Saoud, Yasmine
(2024)
Optical fibre sensor for measuring endotracheal tube cuff contact pressure on tracheal wall.
PhD thesis, University of Nottingham.
Abstract
An endotracheal tube (ETT) is a tube placed through the mouth and into the trachea by an anaesthetist. It provides airway management for unconscious patients by facilitating ventilation and oxygenation. The inflatable cuff located towards the bottom of the ETT is used to provide a seal to prevent liquid and bacteria in the mouth from entering the lungs; it is also used to prevent backflow of air from the ventilator escaping through the mouth. The main problems associated with ETTs are caused by the inflatable cuff. The ETT cuff is generally inflated to an intracuff pressure of 20-30 cmH2O as per hospital guidelines. Over inflation of the cuff exerts high pressures (of over 30 cmH2O) onto the tracheal wall which obstructs the tracheal capillaries, damaging the mucosa and muscle tissue, causing complications like tracheal stenosis, tracheomalacia, and even in some cases tracheoesophageal fistulas. Under inflation of the cuff can cause ventilator associated pneumonia (VAP). VAP not only increases hospital stay but it costs the NHS £10,000 – £20,000 per patient for treatment. It also increases patient morbidity, as 10-20% of patients that are mechanically ventilated go on to develop VAP and increases patient mortality- as between 3,000 and 6,000 people die from VAP every year.
Optical fibre sensors are well suited for use in biological environments as they are biocompatible, extremely small in size, can withstand harsh conditions, and are not affected by electromagnetic interference. Additionally, research shows that Fibre Bragg Grating (FBG) based pressure sensors can be modified to meet the needs of the application. Although initial research into using FBG pressure sensors show great potential for ETT cuff contact pressure sensing, these sensors were calibrated using weighing scales. Calibrating a sensor on a hard and flat surface for their application in a tube-like soft tissue structure (the trachea) may not be the best calibration approach. In addition to this, the sensors were calibrated using pressures of up to 600 cmH2O, much higher than those traditionally used in the trachea (20-30 cmH2O). Since higher pressures were used, it is possible that the sensor cannot sense pressures below a certain value and a better sensitivity may be required for lower pressures. In this thesis, a sensor is optimised for sensing pressures of up to 150 cmH2O.
The aim of this thesis is to optimise a miniature FBG based pressure sensor from literature for better sensitivity to low pressures, to investigate the validity of this sensor through repeatability tests, and to develop a new calibration technique to calibrate the sensors more accurately for measuring tracheal wall pressure.
The first part of this thesis investigated the shortcomings of an FBG based pressure sensor for tracheal wall contact pressure measurements from the literature. Different embedding materials for the FBG were tested for better sensitivity to lower pressures. This was achieved by sandwiching the FBG in between two material samples. Alongside this, a Finite Element Analysis (FEA) model was created of the measurement system and a comparison was made to determine if the model could be used to predict experimental changes. Dragon Skin and SYLGARD performed the best in these initial sandwich tests and were shortlisted for further testing.
The second part of this thesis evaluated the performance and repeatability of embedded sensors using the shortlisted materials. These sensors were created using a new fabrication process that ensured the production of good quality sensors. The FEA model was altered to the new sensor dimensions and utilised to determine methods to optimise the sensitivity of the sensor. Sensors were then optimised by increasing the thickness of the sensor as well as by applying epoxy anchors to either side of the embedding material. The coefficient of variation (CV) was calculated to determine sensor repeatability. Results showed the optimised Dragon Skin sensor had a CV of 0.16% indicating good repeatability and a sensitivity of 3.56 ± 0.0058pm/kPa and the optimised SYLGARD sensor has a sensitivity of 3.75 ± 0.03pm/kPa with a CV of 0.81%. The optimised sensors then went on for testing in a tracheal phantom.
The last part of this thesis described a new calibration approach (tracheal phantom) to test the optimised embedded sensors. This new method involved the use of a tube with a diameter like that of an adult trachea, a load cell to measure wall contact pressure, and an ETT cuff to simulate the applied pressure as it would be applied in an in vivo trachea. Results showed that the sensor needs further optimisation for use in a trachea and that the use of a double cuff affects sensor readings. Challenges like sensor movement during inflation-deflation cycles, the use of a double cuff, sensor orientation, and tube curvature were identified.
Despite these challenges, the findings of this thesis contribute to the growing body of knowledge on the use of FBG based pressure sensors for tracheal wall pressure measurement. This research underlines the necessity for further research and refinement of calibration methods and sensor design to ensure the reliability and accuracy of optical sensors for tracheal applications.
Actions (Archive Staff Only)
 |
Edit View |
Tools
Tools
