Development of a hydraulic circulatory loop and sensorless flow control system for testing a minimally invasive pneumatic heart pump

Abstract

Mechanical support systems have become an important component of medical treatment in patients with acute cardiovascular diseases. Traditionally the performance of the support systems and their controllers is tested in mock circulatory loops that emulate the working environment in which the mechanical support will operate inside the human patient. Yet testing and control systems involving continuously operating percutaneous ventricular assist devices (pVADs) actuated by gas flow do not exist on the market. In this work individual systems for testing and control of various parameters on such a pVAD actuated by helium gas are proposed. The reproducible pulsatile environment dedicated to extensive testing of pVAD prototypes presented here is used for an objective comparison between different heart pump models and as a proof of concept before the start of in-vivo trials. A method for recycling of helium gas streaming from the pVAD is necessary in order to increase safety and allow application in clinical environment, at the same time permitting control of blood flow through the heart pump. Due to inability of direct blood flow measurement, a means to estimate the blood flow through the heart pump is integrated into the flow control system. The pulsatile environment for the heart pump is implemented as a purely hydraulic mock circulation that imitates the behavior of the systemic human circulation with a single active element serving as left ventricle. Based on simulation results, the physical assembly was created and the algorithm for actuation of the left ventricle was implemented. The helium gas is recycled using a gas recirculation in essence consisting of a compressor, a mass flow controller and the pVAD. By combining results from static and dynamic experiments on pneumatic elements, an algorithm for the mass flow control was constructed which ensures the proper behavior of the heart pump. The control system superposes speed and flow control loops onto the mass flow control loop, i.e. the gas recirculation. The flow control loop in its feedback line estimates the blood flow from the latest values of the heart pump¿s speed and mass flow or motor current in case of a pneumatic or motorized unit, respectively. The procedure involved recording of the heart pump¿s static characteristics, creating the lookup tables and applying an algorithm that is able to trace the correct value for blood flow through the pump. Tests with the system proved that the mock circulation is able to replicate healthy pressures in the left ventricle and the aorta (120/80 mmHg) at stroke volumes of 80 ml. The system also permits the replication of conditions with pathological values of ejection fractions, as well as abnormal blood pressure and arrhythmias. The gas recirculation maintains closed loop mass flows between the required 45 to 77.5 gpm of air at pressures that minimize the risk of catheter rupture, simultaneously ensuring adequate of mass flows at sudden changes of the flow reference. The result of the flow estimation provides an instantaneous blood flow through the pump and reaches high accuracy with errors well below 10% of the true value. The systems were verified with 2:1 and 1:1 pVAD prototypes in in-vitro tests, which showed increase in peripheral organ perfusion and stabilization of aortic and left ventricular pressures as a result of the pump¿s operation. The results also demonstrated the correct functionality of the systems developed in this thesis.