Effects of Contact Pressure in Reflectance Photoplethysmography in an In Vitro Tissue-Vessel Phantom

Abstract

With the continued development and rapid growth of wearable technologies, PPG has become increasingly common in everyday consumer devices such as smartphones and watches. There is, however, minimal knowledge on the effect of the contact pressure exerted by the sensor device on the PPG signal and how it might affect its morphology and the parameters being calculated. This study explores a controlled in vitro study to investigate the effect of continually applied contact pressure on PPG signals (signal-to-noise ratio (SNR) and 17 morphological PPG features) from an artificial tissue-vessel phantom across a range of simulated blood pressure values. This experiment confirmed that for reflectance PPG signal measurements for a given anatomical model, there exists an optimum sensor contact pressure (between 35.1 mmHg and 48.1 mmHg). Statistical analysis shows that temporal morphological features are less affected by contact pressure, lending credit to the hypothesis that for some physiological parameters, such as heart rate and respiration rate, the contact pressure of the sensor is of little significance, whereas the amplitude and geometric features can show significant change, and care must be taken when using morphological analysis for parameters such as SpO2 and assessing autonomic responses.

Quantification of the effects of SpO2 accuracy as a function of contact pressure and skin tone

“…Initially, we observed that contact pressure affects the SpO2 readings for all subjects. The readings are closest to the control at 50 mmHg for all skin tones, consistent with a prior study's claim of 35 -50 mmHg as an optimal pressure range for red and infrared PPG signals [7]. This pressure also yielded the least variation (under ± 1) on average, when compared to other pressures, regardless of skin tone.…”

Recent Progress in Flexible and Wearable All Organic Photoplethysmography Sensors for SpO2 Monitoring

“…Hence, an optimal contact pressure determines the maximum pulsatile amplitude. [172][173][174][175] Khan et al studied the reflection-based oximetry on a forearm with applied external pressure of 0.7 kPa. In this experiment, AC signal magnitudes for red and NIR light showed some changes in the baseline values.…”

Photoplethysmography-Based Pulse Rate Variability and Haemodynamic Changes in the Absence of Heart Rate Variability: An In-Vitro Study

“…As explained by Nomoni et al [15] and May et al [10], using this kind of setup has become increasingly useful in the study of PPG signals and light-tissue interaction, since it allows for a dynamic simulation of the circulatory system as well as the investigation of several processes that are difficult to investigate in in-vivo studies due to the large number of uncontrolled variables. Moreover, using these in-vitro setups allows for the investigation of extreme conditions that can be difficult or unethical to simulate in in-vivo circumstances, such as the complete absence of autonomic regulation.…”

“…The main limitation of this study is the restrictions imposed by the in-vitro model. As has been already mentioned, it is not possible to completely simulate the behaviour of the cardiovascular system and the autonomic nervous system in an in-vitro model, limiting the physiological phenomena and anatomical features to be studied [10]. However, having this controlled scenario allowed for understanding how PRV alone would react to haemodynamic changes, and was considered a good first step towards a better understanding of the interaction of PRV and cardiovascular changes.…”

“…Since both HRV and PRV relate to the changes in the duration of cardiac cycles but are measured using signals with different origins [1], there is an intrinsic difficulty in isolating PRV to understand how it is affected by physiological changes and explain the origin of the differences between HRV and PRV using data obtained from in-vivo studies. In recent years, the use of in-vitro models for the simulation of the circulatory system has gained attention, especially since it allows for the investigation of a specific process in the absence of many uncontrolled and confounding variables that are simultaneously occurring in the living human body [10]. Hence, in this study, the development of an in-vitro setup for the simulation of the upper-body circulatory system was proposed, from which PPG signals can be acquired while blood pressure and flow are modified by changing the stroke rate of a commercial pump that replicates the activity of the heart, but without the effects of autonomic modulation, i.e., in the absence of HRV.…”