Perfusion Changes at the Forehead Measured by Photoplethysmography during a Head-Down Tilt Protocol

Photoplethysmography (PPG) signals from the forehead can be used in pulse oximetry as they are less affected by vasoconstriction compared to fingers. However, the increase in venous blood caused by the positioning of the patient can deteriorate the signals and cause erroneous estimations of the arterial oxygen saturation. To date, there is no method to measure this venous presence under the PPG sensor. This study investigates the feasibility of using PPG signals from the forehead in an effort to estimate relative changes in haemoglobin concentrations that could reveal these posture-induced changes. Two identical reflectance PPG sensors were placed on two different positions on the forehead (above the eyebrow and on top of a large vein) in 16 healthy volunteers during a head-down tilt protocol. Relative changes in oxygenated ( Δ HbO 2 ), reduced ( Δ HHb) and total ( Δ tHb) haemoglobin were estimated from the PPG signals and the trends were compared with reference Near Infrared Spectroscopy (NIRS) measurements. Also, the signals from the two PPG sensors were analysed in order to reveal any difference due to the positioning of the sensor. Δ HbO 2 , Δ HHb and Δ tHb estimated from the forehead PPGs trended well with the same parameters from the reference NIRS. However, placing the sensor over a large vasculature reduces trending against NIRS, introduces biases as well as increases the variability of the changes in Δ HHb. Forehead PPG signals can be used to measure perfusion changes to reveal venous pooling induced by the positioning of the subject. Placing the sensor above the eyebrow and away from large vasculature avoids biases and large variability in the measurements

Non-invasive techniques for multimodal monitoring in Traumatic Brain Injury (TBI): systematic review and meta-analysis

Background: Monitoring brain oxygenation and intracranial pressure non-invasively and continuously would be of paramount importance in traumatic brain injury. This review collects the evidence published in the last decade on non-invasive monitoring of intracranial pressure and brain oxygenation in patients with traumatic brain injury. The primary motivation of this study was to identify and provide robust evidence of the most effective techniques for the non-invasive multimodal monitoring for traumatic brain injury. Methods: Two reviewers independently searched PubMed, Embase, Scopus, Cochrane Library and Web of Science between January 15, 2010, and January 22, 2020. Cohort studies assessing correlation or accuracy of non-invasive techniques for ICP and/or brain oxygenation monitoring in TBI patients were included. Newcastle-Ottawa Scale was used to assess the methodological quality of the studies. PROSPERO registration ID is CRD42020164739. Main findings: Eight out of the twelve studies selected focused on the non-invasive measurement of intracranial pressure. Near-Infrared Spectroscopy was the main technology for brain oxygenation, while ultrasound-based techniques were also used for intracranial pressure monitoring. PbtO2 monitoring through Near-Infrared Spectroscopy showed low correlation and limited accuracy in detecting hypoxic events. Most papers investigated severe traumatic brain injury, whereas no study used multimodal monitoring. A meta-analysis on non-invasive intracranial pressure monitoring revealed a strong pooled correlation coefficient of 0.725 (95% CI: 0.450 to 0.874; I2 91.31%) between Transcranial Doppler and the gold standard intracranial pressure monitoring. Conclusion: The current meta-analysis has shown that the two most prominent and widely used technologies for non-invasive monitoring in traumatic brain injury are near-infrared spectroscopy and transcranial doppler. Both techniques could be considered for the future development of a single non-invasive and continue multimodal monitoring device for traumatic brain injury