Morphometric and metabolic profile of the distal segment of the internal mammary artery: caution on its use for coronary anastomoses

OBJECTIVE: Elastic arteries were found to be less prone to intimalhyperplasia than muscular arteries. The internal mammary artery (IMA),which is elastic in its proximal segment, presents a gradual decrease ofmedia elastic fibers along its downstream course. Metabolic andmorphometric studies of the distal end of the IMA with regard to its localsusceptibility to develop intimal changes were undertaken in order toevaluate the reliability of its use as an anastomotic site for bypassgrafting. METHODS: Twenty distal segments of IMA were harvested frompatients who had undergone myocardial revascularization. Histologic,enzyme-histochemical and morphometric studies were undertaken on thesearterial segments. RESULTS: Histologic examinations indicated anelastomuscular structure in 13 patients, a muscular structure in 6 and anelastic structure in 1. Of the 20 IMAs, none was found to have intimalthickening of greater than 25% of the diameter of the lumen. Theenzyme-histochemical profile of the proliferating cells found in theintimal thickening differed from normal contractile smooth muscle medialcells in the loss of myosin and mitochondrial ATPase, plasma membrane 5'nucleotidase, moderately decreased aerobic dehydrogenase and increasedlactate dehydrogenase activity and ribonucleoprotein-linked pyroninophilia.Lysosomal beta-glucuronidase and sulfatase were strongly active. Thisenzyme behavior is unfavorable to contractile function and favorable tocell proliferation and lipid accumulation, two events strongly involved inthe atherogenic process. CONCLUSION: Intimal proliferative changes wereobserved in the distal segment of the IMA. Although there was no histologicevidence of atherosclerotic plaque, the enzyme-histochemical profile ofthis intimal thickening was favorable to cell proliferation and lipidaccumulation. These findings suggest that it may be beneficial to avoidcoronary anastomoses with the distal end of the IMA and to use a moreproximal/elastic segment.