Rates of VGF and associated clinical events post-CABG surgery remain high despite intra-operative measures to prevent VGF include surgical techniques such as avoiding extensive handling during SVG harvesting, selecting the optimal site for the distal anastomosis to ensure a good run-off area, avoiding kinking and flattening of the graft and the no-touch harvesting technique.18 These measures are to preserve the integrity, functionality and viability of the endothelial layer, and to reduce the occurrence of early graft thrombosis and eventual clinical sequalae. However, ischemic injury and associated oxidative damage have been identified as the primary driver of intraoperative endothelial injury that leads to vein graft disease via an IRI mechanism.
IRI is initiated during ischemic episodes through damage caused by oxidative stress; oxidative damage.8,19 Oxidative damage is mediated by the release of ROS from endothelial and other cells in the ischemic organ or tissue and results in chemical modification of cellular and extracellular components including proteins, lipids and nucleic acids. This damage results in overall damage to the molecular integrity of a cell, tissue or organ. The net result is loss of normal cell and matrix components leading to dead, non-functional, structurally perturbed and dysfunctional tissues, cells and matrix. Subsequent reperfusion of the ischemic organ or tissue does not restore normality but instead exacerbates damage incurred during ischemia.8,19 Therefore, prevention of ischemic injury also confers protection later from reperfusion injury. Similarly to allotransplantation, prevention of IRI by storage of the SV graft in a dedicated, biocompatible and protective medium that reduces ischemic injury pre-transplant is key to providing best graft and patient outcomes following transplant/grafting.9
The current study investigated oxidative stress indices in SVG samples stored DuraGraft, an endothelial damage inhibitor designed for the intra-operative graft preservation, versus in the standard of care solutions, PS and HB, which served as controls. Higher OSI levels predispose the graft to higher amounts of oxidative damage during ischemic storage and more oxidative damage predisposes the graft to more severe IRI- mediated VGD which contributes to poor clinical outcomes post CABG. The main finding is that SVGs stored in DuraGraft exhibit a statistically significant lower OSI compared to grafts stored in either PS or HB. Oxidative stress in the grafts reflects a higher level of ROS and other oxidants compared to the levels of available antioxidants. As ROS/oxidant levels increasingly exceed the system’s antioxidant capacity or status, OSI increases and so does oxidative damage to cellular and matrix components.
The lower OSI in DuraGraft stored grafts compared to OSI in grafts stored in PS and HB is attributed to both lower TOS levels and increased TAS levels compared to those levels in grafts stored in PS and HB. The antioxidants in DuraGraft, L-glutathione and L-ascorbic acid are known to inactivate ROS/oxidants through the ability to reduce these molecules; this activity likely explains the lower TOS levels in DuraGraft stored grafts. The higher TAS levels in DuraGraft stored grafts indicate that there is a higher reserve or a surplus of antioxidants in these grafts provided by unused L-glutathione and L-ascorbic acid molecules. Once L-glutathione or L-ascorbic acid inactivates an ROS or oxidant molecule, it becomes inactive itself and levels of antioxidants will become lower as more and more ROS are neutralized. The observation that TAS levels are also higher in DuraGraft stored grafts indicate that there is a greater reserve or supply of unused or available antioxidants compared to levels in grafts stored in PS or HB meaning that the antioxidants in DuraGraft were not depleted by ROS inactivation during graft storage. Overall, the lower OSI in DuraGraft stored grafts compared to grafts stored in PS or HB is predicted to better protect grafts from ischemic damage during ex-vivo storage. Since reperfusion injury exacerbates ischemic injury, preventing or reducing ischemic injury will also reduce or prevent subsequent reperfusion injury thereby mitigating VGD.
The current findings are consistent earlier studies conducted with DuraGraft. In an in-vitro and ex-vivo study that compared heparinized DuraGraft to heparinized PS, human SV segments and isolated pig mammary veins were flushed and submerged in DuraGraft and PS for prespecified times.10 Loss of human SVG cell viability was observed as early as 15 minutes post-exposure to PS whereas viability was maintained up to 5 hours’ exposure to DuraGraft. Histological analyses performed with pig mammary veins demonstrated endothelial damage in pig mammary veins stored in PS. Cytotoxicity assays demonstrated that saline-induced microscopically visible cell damage occurred within 60 minutes. DuraGraft treated cells did not show evidence of damage or reactivity.
In a human clinical study, the effect of storage solutions on SVG early anatomical changes associated with VGD was assessed using multidetector computed tomography angiography at 1, 3, and 12 months post-CABG.15 Within each patient, two SVGs were randomized to either DuraGraft or heparinized PS to exclude differences in patient characteristics as a confounding factor. DuraGraft was found to have a favorable effect on early anatomical markers of VGD such as lesser SVG wall thickness at 12 months, particularly in the proximal segment of the graft where early disease has been shown to most frequently manifest.20 To further assess the performance of DuraGraft, a 3,000 patients registry including patients that underwent isolated CABG as well as combined CABG and valve surgery has been initiated. 14 Enrollment has been completed end of 2019, follow-up is ongoing and the first results are eagerly awaited.
Strengths and limitations of the study
This study was conducted at a single center, all SV were harvested by the same surgeon and all biochemical analysis were performed by a single analyst. This importantly reduces the variation in SV harvesting technique and analysis methodology. Two regularly used storage solutions and one solution specifically developed for graft storage have been tested. Moreover, samples from a large number of patients representative of patients undergoing CABG have been studied. These design elements illustrate the robust design of the study and substantiate the validity of the data. A limitation is that conform the center’s practice no heparin was added to PS, unlike the addition of heparin to DuraGraft and HB. It should be further acknowledged that the pathophysiological and clinical relevance of the observed statistically significant differences need further research.