The effect of innate compliance on the performance of a counterpulsation device

Cardiovascular disease (CVD) continues to be one of the top causes of mortality in the world. World Heart Organization (WHO) reported that in 2004, CVD contributed to almost 30% of death from estimated worldwide death figures of 58 million[1]. Heart failure treatment varies from lifestyle adjustment to heart transplantation; its aims are to reduce HF symptoms, prolong patient survival and minimize risk [2]. One alternative available in the market for HF treatment is Left Ventricular Assist Device (LVAD). Chronic Intermittent Mechanical Support (CIMS) device is a novel (LVAD) heart failure treatment using counterpulsation similar to Intra Aortic Balloon Pumps (IABP). However, the implantation site of the CIMS balloon is in the ascending aorta just distal to aortic valve contrasted with IABP in the descending aorta. Counterpulsation coupled with implantation close to the aortic valve enables comparable flow augmentation with reduced balloon volume. Two prototypes of the CIMS balloon were constructed using rapid prototyping: the straight-body model is a cylindrical tube with a silicone membrane lining with zero expansive compliance. The compliant-body model had a bulging structure that allowed the membrane to expand under native systolic pressure increasing the device’s static compliance to 1.5 mL/mmHg. This study examined the effect of device compliance and vascular compliance on counterpulsating flow augmentation. Both prototypes were tested on a two-element Windkessel model human mock circulatory loop (MCL). The devices were placed just distal to aortic valve and left coronary artery. The MCL mimicked HF with cardiac output of 3 L/min, left ventricular pressure of 85/15 mmHg, aortic pressure of 70/50 mmHg and left coronary artery flow rate of 66 mL/min. The mean arterial pressure (MAP) was calculated to be 57 mmHg. Arterial compliance was set to be1.25 mL/mmHg and 2.5 mL/mmHg. Inflation of the balloon was triggered at the dicrotic notch while deflation was at minimum aortic pressure prior to systole. Important haemodynamics parameters such as left ventricular pressure (LVP), aortic pressure (AoP), cardiac output (CO), left coronary artery flowrate (QcorMean), and dP (Peak aortic diastolic augmentation pressure – AoPmax ) were simultaneously recorded for both non-assisted mode and assisted mode. ANOVA was used to analyse the effect of both factors (balloon and arterial compliance) to flow augmentation. The results showed that for cardiac output and left coronary artery flowrate, there were significant difference between balloon and arterial compliance at p < 0.001. Cardiac output recorded maximum output at 18% for compliant body and stiff arterial compliance. Left coronary artery flowrate also recorded around 20% increase due to compliant body and stiffer arterial compliance. Resistance to blood ejection recorded highest difference for combination of straight body and stiffer arterial compliance. From these results it is clear that both balloon and arterial compliance are statistically significant factors for flow augmentation on peripheral artery and reduction of resistance. Although the result for resistance reduction was different from flow augmentation, these results serves as an important aspect which will influence the future design of the CIMS balloon and its control strategy. References: 1. Mathers C, Boerma T, Fat DM. The Global Burden of disease:2004 update. Geneva: World Heatlh Organization; 2008. 2. Jessup M, Brozena S. Heart Failure. N Engl J Med 2003;348:2007-18

The effect of heat treatment on mechanical properties of pulsed Nd:YAG welded thin Ti6Al4V

Pulsed Nd:YAG has been adopted successfully in welding process of thin (0.7 mm) Ti6Al4V. Laser welding of such thin sheet requires a small focal spot, good laser beam quality and fast travel speed, since too much heat generation can cause distortion for thin sheet weld. The microstructures of Ti6Al4V were complex and strongly affected the mechanical properties. These structures include: a´ martensite, metastable ß, Widmanstätten, bimodal, lamellar and equiaxed microstructure. Bimodal and Widmanstätten structures exhibit a good-balance between strength and ductility. The microstructure of pulsed Nd:YAG welded Ti6Al4V was primarily a´ martensite, which showed the lowest ductility but not significantly high strength. A heat treatment at 950 followed by furnace cooling can transform the microstructure in the weld from a´ martensite structure into Widmanstätten structure

Cell exclusion in couette flow:evaluation through flow visualisation and mechanical forces

Cell exclusion is the phenomenon whereby the hematocrit and viscosity of blood decrease in areas of high stress. While this is well known in naturally occurring Poiseuille flow in the human body, it has never previously been shown in Couette flow, which occurs in implantable devices including blood pumps. The high-shear stresses that occur in the gap between the boundaries in Couette flow are known to cause hemolysis in erythrocytes. We propose to mitigate this damage by initiating cell exclusion through the use of a spiral-groove bearing (SGB) that will provide escape routes by which the cells may separate themselves from the plasma and the high stresses in the gap. The force between two bearings (one being the SGB) in Couette flow was measured. Stained erythrocytes, along with silver spheres of similar diameter to erythrocytes, were visualized across a transparent SGB at various gap heights. A reduction in the force across the bearing for human blood, compared with fluids of comparable viscosity, was found. This indicates a reduction in the viscosity of the fluid across the bearing due to a lowered hematocrit because of cell exclusion. The corresponding images clearly show both cells and spheres being excluded from the gap by entering the grooves. This is the first time the phenomenon of cell exclusion has been shown in Couette flow. It not only furthers our understanding of how blood responds to different flows but could also lead to improvements in the future design of medical devices

In-vitro Evaluation of Physiological Controller Response of Rotary Blood Pumps to Changes in Patient State

Rotary blood pumps (RBPs) have a low sensitivity to preload changes when run at constant speed, which can lead to harmful ventricular suction events. Therefore a control mechanism is needed to adjust RBP speed in response to patient demand, but an appropriate response time for physiological control strategies to these changes in patient demand has not been determined. This paper aims to evaluate the response of a simulated healthy heart with those of different RBP control techniques during exercise simulations and a Valsalva manoeuver. A mock circulation loop was used to simulate the response of a healthy heart to these changes in patient state. The generated data was compared with a simulated RBP (VentrAssist) supported left heart failure condition. A range of control techniques including constant speed, proportional integral (PI) (active control) and a compliant inflow cannula (passive control) were used to achieve restored haemodynamics and evaluate controller response time. Controllers that responded faster (active control) or slower (active control and constant speed mode) than the native heart's response led to ventricular suction. Active control systems can respond both faster or slower than the heart depending on the controller gains. A control system that responded similar to the native heart was able to prevent ventricular suction. This study concluded that a physiological control system should mimic the response of the native heart to changes in patient state in order to prevent ventricular suction

In-Vitro Evaluation of Cardiac Energetics and Coronary Flow with Volume Displacement and Rotary Blood Pumps

Bridge to recovery with left ventricular assist device (LVAD) support has been more prominent with volume displacement pumps (VDPs) than with rotary blood pumps (RBPs), which may be due to VDPs providing greater ventricular unloading and coronary artery flow. To compare ventricular unloading and coronary flow of VDPs and RBPs in a repeatable environment, a physiologic coronary circulation was added to a pre-existing mock circulatory loop. In this study, a physiologic coronary circulation, mimicking a healthy or diseased auto-regulatory response was implemented in a mock circulatory loop. Using the mock circulation loop, a VDP with original (Björk-Shiley) and then replacement (jellyfish) valves was operated in clinically recommended modes and compared to full and partial assist RBP operating at constant speed and rapid speed modulated modes. The Björk-Shiley VDP resulted in increased pressure-volume area, which resulted in greater coronary artery flow when compared to the improved jellyfish valves. Full assist RBP support reduced left ventricular stroke work, pressure-volume area and coronary flow compared to partial assist, whilst the effect of speed modulation modes was not as significant. Of all LVAD operating modes, the counter-pulsed VDP with jellyfish valves demonstrated the greatest reduction in pressure-volume area and improved coronary flow. This study provides a basis for further investigation into RBP speed modulation profiles to match the improved haemodynamic performance of VDPs.</p

Development of a numerical pump testing framework

It has been shown that left ventricular assist devices (LVADs) increase the survival rate in end-stage heart failure patients. However, there is an ongoing demand for an increased quality of life, fewer adverse events, and more physiological devices. These challenges necessitate new approaches during the design process. In this study, computational fluid dynamics (CFD), lumped parameter (LP) modeling, mock circulatory loops (MCLs), and particle image velocimetry (PIV) are combined to develop a numerical Pump Testing Framework (nPTF) capable of analyzing local flow patterns and the systemic response of LVADs. The nPTF was created by connecting a CFD model of the aortic arch, including an LVAD outflow graft to an LP model of the circulatory system. Based on the same geometry, a three-dimensional silicone model was crafted using rapid prototyping and connected to an MCL. PIV studies of this setup were performed to validate the local flow fields (PIV) and the systemic response (MCL) of the nPTF. After validation, different outflow graft positions were compared using the nPTF. Both the numerical and the experimental setup were able to generate physiological responses by adjusting resistances and systemic compliance, with mean aortic pressures of 72.2–132.6 mm Hg for rotational speeds of 2200–3050 rpm. During LVAD support, an average flow to the distal branches (cerebral and subclavian) of 24% was found in the experiments and the nPTF. The flow fields from PIV and CFD were in good agreement. Numerical and experimental tools were combined to develop and validate the nPTF, which can be used to analyze local flow fields and the systemic response of LVADs during the design process. This allows analysis of physiological control parameters at early development stages and may, therefore, help to improve patient outcomes

Development and validation of a low-cost polymer selective laser sintering machine

Due to manufacturer implemented processing parameter restrictions and the cost prohibitive nature of selective laser sintering (SLS) machines, researchers have limited opportunities to explore the processing of new materials using this additive manufacturing (3D printing) process. Accordingly, this article aimed to overcome these limitations by describing the build and operation of a customizable low-cost polymer SLS machine. The machine boasts a three piston powder bed with the center build piston heated by PID controlled ceramic heaters. Thermal energy for powder consolidation was provided via a 2.44 W solid state diode laser which was mechanically traversed using stepper motor driven belt drives. New layers of powder were deposited by a counter-rotating roller system. The SLS machine was controlled by executing G-code in Mach3 allowing full customization of processing parameters. The machine demonstrated the production of parts from polyamide-12 reaching densities of 918 ± 9 kg/m while achieving an elastic modulus of 358.36 ± 3.04 MPa and elongation at break of 11.13 ± 0.02%. With part properties similar to those achievable with a commercial machine, this low-cost SLS machine could be a vital tool in assisting researchers to explore the processing of new materials

Time Course Response of the Heart and Circulatory System to Active Postural Changes

Rotary blood pumps (RBPs) used for mechanical circulatory support of heart failure patients cannot passively change pump flow sufficiently in response to frequent variations in preload induced by active postural changes. A physiological control system that mimics the response of the healthy heart is needed to adjust pump flow according to patient demand. Thus, baseline data is required on how the healthy heart (i.e. heart rate (HR) and cardiac output (CO)) and circulatory system (i.e. systemic vascular resistance (SVR)) respond. This study investigated the response times of the healthy heart during active postural changes (supine-standing-supine) in 50 healthy subjects (27 male / 23 female). Early response times (te) and settling times (ts) were calculated for HR, CO and SVR from data continuously collected with impedance cardiography. The initial circulatory response of HR, CO and SVR resulted in te of 8.8 - 11.7 s when standing up and te of 4.7 - 5.8 s when lying back down. HR, CO and SVR settled in ts of 50.0 - 56.7 s and 46.3 - 66.5 s when standing up and lying down respectively. In conclusion, when compared to active stand up HR, CO and SVR responded significant faster initially when subjects were lying down (

Haemodynamic effect of left atrial and left ventricular cannulation with a rapid speed modulated rotary blood pump during rest and exercise: investigation in a numerical cardiorespiratory model

PURPOSE: The left atrium and left ventricle are the primary inflow cannulation sites for heart failure patients supported by rotary blood pumps (RBPs). Haemodynamic differences exist between inflow cannulation sites and have been well characterized at rest, yet the effect during exercise with the same centrifugal RBP has not been previously well established. The purpose of this study was to investigate the hemodynamic effect of inflow cannulation site during rest and exercise with the same centrifugal RBP. METHODS: In a numerical cardiorespiratory model, a simulated heart failure patient was supported by a HeartWare HVAD RBP in left atrial (LAC) and left ventricular cannulation (LVC). The RBP was operated at constant speed and sinusoidal co- and counter-pulse and was investigated in cardiovascular conditions of steady state rest and 80-watt bike graded exercise. RESULTS: Cardiac output was 5.0 L min-1 during rest and greater than 6.9 L min-1 during exercise for all inflow cannulation sites and speed operating modes. However, during exercise, LAC demonstrated greater pressure-volume area and lower RBP flow (1.41, 1.37 and 1.37 J and 5.03, 5.12 and 5.03 L min-1 for constant speed and co- and counter-pulse respectively) when compared to LVC (pressure-volume area: 1.30, 1.27 and 1.32 J and RBP flow: 5.56, 5.71 and 5.59 L min-1 for constant speed and co- and counter-pulse respectively). CONCLUSION: For a simulated heart failure patient intending to complete exercise, LVC seems to assure a better hemodynamic performance in terms of pressure-volume area unloading and increasing RBP flow