Mass Controller System for Hypoxia and Hyperoxia Testing Huser A J Kreofsky C R Nadler D C Poblocki J R BME 201 200 Department of Biomedical Engineering University of Wisconsin Madison 5 May 2004 Client Brad Hodgeman Instrument Specialist Department of Comparative Biosciences Advisor John G Webster Professor Emeritus Department of Biomedical Engineering Abstract Mass flow controllers are used to regulate the flow of gas through chambers thus controlling the concentrations of gas in an enclosed chamber A system was designed to test the effects of different concentrations of O2 and N2 within mice The system has three main variables as outlined by the client software mass flow controllers and interface for communication A plethora of research has been completed on different types of mass flow controllers mass flow controller manufacturers and different types of communication interfaces A LabView software 2 program has been designed to control hypoxia and hyperoxia testing and is the alpha stage of testing Problem Statement The purpose of this project is to design a system that can create a reproducible and accurate hypoxic hyperoxic environment with the capability of oscillating between various concentrations of oxygen and nitrogen Client Motivation Our client Brad Hodgeman has the following motivations 1 Determine the physiological mechanism of neural respiratory plasticity It is widely believed that neural plasticity is dependent on serotonin 5HT but the whole mechanism is yet to be discovered 2 Purchase new mass flow controllers and develop user friendly software The current mass flow controllers are inaccurate and the software is outdated 3 Increase the automation of the system Currently there are manual aspects of the system that the client would like to eliminate in order to increase efficiency within the system Hypoxia Background The neural respiratory control system s responses to respiratory stresses such as intermittent continuous hypoxia along with hyperoxia are being associated to clinical disorders such as sudden infant death syndrome SIDS apnic sleep disorders and spinal cord injury 3 Links between these and other clinical disorders and hypoxia hyperoxia are being investigated by researchers in hopes of finding the mechanisms behind their correlations Normal respiration includes 21 atmospheric O2 78 N2 and a very small percentage of all other gases A lack of inspired O2 21 can cause a condition called hypoxia where insufficient amounts of O2 reach the tissues of an organism Induced hypoxic conditions are more extreme but analogous to atmospheric oxygen at high altitudes Fig 1 The physiological and morphological effects from hypoxia can be detrimental to the organism if the O2 level is down low enough and is induced for long enough periods of time Figure 1 Phrenic response to Short term hypoxia The steady decline in phrenic response following the short term hypoxic response exhibits no long term facilitation LTF induced from continuous hypoxia From Kinkead et al 1998 Developmental respiratory control in many mammalian species can be heavily influenced by variation in gas concentrations Johnson and Mitchell 2003 Hyperoxia is a condition of ambient O2 levels being above the standard low altitude atmospheric O2 levels of 21 Animal models support the conclusion that perinatal changes in O2 levels induce developmental plasticity lasting changes in the respiratory control system that can be drawn out only during 4 critical periods of development Bavis et al 2003b Carotid body chemoreceptors bathe in the arterial blood and measure the PO2 levels adjusting breathing rate and volume as PO2 changes accordingly Feldman and McCrimmon 2003 Neonatal hyperoxia treated rats when compared to control rats had significantly less carotid body volume Fuller et al 2002 Smaller volume of carotid bodies and attenuated responses to respiratory stresses of hypoxia later in the rat s life 3 months has researchers believing developmental hyperoxia has detrimental effects to postnatal carotid body morphological and functional maturation Bavis et al 2002 Respiratory plasticity is defined as a future change in performance or persistent change in the neural control system based on prior experience Mitchell and Johnson 2003 Intermittent hypoxia and not continuous hypoxia induce long term facilitation LTF the most common and widely studied form of respiratory plasticity Fig 2 LTF is defined as the augmented phrenic burst frequency and amplitude lasting minutes to hours after episodes of intermittent hypoxia Baker and Mitchell 2000 Intermittent hypoxia is necessary to induce but not maintain LTF thus there are other mechanisms behind the increased drive to breathe as seen with the increased phrenic output It is widely accepted among researchers that LTF results from serotonin receptor activation and is maintained with new protein synthesis enhancing synaptic inputs to phrenic motoneurons Fuller et al 2002 Serotonin or 5 hydroxytryptamine 5Ht is a neuromodulator that aids in increasing respiratory drive The exact physiological process in which serotonin elicits LTF is uncertain 5 Figure 2 The phrenic and hypoglossal XII response to 3 episodes of intermittent hypoxia H1 H2 H3 LTF is the amplified response above baseline BL signified at 60 min post intermittent hypoxia From Zabka et al 2001 MFC background In an experimental protocol that involves dynamic entities such as gas flow and control accuracy is of paramount concern In our client s situation this concern is addressed through the technology of mass flow controllers Mass flow controllers MFCs accomplish accuracy through automating gas flow rates and thus gas concentrations to desired levels for use in further testing As a desired gas is fed into the mouth of the MFC it is divided into two different paths A large fraction flows into the bypass of the device creating a pressure drop that shunts the smaller remaining portion usually 5 of the total mass of gas up into the thermal sensor Fig 3 The shunted gas is subjected to a pair of heating coils which measure the change in temperature from the beginning to the end of the tube 6 Figure 3 left About 5 of the gas is shunted through the sensor tube Figure 4 right Temperature rise by adding heat yields mass flow http www sierrainstruments com products pdf 800 20Brochure pdf Once in the sensor the thermal properties of the gas are used to measure the mass flow rate Sierra Instruments 2004 Fig 4 The thermal
View Full Document
Unlocking...