Greater contractility of internal mammary artery bifurcation: Possible cause of low patency rates

Coronary artery bypass grafting using bifurcation of the internal mammary artery (IMA) has been reported to have a poor patency rate. To test the hypothesis that the contractility (tendency for spasm) is greater at the bifurcation than at the main IMA, segments of the bifurcation and the distal section of IMA taken from patients with coronary artery bypass grafts were studied in organ baths. The IMA rings were set up at a physiologic pressure. Concentration-response curves were established for norepinephrine, endothelin-1, U46619, potassium, and glyceryl trinitrate (precontracted with 10 nmoI/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 standardized contraction force was greater in the bifurcation than in the main IMA 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 differences were seen for potassium, U46619, or glyceryl trinitrate, whereas the effective concentration that induced 50% of maximal effect for U46619 was 6.17-fold lower in the bifurcation than in the main IMA (9.14 ± 0.28 versus 8.35 ± 0.09 -log M; p = 0.003), indicating higher sensitivity in the bifurcation. This study demonstrates a greater contractility of the bifurcation than that of the distal section of the main IMA, implying that the occlusion of bifurcation is due not only to its smaller diameter, but maybe more importantly to its greater tendency for contraction or spasm. This characteristic may account for the poor patency rate of the bifurcation.