M. Darowski

Computer simulation of haemodynamic parameters changes with left ventricle assist device and mechanical ventilation.

Left Ventricular Assist Device is used for recovery in patients with heart failure and is supposed to increase total cardiac output, systemic arterial pressure and to decrease left atrial pressure. Aim of our computer simulation was to assess the influence of Left Ventricular Assist Device (LVAD) on chosen haemodynamic parameters in the presence of ventilatory support. The software package used for this simulation reproduces, in stationary conditions, the heart and the circulatory system in terms of pressure and volume relationships. Different circulatory sections (left and right heart, systemic and pulmonary arterial circulation, systemic and pulmonary venous circulation) are described by lumped parameter models. Mechanical properties of each section are modelled by RLC elements. The model chosen for the representation of the Starlings law of the heart for each ventricle is based on the variable elastance model. The LVAD model is inserted between the left atrium and the aorta. The contractility of the heart and systemic arterial resistance were adjusted to model pathological states. Our simulation showed that positive thoracic pressure generated by mechanical ventilation of the lungs dramatically changes left atrial and pulmonary arterial pressures and should be considered when assessing LVAD effectiveness. Pathological changes of systemic arterial resistance may have a considerable effect on these parameters, especially when LVAD is applied simultaneously with mechanical ventilation. Cardiac output, systemic arterial and right atrial pressures are less affected by changes of thoracic pressure in cases of heart pathology.

A modular computational circulatory model applicable to VAD testing and training.

Aim of this work was to develop a modular computational model able to interact with ventricular assist devices (VAD) for research and educational applications. The lumped parameter model consists of five functional modules (left and right ventricles, systemic, pulmonary, and coronary circulation) that are easily replaceable if necessary. The possibility of interacting with VADs is achieved via interfaces acting as impedance transformers. This last feature was tested using an electrical VAD model. Tests were aimed at demonstrating the possibilities and verifying the behavior of interfaces when testing VADs connected in different ways to the circulatory system. For these reasons, experiments were performed in a purely numerical mode, simulating a caval occlusion, and with the model interfaced to an external left-VAD (LVAD) in two different ways: with atrioaortic and ventriculoaortic connection. The caval occlusion caused the leftward shift of the LV p–v loop, along with the drop in arterial and ventricular pressures. A narrower LV p–v loop and cardiac output and aortic pressure rise were the main effects of atrioaortic assistance. A wider LV p–v loop and a ventricular average volume drop were the main effects of ventricular-aortic assistance. Results coincided with clinical and experimental data attainable in the literature. The model will be a component of a hydronumerical model designed to be connected to different types of VADs. It will be completed with autonomic features, including the baroreflex and a more detailed coronary circulation model.

Development of a hybrid (numerical-hydraulic) circulatory model:Prototype testing and its response to IABP assistance

Merging numerical and physical models of the circulation makes it possible to develop a new class of circulatory models defined as hybrid. This solution reduces the costs, enhances the flexibility and opens the way to many applications ranging from research to education and heart assist devices testing. In the prototype described in this paper, a hydraulic model of systemic arterial tree is connected to a lumped parameters numerical model including pulmonary circulation and the remaining parts of systemic circulation. The hydraulic model consists of a characteristic resistance, of a silicon rubber tube to allow the insertion of an Intra-Aortic Balloon Pump (IABP) and of a lumped parameters compliance. Two electro-hydraulic interfaces, realized by means of gear pumps driven by DC motors, connect the numerical section with both terminals of the hydraulic section. The lumped parameters numerical model and the control system (including analog to digital and digital to analog converters) are developed in LabVIEW™ environment. The behavior of the model is analyzed by means of the ventricular pressure-volume loops and the time courses of arterial and ventricular pressures and flows in different circulatory conditions. A simulated pathological condition was set to test the IABP and verify the response of the system to this type of mechanical circulatory assistance. The results show that the model can represent hemodynamic relationships in different ventricular and circulatory conditions and is able to react to the IABP assistance.

Modelling of cardiovascular system: development of a hybrid (numerical-physical) model.

Physical models of the circulation are used for research, training and for testing of implantable active and passive circulatory prosthetic and assistance devices. However, in comparison with numerical models, they are rigid and expensive. To overcome these limitations, we have developed a model of the circulation based on the merging of a lumped parameter physical model into a numerical one (producing therefore a hybrid). The physical model is limited to the barest essentials and, in this application, developed to test the principle, it is a windkessel representing the systemic arterial tree. The lumped parameters numerical model was developed in LabVIEW™ environment and represents pulmonary and systemic circulation (except the systemic arterial tree). Based on the equivalence between hydraulic and electrical circuits, this prototype was developed connecting the numerical model to an electrical circuit - the physical model. This specific solution is valid mainly educationally but permits the development of software and the verification of preliminary results without using cumbersome hydraulic circuits. The interfaces between numerical and electrical circuits are set up by a voltage controlled current generator and a voltage controlled voltage generator. The behavior of the model is analyzed based on the ventricular pressure-volume loops and on the time course of arterial and ventricular pressures and flow in different circulatory conditions. The model can represent hemodynamic relationships in different ventricular and circulatory conditions.

Reproduction of Continuous Flow Left Ventricular Assist Device Experimental Data by Means of a Hybrid Cardiovascular Model With Baroreflex Control

Long-term mechanical circulatory assistance opened new problems in ventricular assist device-patient interaction, especially in relation to autonomic controls. Modeling studies, based on adequate models, could be a feasible approach of investigation. The aim of this work is the exploitation of a hybrid (hydronumerical) cardiovascular simulator to reproduce and analyze in vivo experimental data acquired during a continuous flow left ventricular assistance. The hybrid cardiovascular simulator embeds three submodels: a computational cardiovascular submodel, a computational baroreflex submodel, and a hydronumerical interface submodel. The last one comprises two impedance transformers playing the role of physical interfaces able to provide a hydraulic connection with specific cardiovascular sites (in this article, the left atrium and the ascending/descending aorta). The impedance transformers are used to connect a continuous flow pump for partial left ventricular support (Synergy Micropump, CircuLite, Inc., Saddlebrooke, NJ, USA) to the hybrid cardiovascular simulator. Data collected from five animals in physiological, pathological, and assisted conditions were reproduced using the hybrid cardiovascular simulator. All parameters useful to characterize and tune the hybrid cardiovascular simulator to a specific hemodynamic condition were extracted from experimental data. Results show that the simulator is able to reproduce animal-specific hemodynamic status both in physiological and pathological conditions, to reproduce cardiovascular left ventricular assist device (LVAD) interaction and the progressive unloading of the left ventricle for different pump speeds, and to investigate the effects of the LVAD on baroreflex activity. Results in chronic heart failure conditions show that an increment of LVAD speed from 20 000 to 22 000 rpm provokes a decrement of left ventricular flow of 35% (from 2 to 1.3 L/min). Thanks to its flexibility and modular structure, the simulator is a platform potentially useful to test different assist devices, thus providing clinicians additional information about LVAD therapy strategy.

A new hybrid electro-numerical model of the left ventricle

The paper presents a new project of a hybrid numerical-physical model of the left ventricle. A physical part of the model can be based on electrical or hydraulic structures. Four variants of the model with numerical and physical heart valves have been designed to investigate an effect of a heart assistance connected in series and in parallel to the natural heart. The LabVIEW real time environment has been used in the model to increase its accuracy and reliability. A prototype of the hybrid electro-numerical model of the left ventricle has been tested in an open loop and closed loop configuration.

Virtual respiratory system for education and research: simulation of expiratory flow limitation for spirometry.

Background Due to economic and ethical problems, virtual organs may appear more convenient than experiments on animals or limited investigations on patients. In particular, a virtual respiratory system (VRS) may be useful for tasks such as respirators and support methods testing, education, staff (medical and technical) training, (initial) testing of scientific hypotheses. Methods A comparative study of simulated and real spirometric results for different patient states (healthy lungs, restrictive lung disease, and obstructive lung disease of different localization and degree) was performed. The volume-flow curve and such standard parameters as FEV1, FEV1%VC, MEF75 etc. were analyzed. Results A mathematical description of collapsing bronchi was proposed. All fundamental phenomena present during spirometry also appeared in VRS, especially characteristic dependence between lung volume and air flow for forced expiration. In particular, both airway resistance and the flow limitation were described with one formula derived from commonly known dependence of the resistance on lung volume. Generally there were no significant differences between simulated results and those seen in clinical practice. Only simulation of obstruction in upper airways gave incorrect results, which suggested a different flow limitation mechanism (perhaps wave-speed limitation). Conclusions Our VRS can already be used in medical education, e.g. courses of spirometry, and in some other applications. It seems that the significance of the wave-speed criterion has been overestimated.

A cardiovascular simulator tailored for training and clinical uses

OBJECTIVE In the present work a cardiovascular simulator designed both for clinical and training use is presented. METHOD The core of the simulator is a lumped parameter model of the cardiovascular system provided with several modules for the representation of baroreflex control, blood transfusion, ventricular assist device (VAD) therapy and drug infusion. For the training use, a Pre-Set Disease module permits to select one or more cardiovascular diseases with a different level of severity. For the clinical use a Self-Tuning module was implemented. In this case, the user can insert patients specific data and the simulator will automatically tune its parameters to the desired hemodynamic condition. The simulator can be also interfaced with external systems such as the Specialist Decision Support System (SDSS) devoted to address the choice of the appropriate level of VAD support based on the clinical characteristics of each patient. RESULTS The Pre-Set Disease module permits to reproduce a wide range of pre-set cardiovascular diseases involving heart, systemic and pulmonary circulation. In addition, the user can test different therapies as drug infusion, VAD therapy and volume transfusion. The Self-Tuning module was tested on six different hemodynamic conditions, including a VAD patient condition. In all cases the simulator permitted to reproduce the desired hemodynamic condition with an error<10%. CONCLUSIONS The cardiovascular simulator could be of value in clinical arena. Clinicians and students can utilize the Pre-Set Diseases module for training and to get an overall knowledge of the pathophysiology of common cardiovascular diseases. The Self-Tuning module is prospected as a useful tool to visualize patients status, test different therapies and get more information about specific hemodynamic conditions. In this sense, the simulator, in conjunction with SDSS, constitutes a support to clinical decision - making.

The influence of left ventricle assist device and ventilatory support on energy-related cardiovascular variables

One of the main purposes in using Left Ventricle Assist Devices (LVAD) to assist recovery in patients with heart failure, is to reduce the external work (EW) of the left natural ventricle. The simultaneous presence of mechanical ventilatory support can affect the value of this variable. The aim of our computer simulation was to trace the influence of LVAD on EW, cardiac mechanical efficiency (CME) and pressure volume area (PVA) in the presence of ventilatory support. Pathological conditions of the heart were reproduced. Peripheral systemic arterial resistance (Ras) was also changed to model physiological and pathological states. The influence of mechanical ventilation was introduced by changing levels of mean thoracic pressure. In this way we were able to predict changes of EW, CME and PVA in both ventricles, during ventilatory (mechanical) and cardiovascular (LVAD) support. Our simulation showed that positive thoracic pressure seems to affect the energy-related cardiovascular variables and should be taken into account during the assessment of LVAD effectiveness. Pathological changes of systemic peripheral resistance have a considerable effect on EW, CME and PVA of left ventricle. On the other hand energy-related parameters of the right ventricle are not especially affected by changes in systemic peripheral resistance.

Hybrid (Numerical-Physical) Circulatory Models: Description and Possible Applications

Circulatory models are used for several applications ranging from research, to education and training. They are based on different structures that are essentially numerical or hydraulic. Both structures present advantages and disadvantages which suggested developing a new class of circulatory models, defined as hybrid: hybrid circulatory models permit to merge numerical and physical models. This solution reduces the costs, enhances the flexibility and opens the way to many applications. This paper presents a prototype consisting of a hydraulic model of systemic arterial tree connected to a lumped parameters numerical model of the circulation developed in Lab VIEWtrade. The behavior of the model was verified reproducing two circulatory conditions, physiological and pathological. The latter was assisted by means of an intra-aortic balloon pump. The results show that hybrid modeling permits to build complex networks merging models based on different structures. The hybrid system is able to react to events occurring indifferently in any of the structures forming it