CONTRACTILITY OF THE HUMAN INTERNAL MAMMARY ARTERY AT THE DISTAL SECTION INCREASES TOWARD THE END - EMPHASIS ON NOT USING THE END OF THE INTERNAL MAMMARY ARTERY FOR GRAFTING

The distal section of the internal mammary artery (3 to 4 cm proximal to the bifurcation) is often used for coronary grafting. This part of the artery is more pharmacologically responsive to vasoconstrictor age nts than is its midsection. The present study was designed to test the hypothesis that the reactivity of the distal section of the internal mammary artery is inversely correlated to the diameter of the artery. The distal section of the human internal mammary artery was collected from aorta-coronary bypass grafts and studied in organ baths at a leng th of 3 mm. At the optimal point of the length-tension curves determin ed by a computer-iterative fitting technique, the diameter at 100 mm H g, the maximal contraction forces and effective concentration causing 50% of the maximal response to vasoconstrictor agents U46619, potassiu m chloride, alpha-adrenoceptor agonists norepinephrine, methoxamine, a nd phenylephrine were recorded or calculated. The maximal relaxation a nd 50% response to glyceryl trinitrate in phenylephrine-precontracted internal mammary artery segments were also calculated. The contraction force was standardized by the circumference (grams per millimeter). R egression analysis between contraction force and diameter revealed tha t the contraction force induced by U46619 and potassium chloride was i nversely correlated to diameter (r2 = 0.2, p < 0.05 in U46619-induced contraction and r2 = 0.2, p < 0.01 in potassium chloride-induced contr action). The contraction force induced by norepinephrine also had a tr end inversely correlated to diameter (r2 = 0.2, p = 0.07). Glyceryl tr initrate-induced relaxation was not correlated to diameter. This study demonstrated that the contractility of the distal section of the inte rnal mammary artery is inversely correlated to the diameter; that is, the smaller the diameter, the greater the tendency for spasm to develo p. This suggests that trimming off the distal end of the internal mamm ary artery as much as possible may be the best way to prevent graft sp asm and that superior results of left internal mammary artery grafted to the left anterior descending artery or the use of a ''free graft'' may be related to the shorter length (distal end is trimmed off) and l ess contractility of the graft.