Saphenous veins in coronary artery bypass grafting need external support

Abstract

The saphenous vein is the most commonly used conduit for coronary artery bypass grafting. Arterial grafts are harvested with the outer pedicle intact whereas saphenous veins are harvested with the pedicle removed in the conventional graft harvesting technique. This conventional procedure causes considerable vascular damage. One strategy to improve vein graft patency has been to provide external support. Ongoing studies show that fitting a metal external support improves conventionally harvested saphenous vein graft patency. On the other hand, the no-touch technique of harvesting the saphenous vein provides an improved graft with long-term patency comparable to that of the internal mammary artery. This improvement is suggested to be due to preservation of vessel structures. Interestingly, many of the mechanisms proposed to be associated with the beneficial actions of an artificial external support on saphenous vein graft patency are similar to those underlying the beneficial effect of no-touch saphenous vein grafts where the intact outer layer acts as a natural support. Additional actions of external supports have been advocated, including promotion of angiogenesis, increased production of vascular-protective factors, and protection of endothelial cells. Using no-touch harvesting, normal vascular architecture is maintained, tissue and cell damage is minimized, and factors beneficial for graft patency are preserved. In this review, the significance of external support of saphenous vein grafts in coronary artery bypass grafting is discussed.

Keywords: Coronary artery bypass; saphenous vein; stents; tissue and organ harvesting; vascular patency.

Figure 1.

Histology of conventional saphenous veins at harvest and six months after coronary artery bypass grafting. (a) At harvest the lumen (L) is dilated due to high-pressure distension and the adventitia (ADV) has been partially removed/damaged. (b) Six months after coronary artery bypass grafting, a substantial degree of neointimal hyperplasia (NIH) is seen in the graft at the luminal side of the internal elastic lamina (IEL). The media (M) and neoadventitia of the graft are thicker than at harvesting. Modified from Cox et al. Stranger in a strange land - the pathogenesis of saphenous-vein graft stenosis with emphasis on structural and functional differences between veins and arteries. Prog Cardiovasc Dis 1991;34:45–68.

Figure 2.

Explanted saphenous veins as bypass grafts. Top panel: (A) a no-touch saphenous vein harvested with the surrounding cushion of fat intact, not distended, and with minimal damage to the vein. (B) Conventional saphenous vein that has been distended and with the cushion of fat removed and the adventitia damaged (similar to saphenous vein at harvesting in Figure 1). (C) Conventional saphenous vein with a braided cobalt external stent fitted. Superimposed stent as described in Ben-Gal et al. Expandable external support device to improve saphenous vein graft patency after CABG. J Cardiothorac Surg 2013;8:122. (D) Conventional saphenous vein with a superimposed Dacron external stent fitted. Reproduced from Jeremy et al. On the biology of saphenous vein grafts fitted with external synthetic sheaths and stents. Biomaterials 2007;28:895–908. Lower panel: Transverse histological sections. (A) No-touch saphenous vein with adventitia intact, which has not been distended. (B) Conventionally prepared saphenous vein with the surrounding cushion of fat removed, adventitia damaged, and distended with saline at 300 mm Hg pressure. A and B from Dashwood et al. Retaining perivascular tissue of human saphenous vein grafts protects against surgical and distension-induced damage and preserves endothelial nitric oxide synthase and nitric oxide synthase activity. J Thorac Cardiovasc Surg 2009;138:334–40. (C) Sheep saphenous vein after fitting an external cobalt stent, in which adventitial damage has occurred. Perfusion fixed at 100 mm Hg. Reproduced from Ben-Gal et al. Expandable external support device to improve saphenous vein graft patency after CABG. J Cardiothorac Surg 2013;8:122. (D) Control un-grafted pig saphenous vein with damaged adventitia. Perfusion fixed at 100 mm Hg. Reproduced from Jeremy et al. On the biology of saphenous vein grafts fitted with external synthetic sheaths and stents. Biomaterials 2007;28:895–908.

Figure 3.

Perivascular tissue protects against distension-induced endothelial damage. Top panel: line drawings of saphenous veins harvested by the conventional technique (CT) in which perivascular tissue has been removed and distension used (-PVT DIST); saphenous vein section with perivascular tissue removed but not distended (-PVT nonDIST); saphenous vein with perivascular tissue intact and distended (+PVT DIST) and no-touch (NT) saphenous vein section with perivascular tissue intact and not distended (+PVT nonDIST). The longitudinal axis represents the density of Western blots in arbitrary units. Lower panel: a histogram showing quantitative measurement (mean of 8 Western blots) for CD31 protein levels, as an indicator of endothelial cell integrity. Immunohistochemistry showing representative CD31 immunostaining in which there is reduced endothelial staining (the arrow shows endothelial denudation with a few intact endothelial cells) in CT saphenous vein compared to all other preparations where the endothelium is virtually intact. CD31 protein shows representative CD31 Western blots for all 4 preparations of SV. β-actin = protein control. Modified from Dashwood et al. Retaining perivascular tissue of human saphenous vein grafts protects against surgical and distension-induced damage and preserves endothelial nitric oxide synthase and nitric oxide synthase activity. J Thorac Cardiovasc Surg 2009;138:334–40.

Figure 4.

Computed tomography angiography eight years postoperatively, showing a sequential no-touch (NT) saphenous vein (SV) graft anastomosed to two obtuse marginal branches. There is less risk of kinking despite the excess length and shifting course of the SV graft.

Figure 5.

Adventitial microvessels: a role in graft patency. (A) Microvessels/neovascularization (neoadventitia) at the Extent-outer vessel interface 6 months after implantation. L: lumen; M: media. Reproduced from Jeremy et al. On the biology of saphenous vein grafts fitted with external synthetic sheaths and stents. Biomaterials 2007;28:895–908. (B) Adventitial vasa vasorum in a no-touch saphenous vein (SV) graft (endothelial cells identified by CD34 immunostaining). The interrupted line indicates the media-adventitia border. The vasa vasorum and associated endothelial nitric oxide synthase is more important for saphenous vein than arterial bypass grafts. Reproduced from Dreifaldt et al. The vasa vasorum and associated endothelial nitric oxide synthase is more important for saphenous vein than arterial bypass grafts. Angiology 2013;64:293–9. (C) Immunostaining of microvessels in the neo-adventitial region of an Extent stented vein graft one month after implantation, which indicates endothelial cells lining microvessels and active angiogenesis. Reproduced from Jeremy et al. On the biology of saphenous vein grafts fitted with external synthetic sheaths and stents. Biomaterials 2007;28:895–908. (D) Scanning electron micrograph of damaged vasa vasorum at the adventitia-media border of a conventional saphenous vein. Reproduced from Vasilakis et al. Human saphenous vein and coronary bypass surgery: scanning electron microscopy of conventional and ‘no-touch’ vein grafts. Vasc Dis Prevent 2004;1:133–9.