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

Abay, T., Shafqat, K. & Kyriacou, P. A. ORCID: 0000-0002-2868-485X (2019). Perfusion Changes at the Forehead Measured by Photoplethysmography during a Head-Down Tilt Protocol. Biosensors, 9(2), article number 71. doi: 10.3390/bios9020071


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 (∆HbO2), reduced (∆HHb) and total (∆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. ∆HbO2, ∆HHb and ∆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 ∆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.

Publication Type: Article
Additional Information: © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (
Publisher Keywords: reflectance photoplethysmography; haemoglobin concentrations; Near Infrared Spectroscopy; head-down tilt; venous pooling
Subjects: R Medicine > RC Internal medicine
T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: School of Science & Technology > Engineering
SWORD Depositor:
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Available under License Creative Commons: Attribution International Public License 4.0.

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