Hydraulic Lung Analogue

This paper describes an easy-to-build, hydraulic analogue of the lungs, which has similar mechanical characteristics as an infant lung. It consist of two, clear, U-tubes filled with water (similar to two water-manometers) and connected through two capillary bundles into a T. Since the analogue is passive (it represents an unconscious subject) , it is connected to a ventilator to demonstrate its characteristics. As the air enters through the T, it splits into two pathways (representing the first bifurcation present the trachea), and then passes through two capillary bundles, simulating airway resistance. The other sides of the capillary bundles are connected to the U-tubes, filled with water halfway. Since it is the tendency of the U-tube manometer is to maintain the two water columns at the same level, it provides a recoil effect that simulates lung compliance. The airway resistance can be changed by changing the size and/or number of the small tubes that make the capillary bundle. Lung compliance can be changed by selecting the size of the U-tube, or by placing a solid insert into one or both of U-tubes. This device provides an adequate model for an infant lung provided that the frequency of excitation is not too high. This lung analog is an excellent vehicle for demonstrating the air movement in the respiratory system, since by using clear PVC piping, the water level change is equivalent to the tidal volume. The respiratory system provides excellent example of a dynamic bio-system and a vehicle to model dynamic systems. Introduction The respiratory system is an excellent vehicle for conveying the characteristics of a dynamic system. It provides enough complexity and analogous relationships to make the effort challenging, and also allows the student to work on a system that they are familiar with since they carry it everywhere they go. This exercise offers students the capacity to learn modeling techniques, testing methods, and critical understanding of the respiratory systems. There are several approaches for modeling the respiratory system. These range from simply a tube connected to a balloon, to a system involving bellows. There is also a commercially available device that allows the selection of compliance and resistance in each of its lobes. This paper presents another realization of the respiratory system, by utilizing two U-tube manometer tubes, to produce the recoil behavior present in the alveoli and the abdomen/chest wall. It consist of two, clear, U-tubes filled with water (similar to two water-manometers) and P ge 534.1 connected through two capillary bundles into a T representing the trachea, as shown in Figure 1. Since the analogue represent an unconscious subject, it is normally connected to a ventilator to demonstrate its functioning characteristics. As the air enters into the analogue, it splits into two pathways (representing the first bifurcation present the trachea), and then passes through two capillary bundles, simulating airway resistance. Notice that this allows modeling of asymmetrical resistances. The other sides of the capillary bundles are connected to the U-tubes, filled with water halfway. Since it is the tendency of the U-tube manometer is to maintain the two water columns at the same level, it provides a recoil effect that simulates respiratory system compliance. Figure 1: Schematic of U-tube lung analogue. This analogue offers much value from an educational perspective. It is an excellent vehicle for teaching modeling and simulation of dynamic systems. It also lends itself to something that can be built with inexpensive components, allowing the students to excuse their building ability. Once the system is built and operational, it can provide much insight regarding the dynamics of the respiratory system. It effectively communicates the consequences of any asymmetry. If instrumented and modeled properly, it allows comparison of a real system and its corresponding model. Finally, the resulting analogue, like other devices, provides an excellent training tool for P ge 534.2 respiratory therapists. It is much more desirable for the respiratory therapist to familiarize himself with a new ventilator, first with the lung analogue and subsequently with a real patient. The compliance of each lobe is given by: C D g lobe = π ρ 4 2 2 where D is the inside diameter of tube, D is the density of water and g the acceleration of gravity. This analogue also allows the simulation of a system with asymmetrical lung compliances. This can be done by introducing a cylindrical insert into one of the tubes, as shown in Figure 2. The resulting compliance for this configuration is: C D