Non-invasive photoplethysmography and contact pressure optical fibre sensor

Liu, Chong (2020) Non-invasive photoplethysmography and contact pressure optical fibre sensor. PhD thesis, University of Nottingham.

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In healthcare, measurement of the photoplethysmogram (PPG) can be used for diagnosing diseases (e.g. hypoxaemia, vascular and heart diseases) and measuring a patient’s physiological parameters (blood oxygen saturation and heart rate). For measuring PPG signals, the contact force between the skin and the sensor plays an important role since both insufficient and too high contact pressure will distort the PPG signal. It would therefore be advantageous to develop a sensor that could measure contact pressure and PPG simultaneously.

In this research, a novel optical fibre based pulse oximeter probe was developed that combines the measurements of heart rate (HR), blood oxygen saturation (SPO2) and capillary refill time (CRT) whilst recording the contact pressure under ambient conditions. The probe consists of three 45° angle-tipped plastic optical fibres (POFs) used to deliver and collect light for pulse oximetry and CRT measurement along with dual fibre Bragg gratings (FBGs) inscribed in silica fibre to measure the applied contact pressure. Compared to previous studies of using numerous POFs to measure SPO2 values, this research has reduced the number of fibres applied by increasing the coupling efficiency in order to avoid challenges of manufacturing difficulties associated with large numbers of optical fibres. The coupling efficiency increase of POFs applied in the developed probe was achieved via cleaving the optical fibre tip at 45°. All optical fibres were housed in a biocompatible epoxy patch that reduces motion artefacts in the PPG and transduces transverse loading into an axial strain in the FBG. Meanwhile, the sensitivity of the pressure sensing FBG was increased according to the surrounding material’s low Young’s modulus. A reference FBG written on the silica fibre was protected by a stainless steel tube which was applied to compensate the effects of temperature on the pressure sensing FBG.

It was found that the designed non-invasive sensor could continuously detect the PPG signals for SPO2 measurement from a subject’s finger during repeatable 5 minute recording sessions. The recorded PPG signals were assessed by utilising suitable signal quality indices (SQIs) applied in this research, and then the qualifying PPG signals were used to calculate SPO2. This SPO2 test result was validated against the output of the commercial pulse oximeter (Masimo Radical-7), which demonstrated the SPO2 measurement capability of the designed sensor. By comparing the SPO2 data of 10 volunteers obtained from the sensor under different contact pressures (2 to 45kPa), it was found that a contact pressure ranging from 5 to 15kPa generated the optimal PPG signal, and provided the best SPO2 measurement performance. Data recorded by the sensor also demonstrated the ability of the sensor to detect changes in contact pressure and blood volume in the finger.

Since the designed sensing probe is capable of detecting the reflected light intensity and the contact pressure from a subject, the probe was applied to measure 10 volunteers’ CRTs from their index fingers. The CRT was calculated from the detected light intensity and contact pressure data via signal processing (i.e. refilling signal extraction, normalisation and exponential curve fitting process) whilst poor quality capillary refill signals were rejected if they had a positive gradient for the blood refilling region or had an excessive gradient for the baseline region. By comparing the calculated CRT and the indices of exponential fitting curves, an exponential-like relationship was found between them. Further investigation on using indices of exponential fitting curves to assess the reliability of calculated CRT is highly recommended.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Hayes-Gill, Barrie
Morgan, Steve
Korposh, Serhiy
Correia, Ricardo
Keywords: photoplethysmography, PPG, SpO2, capillary refill time, contact pressure, non-invasive monitor, optical fibre
Subjects: R Medicine > R Medicine (General) > R855 Medical technology. Biomedical engineering. Electronics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 60934
Depositing User: Liu, Chong
Date Deposited: 01 Sep 2020 10:35
Last Modified: 01 Sep 2020 10:45

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