Physical model of the cardiovascular system. Dynasim.

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1 Physical model of the cardiovascular system. Dynasim. Bustamante J., Barros Juan F., Roldán A., García S., Salazar A.F. Escuela de Ingeniería de Antioquia, Instituto de Ciencias de la Salud CES, Universidad Pontificia Bolivariana, Clínica Cardiovascular Santa María, Medellín, Antioquia, Colombia. ABSTRACT Introduction: In the cardiovascular system, different physical variables and fluid mechanics concepts are related, making complex its understanding. Objectives: Design and construct a physical model of the cardiovascular system. Materials and Methods: DYNASIM represents the physiological system including peripheral and pulmonary circulation. It is built with transparent materials to facilitate its observation. A pneumatic pumping system expands two rubber membranes, each in a cylindrical cavity (ventricle), which has two valves: inlet valve and outlet valve. These allow flow of fluid in only one direction, according to physiological valves. The circulatory net, system of plastic tubing, includes compliance and resistance. The first consists of a chamber with one rubber wall. The latter consists of a system that occludes the tube using a gate valve, controlled by a motor. DYNASIM is controlled by a software, from which some parameters are varied. Pressure is measured using electronic sensors. There is an interface that communicates the software with the simulating device, thus all the variables are processed and analyzed in the computer. Results: Repetitive pressure curves were obtained, which clearly show systolic and diastolic period. When compliance is decreased, pressure increases within hoped ranges. Valves and compliance work satisfactorily. A conversion of pulsatile flow at "great arteries" to continuous flow at the "atria" can be seen. Discussion: The compliance is showing a satisfactory result, because there is flow between each pulse. The resistance has a positive result due to the reduction of pulsatility and drop of the pressure. The tubing may have aided in this phenomena. Conclusions: DYNASIM makes the cardiovascular system easier to understand, as it simulates different of its parameters. Several conditions and illnesses can be modeled to study its effects. Its future applications could be related to evaluation of cardiovascular devices developments before experimenting on living beings. INTRODUCTION The dynamics of the heart and the cardiovascular system involve concepts of volume, pressure, fluid mechanics and general physics [1,2]. Cardiovascular simulators have had great application in the process of prosthetic cardiac valve evaluation [3]. These models have been developed according to their needs and based on technological advances that have allowed a closer approach to the cardiovascular system. Few simulators have teaching goals. It could be very useful to the student to interact with the cardiovascular variables and analyze how some of them are affected by different situations or illnesses. The Dynasim model is a research project supported by the Biomedical Engineering program (offered by the Antioquia School of Engineering in agreement with the Health Sciences Institute CES) and the Cardiovascular Dynamics Group from the Pontificia Bolivariana University and the Santa María Cardiovascular Clinic. OBJECTIVES The principal goal of the project is to design and construct a physical model of the cardiovascular system for teaching and research. MATERIALS AND METHODS The device is a hydraulic model (Figure 1) controlled by a computer, in which pressure data from different points of the system, obtained by electronic pressure sensors, is visualized. The model can be divided in two principal areas: mechanical system and instrumentation. Figure 2 shows a block diagram in which the system's organization is illustrated.

2 Figure 1. Physical Model of the Cardiovascular System - Dynasim. Figure 2. Block diagram of the system.

3 Mechanical system Pumping system: compressed air expands two rubber membranes located on the base of two ventricular chambers (cylindrical, acrylic chambers), pushing the fluid towards the circulatory net (Figure 3). There is no auricular systole. Figure 3. Ventricular chamber, atrial chamber and compliance chamber. Valves: there are inlet valves (mitral and tricuspid) and outlet valves (aortic and pumonary). The valves are located on the superior area of the ventricular chamber and consist of cup - shaped structures that contain an acrylic sphere. When the fluid is pushed upwards, the sphere in the inlet valve moves towards the area of the cup with smaller diameter, blocking the conduit's transverse area. The flow from the ventricular chamber to the atrial chamber is then avoided. Meanwhile, the sphere in the outlet valve floats, allowing flow towards the circulatory system. Circulatory system's compliance: The Dynasim model has a net of plastic tubing that decreases its diameter as it is located farther away from the "heart" and returns to its original size as it approaches the atrial chamber. Immediately after the ventricular outlets, two cylindrical chambers are found, which have a rubber wall. These are called compliance chambers (Figure 3), which simulate the distensibility of the great arteries as its rubber wall expands. In this way the arterial walls expand and store energy. As they recoil this energy is dissipated, aiding in the movement of the blood towards the circulatory system. One of the goals of this system, in addition to the arterial resistance, is to convert pulsatile flow coming from the heart to continuous flow at capillary level [1,2]. Circulatory system's resistance: Dynasim's vascular resistance system occludes one of the tubes using a gate valve that is moved by a motor. As it moves downward, the gate occludes the tube, decreasing its diameter. Figure 4 shows the model's vascular resistance.

4 Figure 4. Resistance system. Intrumentation Cardiac frequency control: Dynasim simulates the heart's pulsatile behavior by controlling an electrovalve, which is connected on its inlet with a compressed air source and on its outlet with the ventricular membranes. This solenoid valve, when electrically powered, opens to allow flow of air towards the ventricular membranes (systole). When its power supply is stopped, the valve closes to avoid the entrance of air and expel the air from the membranes to the atmosphere (diastole). The electrovalve is controlled from a computer using the LabVIEW (Laboratory Virtual Instrument Engineering Workbench) computer application. This application has a communication system (Measurements and Automation) between the software and the hardware (physical data acquisition system). LabVIEW works with graphic programming instead of text commands. On normal conditions, the diastole occupies a 70% of the total duration of the cardiac cycle, and the systole occupies a 30 %. However, when cardiac frequency increases, this relationship changes: each event takes 50% of the cycle [2,4,5]. Dynasim considers this change in a linear manner. A software program was developed in LabVIEW that consists of generating a train pulse and sending it to a channel in the data acquisition system to open and close the electrovalve. The channel is connected to an electronic circuit that is part of a communication interface between the software and the physical system. This circuit acts like a switch to power the electrovalve intermittently. Vascular resistance control: A DC motor is used, which moves the gate in the gate valve, occluding a tube from the circulatory net. The motor is controlled by LabVIEW and using an H circuit (a common electronic circuit in motor control), the rotating direction is determined. The resistance level is indicated in terms of percentage of occlusion. A 100% occlusion would totally avoid fluid flow. Data acquisition: The Dynasim model has the possibility of measuring pressure in different points of the system. Two electronic pressure sensors are used, which measure absolute pressure in a range of 0 psi to 60 psi. The program developed in LabVIEW filters the signal, which is shown on a graph of the front panel. Obtained data, in voltage, is processed through a calibration equation to visualize it in mmhg. When the user stops the program, a dialog box opens to save the data as a text file. Experiment Pressure data was taken in the ventricular and atrial chambers. Some measurements were made with only one ventricle working to verify some fluid mechanics phenomena. For these measurements, an air pressure of 10 psi was used. Measurements were also recorded with the two ventricles working: the left ventricle with 10 psi and the right ventricle with 5 psi. All measurements were taken with a cardiac frequency of 70 beats per minute. RESULTS Some fluid mechanics phenomena were seen, such as the change of velocity with respect to the change of

5 diameter of the tube. The compliance chambers distend, synchronized with the ventricular membranes. The valves have an acceptable behavior, even though the inlet valves require high pressure to close. Some results are shown on Figures 5, 6 and 7. Figure 5. Right ventricular pressure and left atrial pressure with the right ventricle working. Figure 6. Right atrial pressure with peripheral occlusion, left ventricle working.

6 Figure 7. Right atrial pressure with decrease in compliance. DISCUSSION The pneumatic pumping system works satisfactorily, as well as the compliance chamber. With little fluid in the system, the conversion from pulsatile flow to continuous flow can be seen. As it is described in fluid mechanics, as the conduit's diameter is decreased, the flow velocity increases. This was verified by injecting an air bubble in the system. However, the great arteries are repeatedly divided as they locate farther away from the heart. Thus, total capillary transverse section area is much larger than the aortic transverse area. This makes the flow velocity less in the capillaries than in the great arteries, facilitating the transport of oxygen from the capillaries to the tissue. Obtained plots have good repetition and consistency. When one of the ventricles works, one can deduct that there is pressure drop along the tubing. This shows that the circulatory tubing has good compliance and flexibility, as it absorbs part of the pumping energy. When compliance is decreased, the mean pressure increases, showing the physiological case: when vessels become rigid, because of smoking or age, the mean arterial pressure increases. If the system is more rigid, there is no energy absorption effect and therefore there is an increase in pressure. Occluding tests were made. When the pulmonary net is occluded, with the left ventricle working, there is an increase in the right side pressure, for the right ventricle does not have an outlet towards the circulatory system. Therefore, fluid arriving from the left ventricle accumulates in the right side and the pressure is increased. This occlusion demonstrates that there exists flow in the system. When the peripheral trajectory is occluded (with the left ventricle working) a hypovolemia condition is simulated: there is no venous return from the peripheral system, which decreases pressure in the right side. CONCLUSIONS The cardiovascular system is a highly complex mechanism, in which there are differences in the cardiac chambers; there is energy transfer from the cardiac muscle to the blood; the reology of blood varies with hematocrit, among other factors; it has an extremely long circulatory net with infinite number of bifurcations, with walls that have unique elastic characteristics and micrometric diameters. However, despite the distance between the inert and the live, interesting results can be achieved, which will be the basis for further advances. The Dynasim model allows an automatic control of the variables, facilitating the results' analysis and its general manipulation. It is necessary to implement bifurcations to the model and to use more elastic materials that are closer to the tissue's characteristics: the cardiac walls have compliance; the veins are more compliant than the arteries; the tubing of the model next to the chambers is rigid. The valves could be better by using the bivalve model, for the ventricular pressure transmits easily to the atrial chambers. The Dynasim model is a continuous project that will keep advancing. It could be a system in which new cardiovascular devices are evaluated and therefore diminish experimenting on animals or on humans. It could be used as the basis for the development of a pumping system for extracorporeal circulation machines or even as an approach to an artificial heart. REFERENCES

7 1. ZAMIR, M. The Physics of Pulsatile Flow. New York: Springer, BUSTAMANTE, John y VALBUENA, Javier. Biomec ánica Cardiovascular. Medellín: Universidad Pontificia Bolivariana, WRIGHT, John T. M. Hydrodynamic evaluation of tissue valves. London: Butterworth, BERNE, Robert M. and LEVY, Matthew N. Cardiovascular Physiology. 8 ed. New York: Mosby, GUYTON, Arthur C. Human Physiology and Mechanisms of Disease. 4 ed. New York: W.B. Saunders Company, Your questions, contributions and commentaries will be answered by the authors in the Medical Informatics list. Please fill in the form and press the "Send" button. Question, contribution or commentary: Name and Surname:: Country:: Argentina Send Erase Top Updating: 10/29/2003