City Research Online

Abstract

Photoplethysmography (PPG) is a photometric technique used for the measurement of volumetric changes in the blood. The recent interest in new applications of PPG has invigorated more fundamental research regarding the origin of the PPG waveform, which since its discovery in 1937, remains inconclusive. A hand full of studies in the recent past have explored various hypotheses for the origin of PPG. These studies relate the PPG to mechanical movement, red blood cell orientation or blood volume variations. Recognising the significance and need to corroborate a theory behind the PPG formation, the present work rigorously investigates the origin of PPG based on a realistic model of light-tissue interactions. A three-dimensional comprehensive Monte Carlo model of finger- PPG was developed and explored to quantify the optical entities pertinent to PPG (e.g., absorbance, reflectance, and penetration depth) as the functions of multiple wavelengths and source-detector separations. Complementary to the simulations, a pilot in vivo investigation was conducted on eight healthy volunteers. PPG signals were recorded using a custom-made multi-wavelength sensor with an adjustable source-detector separation. Simulated results illustrate the distribution of photon-tissue interactions in the reflectance PPG geometry. The depth-selective analysis quantifies the contributions of the dermal and subdermal tissue layers in the PPG wave formation. A strong negative correlation (r = -0.96) is found between the ratios of the simulated absorbances and measured PPG amplitudes. This work quantified for the first time the contributions of different tissue layers and sublayers in the formation of the PPG signal.

Publication Type:	Article
Additional Information:	As the Version of Record of this article is going to be published on a subscription basis, this Accepted Manuscript will be available for reuse under a CC BY-NC-ND 3.0 licence after a 12 month embargo period.
Subjects:	R Medicine > RC Internal medicine T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments:	School of Science & Technology > Engineering
SWORD Depositor:	Symplectic Administrator

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