Project Number: P16084

Project Number: P16084

Physiologically Accurate Representation of the Human Respiratory System for the Demonstration and Analysis of Respiratory Mechanics

Abstract

A physiologically accurate model of the human respiratory system was requested by the Biomedical Engineering Department at the Rochester Institute of Technology, NY.The existing model consisted of a simplified version of the desired model that was constructed from materials that were available at hand, but was difficult to use in a lab setting. In order to improve the ergonomics of the model, custom parts were fabricated to construct a working model that included key components of the respiratory system, such as the rib cage and its contribution to respiration. The project ended successfully because most of the customer requirements were met.

Introduction

The purpose of this project was to design and build a mechanical lung device to model human respiration. Students in the Biomedical Engineering department currently use a balloon in rigid cylinder model for laboratory experiments. This model contains a single latex balloon, representing the lungs, and an elastic diaphragm membrane, all housed in a rigid cylinder representing the chest cavity. Students could not measure meaningful intrapulmonary pressures due to the single lung and excess space surrounding the lung. To increase anatomical accuracy, a rib cage capable of passive movement during respiration was added, two lungs were utilized, and the volume of the intrapleural space (IPS) was decreased. In order to remain within the constraints of a designated budget, the team decided to fabricate a half size (relative to an adult human) model.

Students are expected to collect pulmonary data, namely lung pressure changes and volumetric flow rate, in order to calculate normal airway resistance as well as resistance under stress conditions. The improved design allows students to measure tidal breathing volume, intrapleural pressure as a result of normal breathing, and the force of diaphragm contraction during respiration. Students can also explore how different breathing patterns and pathologies, including asthma, hyperventilation, and hypoventilation, affect respiration. While the model cannot accurately demonstrate the quantitative physiologic effects of abdominal (deep diaphragmatic) versus chest (shallow rib-related) breathing, it is capable of qualitative demonstration.

Design Process

To support student learning, the customer requirements stated that the device must contain representations of the human lungs, diaphragm, chest cavity and ribs. In designing the model, the team was tasked with decreasing the volume between the lungs and chest cavity (IPS). To maintain physiological accuracy the device requires the use of two separate lungs capable of holding and releasing adequate volumes of air. Additional requirements are displayed in Table 1(See Appendix).

The original engineering requirements related to lung volume, respiratory volumes, and airflow rates were set to physiological ranges. In an effort to ensure the device is easy to store, its dimensions were required to be no larger than 2x0.85 ft. The approved budget for this project was $250 for the fabrication on one complete model. Under these constraints, it was best to design a half size model. The relevant engineering requirements were then halved to maintain appropriately scaled metrics during use. Material considerations and the effects of a scaled model will be discussed in further detail in the upcoming sections.

Chest Cavity

In order to create the pressurized vessel, a clear plastic was chosen to allow the students to view the inflation and deflation of the lungs with respect to the diaphragm movement. Initially, a simple PVC cylinder similar to the original model was chosen for its simplicity and availability. Since it was a standard part, we could use the following equation to calculate the volume of air moving in and out of the chest:

Volume of moving air = (1.1)

h1 is the initial height of the piston

h2 is the final height of the piston

r is the radius of the cylinder

In order to refine the shape of the cavity, we modified the contour on the base to accommodate the spine, and modified the equation to be

(1.2)

A is the cross-sectional area of the base

In order to obtain the exact cross-sectional area, Solidworks was used to analyze the CAD model and calculate the volume of air.

Since there is no optically clear plastic material that can be 3D printed directly, a simple molding process was decided upon for the fabrication of the chest. An acrylonitrile butadiene styrene(ABS) mold of the chest to be made was printed and a thermoformer would be used to create the desired cavity out of PETG plastic. This allowed for the formation of multiple chests, while maintaining the desired optical clarity and size. Due to the large height to width ratio of the mold, minimal webbing was observed during the forming process.

Upon completion of thermoforming, the mold was to be taken out of the plastic cavity. The mold would not release from the cavity and a new process had to be created during troubleshooting. The new plan called for a mold made of Styrofoam. Blocks of 7x8 inch Styrofoam were glued together using wood glue. The mold was shaved in order to obtain the desirable shape. The same thermoforming process was used with the new mold.

Important features of the chest cavity include access to intrapleural pressure (IPP), intrapulmonary access, and the ability to open the intrapleural space to air. A gas sampling port was incorporated into the top of the cavity to allow the user to remove air and create a negative intrapleural pressure relative to pulmonary (lung) pressure. Intrapulmonary pressure access was provided through the use of a barbed nylon y-connector. A tubing connector was used to allow IPP measurements. To aid in the simulation of a collapsed lung scenario, the tubing can be disconnected, depressurizing the chest cavity.

Diaphragm

The original design of the diaphragm was fabricated as a piston type structure to inflate and deflate the lungs, and achieve the desired differential pressure values. The plan was to use 3D modeling and printing techniques in order to form an ergonomic design that would allow for easy interfacing with existing equipment.

A novel flexible 3D printing material called Ninjaflex was used to create the pressure seal. Since the contour of the piston head was not a geometric shape, 3D printing the shape was the easiest way to ensure adequate tolerancing to ensure a sealed vessel. In order to further improve the aesthetics and recognizability of the model, the color of material was chosen to be pink plastic.

As seen in equation (1.2), the required volume of moving air was determined theoretically, and the area of the base contour is as follows:

Area from the model = 26.46 in2

Height of the piston from the model = 2.49 in

Therefore, volume of moving air = 65.885 in3

Due to the issues faced when fabricating the chest cavity, the diaphragm design was altered. The original piston design required alterations upon chest cavity completion. The limited time available after the final thermoform did not allow for edits and re-prints. Therefore, a new design was developed in which a sheet of Ninjaflex was 3D printed. The design is similar to that of the previously used model, but with a more robust material. A handle was 3D printed and attached to the Ninjaflex sheet. When the student pulls down on the handle, the lungs are meant to inflate with air.

Lungs

The average human can hold up to 6 L of air in their lungs [2]. The team proceeded with the design to follow that of a half-sized human model so the total lung volume was 3 L. Upon construction, the lungs were too large for the finalized chest cavity and the team decided to half the size again. Therefore, the completed model follows that of a quarter-sized human lung, 1.5 L. The finalized design utilizes two rubber sheets which are glued together which allowed for shape and volume control. As opposed to the previously used model, the new model incorporates the usage of two lungs.

In the human system, the body maintains a volume in the lungs which is not meant to deplete, even after forced expiration. This volume, referred to as residual volume, equates to about 1.2 L [2]. In the quarter-sized model, the residual volume equates to a total of 0.3 L, or 0.15 L per lung. In order to maintain residual volume in the model, bubble wrap was utilized. The bubble wrap is glued into the lining of the lung so that it is secured. By representing the residual volume as a solid, it is ensured that the necessary volume will not be depleted.

In order to maintain physiological accuracy, the color of the lung needed to the altered. The material is only available for order in black and this was undesirable. The decision was made to spray paint the lungs, utilizing a rubber safe option, so that the color can be controlled. The resultant color is pink.

Figure 1. Physiological pressure values, in mmHG, for inspiration and expiration [2]

As previously mentioned, a goal of the proposed laboratory procedure is to have students collect the pressures produced by the model during inspiration and expiration. The physiological pressures are displayed in Figure 1 as well as outlined in Table 1 (See Appendix). The top line of the figure, PA, represents the alveolar pressure which are the desired results for the collected lung pressures. Therefore, the model output should report values below 1 atm during inspiration and below during expiration.

Rib system

The design choice was made to use 3D printed ribs to more closely visually represent human ribs. ABS was chosen for the purpose of material consistency with available 3D printers. To represent the cartilage connecting the ribs, flexible inserts fabricated from NinjaFlex filament was used. These inserts were printed as well. Designs for the ribs were obtained from an open source. The spine was based on a scoliosis case. This was an acceptable compromise since the file provided the highest resolution. Due to gaps in the spine, multiple cylinders were inserted into the file to improve the stability of the print.

During normal human respiration, intercostal muscles lining the ribs aid in lifting and expanding the rib cage. In order to achieve proper rib movement, a system of eye hooks was incorporated into the stand design. Floating ribs of the model were linked together via twine. The highest floating ribs on either side of the spine were attached to twine and fed through the eye hooks affixed to an external attachment point. The free end of the wire can be pulled by a student in order to simulate rib movement during breathing.

Model improvements

Main issues that were identified in the previous version were:

Table 2: Model comparison

Previous Model / New Model
Usage / Needed to be sandwiched between lab tables during use. This resulted in broken and cracked models. / Created a wooden stand to hold the new model in place. No outstanding risks of chemical damage in dry lab.
Lungs / latex diaphragm and lungs / Rubber sheet used to create more durable components.Reduce frequency of part replacement.
Ribs / No ribs / Ribs as well as associated movement during inhalation/exhalation
Intrapleural space / Very large intrapleural space/volume / Substantially reduced the intrapleural volume, improving measurement accuracy

Results and discussion

The project was not completed until the end of the semester so not all of the tests could be run. Those that were run tested: IPS pressure, lung pressure, tandem movement, and volumetric flow rate. The two pressures were tested by attaching the model to the desired instrumentation and utilizing Capstone software in the BME lab. The IPS pressure differential equated to less than 1 kPa. The lung pressure differential equated to 1 kPa depending on the intensity of diaphragm pumping on the part of the volunteer. The IPS and lung pressures are depicted in Figure 1. Comparing the values obtained to the values in Figure 1, the minimal difference in pressure does relate to the pressures created within the body during inspiration. The diaphragm is not very compliant, stretchy, which allowed for the low pressure differential. Therefore, the finalized diaphragm design is advantageous when compared to the latex.

Although the tandem movement, visual aesthetics, and ease of use were not tested by a panel as proposed, the tandem movement is evident. When the diaphragm is pumped, the pressure differential causes inhalation and exhalation. A last minute decision was to have the rib movement be separate from the pumping of the diaphragm so the ribs are no longer considered when observing tandem movement.

Lastly, the volumetric flow rate was tested by using LabChart software and a compatible flow head. The model was exercised and the tidal volume was calculated. The final, calculated tidal volume equaled 600 mL which is equivalent to that of a normal sized adult. The student is in charge of pumping the diaphragm and is in complete control of the number of breathes to be recorded per minute. Therefore, the desired 60 breaths per minute can be simulated as well as a hyperventilation case.

Conclusions and recommendations

Ultimately, the project ended up being a success. The improvements made allow for a more physiologically accurate model. Also, the measured values are within adequate physiological ranges. In terms of recommendations, more advanced testing should be done as there was not enough time to assemble panels to test the model at the end of the semester. Also, the diaphragm was difficult to secure so changing the lungs, although possible, may be too difficult to accomplish during a lab period. Lastly, the design was revised near the end of the semester and the rib movement system had to be altered. Therefore, the ribs do not move as desired and cannot be measured via the load cell attached to the diaphragm. The capability is present for this process to work but more time will need to be spent making the necessary changes.

References

[1] Anderson J, Goplen C, Murray L, Seashore K, Soundarrajan M, Lokuta A, Chesler N. Human respiratory mechanics demonstration model. AdvPhysiolEduc 2009;

[2]Feher, J. J., 2012, Quantitative Human Physiology, Elsevier, Waltham,MA, Chap.6

[3]Anderson J, Goplen C, Murray L, Seashore K, Soundarrajan M,

Lokuta A, Strang K, Chesler N. Human respiratory mechanics demonstration

model. AdvPhysiolEduc 33: 53–59, 2009.

Acknowledgments

We would like to thank the BME department for the opportunity to create the model, as well as Dr. Jennifer Bailey for her cooperation and assistance in the design and completion of this project.

We would also like to thank the Multidisciplinary Senior Design team that is part of the Kate Gleason College of Engineering and all the staff and subject matter experts that contributed to the learning experience and made it possible to deliver a product that met all expectations. We would especially like to thank Prof. John Kaemmerlen for supporting us and proving to be an immensely effective resource regarding every aspect of this project.

Appendix

Table 1. Engineering Requirements