Effect of chronic native flow competition on internal thoracic artery grafts

Abstract

Residual competitive flow from the native coronary artery has been proposed as a mechanism that reduces flow in an internal thoracic artery graft (ITA), resulting in narrowing and ultimately failure of the graft. Results from acute experiments have indicated that competitive flow from a fully patent native artery did not abolish ITA graft flow. The present study was designed to examine the consequences of dynamic flow competition between the native vessel and the ITA graft in a chronic model. Fifteen mongrel dogs underwent coronary artery bypass grafting using the pedicled left ITA anastomosed to the normal, fully patent circumflex (CFX) coronary artery. The procedure was performed through a sterile thoracotomy, without systemic cardiopnlmonary bypass, using a brief local occlusion to construct the anastomosis. Intraoperatively, ITA flow was measured in situ on the chest wall, before the pedicle was mobilized. Internal thoracic artery graft and distal CFX flow were measured after the anastomosis was completed, with and without brief occlusion of the proximal CFX. Angiography was performed 72 hours, 4 weeks, and 8 weeks later; graft patency and diameter were evaluated. After 8 weeks, open-chest direct flow measurements comparable with the intraoperative assessment were obtained. Two grafts (13%) occluded early, the technical result of poor anastomotic construction. In the 13 remaining animals, all grafts were widely patent at all time points. Internal thoracic artery flow in situ averaged 10.9 ± 7.8 mL/min (mean ± standard deviation), and was maintained after grafting (11.5 ± 4.4mL/min; p = not significant). Internal thoracic artery graft flow represented 38.7% of total distal CFX flow (11.5/29.7) acutely. Comparable flows were obtained after 8 weeks of chronic competitive flow (11.9/ 30.4; 39.1%; p = not significant). Even after 8 weeks, when the proximal CFX was occluded transiently, flow increased in the ITA graft to provide 95% of the distal CFX requirements (30.4 ± 7.8 mL/min with proximal CFX open versus 28.9 ± 11.6 mL/min with proximal CFX occluded). The results demonstrate that even after 2 months of maximal chronic flow competition from a fully patent native artery, ITA graft flow was maintained above in situ levels, and a recruitable flow reserve from the ITA graft could be demonstrated when the native vessel was occluded. The data suggest that ITA grafts are dynamic and may remain patent despite significant residual flow in the native vessel.