Abstract
Background
Long term outcomes of coronary artery bypass graft surgery (CABG) are directly affected by gross and microscopic conduit morphology. Although the ulnar artery (UA) has been demonstrated to be a safe conduit, its use is limited in favour of the radial artery. The detailed morphology of the UA has not been well studied and this could be one potential reason for its limited adoption in CABG surgery.
Aims
The overall purpose of this study was to evaluate the gross and microscopic morphology of the UA to assess its viability as a conduit for CABG. Specific aims were to: document the length and luminal diameter of potential UA conduits; quantify tunica media size and composition; and determine the expression of vasoconstrictive receptors, across different segments of the artery.
Methods
The UA in 20 embalmed cadaveric forearms was exposed through dissection, and potential conduit lengths (maximum and ideal) were obtained. Arteries were fixed under physiological pressure, and histological sections then harvested from the proximal, middle and distal segments of the artery. Sections were stained with Verheoff van Gieson, to examine the elastin, and with antibodies against Endothelin-1 and Endothelin A receptors. Luminal diameter and tunica media size were measured in QuPath and a machine learning algorithm was developed to quantify the amount of smooth muscle and elastin in the tunica media. The degree of intimal hyperplasia was measured by calculating the intimal thickness index. The expression of endothelin receptors and ligands in the three segments were quantified by measuring staining intensity. Comparisons between the different segments were made using a repeated measures ANOVA or non-parametric Friedman test, with significance set at p < 0.05.
Results
The mean length of the maximum potential conduit was 18.5 ± 1.9 cm, and the ideal conduit, which preserves the maximum collateral supply to the hand was 15.8 ± 2.6 cm. The lengths of both conduits were significantly correlated with forearm length. Mean luminal diameter was 2.00 ± 0.33 mm, and the proximal diameter was significantly larger than the middle (p = 0.041) and distal (p = 0.035) segments. The tunica media had a mean cross-sectional area of 1.31 ± 0.37 mm^2 and was greatest in the proximal and distal segments. Elastin occupied a mean of 2.38 ± 1.49% of the tunica media, while a mean of 47.75 ± 19.26% was occupied by smooth muscle. Both elastin and smooth muscle content decreased distally in the artery. The UA tunica intima cross-sectional area was 0.36 ± 0.24 times that of the tunica media. This ratio was least in the proximal segment. Both endothelin-1 ligands and endothelin A receptors were present down the length of the UA, and endothelin A concentration was greatest in the middle segment.
Conclusion
This study has demonstrated that the maximum UA conduit has adequate length and diameter to effectively bypass most coronary arteries when grafted to the aorta. Microscopically, the UA is characteristic of a muscular artery, with a high concentration of smooth muscle in the tunica media, introducing the potential for conduit spasm. It appears that the risk of spasm should decrease distally, down the arterial length, potentially allowing a lower risk bypass if a short conduit is required. The UA, however, had a high level of intimal hyperplasia, in these specimens, a parameter which would need to be assessed carefully before harvest. Overall, the morphology of the UA is similar to other frequently used arterial conduits such as the radial artery, and in some instances offers advantages over other conduits, indicating it should be safe for use in CABG.