GREATER CONTRACTILITY OF INTERNAL MAMMARY ARTERY BIFURCATION - POSSIBLE CAUSE OF LOW PATENCY RATES

Coronary artery bypass grafting using bifurcation of the internal mamm ary artery (IMA) has been reported to have a poor patency rate. To tes t the hypothesis that the contractility (tendency for spasm) is greate r at the bifurcation than at the main IMA, segments of the bifurcation and the distal section of IMA taken from patients with coronary arter y bypass grafts were studied in organ baths. The IMA rings were set up at a physiologic pressure. Concentration-response curves were establi shed for norepinephrine, endothelin-l, U46619, potassium, and glyceryl trinitrate (precontracted with 10 nmol/L U46619). Contraction forces were standardized (gram per mm circumference) at a pressure of 100 mm Hg. The diameter was 1.50 +/- 0.08 mm (n = 38) for the bifurcation and 2.03 +/- 0.07 (n = 42) for the main IMA (p < 0.0001). The standardize d contraction force was greater in the bifurcation than in the main IM A for norepinephrine (0.82 +/- 0.06 versus 0.54 +/- 0.1; p = 0.02) and endothelin-1 (1.07 +/- 0.11 versus 0.69 +/- 0.07; p = 0.02). No diffe rences were seen for potassium, U46619, or glyceryl trinitrate, wherea s the effective concentration that induced 50% of maximal effect for U 46619 was 6.17-fold lower in the bifurcation than in the main IMA (9.1 4 +/- 0.28 versus 8.35 +/- 0.09 -log M; p = 0.003), indicating higher sensitivity in the bifurcation. This study demonstrates a greater cont ractility of the bifurcation than that of the distal section of the ma in IMA, implying that the occlusion of bifurcation is due not only to its smaller diameter, but maybe more importantly to its greater tenden cy for contraction or spasm. This characteristic may account for the p oor patency rate of the bifurcation.