Mock circulatory loops (MCLs) are usually developed for assessment of ventricular assist devices and consist of abstracted anatomical structures represented by connecting tubing pipes and controllable actuators which could mimic oscillating flow processes. However, with increasing use of short-term peripheral mechanical support (extracorporeal life support [ECLS]) and the upcoming evidence of even counteracting flow processes between the failing native circulation and ECLS, MCLs incorporating the peripheral vascular system and preserved anatomical structures are becoming more important for systematic assessment of these processes. For reproducible and standardized fluid-mechanical studies using magnetic resonance imaging, Doppler ultrasound, and computational fluid dynamics measurements, we developed a MCL of the human circulation. Silicon-based life-sized dummies of the human aorta and vena cava (vascular module) were driven by paracorporeal pneumatic assist devices. The vascular module is placed in a housing with all arterial branches merging into peripheral resistance and compliances modules, and blood-mimicking fluid returns to the heart module through the venous dummy. Compliance and resistance chambers provide for an adequate simulation of the capillary system. Extracorporeal life support cannulation can be performed in the femoral and subclavian arteries and in the femoral and jugular veins. After adjusting vessel diameters using variable Hoffmann clamps, physiologic flow rates were achieved in the supraaortic branches, the renal and mesenteric arteries, and the limb arteries with physiologic blood pressure and cardiac output (4 L/min). This MCL provides a virtually physiologic platform beyond conventional abstracted MCLs for simulation of flow interactions between the human circulation and external circulation generated by ECLS.

Aims Extracorporeal life support (ECLS) during acute cardiac failure restores haemodynamic stability and provides life-saving cardiopulmonary support. Unfortunately, all common cannulation strategies and remaining pulmonary blood flow increase left-ventricular afterload and may favour pulmonary congestion. The resulting disturbed pulmonary gas exchange and a residual left-ventricular action can contribute to an inhomogeneous distribution of oxygenated blood into end organs. These complex flow interactions between native and artificial circulation cannot be investigated at the bedside: only an in vitro simulation can reveal the underlying activities. Using an in vitro mock circulation loop, we systematically investigated the impact of heart failure, extracorporeal support, and cannulation routes on the formation of flow phenomena and flow distribution in the arterial tree. Methods and results The mock circulation loop consisted of two flexible life-sized vascular models (aorta and vena cava) driven by two paracorporeal assist devices, resistance elements, and compliance reservoirs to mimic the circulatory system. Several large-bore antegrade and retrograde access ports allowed connection to an ECLS system for extracorporeal support. With four degrees of extracorporeal support-that for cardiac failure, early recovery, late recovery, and weaning-we investigated aortic blood flow velocity, blood flow, and mixing zones using colour-coded Doppler ultrasound in the aorta and its corresponding branches. Full retrograde extracorporeal support (3-4 L/min) perfused major portions of the aorta but did not reach the supra-aortic branches and ascending aorta, resulting in an area in the thoracic aorta demonstrating nearly stagnant blood flow velocities during cardiogenic shock and early recovery (0 ± 4 cm/s; À10 ± 15 cm/s, respectively) confined by two watersheds at the aortic isthmus and renal artery origin. Even increased ECLS flow was unable to shift the watershed towards the aortic arch. Antegrade support resulted in homogeneous flow distribution during all stages of cardiac failure but created a markedly negative flow vector in the ascending aorta during cardiogenic shock and early recovery with increased afterload. Conclusions Our systematic fluid-mechanical analysis confirms the clinical assumption that despite restoring haemodynamic stability, extracorporeal support generates an inhomogeneous distribution of oxygenated blood with an inadequate supply to end organs and increased left-ventricular afterload with absent ventricular unloading. End-organ supply may be monitored by near-infrared spectroscopy, but an obviously non-controllable watershed emphasizes the need for additional measures: pre-pulmonary oxygenation with a veno-arterial-venous ECLS configuration can allow a transpulmonary passage of oxygenated blood, providing improved end-organ supply.

Diseases of the cardiovascular system account for nearly 42% of all deaths in the European Union. In Germany, approximately 12,000 patients receive surgical replacement of the aortic valve due to heart valve disease alone each year. A three-dimensional (3D) numerical model based on patient-specific anatomy derived from four-dimensional (4D) magnetic resonance imaging (MRI) data was developed to investigate preoperatively the flow-induced impact of mounting positions of aortic prosthetic valves to select the best orientation for individual patients. Systematic steady-state analysis of blood flow for different rotational mounting positions of the valve is only possible using a virtual patient model. A maximum velocity of 1 m/s was used as an inlet boundary condition, because the opening angle of the valve is at its largest at this velocity. For a comparative serial examination, it is important to define the standardised general requirements to avoid impacts other than the rotated implantation of the prosthetic aortic valve. In this study, a uniform velocity profile at the inlet for the inflow of the aortic valve and the real aortic anatomy were chosen for all simulations. An iterative process, with the weighted parameters flow resistance (1), shear stress (2) and velocity (3), was necessary to determine the best rotated orientation. Blood flow was optimal at a 45° rotation from the standard implantation orientation, which will offer a supply to the coronary arteries.

OBJECTIVES Limb ischaemia during extracorporeal life support (ECLS) using femoral artery cannulation is frequently observed even in patients with regular vessel diameters and without peripheral arterial occlusive disease. We investigated underlying pathomechanisms using a virtual fluid-mechanical simulation of the human circulation. METHODS A life-sized model of the human aorta and major vascular branches was virtualized using 3-dimensional segmentation software (Mimics, Materialise). Steady-state simulation of different grades of cardiac output (0–100%) was performed using Computational Fluid Dynamics (CFX, ANSYS). A straight cannula [virtualized 16 Fr (5.3 mm)] was inserted into the model via the left common femoral artery. The ECLS flow was varied between 1 and 5 l/min. The pressure boundary conditions at the arterial outlets were selected to demonstrate the downstream vascular system. Qualitative and quantitative analyses concerning flow velocity and direction were carried out in various regions of the model. RESULTS During all simulated stages of reduced cardiac output and subsequently adapted ECLS support, retrograde blood flow originating from the ECLS cannula was observed from the cannulation site up to the aortic bifurcation. Analysis of pressure showed induction of zones of negative pressure close to the cannula tip, consistent with the Bernoulli principle. Depending on cannula position and ECLS flow rate, this resulted in negative flow from the ipsilateral superficial femoral artery or the contralateral internal iliac artery. The antegrade flow to the non-cannulated side was generally greater than that to the cannulated side. CONCLUSIONS The cannula position and ECLS flow rate both influence lower limb perfusion during femoral ECLS. Therefore, efforts to optimize the cannula position and to avoid limb malperfusion, including placement of a distal perfusion cannula, should be undertaken in patients treated with ECLS.

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In Germany in 2016 17,085 patients received TAVI operations and 9,579 had conventional aortic valve surgery. The ‘Heart Team’ uses established scoring systems (EuroSCORE, STS, German AV Score) to evaluate operation risks and which technique to use. However, such risk grading fails to consider patient morphology and possible long-term behavior of the replacement valve chosen. Therefore, preoperative simulation of the dynamic loading on the valve leaflets after TAVR provides information vital for the selection of the appropriate aortic valve therapy - interventional versus conventional. Individual aorta used in this study was captured by MRI. Segmentation and data processing were done with Mimic In-novation Suite. The available biological aortic valves prostheses were reverse engineered to create a 3D CAD model. Simulations combined bi-directional fluid structure interaction (FSI) with a first order Ogden model of the hyperelastic behavior of aortic leaflets from bovine pericardium. Movements induced by flow and the resultant tension on the biological leaflets were computed with developed simulation model. Stress analyses of the leaflets showed behavior attributable to their particular structure. Both valves showed two stress peaks within the initial 0.3 s. Maximum stress occurred, however, at other time points. Furthermore, the initial increase in stress showed a delayed onset. The patterns of movement were also significantly different. So, at opening of the valve, the freely perfused area of the valve, the freedom of leaflet movement and symmetry at closure were different in the two valves. Simulated movement of valve leaflets corresponds well with reality. The estimated stresses clearly lie below thresholds published in the literature for bovine pericardium. It is planned to further develop the current workflow to increase stability and optimize processing time, with the intention of providing the ‘Heart Team’ with a tool for incorporating individual anatomy when selecting the aortic valves.

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An in silico investigation of modelled Extracorporeal Life Support (ECLS) via a femoral arterial cannula revealed the existence of both a defined separation zone between the opposing flows (ECLS, native flow) and different ranges dependent on flow distribution. The interaction between pulsating native circulation and constant ECLS flow is dynamic. A transient simulation model was developed to investigate the dynamic influence on this fluid mechanical interaction. The in silico model is based on a CT-generated 3D model derived from a life-sized silicon aorta. A geometric standard cannula (16Fr) is inserted femoral. Inlet boundary conditions such as the temporal flow profile of a subject from the left ventricle (native circulation) and the flow from the femoral cannula are varied such that during transient simulations the summed flow (total perfusion) is 5.5 l/min. The outlet pressure boundary conditions at the branching arteries are selected such as to model the downstream vascular system. Transient simulations revealed the dynamic effects of different flow fractions (Heart - ECLS) on the flow. Stationary simulations show a separation zone between the two flows, the position of which respectively the ECLSrange, oscillates dependent of the native circulation. Furthermore, it was noted that a raised pulse was impedimental to ECLS. This can be partly compensated by increasing the length of cannula inserted. At the same time the ECLS supply for the brain can improve at the cost of performance post-bifurcation. Increasing the ECLS fraction to above 50% flow led to retrograde flow combined with blood suction from the femoral artery. The EMPAC project model has been further developed to include investigation of the dynamic effects of blood flow. This has made it possible for the first time to analyse in detail and evaluate the temporal effects of both opposing flows streams. A subsequent investigation explains whether aortic elasticity plays a significant role.