Scattering-driven PPG signal model

Abstract

This article discusses the origin of PPG signals. Two plausible hypotheses are analyzed: the volumetric hypothesis and a model wherein the PPG is driven by the RBC aggregation process. To verify the model predictions, the PPG signals at the fingertip were measured. External pressure was applied to the fingertip, presumably reducing the blood flow. The results expressed in terms of gamma, used in pulse-oximetry, agree with the aggregation model. In addition, the oscillometric signal and the PPG signal amplitude were simultaneously measured in the fingertip. All of the experimental results favor the proposed aggregation mechanism as responsible the PPG signal.

Fig. 1.

RBC aggregate size vs. shear rate.

Fig. 2.

Rouleaux approximated by equivalent sphere.

Fig. 3.

(a) Volumetric model. Gamma)670 nm, 940 nm). (b) Gamma (590 nm, 940 nm)

Fig. 4.

SDM. (a) Gamma (670 nm, 940 nm). (b) Gamma (590 nm, 940 nm)

Fig. 5.

Reflection model. (a) Gamma (670 nm, 940 nm). (b) Gamma (590 nm, 940 nm).

Fig. 6.

Measuring system diagram consisting of optoelectronic and pneumatic sub-systems.

Fig. 7.

(a) PPG and Oscillometric signals 71 Torr. (b) PPG and Oscillometric signals at 98 Torrs. (c) Delay between Oscillometric and PPG signals.

Fig. 8.

(a) PPG vs. Oscillometric pulse (50 Torrs). (b) PPG vs. Oscillometric pulse (130 Torrs).

Fig. 9.

(a) Experimental gamma (670 nm, 940 nm). (b) Experimental gamma (590 nm, 940 nm).