Vein graft analysis at 6 weeks
- Saphenous vein graft (SVG) represents an autologous transplantation, as it is explanted, preserved in storage solution into a cup at room temperature and then implanted in arterial system. In clinical practice, the duration of vein graft storage corresponding to the warm ischemia tissue was difficult to evaluate because no data are published. Some surgeons do not store SVG before creating the anastomoses and develop the concept of “no-touch technique” with excellent graft patency [
23]. In contrast to what is done for solid organs destined for allotransplantation, where different preservation solutions were developed such as University of Wisconsin solution for pancreas or Celsior solution for heart, the normal saline solution was the first storage solution for SVG. O’Connel et al. demonstrated that 2 h of saline infusion produced an IH [
24]. Saline solution has negative effects on the endothelial layers and therefore may compromise graft patency [
25]. However, different in-vitro studies cannot conclude on the superiority of autologous whole blood, alternatives solutions such as storage solutions like Tiprotec™ or Somaluthion™ remain the subject of further clinical trial [
25]. Other study showed that autologous whole blood had some clear advantages compared to saline solution, this difference does not persist post-arterialization [
26]. In the present study, the duration of vein storage in the different solution was high as to allow preparation of the recipient rat following vein graft harvesting but no relation was found between storage duration and importance of IH. Thatte et al. showed that duration of storage time in GALA solution (from 60 min to 1440 min) did not alter smooth muscle or endothelial cell function [
10]. None of the storage solutions used in this study has reduced the intimal hyperplasia. In fact, we observed a significant increase of the wall thickness and histological signs of fibrosis.
- Among the occluded veins, only an important IH could explain the vein graft occlusion. In order to obtain an arterialized vein graft, we analyzed the conduit at 6 weeks when the IH started its process. Similar results were observed in the study published by Wong et al. who performed arteriovenous fistula in a murine model and then collected for analysis at 7, 14 and 28 days postoperatively [
27]. They found significant changes in the intima at 7 days, a significant hyperplasia was observed at 14 days and the patency rates at 28 days were 50%. Sun et al. investigated the efficacy of oral administration of hydrogen-rich water (HW) for prevention of intimal hyperplasia concerning inferior vena cava placed as an interposition graft in the abdominal aorta. Six weeks after bypass procedure, all vein grafts presented SMC and collagen deposits, macrophage infiltration but significantly less in the rats that consumed HW [
28]. In our study, the rate of occluded vein grafts among them was greater in the GALA group than AHB and saline solution groups. GALA solution is based on a physiological salt solution and contains glutathione and L-ascorbic acid, antioxidant and arginine, a substrate for NOS (Nitric Oxide Synthetase) in EC to protect the endothelium against ischemic injury during storage. The analysis of GALA effects on EC showed a protecting endothelial structure and function [
10]. According to our results, we cannot conclude that GALA solution protects the endothelial layer when the vein graft was arterialized in this rat model. These results should be interpreted with caution if we extrapolated them in human. The occluded vein graft rate at 6 weeks is low after coronary bypass but probably storage solutions did not limit vascular remodeling during the chronic intimal hyperplasia (stage III), their benefits intervening during the first weeks. Haime M et al. compared the impact of intraoperative preservation of SVG in GALA solution (Duragraft®) versus heparinized saline on vein-graft failure related outcomes after CABG in 2436 consecutive patients. In this retrospective study, the intraoperative treatment of SVGs with Duragraft was associated with a lower risk of nonfatal myocardial infarction, revascularization [
29]. However, these encouraging results must be interpreted with caution and only a well-constructed randomized study will answer the real question “which solution best preserves endothelial integrity from the vein harvesting to the arterial implantation” [
17].
Many studies showed that intimal hyperplasia is an adaptive mechanism following arterialization of the vein and is unlikely linked to vein graft thrombosis [
30]. Arterialization of a vein appears when the graft is exposed to arterial flow. Contractile SMC become secretory and proliferative [
16]. EC cannot produce NO and endothelial layer damage produces growth factors and proinflammatory cytokines (IL-6, IL-8) [
31]. This process promotes leukocytes recruitment and thrombin formation [
32].
- In the present study, regardless of the storage solution, no endothelium-dependent dilatation was observed when the vein grafts were mounted on a wire-myograph attesting to endothelial injury. The same findings were showed in the control vein and aorta groups. However, the SMC layer viability was observed in the control vein group, the preservation solutions maintained the SMC functionality from 88 to 106 min of storage.
The implication of the shear stress in vascular biology and inflammation was described by Touys et al. [
33]. All vascular cell types produce reactive oxygen species (ROS) that regulate vascular function by modulating cell growth, apoptosis, migration, inflammation and extracellular matrix protein production. Oxidative stress and associated oxidative damage are mediators of vascular injury and inflammation and constitute the first steps of atherosclerosis development [
13]. In our study, we observed an important oxidative stress in the different vein graft segments analyzed. The inflammatory response into the vein wall due to shear stress was also observed in arterial samples explaining the vascular function damage. These results should be interpreted with caution because of the small number of permeable vessels at 6 weeks. In the presence of high levels of sodium nitroprusside, a dilatation was observed in the control vein group. Saad Enouri et al. studied characteristics of myogenic reactivity in isolated rat mesenteric veins. The integrity of the endothelium was assessed by a dilator response to acetylcholine in phenylephrine-preconstricted veins. The veins were able to develop significant myogenic tone that appears greater over the low-to-intermediate pressure ranges, but mesenteric veins did not demonstrate myogenic responses unlike mesenteric arteries [
34]. In our study, the control vein presented an endothelium-dependent relaxation injury with a preservation of smooth muscle cell (SMC) activity (endothelium-independent relaxation) while the vein graft was no vascular reactivity because of endothelial injury but conserved a low relaxation mediated by SMC for high dose of sodium nitroprusside.
Focus of the model
Models of venous grafting in the arterial position have been used to improve the patency of grafts in humans [
35,
36], which is especially relevant in CABG. Goldman et al. [
37] reported saphenous vein graft patency of 60% or more at 10 years postoperatively in humans. Animal models are useful to analyze the pathology of vein graft disease and to test therapeutic strategies in vivo [
38]. In large animals such as pigs and dogs, the model consists of an interposition of saphenous and jugular veins into common carotid artery. Wan et al. reported rate patency at 4 weeks [
39] with saphenous vein-common carotid artery interposition model in large white swine. In rats, different models of inferior vena cava into abdominal aorta interposition have been described as superficial epigastria vein into femoral artery interposition or ileolumbar vein into the abdominal aorta interposition [
28]. In the surgical model proposed by Sun et al., the vein graft was interposed into the recipient’s abdominal aorta using microsurgical techniques in a latero-terminal configuration, and the aorta of the recipients was then ligated between the two anastomoses [
28] while we developed an end-to-end anastomosis technique aligning vein and arterial endothelial cells in direction of arterial blood flow, therefore minimizing the effects of shear stress. Vein graft failure is characterized by an inflammatory response with leukocytes recruitment from circulating blood cells as we did not observe on the aorta segments. Like saphenous vein graft during CABG, our vein graft was submitted to a systolic flow. The vena cava was harvested using state-of-the-art and optimal handling techniques (same surgeon, a traumatic surgical technique, avoiding excessive handling and distortion) in order to reduce traumatic damage to the endothelium layer, which is the first step of the graft failure. The main difficulty in vein graft transplantation was the vein wall’s thickness. In fact, the texture of the vein issued from its storage solution makes the wall extremely fine.