The influence of inflow boundary conditions on intra left ventricle flow predictions

The combination of computational fluid dynamics (CFD) and magnetic resonance imaging (MRI) offers a promising tool that enables the prediction of blood flow patterns in subject-specific cardiovascular models. The influence of the model geometry on the accuracy of the simulation is well recognized. This paper addresses the impact of different boundary conditions on subject-specific simulations of left ventricular (LV) flow. A novel hybrid method for prescribing effective inflow boundary conditions in the mitral valve plane has been developed. The detailed quantitative results highlight the strengths as well as the potential pitfalls of the approach.     

The influence of out-of-plane geometry on the flow within a distal end-to-side anastomosis

This paper describes a computational and experimental investigation of flow in a proto-type model geometry of a fully occluded 45 deg distal end-to-side anastomosis. Previous investigations have considered a similar configuration where the centerlines of the bypass and host vessels lie within a plane, thereby producing a plane of symmetry within the flow. We have extended these investigations by deforming the bypass vessel out of the plane of symmetry, thereby breaking the symmetry of the flow and producing a nonplanar geometry. Experimental data were obtained using magnetic resonance imaging of flow within perspex models and computational data were obtained from simulations using a high-order spectral/hp element method. We found that the nonplanar three-dimensional flow notably alters the distribution of wall shear stress at the bed of the anastomosis, reducing the peak wall shear stress peak by approximately 10 percent when compared with the planar model. Furthermore, an increase in the absolute flux of velocity into the occluded region, proximal to the anastomosis, of 80 percent was observed in the nonplanar geometry when compared with the planar geometry.     

The influence of out-of-plane geometry on pulsatile flow within a distal end-to-side anastomosis

We present an experimental and computational investigation of time-varying flow in an idealized fully occluded 45 degrees distal end-to-side anastomosis. Two geometric configurations are assessed, one where the centerlines of host and bypass vessels lie within a plane, and one where the bypass vessel is deformed out of the plane of symmetry, respectively, termed planar and non-planar. Flow experiments were conducted by magnetic resonance imaging in rigid wall models and computations were performed using a high order spectral/hp algorithm. Results indicate a significant change in the spatial distribution of wall shear stress and a reduction of the time-averaged peak wall shear stress magnitude by 10% in the non-planar model as compared to the planar configuration. In the planar geometry the stagnation point follows a straight-line path along the host artery bed with a path length of 0.8 diameters. By contrast in the non-planar case the stagnation point oscillates about a center that is located off the symmetry plane intersection with the host artery bed wall, and follows a parabolic path with a 0.7 diameter longitudinal and 0.5 diameter transverse excursion. A definition of the oscillatory shear index (OSI) is introduced that varies between 0 and 0.5 and that accounts for a continuous range of wall shear stress vector angles. In both models, regions of elevated oscillatory shear were spatially associated with regions of separated or oscillating stagnation point flow. The mean oscillatory shear magnitude (considering sites where OSI>0.1) in the non-planar geometry was reduced by 22% as compared to the planar configuration. These changes in the dynamic behavior of the stagnation point and the oscillatory shear distribution introduced by out-of-plane graft curvature may influence the localization of vessel wall sites exposed to physiologically unfavorable flow conditions.     

Experimental analysis of the influence of stenotic geometry on steady flow.

We studied the flow behavior under steady flow conditions in four models of cylindrical stenoses at Reynolds numbers from 150 to 920. The flow upstream of the constrictions was always fully developed. The constriction ratios of the rigid tubes (D) to the stenoses (d) were d/D = 0.273; 0.505; 0.548; 0.786. The pressure drop at various locations in the stenotic models was measured with water manometers. The flow was visualized with a photoelasticity apparatus using an aqueous birefringent solution. We also studied the flow behavior at pulsatile flow in a dog aorta with a constriction of 71%. The flow through stenotic geometries depends on the Reynolds number of the flow generated in the tube and the constriction ratio d/D. At low d/D ratios, (with the increased constriction), the flow separation zones (recirculation zones, so-called reattachment length) and flow disturbances increased with larger Reynolds numbers. At lower values, eddies were generated. At high Re, eddies were observed in the pre-stenotic regions. The pressure drop is a function of the length and internal diameter of the stenosis, respective ratio of stenosis to the main vessel and the Reynolds numbers. At low Re-numbers and low d/D, distinct recirculation zones were found close to the stenosis. The flow is laminar in the distal areas. Further experiments under steady and unsteady flow conditions in a dog aorta model with a constriction of 71% showed similar effects. High velocity fluctuations downstream of the stenosis were found in the dog aorta. A videotape demonstrates these results.     

The use of the long head of triceps interposition muscle flap for treatment of massive rotator cuff tears

Massive rotator cuff tears present a difficult problem for orthopedic surgeons. To address this problem, a long head of triceps muscle interposition flap was proposed. Ten patients underwent repair of massive rotator cuff tears using the triceps muscle flap. The patients' strength and range of motion were tested preoperatively, and a University of California, Los Angeles, shoulder score was assigned. Similar testing was performed 1 year later. Postoperatively, the patients showed significant improvement in the overall shoulder score and in the pain and function components of the score. There was no significant improvement in shoulder range of motion, except for shoulder flexion. An important finding was that there was no loss of strength in elbow extension following the loss of the long head of triceps muscle. It was concluded that the long head of triceps interposition flap is useful in the reconstruction of the massive rotator cuff tear.     

Detailed flow patterns in the nasal cavity.

The human nasal cavity filters and conditions inspired air while providing olfactory function. Detailed experimental study of nasal airflow patterns has been limited because of the complex geometry of the nasal cavity. In this work, particle image velocimetry was used to determine two-dimensional instantaneous velocity vector fields in parallel planes throughout a model of the nasal cavity that was subjected to a nonoscillatory flow rate of 125 ml/s. The model, which was fabricated from 26 computed tomography scans by using rapid prototyping techniques, is a scaled replica of a human right nasal cavity. The resulting vector plots show that the flow is laminar and regions of highest velocity are in the nasal valve and in the inferior airway. The relatively low flow in the olfactory region appears to protect the olfactory bulb from particulate pollutants. Low flows were also observed in the nasal meatuses, whose primary function has been the subject of debate. Comparison of sequentially recorded data suggests a steady flow.     

Prey specialization may influence patterns of gene flow in wolves of the Canadian Northwest

This study characterizes population genetic structure among grey wolves (Canis lupus) in northwestern Canada, and discusses potential physical and biological determinants of this structure. Four hundred and ninety-one grey wolves, from nine regions in the Yukon, Northwest Territories and British Columbia, were genotyped using nine microsatellite loci. Results indicate that wolf gene flow is reduced significantly across the Mackenzie River, most likely due to the north-south migration patterns of the barren-ground caribou herds that flank it. Furthermore, although Banks and Victoria Island wolves are genetically similar, they are distinct from mainland wolf populations across the Amundsen Gulf. However, low-level island-mainland wolf migration may occur in conjunction with the movements of the Dolphin-Union caribou herd. Whereas previous authors have examined isolation-by-distance in wolves, this study is the first to demonstrate correlations between genetic structure of wolf populations and the presence of topographical barriers between them. Perhaps most interesting is the possibility that these barriers reflect prey specialization by wolves in different regions.     

Ventricular flow dynamics with varying LVAD inflow cannula lengths: In-silico evaluation in a multiscale model

Left ventricular assist devices are associated with thromboembolic events, which are potentially caused by altered intraventricular flow. Due to patient variability, differences in apical wall thickness affects cannula insertion lengths, potentially promoting unfavourable intraventricular flow patterns which are thought to be correlated to the risk of thrombosis. This study aimed to present a 3D multiscale computational fluid dynamic model of the left ventricle (LV) developed using a commercial software, Ansys, and evaluate the risk of thrombosis with varying inflow cannula insertion lengths in a severely dilated LV. Based on a HeartWare HVAD inflow cannula, insertion lengths of 5, 19, 24 and 50 mm represented cases of apical hypertrophy, typical ranges of apical thicknesses and an experimental length, respectively. The risk of thrombosis was evaluated based on blood washout, residence time, instantaneous blood stagnation and a pulsatility index. By introducing fresh blood to displace pre-existing blood in the LV, after 5 cardiac cycles, 46.7%, 45.7%, 45.1% and 41.8% of pre-existing blood remained for insertion lengths of 5, 19, 24 and 50 mm, respectively. Compared to the 50 mm insertion, blood residence time was at least 9%, 7% and 6% higher with the 5, 19 and 24 mm insertion lengths, respectively. No instantaneous stagnation at the apex was observed directly after the E-wave. Pulsatility indices adjacent to the cannula increased with shorter insertion lengths. For the specific scenario studied, a longer insertion length, relative to LV size, may be advantageous to minimise thrombosis by increasing LV washout and reducing blood residence time.     

Small artery anastomosis, using a cuff of dura mater and a tissue adhesive.

A new and simple technique for the anastomosis of small arteries, using a lyophilized dural cuff and a tissue adhesive, is presented. The feasibility of this new technique was demonstrated on 26 carotid arteries in rats. Possible indications for it are noted.     

The influence of adrenal vein occlusion on whole-kidney hemodynamics in the spontaneously hypertensive rats.

It has been shown that occlusion of the adrenal vein causes an increase in renal vascular resistance in the ipsilateral kidney in Wistar Kyoto rats (WKY). The most probable mechanism of this phenomenon is the direct inflow of adrenal catecholamines to the kidney by the adrenal renal portal circulation (ARPC). As the number of vessels of the ARPC is bigger and the tonic sympathetic activity is higher in spontaneously hypertensive rats (SHR), the aim of the current study was to compare the effect of adrenal vein occlusion on renal vascular resistance between SHR and WKY. Mean arterial blood pressure and renal blood flow (RBF) were measured and renal vascular resistance (RVR) was calculated before and after closure of the adrenal vein. Occlusion of the adrenal vein significantly reduced RBF and increased RVR in both strains of rats. The rise of the RVR was significantly higher in SHR than in WKY. Therefore we assume that the hemodynamic responsiveness of the kidney due to increase in blood flow through ARPC is greater in SHR and may contribute to the development of arterial hypertension in this strain of rat.     

Iatrogenic chorioretinal venous anastomosis in the treatment of central retinal vein occlusion

This paper discusses the clinical application of an iatrogenic chorioretinal venous anastomosis for experimental treatment of non-ischemic central retinal vein occlusions (CRVO). The creation of an iatrogenic chorioretinal venous anastomosis offers a potential means of by-passing the obstructed central retinal vein and allowing venous blood to exit the retinal circulation through the choroid. A successful anastomosis may allow improved venous drainage and faster resolution of macular edema. This procedure may also prevent the conversion of a non-ischemic CRVO to the ischemic type. The natural history of a CRVO, the procedure for anastomosis creation, the patients most likely to benefit, and complications of chorioretinal anastomosis are discussed.     

Experimental studies in the survival of venous island flaps without arterial inflow

The authors have studied the effects of various circulatory settings on flap survival. The dog model was used to study the survival of venous flaps without arterial inflow both as island and free flaps. Venous flaps were compared with arterial flaps without venous outflow and standard island flaps with arterial inflow and venous outflow. Attempts were made to study their vascular morphology and blood gas changes. The venous flaps without arterial inflow survived with normal hair growth and wound healing, as did the standard island flaps. These observations suggest that capillary diffusion can occur without the continuous flow of blood through a capillary. Several possible mechanisms to explain survival of the venous flaps without arterial inflow were discussed. These observations could be important in providing an animal model to study microcirculation and a possible new area for microsurgical transfer of a skin flap.     

Blood flow in a brachial artery compressed externally by a pneumatic cuff.

Blood flow in human brachial arteries, compressed by a pneumatic cuff for blood pressure measurement, is examined using several different noninvasive techniques. From the experimental results it is shown that, when arterial pressure distal to the cuff is always lower than cuff pressure, flow in the artery under the cuff becomes supercritical near the cuff downstream margin and no reflection occurs there and the reflected wave from the peripheral vascular system of the arm does not propagate beyond the cuff downstream margin. Therefore an unsteady transition from supercritical to subcritical flow occurs near the cuff downstream margin. When the peak value of distal arterial pressure exceeds cuff pressure, a reflection occurs near the cuff downstream margin. The reflection becomes stronger corresponding to decrement of the cuff pressure and the whole artery segment under the cuff inflates fully at systole. At diastole, however, the artery segment under the cuff does not collapse completely and, hence, the phenomenon becomes that of pressure wave propagation in a partially collapsed artery segment.     

The role of lone pair and dipolar interactions in the non-planarity of 1,3-dioxolane and 1,3-dioxole

1,3-Dioxole has been shown to be non-planar by infrared and Raman spectroscopy. An MM3 study of this molecule enabled the investigators to suggest that this non-planarity was due to the anomeric effect. Subsequently, an ab initio theoretical study of this molecule was performed, which also concluded that the non-planarity of 1,3-dioxole was due to the anomeric effect and not to dipole-dipole interactions. Neither study used rigorous methods for assessing the role of dipolar interactions in the geometry of 1,3-dioxole. A new study of 1,3-dioxole, 1,3-dioxolane, tetrahydrofuran, cyclopentane, and some related molecules using the new QVBMM (molecular mechanics) force field shows conclusively that the non-planarity of 1,3-dioxole and 1,3-dioxolane is due primarily to torsional and dipolar effects, and not secondary molecular orbital overlap interactions.