Perfusion Changes at the Forehead Measured by Photoplethysmography during a Head-Down Tilt Protocol
Open accessAbstract
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.
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The finger is arterially dominated while the forearm has a larger venous component.
“The finger is arterially dominated while the forearm has a larger venous component [120].”
From Sources of inaccuracy in photoplethysmography for continuous cardiovascular monitoring (2021) · cites this paper as [120]
Shifts in blood volume in the venous system due to changes in respiratory behavior will cause a corresponding change to the baseline amplitude of the PPG signal as blood volume increases and decreases.
“Shifts in blood volume in the venous system due to changes in respiratory behavior will cause a corresponding change to the baseline amplitude of the PPG signal as blood volume increases and decreases [120].”
From Sources of inaccuracy in photoplethysmography for continuous cardiovascular monitoring (2021) · cites this paper as [120]
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- Authors
- Ysehak Abay, T., Shafqat, K., & Kyriacou, P. A.
- Affiliations
- Research Centre for Biomedical Engineering, City, University of London, London EC1V 0HB, UK
- City, University of London (GB)
- Venue
- Biosensors
- Publisher
- MDPI AG
- Published
- 2019-05-27
- Volume
- 9
- Issue
- 2
- Pages
- 71
- ISSN
- 2079-6374 (ISSN-L)
- PMC
- PMC6628167
- OpenAlex
- W2947005341
- Cited by
- 22
- References
- 27
- Citations by year
- 2026: 1 · 2025: 2 · 2024: 3 · 2023: 3 · 2022: 6 · 2021: 7
- FWCI
- 1.14
- Type
- journal article
- Language
- en
- Open access
- gold
- dx.doi.org
- Licence
- Locations
- Biosensors (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- City Research Online (City University London) (acceptedVersion)
- DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
- Funders
- Barts and The London NHS Trust (Barts Charity) (832/1716)
- Topics
- Physical Sciences › Engineering › Biomedical Engineering › Non-Invasive Vital Sign Monitoring
- Health Sciences › Medicine › Radiology, Nuclear Medicine and Imaging › Optical Imaging and Spectroscopy Techniques
- Health Sciences › Medicine › Surgery › Hemodynamic Monitoring and Therapy
- Keywords
- head-down tilt, venous pooling, oxygenated haemoglobin, total hemoglobin, near-infrared spectroscopy, sensor placement, arterial oxygen saturation
- MeSH terms
- Forehead, Hemoglobinometry, Humans, Biosensing Techniques, Photoplethysmography, Head-Down Tilt
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