Background: Photoplethysmography (PPG) is a non-invasive optical technique that measures variations in the propagation of light through tissue driven by cardiac cycles. PPG is well-known for its application in pulse oximetry and heart rate estimation in wearables. However, despite the widespread use of PPG, its origin remains uncertain. Several hypotheses have been established in an attempt to explain the origin of the PPG signal. Currently there are three leading hypotheses: 1) blood volume variations in the probed vascular bed during each cardiac cycle [1], 2) variations of optical properties of blood [2] and 3) deformation of surrounding tissue by pulsating blood pressure [3].
Objective: This study introduces an experimental method by which the second hypothesis can be tested for a wide range of wavelengths in relation to the PPG signal.
Methods: We developed an experimental setup that accurately controlled and varied the fluid flow rate. Using this flow system, a fluid can be infused and withdrawn in a controlled manner. The flow system was subsequently used in conjunction with a setup that allowed optical transmission measurements (400-800 nm). Experiments were carried out on human whole blood flowing through a rigid glass tube (1 mm inner diameter). During these measurements, the flow rate was varied (from 0 to 18 mL/min) and it was examined how blood flow rate (through the associated shear rate) influenced optical transmittance. Using this experiment, a potential dependence of optical properties on flow rate (and in particular the associated shear rate) was examined for a range of wavelengths.
Results: The calculated normalized transmittance spectra indicate a dependency on flow rate (n = 3). The behaviour of integrated transmittance spectra reveals that three flow intervals can be distinguished, corresponding to low, medium and high flow rates, each with a different dependency.
Conclusion: Based on the results, it can be concluded that hypothesis 2, concerning the variation in optical properties of blood, was verified, as the normalized transmittance spectra demonstrated variation with flow rate.
Significance: Our findings suggest that changes in the optical properties of flowing blood during cardiac cycles contribute to the measured PPG signal. Understanding the origin of the PPG signal can aid in the development of new biomedical applications and medical devices.

References:
[1] Moço AV, Stuijk S, de Haan G. New insights into the origin of remote PPG signals in visible light and infrared. Sci Rep. 2018 May 31;8(1):8501. doi: 10.1038/s41598-018-26068-2. PMID: 29855610; PMCID: PMC5981460.
[2] Schmid-Schönbein H, Volger E, Klose HJ. Microrheology and light transmission of blood. II. The photometric quantification of red cell aggregate formation and dispersion in flow. Pflugers Arch. 1972;333(2):140-55. doi: 10.1007/BF00586913. PMID: 5065509.
[3] Volkov MV, Margaryants NB, Potemkin AV, Volynsky MA, Gurov IP, Mamontov OV, Kamshilin AA. Video capillaroscopy clarifies mechanism of the photoplethysmographic waveform appearance. Sci Rep. 2017 Oct 16;7(1):13298.
doi: 10.1038/s41598-017-13552-4. PMID: 29038533; PMCID: PMC5643323.