Development of a Low Cost Low Volume Flow Rate Sensor for Neonatal Resuscitators

Abstract

Nearly 10 million newborns worldwide have complications that require resuscitation immediately after birth in order to initiate and support their breathing process. The resuscitation process is performed using mechanical or manual ventilators. The ventilation process is prone to ventilator induced lung injuries (VILI) due to excessive or insufficient air being supplied. Injury due to excessive volume supply is known as volutrauma which causes overdistension of the lungs and could lead to fatal consequences. Studies show that measuring the expired volume of a patient is a good indication of the air supplied from a resuscitator. Therefore an accurate flow sensor is a key aspect to measure the expired breath of a neonate which can help determine and regulate the tidal volume (TV) supplied. This thesis focuses on developing a low cost volume flow rate sensor. The low cost will reduce the need for constant sterilisation and create a disposable ‘one-off’ flow rate sensor. The volume flow rate sensor is to be used in conjunction with The Next Step Neonatal Resuscitator by KM Medical. The Next Step device is a portable resuscitator that supplies TV at various flow rates. The implementation of a flow rate sensor will allow the resuscitation device to regulate the volume supply. Three sensor concepts, venturi, optical and intrinsically conductive polymer (ICP) were tested in this thesis. All sensors used the same test bed which consisted of a flow generator capable of creating low flow rates similar to the ones exhibited by a neonate. The venturi sensor is a small venturi tube which is designed to fit into the exhaust vent of the Next Step resuscitator. It uses a pressure sensor to determine the pressure difference at the inlet and the throat to determine flow rate passed through it. The sensor exhibited an accuracy of 6% with a good response to low flow rates from 1.8 ml/s. The high resolution of the pressure indicator (0.01Pa) allowed it to sense 0.1 ml/s changes in flow rate. The optical sensor uses a linear optical array, a light source and a thin polyester cantilever. The cantilever has a flag at the free end which blocks the light when it is deflected by low flows. This creates an output from the optical array. The optical sensor measures flow rates from 10 ml/s and is able to do so at a high resolution (0.1ml/s) and accuracy (3.3%). The sensor poses instabilities due to the mass of the flag, lack of stiffness and low damping exhibited by the polyester cantilever. The ICP concept uses polypyrrole (PPy) as the conductive polymer. The PPy is deposited on the surface of a natural rubber substrate and changes resistance when surface strains are experienced. Low flow trials proved that the PPy film showed low sensitivity towards strains experienced by 35 or 50 μm thick rubber strips. A bending rig was used to quantify the sensor’s response to angular deflection. The 1.5 mm thick sensor set-up produced linear outputs with increasing angular deflection. Comparative analysis revealed that the venturi sensor is the best solution of the three sensors for detecting low flow rates of a neonate. Further developments are required for the additional two sensors to be feasible flow rate sensors.