Numerical simulation of thrombus aspiration in two realistic models of catheter tips.

Thrombus aspiration catheters are devices used to remove a blood clot from a vessel, usually prior to angioplasty or stent implantation. However, in vitro results showed that the use of different commercial devices could produce very different thrombus removals, suggesting a primary dependence on the distal tip configuration of the catheter. A computational methodology based on realistic catheter tip modeling was developed to investigate the factors affecting the thrombus suction. Two different designs were considered, either with a single central lumen or a combination of central and side holes. First, steady-state aspiration of distilled water from a reservoir was simulated and compared with experimental tests. Subsequently, the aspiration of a totally occlusive thrombus, modeled as a high viscous fluid, was simulated solving a complex two-phase (blood and thrombus) problem. In particular, the benefit of additional openings was investigated. Good matching between the steady-state experimental and numerically simulated hydraulic behaviors allowed a validation of the numerical models. Numerical results of thrombus aspiration showed that the catheter with central and side holes had a worse performance if compared with the single central lumen catheter. Indeed, the inlets in contact with both blood and thrombus preferentially aspirate blood due to its much lower viscosity. This effect hindered the aspiration of thrombus. The amount of aspirated thrombus highly depends on the complex, two-phase fluid dynamics occurring across the catheter tips. Results suggested that location of additional holes is crucial in the catheter aspiration performance.

Management of a stenotic right ventricle-pulmonary artery shunt early after the Norwood procedure.

BACKGROUND: Inadequate pulmonary blood flow through a right ventricle-to-pulmonary artery (RV-PA) shunt early after the Norwood operation can be remedied by adding a modified Blalock-Taussig (mBT) shunt. We used multiscale computational modeling to determine whether the stenotic RV-PA shunt should be left in situ or removed. METHODS: Models of the Norwood circulation were constructed with (1) a 5-mm RV-PA shunt, (2) a RV-PA shunt with 3- or 2-mm stenosis at the RV anastomosis, (3) a stenotic RV-PA shunt plus a 3.0- or 3.5-mm mBT shunt, or (4) a 3.5-mm mBT shunt. A hydraulic network that mathematically describes an entire circulatory system with pre-stage 2 hemodynamics was used to predict local dynamics within the Norwood circulation. Global variables including total cardiac output, mixed venous oxygen saturation, stroke work, and systemic oxygen delivery can be computed. RESULTS: Proximal stenosis of the RV-PA shunt results in decreased pulmonary blood flow, total cardiac output, mixed venous saturation, and oxygen delivery. Addition of a 3.0- or 3.5-mm mBT shunt leads to pulmonary overcirculation, lowers systemic oxygen delivery, and decreases coronary perfusion pressure. Diastolic runoff through the stenotic RV-PA shunt dramatically increases retrograde flow into the single ventricle. Removal of the stenotic RV-PA shunt balances systemic and pulmonary blood flow, eliminates regurgitant flow into the single ventricle, and improves systemic oxygen delivery. CONCLUSIONS: Adding a mBT shunt to remedy a stenotic RV-PA shunt early after a Norwood operation can lead to pulmonary overcirculation and may decrease systemic oxygen delivery. The stenotic RV-PA shunt should be taken down. Conversion to an optimal mBT shunt is preferable to augmenting a stenotic RV-PA shunt with a smaller mBT shunt.

Toward the development of a fully elastic mitral ring: preliminary, acute, in vivo evaluation of physiomechanical behavior.

OBJECTIVES: The optimal repair of functional mitral regurgitation is still debated. No device is able to simultaneously abolish mitral regurgitation and replicate natural mitral annular dynamics. We have tested a fully elastic mitral ring in an acute animal study with the purpose of evaluating (1) ring design and implantation technique, (2) elastic performance, and (3) acute effects on the native mitral annulus. METHODS: Ten healthy sheep underwent surgical implantation of mitral devices, the elastic component of which is represented by a helicoid metallic spring. Preimplantation and postimplantation echocardiographic parameter measurements to evaluate annular dynamics and ventricular function comprise mitral annular motion, systolic tissue Doppler imaging peak wave, transmitral pressure gradient, peak transmitral flow velocity, and ejection fraction. Postimplantation angiographic analysis allowed measurement of the mitral annular area and perimeter variations by means of segmentation of the radiopaque mitral device contour. RESULTS: No significant difference in terms of ejection fraction (P = .13) and systolic tissue Doppler imaging peak wave (P = .87) was found before and after implantation. Mitral annular motion (1.16 cm) was preserved. The percentage of systolic annular reduction derived from angiographic analysis was 14.1% (range, 7.7%-19.7%) in terms of area and 7.2% (range, 4.9%-10.0%) in terms of perimeter. CONCLUSIONS: A mitral elastic ring, implantable by using a standard technique, acutely preserves mitral annular dynamics, allowing area and perimeter changes. Further chronic study is needed to verify the biocompatibility and durability of the device.

Combined computational study of mechanical behaviour and drug delivery from a porous, hydroxyapatite-based bone graft.

This paper presents a numerical model of a porous, hydroxyapatite-based bone graft also suitable as a drug delivery device. The graft was positioned in different sites and with different porosities inside a human femur model. The structural analyses were carried out to verify the graft mechanical strength, using the Tsai-Wu criterion, and the maximum porosity at which static failure does not occur. A local stress shielding risk was also calculated as the ratio between the bone stress in the intact condition and the stress after implantation of the graft. Drug release kinetics was calculated by means of the finite element method. High porosity grafts were found to fail in all implantation sites. Lower porosity grafts showed to have adequate strength if implanted in some positions, while provided insufficient resistance for other implantation sites. Drug release kinetics was found to be strongly dependent both on the porosity of the graft and the bone density near the bone-graft interface.

Pulmonary regurgitation: the effects of varying pulmonary artery compliance, and of increased resistance proximal or distal to the compliance.

BACKGROUND: Pulmonary regurgitation is common after repair of tetralogy of Fallot, predisposing to right ventricular dilatation and potentially fatal arrhythmias. Magnetic resonance studies of such patients led us to hypothesize that the amount of regurgitation, in the absence of an effective valve, depends on pulmonary arterial compliance and on the location of resistance relative to the compliance. METHODS AND RESULTS: Using a pre-existing mathematical model representing the cardiovascular system, removal of the virtual pulmonary valve gave a triphasic pulmonary artery flow curve similar in shape to those recorded in patients with free regurgitation, with a regurgitant fraction of 30%. There was no reversal of flow at pulmonary capillary level, the regurgitant volume originating entirely from the compliance of the virtual pulmonary arteries and arterioles. Doubling their compliance increased regurgitation to 35%, whereas halving it decreased regurgitation to 23%. Doubling the total pulmonary vascular resistance by increasing arteriolar resistance increased regurgitation to 46%, whereas doubling it by simulating pulmonary annular stenosis proximal to the compliance limited regurgitation to 10%, but at the cost of a 32 mmHg peak systolic pressure drop. CONCLUSIONS: The model supported our hypotheses, indicating the relevance to pulmonary regurgitation of previously overlooked variables. The virtual pulmonary regurgitation was exacerbated by pulmonary artery compliance and by elevated resistance distal to it, but was limited by more proximal resistance. These relationships merit careful clinical investigation as they would have implications for the initial management, subsequent investigation and decisions on re-intervention in patients with pulmonary regurgitation.

Use of mathematic modeling to compare and predict hemodynamic effects of the modified Blalock-Taussig and right ventricle-pulmonary artery shunts for hypoplastic left heart syndrome.

OBJECTIVE: Stage one reconstruction (Norwood operation) for hypoplastic left heart syndrome can be performed with either a modified Blalock-Taussig shunt or a right ventricle-pulmonary artery shunt. Both methods have certain inherent characteristics. It is postulated that mathematic modeling could help elucidate these differences. METHODS: Three-dimensional computer models of the Blalock-Taussig shunt and right ventricle-pulmonary artery shunt modifications of the Norwood operation were developed by using the finite volume method. Conduits of 3, 3.5, and 4 mm were used in the Blalock-Taussig shunt model, whereas conduits of 4, 5, and 6 mm were used in the right ventricle-pulmonary artery shunt model. The hydraulic nets (lumped resistances, compliances, inertances, and elastances) were identical in the 2 models. A multiscale approach was adopted to couple the 3-dimensional models with the circulation net. Computer simulations were compared with postoperative catheterization data. RESULTS: Good correlation was found between predicted and observed data. For the right ventricle-pulmonary artery shunt modification, there was higher aortic diastolic pressure, decreased pulmonary artery pressure, lower Qp/Qs ratio, and higher coronary perfusion pressure. Mathematic modeling predicted minimal regurgitant flow in the right ventricle-pulmonary artery shunt model, which correlated with postoperative Doppler measurements. The right ventricle-pulmonary artery shunt demonstrated lower stroke work and a higher mechanical efficiency (stroke work/total mechanical energy). CONCLUSIONS: The close correlation between predicted and observed data supports the use of mathematic modeling in the design and assessment of surgical procedures. The potentially damaging effects of a systemic ventriculotomy in the right ventricle-pulmonary artery shunt modification of the Norwood operation have not been analyzed.

Effects of different stent designs on local hemodynamics in stented arteries.

Following the deployment of a coronary stent and disruption of an atheromatous plaque, the deformation of the arterial wall and the presence of the stent struts create a new fluid dynamic field, which can cause an abnormal biological response. In this study 3D computational models were used to analyze the fluid dynamic disturbances induced by the placement of a stent inside a coronary artery. Stents models were first expanded against a simplified arterial plaque, with a solid mechanics analysis, and then subjected to a fluid flow simulation under pulsatile physiological conditions. Spatial and temporal distribution of arterial wall shear stress (WSS) was investigated after the expansion of stents of different designs and different strut thicknesses. Common oscillatory WSS behavior was detected in all stent models. Comparing stent and vessel wall surfaces, maximum WSS values (in the order of 1Pa) were located on the stent surface area. WSS spatial distribution on the vascular wall surface showed decreasing values from the center of the vessel wall portion delimited by the stent struts to the wall regions close to the struts. The hemodynamic effects induced by two different thickness values for the same stent design were investigated, too, and a reduced extension of low WSS region (<0.5Pa) was observed for the model with a thicker strut.

Multiscale modelling in biofluidynamics: application to reconstructive paediatric cardiac surgery.

Multiscale computing is a challenging area even in biomechanics. Application of such a methodology to quantitatively compare postoperative hemodynamics in congenital heart diseases is very promising. In the treatment of hypoplastic left heart syndrome, which is a congenital heart disease where the left ventricle is missing or very small, the necessity to feed the pulmonary and systemic circulations is obtained with an interposition shunt. Two main options are available and differ from the sites of anastomoses: (i) the systemic-to-pulmonary conduit (Blalock-Taussig shunt known as the Norwood Operation (NO)) connecting the innominate artery (NO-BT) or the aorta (NO-CS) to the right pulmonary artery and (ii) the right ventricle to pulmonary artery shunt (known as Sano operation (SO)). The proposition that the SO is superior to the NO remains controversial. 3-D computer models of the NO (NO-BT and NO-CS) and SO were developed and investigated using the finite volume method. Conduits of 3, 3.5 and 4 mm were used in the NO models, whereas conduits of 4, 5 and 6 mm were used in the SO model. The hydraulic nets (lumped resistances, compliances, inertances and elastances) which represent the systemic, coronary and pulmonary circulations and the heart were identical in the two models. A multiscale approach was adopted to couple the 3-D models with the circulation net. Computer simulation results were compared with post-operative catheterization data. Results showed that (i) there is a good correlation between predicted and observed data: higher aortic diastolic pressure, decreased pulmonary arterial pressure, lower pulmonary-to-systemic flow ratio and higher coronary perfusion pressure in SO; (ii) there is a minimal regurgitant flow in the SO conduit. The close correlation between predicted and observed clinical data supports the use of mathematical modelling, with a mandatory multiscale approach, in the design and assessment of surgical procedures.

Multiscale modeling of the cardiovascular system: application to the study of pulmonary and coronary perfusions in the univentricular circulation.

The objective of this study is to compare the coronary and pulmonary blood flow dynamics resulting from two configurations of systemic-to-pulmonary artery shunts currently utilized during the Norwood procedure: the central (CS) and modified Blalock Taussig (MBTS) shunts. A lumped parameter model of the neonatal cardiovascular circulation and detailed 3-D models of the shunt based on the finite volume method were constructed. Shunt sizes of 3, 3.5 and 4 mm were considered. A multiscale approach was adopted to prescribe appropriate and realistic boundary conditions for the 3-D models of the Norwood circulation. Results showed that the average shunt flow rate is higher for the CS option than for the MBTS and that pulmonary flow increases with shunt size for both options. Cardiac output is higher for the CS option for all shunt sizes. Flow distribution between the left and the right pulmonary arteries is not completely balanced, although for the CS option the discrepancy is low (50-51% of the pulmonary flow to the right lung) while for the MBTS it is more pronounced with larger shunt sizes (51-54% to the left lung). The CS option favors perfusion to the right lung while the MBTS favors the left. In the CS option, a smaller percentage of aortic flow is distributed to the coronary circulation, while that percentage rises for the MBTS. These findings may have important implications for coronary blood flow and ventricular function.