Flow capacity of the canine internal mammary artery

The internal mammary artery has excellent long-term patency when used as a conduit for coronary artery bypass, and clinical and experimental studies have shown that blood flow through an internal mammary artery graft is satisfactory for most coronary artery branches. Multiple distal anastomoses from a single internal mammary artery, either with sequential anastomoses or with a Y-graft, might require additional blod flow through the vessel, and there has been concern that the flow capacity of the internal mammary artery is insufficient with these techniques. To better define the immediate postoperative flow capacity and pattern of the internal mammary artery, we performed experiments in seven dogs in which the left internal mammary artery was anastomosed to the circumflex coronary artery. In situ, blood flow in the internal mammary artery was 27 ml/min. Blood flow was 63 ml/min in the circumflex coronary artery and 42 ml/min in the left anterior descending coronary artery. After anastomosis of the left internal mammary artery to the circumflex coronary artery, the left main coronary artery was ligated; flow through the bypass graft increased to 92 ml/min, and systemic hemodynamics remained stable. Isoproterenol stimulation further increased flow through the left internal mammary artery graft to 160 ml/min. This study suggests that the canine internal mammary artery is capable of substantial early increase in flow and can, in fact, support the entire left coronary circulation.