 cm. The second study assessed the central hypothesis of this research by recording PPG signals from the phantom’s brain at different intracranial pressure levels and implementing ML models utilising pertinent features from the PPG. Results from the second study showed a correlation coefficient of 0.86, mean absolute error of 3.7 mmHg, and limits of agreement of ±4 mmHg, which suggest that NIRS-PPG signals could estimate ICP non invasively. Finally, a third study demonstrated the sensor’s response to in vitro changes in blood oxygenation levels, with less than 33.8% error in half the measurements compared to the reference. This final implementation of spatially resolved spectroscopy algorithms actualize the proposed non-invasive multimodal monitoring sensor for traumatic brain injury. The novel technological developments and the new knowledge acquired from this research paves the way for the development of a transformative non-invasive optical sensor technology for the continuous monitoring of ICP and cerebral oxygenation in TBI patients and beyond

Intra-operative optical monitoring of bowel tissue viability based on photoplethysmography and laser doppler flowmetry

Determination of bowel viability in patients undergoing bowel resection is essential in gastrointestinal surgery. One of the most common operations in gastrointestinal surgery is bowel resection for patients who have different kinds of bowel cancer or any other occlusion in which anastomosis has to be carried out following the removal of an unhealthy segment of the bowel. Monitoring blood flow in abdominal surgery especially intraoperatively would be a valuable tool for prevention of a postoperative anastomosis complication (e.g. anastomotic leak, which is the main complication after colorectal resection). The development of a continuous method for monitoring perfusion of bowel tissue would assist in early detection of inadequate blood supply which then help to reduce the occurrence of an anastomosis complication. Although various monitoring techniques have been proposed to assess intestinal viability intraoperatively, none of these techniques have proved to be reliable enough to replace visual observation. Therefore, to date there is no widely accepted and readily available intraoperative technique to reliably assess the viability of bowel tissue. The aim of this study was to combine the established techniques, laser Doppler flowmetry (LDF) and Photoplethysmography (PPG), into one probe intended for assessment of perfusion in abdominal tissue during bowel resection intraoperatively. In PPG, changes in transmission of light through tissue due to pulsation of small arteries can be monitored whereas in LDF microcirculatory blood cell velocity and flux can be studied. Such a probe could alert the surgeon immediately of any compromise in blood flow so further investigation and, if necessary, therapeutic steps can be applied immediately to prevent severe consequences. Therefore, custom reflectance PPG along with LDF sensor was designed and built in the form of a probe to investigate the changes in blood volume, blood flow and arterial oxygen saturation in patients undergoing bowel resection. The instrumentation was designed successfully and the data was saved for the further analysis. Twenty-four patients undergoing bowel resection were recruited for monitoring of perfusion and blood flowintraoperatively; twenty had undergone laparoscopy and the remainder had a laparotomy operation. Eight different measurements were performed during each trial. The results revealed that the probe could be an indicator of evaluating perfusion and blood flow changes at different stages of the surgery. The results also suggest that laser Doppler is more sensitive to artefact compared to PPG. Differences in amplitude of PPG between different measurements reveal that the sensor does detect changes in blood volume and flow confirming that it has the ability to verify that pulsatile flow is being preferentially preserved at the last step of the resection procedure (at the edges of the anastomosis sites after anastomosis is been constructed