Human Respiratory Mechanics Demonstration Unit Final Design Report December 11th, 2007 Team Members: Janelle Anderson – Co-Team Leader Malini Soundarrajan - Co-Team Leader Chris Goplen - Communicator Lynn Murray – BWIG Kristen Seashore - BSAC Clients: Dr. Kevin Strang & Dr. Andrew Lokuta Department of Physiology Advisor: Professor Naomi Chesler1 TABLE OF CONTENTS Abstract…………………………………………………………………..2 Background Problem Statement………………………………………………...2 Problem Motivation…………………………………………..…...2 Respiratory Physiology……………………………………………3 Design Constraints………………………………………………...3 Competition and Current Devices……………………………...….4 Alternate Designs Design 1: Hinged Door Design………………….…………………5 Design 2: Rib Membrane Design…………………………………..6 Design 3: Quarter Section Design ……………………….………...8 Design Matrix………………………………………………………….….9 Materials……………………………………..…………………………....9 Final Design……………………………………………………………….10 Testing…………………………………………………….………….…...11 Future Work…..………………………………………………………..….12 Appendixes References…………………………………………………………A Schematics of Final Design………………………………………..B Bill of Materials…………………………………………………....C Product Design Specifications……………………………………..D2ABSTRACT Human respiratory models help students visualize alveolar and intrapleural pressure changes that occur while breathing. However, several problems exist with current respiratory models: their life-spans are short, the scaling of parts is physiologically inaccurate, and the rib cage expansion is not demonstrated. Our goal is to design and build an adequate mechanical respiratory model for class instruction purposes. We developed three preliminary designs, and decided to construct the Rib Membrane Design using acrylic for the enclosure. This design utilizes a membrane flange mechanism and a piston to model rib and diaphragm expansions, respectively. Various elastic materials, including latex, Theraband®, and gum rubber, were tested for maximum load and extension characteristics. The pure gum rubber had the highest maximum load and extension (3.2kg, 207.1mm), thereby making it a suitable material for use as the rib cage membrane. Tensile testing of RTV-sealed seams in latex and Theraband® materials revealed that Theraband® material is more appropriate for use as the lungs in the model. Our next steps for this design are to find a transparent rib cage membrane; develop an easily replaceable rib membrane and lungs; and integrate our device with BioPac® software, allowing for real-time visualization of pressure changes occurring during breathing. BACKGROUND Problem Statement Our goal is to design and build an adequate mechanical respiratory model for class instruction. This model should demonstrate pressure differences between alveolar and intrapleural spaces. It must further demonstrate the expansion of the thoracic cavity from the rib cage as well as the diaphragm, thereby displaying a 3-D expansion. The size of the lungs relative to the size of the thoracic cavity enclosure should be scaled to represent the human anatomy. The lungs in the current model inflate to fill roughly 1/15 of the thoracic cavity. In actual humans the lungs inflate to fill nearly the whole cavity with the exception of the space occupied by the heart and major blood vessels [1]. The device must also be portable and small enough to use with a document camera. Problem Motivation Though simple homemade models and basic commercial Plexiglas® lung models are available, they have short life-spans and parts that are difficult to replace. For most modes, when one portion of the model fails, the entire unit must be replaced; this is both inconvenient and expensive. Our clients had been using a basic lung model; however, since components wore out, their model was no longer useable. Furthermore, currently available models do not demonstrate rib cage movements or display pulmonary pressures, which make it difficult for students to visualize the forces driving gas exchange between the lungs and the atmosphere. Also, in current models, balloons used to model the lungs are much smaller than the thoracic cavity and are not scaled to reflect physiological conditions. Hence, a physiologically scaled model of the lungs which demonstrates the movement of the ribcage and diaphragm along with pressure displays would be a valuable teaching aid.3 Respiratory Physiology The main components of the human respiratory system are situated in the thoracic cavity. This space includes the ribs, heart, trachea, lungs, and diaphragm. When breathing, the alveolar and intrapleural pressures change, as shown in Figure 1. Alveolar pressure (Palv) describes the pressure inside the lungs, while intrapleural pressure (Ppl) describes the pressure in the space between the lungs and the pleural membrane (intrapleural space). At rest, Palv is 0 cm H2O and Ppl is -5 cm H2O [2]. When the diaphragm contracts, the intrapleural space increases and creates a negative pressure. This negative pressure expands the lungs and decreases the alveolar pressure, drawing air into the lungs from the atmosphere. During exhalation, the diaphragm relaxes and the pressures return to their resting states, forcing air out of the lungs. These pressure changes affect the volume of air contained within the lungs. The difference between Palv and Ppl in combination with the lung’s elastic properties influence this volume. Contraction of the diaphragm and expansion of the rib cage by the intercostal muscles control the changes in these two pressures (Figure 2). Both the diaphragm and the intercostals function together during inhalation and increase the thoracic cavity space, which consequently causes Palv and Ppl to become more negative. Design Constraints The final design should have considerations
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