Samsung Advanced Institute of Technology

Photoplethysmography (PPG) is a well-studied optical measurement technique, utilised for the estimation of blood oxygen saturation (SpO2) and the calculation of heart rate. With the continued development and rapid growth of wearable technologies, PPG has become increasingly more common in every-day consumer devices such as smart phones and watches. There is however minimal knowledge on the effect of the contact pressure exerted by the PPG sensor 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 PPG features) from an artificial tissue-vessel phantom across a range of simulated blood pressure values. Four ranges of blood pressures were recreated corresponding to Hypotensive, Normotensive, Stage 1 Hypertensive and Stage 2 Hypertensive states, confirmed with an inline pressure transducer. Reflectance red and infrared PPG signals were recorded during each blood pressure state and the SNR was calculated.  The protocol confirmed that for reflective PPG signal measurements for a given anatomical model there exists an optimum probe contact pressure when the SNR measurement is at a maximum for red or infrared signals (between 35.1 mmHg and 48.1 mmHg). In this protocol it cannot be concluded that the maximum pressure needed is dependant of the blood pressure state, but may in fact be purely mechanical, and depends on the tissue and vessel properties. Furthermore, 17 morphological features of the PPG signal were investigated and ranked according to which features were less affected by the increase in contact pressure. 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 or compensation applied when using morphological analysis for parameters such as SpO₂ and PPG amplitude variability analysis, usually applied for assessing autonomic responses. Further experimentation is needed to test other morphological features, and different phantom properties, namely vessel elasticity and tissue softness, to see whether significant differences exist between the blood pressure states for phantoms that simulate vessel stiffening or decreased elastic properties. Overall, in vitro modelling of PPG signals is a robust and valuable technique, capable of rigorously testing experimental setups involving different tissue geometries, sensor setups and signal properties.