Download HYBRID CARDIOVASCULAR SIMULATOR PERFORMANCE

HYBRID CARDIOVASCULAR SIMULATOR PERFORMANCE
EVALUATION
Jeison Fonseca1,2,3, Bruno Utiyama1,4, Aron Andrade1,3, Beatriz Uebelhart1,4, Juliana
Leme1, Cibele Silva1, José F. Biscegli1, Julio Lucchi2,3
1
2
Department of Bioengineering, Institute Dante Pazzanese of Cardiology, Sao Paulo (SP), Brazil
Department of Electronic and Computer Engineering, Technological Institute of Aeronautics, Sao Jose
dos Campos (SP), Brazil
3
Department of Electrical Engineering, University São Judas Tadeu, Sao Paulo (SP), Brazil
4
Department of Mechanical Engineering, University of Campinas, Campinas (SP), Brazil
E-mail: [email protected]
Background: A mock circulatory system (MCS) is an equipment capable to simulate the cardiovascular
system. MCS main goal is to perform simulation of physiological circulatory conditions and to minimize
number of “in vivo” animal tests. Hybrid Cardiovascular Simulator (HCS) is a system composed by two
sections: 1) numeric section: comprising seven compartments – right heart, lungs, head, arms, trunk, legs
and vena cava; 2) physical section: a hydraulic apparatus consisting of an electromechanical pump – as
left heart, compliance chamber - simulating arteries and a proportional hydraulic valve – simulating
Systemic Vascular Resistance (SVR). Purpose: This paper presents a performance evaluation of our HCS
in mimicking different human circulatory conditions. Methods: LabVIEW software is used in the
numerical section, for physical section control and for parameters visualization. An acquisition board is
connected to physical section as interface between the computer and hydraulic system. Baroreflex is
simulated by changing left pump frequency and SVR. HCS evaluation was made with physiological and
pathological conditions, and also, pathological condition with Left Ventricular Assist Device (LVAD).
Results: The results demonstrate HCS ability to reproduce different circulatory conditions and their
response to LVAD actuation.
Keywords: Mock Circulatory System, Hybrid Cardiovascular Simulator, Left
Ventricular Assist Device, LabVIEW.
1.
INTRODUCTION
Cardiovascular simulators development has grown mainly because of the
necessity to decrease the number of in vivo tests to evaluate ventricular assist devices
(VADs) (Patel, 2003).
Simulators of the cardiovascular system are not intentioned to replace in vivo
tests, because in these experiments, some physiological conditions are evaluated which
will not be possible in the simulation environment. However, the use of cardiovascular
simulation in a controlled environment with a reliable simulation tool, turn possible that
many variables can be observed and controlled. Also, the execution of some tests is
easier, and sometimes possible, to be accomplished than an in vivo test.
In the literature, there are many examples of cardiovascular computer simulators
which are mathematical or electrical analog models of the cardiovascular system
(Avanzolini, 1991). The goal of a computer cardiovascular simulator, modeled by
electrical analog is to evaluate VAD performance where it is possible to change
cardiovascular parameters such resistances, compliances and elastances in order to
observe the support provided by VADs in these situations.
Numeric simulators are low cost, have high flexibility to allow modifications,
and also are possible to evaluate various physiological signals.
Although these simulators show reliable and flexible methods to reproduce the
cardiovascular system, in order to evaluate VADs is necessary to model the device to be
studied.
The use of VAD model interacting to computer cardiovascular model, give us an
approximation of real entire system behavior. Although, is much better if rather than use
a VAD model, we could connect a real device directly to the simulator.
On the other hand, physical simulators has advantage in connecting real devices
under evaluation, however if system modifications are necessary, sometimes it needs to
be entirely disassembled. Also, it demands high cost (Felipini, 2008), (Legendre, 2008).
In this context we assembled a Hybrid Cardiovascular Simulator, which
combines the positive characteristics of numeric and physical simulators (Fonseca,
2010). HCS provides a flexible system where it is possible to connect a LVAD
evaluating its control and performance in different cannulation sites. HCS is composed
by two sections, Fig. 1: 1) physical section: left ventricle (LV), aorta compliance,
systemic vascular resistance; 2) numeric model section concentrates lumped parameters
of right heart, pulmonary circulation and systemic circulation. Also, in numeric section
the coronary circulation and brachial shunt is modeled.
Figure 1. Hybrid Cardiovascular Simulator diagram.
2.
MATERIALS AND METHODS
2.1
Description of the assembled HCS
Figure 2 shows the assembled system. A brushless direct current motor was used
to pump the fluid. A mechanism composed of a roller screw and a pusher plate changes
the rotational movement of the rotor to linear movement of screw. Inside the pumping
compartment there is a mechanical valve prosthesis to drive the flow. A polyurethane
diaphragm is used like an interface between the pusher plate and the fluid. This
electromechanical set is driven by a computer through an electronic controller.
From the reservoir, modeling the left atria passively, the fluid passes to a
pumping chamber through a mechanical valve (like the mitral valve).The chamber is
passively filled by a diaphragm displacement. Pumping phase occurs depending on
synchronism signal provided by computer through electronic interface. When that
happens, the electronic driver turns the motor on in order to eject the fluid starting the
systolic phase of the left ventricle. The fluid goes to the arterial system through another
mechanical valve (aortic valve). Closed to ventricle outflow, the fluid flow passes
through the adjustable compliance chamber to the systemic system where there is a
proportional valve (EPV-375B, Hass Manufacturing Co., New York, USA), controlled
by computer, to adjust the peripheral resistance. After that, the fluid returns to the
reservoir.
Figure 2. Hybrid Cardiovascular Simulator. [1] Reservoir (left atrium); [2] Pumping
chamber (left ventricle); [3] Pumping-chamber internal pressure probe. [4] Systemic
Vascular Resistance; [5] Adjustable compliance chamber; [6] Systemic pressure sensor;
[7] Mechanical valves; [8] LVAD Cannulation Sites.
The acquired signals from physical section of HCS are: left atrial pressure (pre
load), intraventricular pressure, systemic pressure, systemic flow and diaphragm
displacement. Pressure signals are acquired by TrueWave pressure transducers
(Edwards Lifesciences, Irvine, CA, USA) and amplified by an electronic module
assembled in our laboratories. An ultrasound flowmeter (HT110,Transonic, Ithaca, NY,
USA) was used for the acquisition of the systemic flow. A linear variable differential
transformer (LVDT) (DC-SE 1000, Schaevitz Sensors, Hampton, VA, USA) was used
for diaphragm displacement acquisition to measure the instantaneous volume. All
signals are sent to an acquisition board (USB-6229, National Instruments, Austin, TX,
USA), which is connected to a microcomputer PC by USB interface. An interface
programmed in LabVIEW shows the results.
Another interface between the computer and the physical apparatus (PCI-6036E,
National Instruments, Austin, TX, USA) drives the events: systole/diastole; motor
speed; air volume inside compliance chamber; proportional valve aperture and left
ventricle preload.
The baroreflex was accomplished by numerical section measuring the arterial
mean pressure and actuating in the motor speed controlling the heart rate. Also, this
mechanism actuates in the proportional valve to adjust the systemic vascular resistance
(SVR). Both adjusts are realized in according to a table (Lucchi, 1999).
2.2
Evaluation of HCS
In order to evaluate the HCS, some tests were performed. Firstly, LV preload
was changed (10, 15 and 20mmHg). After that, the ventricle contractility was modified
(0.6, 1.0 and 1.6mL/mmHg) keeping a fixed preload (Ferrari, 1994).
HCS assessment with LVAD connected was performed: a rotary pump was
connected through LV to aorta (LV apex to aorta); a pulsatile pump was connected
between LV chamber to aorta (LV apex to aorta). In both cases previous descripted, the
LV contractility was kept in 1 mL/mmHg simulating a LV disease.
3.
RESULTS AND DISCUSSION
Figure 3 shows the LV cardiac cycle (pressure vs volume loop) in three different
situations: (a) displacements when atrial pressure changes (10, 15, 20 mmHg) with
constant arterial pressure and contractility. Is possible to observe that the end systolic
volume remains approximately unchanged (110mL) while end diastolic volume
changes; (b) changes in contractility state (1/Ees=0.66, 1, 1.66 mL/mmHg), where Ees is
the end systolic elastance, with fixed atrial pressure. It shows the change in the end
systolic volume while end diastolic volume remains unchanged; (c) effect on the cardiac
cycle of the rest volume variations, contractility remains unchanged and its volume
shifts according to the rest volume variations – arterial pressure remains constant.
The use of pressure vs volume loop has been emphasized, because it makes
possible the evaluation of the left cardiac pump (Li, 2004).
(a)
(b)
(c)
Figure 3. Experimental results obtained by HCS. (a) displacements of the work cycle for
three different values of atrial pressure (10, 15 and 20 mmHg) at fixed heart
contractility and arterial pressure; (b) three different values of heart contractility (1/Ees =
0.6, 1 and 1.6 mL/mmHg) and fixed atrial pressure; (c) displacements of the work cycle
for three different values of ventricular rest volume (49, 59 and 69mL) and fixed heart
contractility and atrial pressure.
Figure 4 shows the HCS behavior when a rotary blood pump is connected
between LV chamber and aorta. This test was performed at cardiac disease situation
which the LV contractility was kept at 1/Ees=1mL/mmHg – in a healthy LV, 1/Ees 
0.26 mL/mmHg. Firstly, the pump speed was kept at 900 rpm in order to not allow
backflow into LV, thus a step increasing the speed to 1800 rpm was applied as shown in
Fig. 4(a). As result, heart rate decreased from 120bpm to 75bpm, Fig. 4 (b).
(a)
(b)
Figure 4. HCS connected to rotary LVAD between LV chamber and aorta. (a) LV
pressure and aortic pressure; (b) After speed step, heart rate is automatically decreased
by baroreflex actuation.
In Fig. 5, an electromechanical pulsatile blood pump as LVAD was connected
between LV chamber and aorta of HCS. The LV contractility was kept at
1/Ees=1mL/mmHg. Figure 5(a) show the aortic and LV pressures and Figure 5(b) the
heart rate behavior when the LVAD was turned on.
(a)
(b)
Figure 5. Pulsatile LVAD between LV chamber and aorta connected to HCS. (a) LV
pressure and aortic pressure; (b) After LVAD turned on, heart rate was decreased.
4.
CONCLUSION
HCS test results showed in Fig. (3) prove that it follows the Frank Starling law.
Also, comparing to Ferrari studies (Ferrari, 1994), the obtained HCS behavior are
similar to that showed in the article.
HCS behavior when connected to a rotary or pulsatile LVAD is similar to
simulations using electrical analog model presented by Lucchi (1999).
Also, HCS demonstrated that a rotary pump as LVAD, Fig. (4), can help the
diseased heart, however the pump needs to have a careful rotational speed control to
avoid natural heart injuries. Application of an electromechanical pulsatile pump as
LVAD, Fig. (5), runs synchronized to HCS, decreasing the left ventricle afterload
helping to maintain physiological mean aortic pressure.
Also, in case of LVAD failure, the pulsatile device rather than continuous pump
do not provoke natural heart acceleration.
ACKNOWLEDGMENTS
The authors would like to thanks the Adib Jatene Foundation (FAJ), Heart
Hospital (HCor) and FAPESP for partially supporting this research.
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