Research On The Cardiovascular System Using A Biventricular Assist Device

Biventricular assist devices have been an important clinical treatment for end-stage biventricular heart failure.The therapy using ventricular assist can improve the system's hemodynamics and promote recovery of cardiac function.The control strategy of the mechanical assist device affects the hemodynamics of the cardiovascular system in patients with heart failure.Therefore,in this dissertation,numerical simulation is used to study the cardiovascular system assisted by a biventricular assist device and the effect of assist mode on the hemodynamics of the system was investigated.The main research contents are as follows:A lumped parameter model of the cardiovascular system is established based on physiological basis.Compared with the left cardiovascular model,the model includes the right heart and the pulmonary circulation systems,and can be used to study the interaction between the biventricular assist device and the cardiovascular system systematically.The model can mimic the characteristics of hemodynamic changes under different physiological states.The numerical data are consistent with clinical physiological characteristics,confirming that the model is feasible.Combining with the mathematical model of the rotary heart pump proposed by the research group earlier,a cardiovascular-heart pump coupled model is developed.Using the coupled model,the effects of unloading level of left ventricular assist and assist mode on the cardiovascular system hemodynamics and biventricular pulsation synchronization are firstly studied.The results show that as the unloading level increases,both pressure and volume of left ventricle decrease while increase in right ventricle.If left ventricle is excessively unloaded,the load of right ventricle increases,raising the risk of right heart failure.Considering hemodynamic changes,continuous mode can achieve maximum left ventricular unloading and maximum load on right ventricle.It also significantly reduces the pulsatility of aortic pressure and aortic flow,which may reduce arterial compliance and result in hardening of blood vessels if the low physiological pulsatility remains for a long time.In the pulse mode,the increaseof the load on right ventricle reaches minimum,and the pulsatility of aortic pressure and flow increase.Among the three assist modes,the duration of left ventricle systolic phase in the continuous mode is significantly reduced,resulting in asynchronization of left and right ventricular contractions.The counter-pulse mode significantly corrects the duration of left ventricle systolic phase by reducing the rotation speed of the systole,which is beneficial to achieve synchronization between left and right ventricles and prevent right ventricular dysfunction.In conclusion,the counter-pulse mode has advantages in maintaining the synchronization of the biventricular beats,while the pulse mode can avoid overload on the right ventricle and can enhance the system pulsatility.Secondly,the effect of the assist mode of the biventricular assist device on the hemodynamics of the cardiovascular system is investigated.The results show that although constant-speed assist model can improve blood perfusion of patients with heart failure,it reduces the system's pulsatility,and cannot automatically adjust the pump speed to adapt to perfusion needs according to the change of the patient's physiological state.Variable speed assist mode can quickly respond to blood perfusion needs when the patient's activity level changes,and can maintain the pulsatility of arterial blood pressure and blood flow when the patient's activity level changes.In addition,this control mode can automatically adjust the pump speed following the characteristics of ventricular systole and diastole to avoid the device's regurgitation and ventricular suction.Finally,the asynchronous assist mode of the biventricular assist device is developed to explore the influence of the asynchronous assist mode on the system.The delay of the pump speed is used to characterize the asynchrony.The results show that with asynchronous assistance,as the delay time of the pump speed increases during of the cardiac cycle,the average pump flow gradually decreases,and the cross-valve flow gradually increases,while the total cardiac output remains at a normal level.The reason is that the longer the delay time of the pump speed in the cardiac cycle,the longer the duration of ejection of left and right ventricles,and thus the more blood is pumped by the ventricles.Therefore,delaying the pump speed appropriately can keepventricles maintain a certain degree of autonomous pumping function and thus maintain the orderly opening and closing of the arterial valves,which can maintain the biological activity of the valves and prevent commissural fusion.