A peripheral vascular graft is used for the treatment of peripheral arterial disease. Restenosis in the distal anastomosis is the main reason of occlusion and is related to haemodynamics. Single spiral flow is a normal feature in vessels. A graft designed to generate a single spiral in its outflow (VFT Ltd, Dundee, UK) has been introduced in clinical practice. This study compared the spiral graft with a control non-spiral using image-guided modelling.
Both grafts were housed in ultrasound flow phantoms. Anastomotic angle θ was applied at 20°, 40°, 60° and 80°. The phantoms were scanned with CT (Biograph mCT, SIEMENS, Germany) and the graft-vessel mimic lumen geometry was extracted with Amira (FEI Visualization, France). Based on these geometries volume meshes were created (ICEM CFX, ANSYS, Canonsburg, USA), which consisted of tetrahedral cells in the core and prismatic cells in the wall boundary. Mesh independence tests were applied based on maximum wall shear stress and velocity.
The blood was assumed Newtonian, homogeneous and incompressible, the walls rigid and the inflow a steady parabola (Reynolds 620, 935). The Navier-Stokes governing equations of flow were solved with ANSYS CFX.
Fluid dynamic parameters were compared between the spiral and corresponding non-spiral models focusing on the flow downstream of the anastomosis.
The vortical structures at cross-flow patterns 1-4 had previously been studied experimentally with ultrasound vector Doppler imaging, which was used for validation.
The presented results are for θ = 40°.
A single spiral was the main characteristic in the outflow of the spiral graft and a double or triple spiral in the outflow of the control.
The maximum in-plane velocity (perpendicular to flow direction) at cross-flow planes 1 – 4 was constantly higher for the spiral graft model.
The total circulation in cross-flow planes 1 – 4 was higher for the spiral graft model particularly for increased Reynolds.
Helicity in the volume between cross-flow plane 1 and 4 was higher for the spiral model.
The pressure drop over length from the graft inlet to cross-flow plane 4 was reduced for the non-spiral graft model.
The wall shear stress (WSS) was examined in proximal and distal locations of the floor and toe wall centrelines. The WSS was higher for the spiral graft model in all tested locations.
The results from θ = 20°, 60°, 80° were comparable.
The flow pattern generated by the spiral graft was related to less flow separation, stagnation and instability than that induced by the control graft. The increased in-plane velocity, circulation and helicity of the spiral device showed increased in-plane mixing, which has been reported to protect endothelial function. Pressure drop is not desirable. The detected difference in pressure loss can be assumed negligible because the physiologic pressure is in the range of 1 – 20 × 104 Pa. Increased WSS is considered atheroprotective, although this may not apply in the proximal floor where the blood impinges abnormally on the wall of the host vessel.
The spiral graft was able to reintroduce a single spiral pattern in its outflow, associated with flow coherence downstream of the host vessel and high intensity cross-flow phenomena. Such local haemodynamics are known to prevent neointimal hyperplasia and thrombosis. These results support the hypothesis that spiral grafts may improve the patency rates in patients.
In healthy individuals flow patterns in the arterial tree have a spiral vector (Spiral laminar flow, SLF) which is attributed to the eccentric myocardial action and the spiral nature of the aortic arch. (more…)
Access vascular (AV) prostheses are commonly used for haemodialysis. Their low patency rates remain a challenge with restenosis in the distal anastomosis being the main reason of failure. (more…)
The primary patency rate of arteriovenous (AV) grafts is limited by distal venous anastomosis stenosis or occlusion due to intimal hyperplasia associated with distal graft turbulence. The normal blood flow in native arteries is spiral laminar flow. Standard vascular grafts do not produce spiral laminar flow at the distal anastomosis. Vascular grafts which induce a spiral laminar flow distally result in lower turbulence, particularly near the vessel wall. This initial study compares the hemodynamic effects of a spiral flow-inducing graft and a standard graft in a new AV carotid to jugular vein crossover graft porcine model.
Four spiral flow grafts and 4 control grafts were implanted from the carotid artery to the contralateral jugular vein in 4 pigs. Two animals were terminated after 48 hours and 2 at 14 days. Graft patency was assessed by selective catheter digital angiography, and the flow pattern was assessed by intraoperative flow probe and colour Doppler ultrasound (CDU) measurements. The spiral grafts were also assessed at enhanced flow rates using an external roller pump to simulate increased flow rates that may occur during dialysis using a standard dialysis needle cannulation. The method increased the flow rate through the graft by 660 ml/min. The graft distal anastomotic appearances were evaluated by explant histopathology.
All grafts were patent at explantation with no complications. All anastomoses were found to be wide open and showed no significant angiographic stenosis at the distal anastomosis in both spiral and control grafts. CDU examinations showed a spiral flow pattern in the spiral graft and double helix pattern in the control graft. No gross histopathological effects were seen in either spiral or control grafts.
This porcine model is robust and allows haemodynamic flow assessment up to 14 days post-implantation. The spiral flow-inducing grafts produced and maintained spiral flow at baseline and enhanced flow rates during dialysis needle cannulation, whereas control grafts did not produce spiral flow through the distal anastomosis. There was no deleterious effect of the spiral flow-inducing graft on macroscopic and histological examination. The reducing effect of spiral flow on intima hyperplasia formation will be the subject of further study using the same AV graft model at a longer period of implantation.
To study the sequential shortening of Torrent-Guasp’s ‘rope-heart model’ of the muscular band, and analyse the structure—function relationship of basal loop wrapping the outer right and left ventricles, around the inner helical apical loop containing reciprocal descending and ascending spiral segments.
In 24 pigs (27—82 kg), temporal shortening by sonomicrometer crystals was recorded. The ECG evaluated rhythm, and Millar pressure transducers measured intraventricular pressure and dP/dt.
The predominant shortening sequence proceeded from right to left in basal loop, then down the descending and up the ascending apical loop segments. In muscle surrounded by the basal loop, epicardial muscle predominantly shortened before endocardial muscle. Crystal location defined underlying contractile trajectory; transverse in basal versus oblique in apical loop, subendocardial in descending and subepicardial in ascending segments. Mean shortening fraction average 18 ± 3%, with endocardial exceeding epicardial shortening by 5 ± 1%. Ascending segment crystal displacement followed descending shortening by 82 ± 23ms, and finished 92 ± 33 ms after descending shortening stops, causing active systolic shortening to suction venous return; isovolumetric relaxation was absent.
Shortening sequence followed the rope-like myocardial band model to contradict traditional thinking. Epicardial muscle shortened before endocardial papillary muscle despite early endocardial activation, and suction filling follows active systolic unopposed ascending segment shortening during the ‘isovolumetric relaxation’ phase.
Physiological blood flow patterns are themselves poorly understood despite their impact on arterial disease. Stable spiral (helical) laminar flow has been observed in normal subjects. (more…)
In 1932, Bremer of Harvard filmed the blood in the very early embryo circulating in self-propelled mode in spiralling streams before the heart was functioning. (more…)