Human Respiratory Mechanics Demonstration Model Anderson, Janelle; Goplen, Chris; Murray, Lynn; Seashore, Kristen; Soundarrajan, Malini ABSTRACT Currently existing human respiratory mechanics models are limited in their abilities to demonstrate effects of the rib cage movement on alveolar and intrapleural pressures and do not display any pressures. We have developed a model that can be used in both large and small classroom settings. The model also contains digital pressure displays and computer integration for real-time pressure data to visually demonstrate pressure changes that correspond to the different phases of breathing. Moving the diaphragm and rib cage causes a volume change which results in pressure changes visible on the digital sensors and computer display. Device testing affirmed the model’s ability to accurately demonstrate pressure changes in proportion to physiological values. Classroom testing showed improved understanding of respiratory concepts in 369 surveyed students. INTRODUCTION Our goal was to design and build an adequate mechanical model of human respiratory physiology for class instruction. The model demonstrates pressure differences between alveolar and intrapleural spaces. It also demonstrates the expansion of the thoracic cavity by the rib cage and diaphragm, displaying a three dimensional expansion. The device is small enough to use with a document camera or demonstrate close-up with smaller class sections. Though simple homemade models and basic commercial Plexiglas® lung models are available, they have short life-spans and parts that are difficult to replace. Currently available models do not display pulmonary pressures, making it difficult for students to visualize the forces driving gas exchange between the lungs and the atmosphere. No currently available physical models illustrate the expansion of the rib cage. Though most of the lung’s volume change is due to the diaphragm’s contractions, the rib cage movement contributes between 5 and 42 percent of the lung’s total volume change (Faithfull, 1979). To determine the efficacy of our model, a research plan outlining a series of surveying and analysis procedures was conducted after receiving approval from the Social and Behavioral Sciences Institutional Review Board (SBS IRB). Additionally, a series of physical tests were done to ensure the pressures generated could effectively demonstrate the actual mechanics of the respiratory system. The device is compatible with BioPac®, a commonly used physiology software package which we customized to graph alveolar and intrapleural pressures in real-time. When a BioPac® system is unavailable, the model’s digital pressure sensors will still display the instantaneous pressures generated. DESIGN, FABRICATION, & COST Requirements The device should contain both intrapleural and alveolar pressure displays to demonstrate pressure relationships during inspiration and expiration. To accommodate different classroom settings, the model should be functional in a small classroom as well as a large lecture hall. Because document cameras are frequently used in lecture halls to present information to students, the device must fit under a typical 13x17” document camera. The device should be compatible with BioPac® software and operable by a single user. The container housing the lungs should betransparent such that the inner components of the model are visible. To allow for transport, the device should weigh no more than twenty pounds. One of the major concerns with previous models is the difficulty of replacing components. Therefore, components under frequent stress should be made more durable and easily replaceable. Mechanical Design The respiratory demonstration model consists of a sealed transparent chamber in which pressures can be changed using the piston and elastic membranes to inflate and deflate balloons representing the lungs (Figure 1). The container, which corresponds to the thoracic cavity, was constructed of transparent polycarbonate to allow a clear view of the lungs. Polycarbonate was chosen over acrylic and other transparent materials for ease of construction. The container was designed as a rectangular box (7.25”x7.25”x10”) with a curved front panel. The box provides a flat back such that the model can be used on a document camera or overhead projector while the curved front panel allows a wider viewing angle. In order to mimic the intrapleural space, a constant negative pressure must be maintained within the container. A plug in one side panel of the model can be removed to apply a residual negative pressure to reflect functional residual capacity in vivo. In addition, the plug can be removed after lung inflation to demonstrate pneumothorax. Volume changes are produced by two distinct methods: a diaphragm piston and rib membranes. These two different mechanisms were selected to clearly differentiate between rib and diaphragm effects. In the body, the diaphragm muscle provides at least 58 % of the lung’s volume change, with rib expansion contributing the rest. Similarly, our model’s diaphragm piston provides a larger volume change than the rib membranes. The 5” diameter diaphragm piston is located on the bottom of the model and mimics the function and location of the diaphragm muscle in the human body. By pulling out the piston, the volume in the container increases, causing the pressure inside to decrease and the lungs to expand. The piston can be removed to provide access to the interior of the container for part replacement when needed. The rib membranes represent chest expansion and are located on both side panels of the model. Sections of gum rubber, selected for its durability and elasticity, are stretched over holes in the side panels that increase the internal volume when pulled outwards. The gum rubber is attached to the panels by a flange which was screwed on to create a leak proof seal while allowing easy replacement of the membrane material. The small hole in the container beneath the membrane side panel allows air flow when the rib membrane is stretched, but keeps it from collapsing inward when negative pressure is created inside the container. Handles are attached to both the piston and rib membranes for easy manipulation by the user. Elastic lungs are located within the model chamber and inflate or deflate according to the internal volume and pressure changes. Standard latex balloons were selected for
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