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Abstract:The modified Blalock-Taussig shunt is a palliative operation for some congenital heart diseases. An artificial conduit with antithrombotic surface placed between the subclavian (or innominate) and the pulmonary artery supplies blood to the lungs in defects with decreased pulmonary flow. Clotting of the graft is the main cause of its failure. Stenosis of the arteries is also observed. The objective of the present study was to investigate the flow pattern in the graft to investigate the possibility that clotting is initiated by the stimulation of platelets by high shear stress, and the possible effect of the pathological wall shear stress on the stenosis formation. The model included the left subclavian artery (LSA), the left pulmonary artery (LPA), and the graft. The three-dimensional relative position and size of the arteries was obtained from a CT scan of real anatomy. Four different types of the graft with two different diameters (3 and 4 mm) and two different shapes (straight and curved) of the pipe, and one variable diameter pipe were inserted in the model. A pulsatile flow of 0.81 L/min on average was assumed at the inlet to LSA, and 80% of the flow was directed through the graft. Computer simulations demonstrated a complex flow pattern with eddies and low velocity regions in the arteries at the anastomoses with the graft in all five models. An eddy was also found inside the straight 4 mm graft. A high pathological shear rate was present within the graft, with higher values in the 3 mm grafts. The fractional volume with a high ( > 2500 L/s) shear rate was between 2.5% and 4.5%, and that with a very high ( > 7500 L/s) shear rate between 0 and 1.5% of the model volume, and depended on the graft geometry and the phase of the cardiac cycle. Pathologically high ( > 3.5 Pa) and pathologically low ( < 1.0 Pa) wall shear stress, which may induce neointimal growth, was found in LSA and LPA. We conclude that the activation of platelets by high shear rate is possible within the graft, followed by their subsequent aggregation in the eddies with a low flow rate. Flow-induced changes of the vessel wall thickness (stenosis) can also appear, especially in the pulmonary artery.

We describe a case of delayed pulmonary artery perforation with the anchoring hooks of the Amplazer Cardiac Plug (ACP) (St Jude Medical, MN), which occurred 17 days after its proper and uncomplicated implantation. The patient was successfully treated with a surgical procedure. Technologic improvement of the ACP anchoring system seems to be most rational solution necessary to eliminate the risk of the adverse event described here.

The aim of this work was the application of computer and physical in vitro simulation methods for estimating surgery procedure hemodynamics. The modified Blalock-Taussig (mB-T) palliative surgical procedure is performed to increase the pulmonary blood flow in children with congenital heart defects. Such a systemic-to-pulmonary shunt yields substantial modification in the blood flow within the large blood vessels. The objective of the present study was to investigate basic characteristics of the flow, flow pattern and pressure-flow efficiency, before and after opening of the mB-T graft. Methods The model was based on the vessel geometry obtained from the Visible Human Project and included the arch of aorta, the three arteries branching from the arch, the pulmonary trunck, and the left and right pulmonary arteries. The graft was added between the left subclavian artery and the left pulmonary artery. The glass model of the vessels was produced and investigated in a physical model of the cardiovascular system with an artificial ventricular device as the blood pump. Flow rate and hydrostatic pressure were measured at the inlet to and outlets from the glass model and in a few points within the system. Laser flow visualization was also performed. Computer simulations were done using the boundary conditions from the physical model. Results The opening of the mB-T graft changed flow distribution in all branches (including inflow). A complex flow pattern with large eddies and channelling of the flow in the vicinity of the graft and within it was observed in flow visualization and in computer simulations. Because of that complexity the local measurements of hydrostatic pressure at the vessel wall could not predict the average flow rate. The reversed flow in the graft was observed during the systole. Conclusions The complex flow pattern developed in the physical model of the mB-T graft. The channelling of the flow and the formation of large eddies may yield high shear stress and modify blood properties. The rigid wall model can describe only some flow characteristics observed in vivo. Computer simulation is a very fast and accurate method which permits earlier qualification of cardiac surgeons on how to change cardiac vascular blood flow after operations.

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