Alterations of blood flow pattern after triple stent endovascular treatment of saccular abdominal aortic aneurysm : a porcine model

Objective: to determine the blood flow pattern changes after endovascular treatment of saccular abdominal aortic aneurysm with triple stent. Methods: we conducted a hemodynamic study of seven Landrace and Large White pigs with saccular aneurysms of the infrarenal abdominal aorta artificially produced according to the technique described. The animals were subjected to triple stenting for endovascular aneurysm. We evaluated the pattern of blood flow by duplex scan before and after stent implantation. We used the non-paired Mann-Whitney test for statistical analysis. Results: there was a significant decrease in the average systolic velocity, from 127.4cm/s in the pre-stent period to 69.81cm/s in the post-stent phase. There was also change in the flow pattern from turbulent in the aneurysmal sac to laminate intra-stent. Conclusion: there were changes in the blood flow pattern of saccular abdominal aortic aneurysm after endovascular treatment with triple stent.


INTRODUCTION
T he endovascular treatment of abdominal aortic an- eurysms has become an increasingly used surgical alternative instead of traditional surgery 1 .Treatment is successful when there is full suppression of blood flow and systemic pressure in the aneurysmal sac, which prevent rupture [2][3][4] .Although we have achieved significant technological advances since the first generation of aortic stents, recent information from late postoperative follow-up have shown a significant percentage of complications requiring surgical reintervention.The reason for treatment failure, in many cases, is still connected to the endoprosthesis used (type III leakage) [5][6][7] .
Studies have focused on the deterioration of the currently marketed prosthetic material, which is made by the association of a metal structure with PTFE or polyester coating [8][9][10] .Thus arises a new therapeutic method, based on experimental haemodynamic studies, the multilayered stents: uncoated metal prostheses, capable of promoting redirection of blood flow within the aneurysm sac, preventing its expansion and rupture, even without total flow abolition in the aneurysmal sac [8][9][10][11][12][13][14] .
The aim of this study is to experimentally analyze the effects of the implant, in pigs, of three stents with the same design (triple stent) on the redirection of blood flow of the aneurysmal sac, and to determine possible changes in the abdominal aorta blood flow pattern after treatment.

METHODS
The research project was approved by the Ethics in Research Committee of the Universidade Estadual de Ciências da Saúde de Alagoas (UNCISAL) Protocol 61-A, and strictly followed the ethical principles of animal experimentation of the Colégio Brasileiro de Experimentação Animal (COBEA), based on Resolution 714/02 of the Conselho Federal de Medicina Veterinária 15,16 .
The sample consisted of seven female pigs from the crossing of Landrace and Large White, weighing 20 to 25 kg, supplied by the same producer, proper-ly vaccinated and dewormed according to the age.We submitted them to the artificial production of infrarenal abdominal aortic aneurysms according to the modified technique of Perini 17 as described: transabdominal exposure of the abdominal aorta, with xyphopubic median incision, followed by aortic exposure through circumferential dissection between the renal arteries and the distal trifurcation (common iliac arteries and internal iliac artery trunk); election of a 3cm segment for making of the aneurysm, the branches repaired with 3.0 linen thread, and administered intravenous heparin (100IU/kg); clamping of the aorta proximal and distal to the chosen segment and conduction of a longitudinal arteriotomy followed by the patch suture with bovine pericardium in the form of a previously prepared bag of 3x3 cm, 6.0 polypropylene continuous sutures (Figure 1); Fifteen days after surgery, we performed Duplex Scan evaluation to confirm the aneurysms patency, as well as analyze the pre-stenting blood flow parameters.Then they underwent triple stent implantation and reevaluation of blood flow to assess the variables.Anesthesia was performed following the CCEB/ UNCISAL general anesthesia protocol for porcine.

Stents Implantation
Vascular access to aortography was obtained by surgical dissection of the right femoral artery, and the arterial puncture performed under direct vision with a 16 Jelco catheter.After the advance of a 0.035x260 cm angled tip hydrophilic guidewire, we introduced a 11 cm 5F sheath.We advanced the sheath under fluoroscopic control until the common iliac artery.Further, we performed the aortography with a 100 cm 5F MP angiographic catheter to identify the aneurysm, follow by the positioning of the catheter in the thoracic aorta, above the aneurysm site.
We later performed the vascular access for the implantation of stents through surgical dissection of the right carotid artery, the arterial puncture being performed under direct vision with a 16 Jelco.After the advance of 0.035x260 cm angled tip hydrophilic guidewire, we introduced a 11 cm 7F sheath.We advanced the sheath under fluoroscopic control until the aortic arch, and positioned the guidewire in the thoracic aorta with the aid of an IM catheter.
After positioning of the femoral catheter within the aneurysmal sac to angiographic control, we proceded to the sequential insertion with deployment of the stents through the right carotid artery, in the following order of sizes: 8x40 mm, 9x40 mm and 10x40 mm, starting the deployment from the porcine aorta trifurcation.Then, we carreid out a control aortography with a 5F MP angiographic catheter via the right carotid artery (Figure 2).Scan 30 minutes before the stents implantation and 30 minutes after the procedure.
We evaluated the images as for the flow pattern change, turbulent and laminar, as well as for the peak systolic velocity obtained in the aneurysmal aorta.
We also evidenced the change of flow pattern from turbulent to laminar after the stent release by aortography.

Primary Variable
Change in the blood flow pattern after endovascular treatment of saccular aortic aneurysm.We quantified the flow by vascular ultrasound, using a portable device.

Secondary Variables
Average blood flow velocity in the aorta; frequency of thrombosis in the aneurysmal sac.
As additional information, we evaluated the averages of the animals ages, of their weights and of the procedure time.
We performed the statistical analysis using the unpaired Mann-Whitney test and calculating the confidence interval (CI) of 95% for each point estimate.We evaluated the statistical hypotheses H0 (pre-stent flow equals poststent flow) and H1 (pre-stent flow is not equal post-stent flow) with the GraphPad Instat Prism 5 (2012) statistical program for Windows.The sample size was based on previous studies on the subject conducted in pigs [18][19][20][21] .

RESULTS
All seven animals were successfully submitted to implantation of stents without technical difficulty.The surgical procedure time was 190 minutes, and we did not observe any complications such as malposition, migration or inadequate expansion of stents.
In table 1, we describe the quantitative variables that constitute Systolic Peak velocity (PSV) and resistance index (RI) before and after the triple stenting, which revealed significant changes, i.e., loss of systolic velocity after placement of stents, as well as the fall in resistance index in many animals.Regarding systolic velocity, we observed a significant decrease with an average pre-stent velocity 127.4cm/s (95% CI 79.93±0174.

DISCUSSION
The treatment of abdominal aortic aneurysms by endovascular technique has been performed with increasing frequency.This has lead to the increased observation of leaks related to the endoprosthesis 6 , usually due to early defects of components or material fatigue.Recently, a new kind of stent  In this pioneering study, we used an experimental model in pigs to assess the changes in the flow pattern with duplex scan after triple stent implantation for the treatment of aortic aneurysm in animals previously submitted to the making of aneurysmal sac with bovine pericardium by the modified Perini technique 17 .
Changes in blood flow pattern in this study were evaluated through duplex scan analysis.Two analyzes were carried out, one in the animal with the aneurysm before the stents implantation, the and other after implantation of stents.We obtained the blood flow pattern analysis and also the parameters systolic velocity and resistance index.
The systolic velocity showed a significant decrease, with an average pre-stent velocity of 127.In conclusion, this study showed changes in the blood flow pattern of saccular abdominal aortic aneurysms after endovascular treatment with the triple stent.

Oliveira
Alterations of blood flow pattern after triple stent endovascular treatment of saccular abdominal aortic aneurysm: a porcine model
8) and a mean post-stent velocity 69,81 cm/s (95% CI 40.18±99.43),confirming the change in blood flow patterns.The qualitative variables, which represent the change in the flow pattern from turbulent in the aneurysmal sac to laminar intrastent after treatment with triple stent, were demonstrated through the duplex scan (Figures 3 and 4).
4cm/s (95% CI 79.93±174.8)and a mean post-stent velocity of 69.81cm/s (95% CI 40.18±99.43),confirming the change in blood flow patterns.Images by duplex scan showed change in the flow pattern, ie, from turbulent in the aneurysmal sac to laminar intrastent, the aneurysmal sac presenting with blood flow or excluded.Doppler ultrasonography in pigs showed the possibility to analyze not only the presence of flow within the aneurysmal sac, but also the hemodynamic features of such flow, with more information.