Cardiac pacing induced flow responses in internal thoracic artery and saphenous vein coronary artery bypass grafts.

The flow reactivity of an internal thoracic artery (ITA) graft and a vein graft for multiple coronary beds in response to different modes of cardiac pacing remains unclear. These experiments were conducted in 14 anesthetized dogs with the ITA or the vein grafted to the circumflex coronary artery, off pump, using a brief local occlusion. The left main coronary artery was occluded, rendering the entire left ventricle totally dependent upon the ITA graft or the vein graft. When the left main coronary artery was occluded and the heart rate was 120 beats per min, graft flow was 93.4 ± 9.6 ml per min in the ITA, and 96.1 ± 10.4 ml per min in the vein graft. Atrial pacing to increase heart rates 25 % to 150 beats per min increased both the ITA graft flow (110.3 ± 9.7 ml per min, p < 0.05 versus flow in sinus rhythm) and the vein graft flow (109.8 ± 7.9 ml per min, p < 0.05 versus flow in sinus rhythm). The increases in flow in both cases were not attributable to changes in perfusion pressure. In contrast, ventricular pacing to the same heart rate decreased systemic pressure slightly, but insignificantly. Despite the slight decrease in perfusion pressure, ventricular pacing increased ITA flow (107.9 ± 8.4 ml per min, p < 0.05 versus flow in sinus rhythm), but the increase in vein graft flow was not significant compared with flow in sinus rhythm (102.1 ± 7.3 ml per min, p = ns versus flow in sinus rhythm). Both pacing modes increased ITA flow equally, but atrial pacing augmented vein graft flow slightly but statistically more than ventricular pacing (p < 0.05). When ITA flow was measured on the chest wall before grafting, neither atrial nor ventricular pacing influence the ITA in situ flow. In conclusion, when heart rate was increased to 150 beats per min by atrial pacing, ITA grafts and vein grafts can increase the graft flow in response to myocardial oxygen demands. However, vein grafts demonstrated a relative flow deficit after ventricular pacing, compared with the ITA graft in the same situation. Based upon the present studies, there may be some disadvantages with a high rate of ventricular pacing (25 % above baseline), suggesting the potential for relative ischemia in a vein graftdependent region of the canine heart