Accurate length adjustment of aortocoronary saphenous vein bypass grafts

Hemodynamics and Wall Mechanics of Vascular Graft Failure

Blood vessels are subjected to complex biomechanical loads, primarily from pressure-driven blood flow. Abnormal loading associated with vascular grafts, arising from altered hemodynamics or wall mechanics, can cause acute and progressive vascular failure and end-organ dysfunction. Perturbations to mechanobiological stimuli experienced by vascular cells contribute to remodeling of the vascular wall via activation of mechanosensitive signaling pathways and subsequent changes in gene expression and associated turnover of cells and extracellular matrix. In this review, we outline experimental and computational tools used to quantify metrics of biomechanical loading in vascular grafts and highlight those that show potential in predicting graft failure for diverse disease contexts. We include metrics derived from both fluid and solid mechanics that drive feedback loops between mechanobiological processes and changes in the biomechanical state that govern the natural history of vascular grafts. As illustrative examples, we consider application-specific coronary artery bypass grafts, peripheral vascular grafts, and tissue-engineered vascular grafts for congenital heart surgery as each of these involves unique circulatory environments, loading magnitudes, and graft materials.

Biomechanics and Mechanobiology of Saphenous Vein Grafts

Within several weeks of use as coronary artery bypass grafts (CABG), saphenous veins (SV) exhibit significant intimal hyperplasia (IH). IH predisposes vessels to thrombosis and atherosclerosis, the two major modes of vein graft failure. The fact that SV do not develop significant IH in their native venous environment coupled with the rapidity with which they develop IH following grafting into the arterial circulation suggests that factors associated with the isolation and preparation of SV and/or differences between the venous and arterial environments contribute to disease progression. There is strong evidence suggesting that mechanical trauma associated with traditional techniques of SV preparation can significantly damage the vessel and might potentially reduce graft patency though modern surgical techniques reduces these injuries. In contrast, it seems possible that modern surgical technique, specifically endoscopic vein harvest, might introduce other mechanical trauma that could subtly injure the vein and perhaps contribute to the reduced patency observed in veins harvested using endoscopic techniques. Aspects of the arterial mechanical environment influence remodeling of SV grafted into the arterial circulation. Increased pressure likely leads to thickening of the medial wall but its role in IH is less clear. Changes in fluid flow, including increased average wall shear stress, may reduce IH while disturbed flow likely increase IH. Nonmechanical stimuli, such as exposure to arterial levels of oxygen, may also have a significant but not widely recognized role in IH. Several potentially promising approaches to alter the mechanical environment to improve graft patency are including extravascular supports or altered graft geometries are covered.

Epicardial dissection may solve the problem of saphenous vein graft length

Saphenous vein graft length may be not enough in some cases in aortocoronary bypass surgery. If the graft length is not enough, cardiac surgeons can use epicardial and fatty dissection to release tightening saphenous vein graft, especially in the right coronary artery fashion.

Transmural pressure and axial loading interactively regulate arterial remodeling ex vivo

Physiological axial strains range between 40 and 60% in arteries, resulting in stresses comparable to those due to normal blood pressure or flow. To investigate the contribution of axial strain to arterial remodeling and function, porcine carotid arteries were cultured for 9 days at physiological and reduced axial stretch ratios in the presence of normotensive and hypertensive transmural pressures by ex vivo perfusion techniques. Consistent with previous in vivo studies, vessels cultured with physiological levels of axial strain and exposed to hypertensive pressure had greater mass, wall area, and outer diameter relative to those cultured at the same axial stretch ratio and normotensive pressure. Reducing the amount of axial strain resulted in mass loss and decreased cell proliferation. Culture in a hypertensive pressure environment at reduced axial strain produced arteries with greater contractility in response to norepinephrine. Arteries cultured at reduced axial strain with the matrix metalloproteinase inhibitor GM6001 maintained their masses over culture, indicating a possible mechanism for this model of axial stretch-dependent remodeling. Although not historically considered one of the primary stimuli for remodeling, multiple linear regression analysis revealed that axial strain had an impact similar to or greater than transmural pressure on various remodeling indexes (i.e., outer diameter, wall area, and wet mass), suggesting that axial strain is a primary mediator of vascular remodeling.

Serial quantitative coronary analyses for the evaluation of one-year change in saphenous vein grafts

BACKGROUND

A paucity of data exists with respect to changes in whole saphenous vein grafts (SVGs) despite accelerated atherosclerosis within grafted saphenous vein conduits. In the present study, we evaluated the one-year change in SVGs by means of quantitative coronary analysis.

METHODS

This study enrolled consecutive 52 patients with 109 SVGs, who underwent coronary artery bypass graft surgery successfully. A follow-up study was performed in 33 patients with 65 SVGs after one year because 16 SVGs were obstructed (baseline, 8; follow-up period, 8), and 15 patients with 28 SVGs dropped out within one year.

RESULTS

Both minimal and mean lumen diameters decreased significantly (3.17 +/- 0.64 mm vs 2.41 +/- 0.57 mm, p < 0.001; 3.70 +/- 0.69 mm vs 2.92 +/- 0.70 mm, p < 0.001; respectively). Graft length also decreased significantly (107.1 +/- 25.8 vs 100.6 +/- 25.2 mm, p < 0.001). The graft shortening rate (graft shortening length/baseline graft length x 100) was greater than 5% in 33 vessels (51%) and greater than 10% in 23 vessels (35%). Coronary risk factors (smoking, diabetes mellitus, hypertension, dyslipidemia) did not reveal significant relationship with late loss of minimal and mean lumen diameters.

CONCLUSIONS

The present study showed a considerable and uniform lumen loss of SVGs after one year, irrespective of coronary risk factors. Graft length shortening was seen more than elongation.

High early patency of saphenous vein graft for coronary artery bypass harvested with surrounding tissue

BACKGROUND

Surgical trauma to the saphenous vein, used as a conduit for coronary artery bypass grafting, affects their occlusion rate. This study evaluates the early patency of saphenous vein grafts harvested with a pedicle of surrounding tissue that protects the vein from spasm and trauma.

METHODS

Fifty-two patients underwent coronary artery bypass grafting with saphenous veins harvested with surrounding tissue. Forty-five patients, who received a total of 124 vein grafts and 42 left internal mammary arteries, underwent angiographic follow-up at a mean of 18 months (9 to 24 months).

RESULTS

Patency for saphenous vein grafts was 95.4% and for left internal mammary arteries, it was 93.3%. Twenty-nine of 30 (96.7%) vein grafts anastomosed to arteries 2.0 mm or more, 65 of 67 (97%) grafts to 1.5 mm, and 10 of 13 (77%) anastomosed to 1-mm arteries were patent. Nineteen of 22 (86.4%) vein grafts with flow rates 20 mL/min or less, 32 of 34 (94.1%) with flow between 20 and 40 mL/min, and 50 of 51 (98%) with flow more than 40 mL/min were patent. Other registered surgical and clinical factors did not contribute to vessel occlusion.

CONCLUSIONS

Early patency rate of saphenous veins harvested with surrounding tissue is very high, even in saphenous vein grafts demonstrating low blood flow. Preservation of graft endothelium using our harvesting technique may be the explanation of this success.