Maurizio Arabia

Left ventricle load impedance control by apical VAD can help heart recovery and patient perfusion: a numerical study.

The aim of this work is to investigate the dependence between left ventricular load impedance control by an apical ventricular assist device (VAD) and the consequent benefits for pathological heart recovery. A pathological left ventricle with 34% contractility has been simulated in the assisted and nonassisted conditions. By means of an extended Kalman filter, left ventricular pressure-volume loops have been partially estimated and ventricular as well as circulatory quantities inferred. The heart operation mode, based on cardiac energetic criteria, is imposed by controlling the VAD filling phase. In the assisted condition, results show that the left ventricle end-diastolic volume, left atrial pressure, and wall stress all decrease; stroke volume, ejection fraction, ventricular efficiency, aortic pressure, and cardiac output all increase. Benefits are also evident for the right ventricle and systemic and pulmonary circulation. The strategy outlined in this work also shows that good results for heart recovery are achievable and a possible way to improve the functional properties of commercial pulsatile VADs.

Left Ventricle Afterload Impedance Control by an Axial Flow Ventricular Assist Device: A Potential Tool for Ventricular Recovery

Ventricular assist devices (VADs) are increasingly used for supporting blood circulation in heart failure patients. To protect or even to restore the myocardial function, a defined loading of the ventricle for training would be important. Therefore, a VAD control strategy was developed that provides an explicitly definable loading condition for the failing ventricle. A mathematical model of the cardiovascular system with an axial flow VAD was used to test the control strategy in the presence of a failing left ventricle, slight physical activity, and a recovering scenario. Furthermore, the proposed control strategy was compared to a conventional constant speed mode during hemodynamic changes (reduced venous return and arterial vasoconstriction). The physiological benefit of the control strategy was manifested by a large increase in the ventricular Frank-Starling reserve and by restoration of normal hemodynamics (5.1 L/min cardiac output at a left atrial pressure of 10 mmHg vs. 4.2 L/min at 21 mmHg in the unassisted case). The control strategy automatically reduced the pump speed in response to reduced venous return and kept the pump flow independent of the vasoconstriction condition. Most importantly, the ventricular load was kept stable within 1%, compared to a change of 75% for the constant speed. As a key feature, the proposed control strategy provides a defined and adjustable load to the failing ventricle by an automatic regulation of the VAD speed and allows a controlled training of the myocardium. This, in turn, may represent a potential additional tool to increase the number of patients showing recovery.

Hybrid test bench for evaluation of any device related to mechanical cardiac assistance.

Hydraulic mock circulatory systems have low flexibility to allow tests of different cardiovascular devices and low precision when a reference model must be reproduced. In this paper a new bench is described. It combines the computer model of the environment in which the device will operate and the electro-hydraulic interfaces by which device and computer are connected. A models library provided with basic functions allows implementing many layouts of the bench, which in turn depend both on the device properties and the desired experiment. In case of an apical LVAD evaluation, the bench can reproduce right and left ventricles, pulmonary and systemic circulations, inlet and outlet LVAD cannulas. An interface forces the instantaneous calculated flow at the VAD input and feeds back the measured pressure to the computer; another interface works in a similar -but complementary- way at the VAD output. The paper focuses on the operating principle of the electro hydraulic interfaces which represent a relevant component of the bench, on the RT-Linux-based software architecture, on the models of the basic elements of the bench. A patent is under preparation. At the moment, only a portion of the bench has been developed. It consists of a piston-cylinder mechanism, which mimics the elastance-based mechanism of a natural ventricle, and a hydraulic circuit representing the arterial load according to a modified windkessel model and the venous return according to the Guytons model. The pump is driven by a real-time simulation of the cardiovascular system. This preliminary layout allowed testing the piston-cylinder mechanism, its control, and the software. This electro-hydraulic interface has been used to reproduce a pulsatile pump working in different modes. The hybrid model approach can support the development of new cardiac assist devices from their computer model to their manufacture.

A modified elastance model to control mock ventricles in real-time: numerical and experimental validation.

This article describes an elastance-based mock ventricle able to reproduce the correct ventricular pressure-volume relationship and its correct interaction with the hydraulic circuit connected to it. A real-time control of the mock ventricle was obtained by a new left ventricular mathematical model including resistive and inductive terms added to the classical Suga-Sagawa elastance model throughout the whole cardiac cycle. A valved piston pump was used to mimic the left ventricle. The pressure measured into the pump chamber was fed back into the mathematical model and the calculated reference left ventricular volume was used to drive the piston. Results show that the classical model is very sensitive to pressure disturbances, especially during the filling phase, while the modified model is able to filter out the oscillations thus eliminating their detrimental effects. The presented model is thus suitable to control mock ventricles in real-time, where sudden pressure disturbances represent a key issue and are not negligible. This real-time controlled mock ventricle is able to reproduce the elastance mechanism of a natural ventricle by mimicking its preload (mean atrial pressure) and afterload (mean aortic pressure) sensitivity, i.e., the Starling law. Therefore, it can be used for designing and testing cardiovascular prostheses due to its capability to reproduce the correct ventricle-vascular system interaction.

Control of a rotary blood pump for defined ventricular unloading: A potential tool for ventricular recovery

Complementary to an already available pump control for physiological demand of rotary blood pump recipients, a control for a defined and controllable hydraulic load for the failing ventricle was developed. Such explicitly definable loading condition may be preferable for training the heart towards recovery.

A PRELOAD AND AFTERLOAD SENSITIVE ARTIFICIAL VENTRICLE TO TEST CARDIOVASCULAR PROSTHESES

Abstract In the last decades cardiovascular diseases greatly increased worldwide, and bioengineering provided new technologies and cardiovascular prostheses to medical doctors and surgeons. Ventricular assist devices aroused notable interests. As a consequence it is important to faithfully reproduce the interaction between the prostheses and the cardiovascular system when in-vitro experiments are performed. For this reason, a new and improved kind of test benches become necessary. In this paper an artificial ventricle connected to a hydraulic circuit is described. The ventricles control architecture is based on the estimation of some characteristic parameters. It is showed how this control strategy leads to the mutual interaction between the artificial ventricle and the hydraulic circuit and a correct mean atrial (preload) and aortic (afterload) pressure sensitivity and hydrodynamics.