Discussion
The open operation of aortic disease results in severe trauma and many perioperative complications; thus, open surgery is not appropriate for elderly patients or patients with multiple complications because they cannot tolerate the trauma of surgery. With the advancement of interventional techniques, TEVAR surgery has been widely used due to its advantages of being less invasive and having a rapid recovery and a low rate of mortality and morbidity. However, TEVAR surgery requires a sufficient length of normal blood vessels as the anchoring area; the presence of aortic branch vessels often results in an insufficient length of the anchoring area and limits TEVAR surgical use. In response to this problem, many solutions have been introduced, such as chimney technology, in situ fenestration technology, custom-made fenestration technology, branched stents, and multilayer bare stents [
1].
Among these solutions, the most convenient one is a multilayer bare stent (known as a multilayer flow modulator, MFM). The haemodynamic changes after implantation of the MFM are aneurysms and penetrating ulcers, and turbulent flow in the aortic dissection can become laminar, making the blood flow parallel to the laminar flow of the aneurysm wall, thus eliminating turbulence, reducing pressure on the aortic wall and reducing the risk of rupture. The blood flow velocity of the aortic aneurysm or penetrating ulcer is slowed down and prone to thrombosis. The buffering effect of the thrombus further reduces the pressure on the aortic wall, and the reduction in aortic wall pressure promotes the reconstruction of the middle layer of the artery. The MFM metal stent has a mesh hole, laminar flow parallel to the branch blood vessel can be retained in the branch blood vessel, and endothelialization of the stent mesh can be inhibited. The blood flow of the branch blood vessel can be effectively preserved, where there is no branch blood vessel. It can slow the blood flow rate, facilitate the adhesion and growth of endothelial cells, and promote endothelialization of the stent mesh to isolate the tumour cavity. Haemodynamic changes after implantation of the MFM promote thrombosis in the aneurysm cavity, reduce the pressure on the aneurysm wall, and preserve branch vascular patency. TBAD can also seal the endometrial intimal opening, enlarge the true cavity, compress the false lumen and promote the thrombosis of the pseudocatheter; these mechanisms can promote the reconstruction of the aorta [
2,
3].
Our new blood flow regulator produced similar haemodynamic changes in 14 patients. The technical success rate was 100%, and there were no serious liver function injuries or instances of renal failure, gastrointestinal ischaemia, or other organ perfusion symptoms in the early postoperative period. There were no acute subclavian artery ischaemia complications, such as limb pain and dizziness, and there were no central nervous system complications, such as paraplegia and apoplexy. There were 2 patients who died after 14 months of follow-up: one patient died due to abdominal aortic rupture, and the other cause was unknown (survival rate was 86% (12/14)). There were 2 cases of type-III endoleak and 1 case of aortic-related adverse events, 1 case of renal dysfunction, 1 case of repeated TIA, and 1 case of upper limb dysfunction; however, there were no complications such as stroke, paraplegia or visceral ischaemia. During the follow-up period, 90% (9/10) of patients with TBAD were treated with false lumen thromboembolism. The short-term curative effect was satisfactory, and there were no complications of covered branch embolization.
The new blood flow regulator is a commercialized product that can be used in emergency surgery without customization. It can address branch vessels via balloon dilation without implanting stent(s) into branch vessels, which avoids the bracket damage caused by the friction between pluralities of brackets in chimney technology. The blood flow regulator does not block the blood flow of the branch vessels, avoiding the ischaemic time before the fenestration of the branch vessels in the in situ fenestration technique and effectively reducing ischaemia-related complications. The blood flow regulator has the advantages of simple implantation, a low accuracy of localization, low technical difficulty, a small learning curve, a short operation time, a small dose of radiation to patients and physicians, and a small dosage of contrast agent.
The new blood flow regulator is a stent graft with a small mesh and demonstrates changes similar to the haemodynamic changes observed in aortic aneurysms with multilayer bare stents. Compared with multilayer bare stents, the new blood flow regulator has many advantages in terms of practical applications. (1) Multilayer bare stents promote thrombosis by slowing down the blood flow in the lumen of the isolated aneurysm. The thrombus of the aneurysm into the branch vessel can cause branch vessel embolization; the new type of blood flow regulator can dilate the reticular capping of branch vessels and avoid the risk of endovascular thromboembolism in isolated aneurysms after the implantation of the stent in the branch vessels. (2) The multilayer bare metal stent is not suitable for implantation in the area with thrombus because it may cause the thrombus to break off, whereas the new blood flow regulator is a mesh-shaped stent that does not cut the thrombus and isolates the thrombus from the lumen of the vessel to avoid thrombus loss. (3) The length of the multilayer bare stent is related to the diameter. Furthermore, the difficulty of accurately locating the distal end of the stent may lead to the difficulty of incomplete release, while the length of the new type of blood flow regulator is fixed after release, and the location of the distal end is accurate [
4].
Blood flow through the stent will produce shear force on the stent, which requires balanced friction between the stent and the vascular wall to ensure that the stent does not shift. The friction between the stent and the vascular wall is proportional to the radial support of the stent, and the pressure generated by the radial support of the stent is inversely proportional to the length of the tumour neck. Therefore, a shorter neck must be subjected to high pressure to ensure that it is not displaced from the covered stent, and a higher pressure on the neck may lead to long-term neck dilation or even to iatrogenic aortic rupture or reverse avulsion aortic dissection. The new blood flow regulator reduces the pressure on the anchoring area by extending the proximal anchoring area, avoiding further dilation of the neck of the tumour and reducing iatrogenic aortic injury [
5].
There was no paraplegia in this group of patients and no paraplegia or other spinal cord ischaemia after covering the intercostal artery, as permanent paraplegia after TEVAR is associated with the length of the thoracic aorta covered. Feezor et al. [
6] found that spinal cord ischaemia after TEVAR occurred when the distance between the stent graft and the proximal end of the abdominal cavity was 17.3 ± 21.8 mm, whereas no ischaemia was observed when the distance was 63.1 ± 62.9 mm (
p = 0.0006). The coverage length threshold was 205 mm, and the sensitivity and specificity of predicting spinal cord ischaemia after TEVAR were 80 and 92.5%, respectively. The effect of the new blood flow regulator on preventing spinal cord ischaemia was significantly better than that of the stent graft. This was a chronic occlusion of the spinal cord artery after the blood flow regulator covered the intercostal artery, and the chronic occlusion provided sufficient time for the establishment of the spinal artery collateral circulation [
7]. Some studies have also suggested that planned, temporary endoleak after TEVAR can maintain spinal cord blood supply to reduce the incidence of paraplegia [
8].
The new blood flow regulator can also be used in combination with stent grafts in some special cases. (1) When the length of the anchoring area is insufficient, the blood flow regulator is first placed into the normal vessel wall, the anchoring area is expanded, and then the stent graft is implanted in the flow regulator. (2) When the large tumour cavity or endometrial rupture is adjacent to the branch vessel, the blood flow regulator can be implanted first, and then the stent graft is implanted in the vicinity of the branch vessel opening, which can partially close the tumour cavity or break and can reduce the permeation stent. The amount of internal leakage promotes thrombosis in the tumour cavity or pseudocavity and promotes vascular remodelling. There were 5 patients in this group. First, a blood flow regulator was placed in the aortic arch, and then a stent graft was placed in the blood flow regulator. During the follow-up, the tumour cavity was thrombotic, and the branch vessels were patent. The combination of blood flow regulators and stent grafts can expand the surgical indications of TEVAR and achieve good results.
However, there are some defects of this new blood flow regulator, such as type III endoleak and thromboembolism. The occurrence of endoleak is mainly related to the mesh-like lamella, the size of the aortic aneurysm and the size of the aortic dissection. After the operation, the type III endoleak can be directly closed by implanting sent grafts. Otherwise, another blood flow regulator can be superimposed to reduce the mesh size. To reduce type III endoleaks, permeation stents with different mesh sizes can be applied according to the size of different tumour cavities. The mesh-like mulch covering the opening of the branch vessel may result in thrombosis in the early stage. To solve this problem, aspirin was used orally (100 mg qd) to prevent thrombosis. After the transmembrane stent was covered with endothelial cells, the probability of thrombosis was significantly reduced. Antiplatelet therapy is thought to affect the thrombosis of the tumour cavity or pseudocavity and aortic remodelling, but it can also increase the patency of branch collateral vessels. Our results showed that thromboembolism occurred when the blood flow regulator was applied to the arcuate vessels. Therefore, anticoagulation is necessary after the blood flow regulator is implanted.
The limitation of this study is the small number of patients and the short follow-up time. The effect of the regulator is good according to recent results, but further studies are needed to assess the long-term effect. Major long-term observations include the time of thrombosis of the pseudocavity or tumour cavity, the reconstruction of the aorta, the patency of branch vessels, and changes in the neck pressure. Most patients in recent studies were treated in the subacute phase. At this time, the inflammation of the aorta is lower than that in the acute phase, and the toughness of the endometrium is increased but not rigid. After TEVAR, the true cavity is fully enlarged, and the pseudocavity is compressed. The middle layer is closely adhered to the outer membrane to achieve a better therapeutic effect. The treatment of chronic interlayers remains to be addressed.
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