Distal Flow Determinants in Canine Myocardium Perfused Through Internal Thoracic Artery Bypass Grafts

The dynamic reactivity and the acute, recruitable flow capacity of an internal thoracic artery (ITA) graft remains unclear. These experiments were conducted in 20 anesthetized dogs with the left ITA grafted to the circumflex artery, off pump, using a brief local occlusion. The left main coronary artery was occluded, rendering the entire left ventricle, including anterior descending artery and circumflex regions, totally dependent on the ITA graft. When the left main coronary artery was occluded, the ITA flow immediately increased more than fivefold (93.4 ± 9.6 mL/min; mean ± standard deviation), representing an absolute flow value three times higher than ITA flow measured in situ on the chest wall (27.5 ± 9.6 mL/min; p < 0.05 versus control), and the ITA graft provided total resting flow requirements (93.4 ± 9.6 mL/min) for both left anterior descending and circumflex coronary artery perfusion territories at levels comparable with measured native flow values (y = (0.9555)x + 21.9272; r = 0.976; p < 0.05). Pharmacologic challenge with adenosine (0.2 mg · kg−1 · min−1 intravenously) significantly increased the graft flow (120.3 ± 18.7 mL/min; p < 0.05 versus control), but also significantly decreased the mean arterial pressure (85.4 ± 5.0 versus 74.6 ± 6.1 mm Hg; p < 0.05). Phenylephrine (0.003 mg · kg−1 · min−1 intravenously) significantly decreased ITA graft flow (81.2 ± 9.0 mL/min; p < 0.05 versus control) despite significantly increased perfusion pressure (84.8 ± 6.3 versus 108.2 ± 8.6 mm Hg; p < 0.05 versus control). Physiologic stimulation of myocardial oxygen consumption with ventricular pacing (heart rate, 150 versus 120 beats/min for control) increased ITA graft flow to 107.9 ± 8.4 mL/min (p < 0.05 versus control), which was similar to changes observed with atrial pacing (110.3 ± 9.7 mL/min; p < 0.05 versus control) but was not attributed to changes in perfusion pressure. After a 10-second occlusion and release of the ITA graft, the hyperemic graft flow peaked at 197.6 ± 38.7 mL/min (p < 0.001 versus control). However, neither myocardial pacing nor short occlusion produced flow changes when assessed in the ITA in situ on the chest wall. Despite initially high flow demands, the canine ITA bypass graft is capable of very large recruitable flow and retains considerable dynamic reactivity to pharmacologically driven pressor responses and physiologically stimulated changes in myocardial oxygen demands.