Transmural Gradient of Retrograde Collateral Blood Flow in Acutely Ischemic Canine Myocardium

The contribution of retrograde coronary flow to total collateral flow was examined transmurally in anesthetized, open-chest dogs subjected to acute obstruction of the left anterior descending coronary artery. Regional coronary flow in normal and ischemic tissue was reflected by the tissue content of radioisotopes of potassium and rubidium and by radioactive microspheres. To measure total collateral flow, one isotope was administered when the retrograde cannula was clamped. The other isotope was administered when the cannula was opened to divert retrograde coronary flow away from acutely ischemic myocardium. Significant quantities of this isotope reached the ischemic region despite diversion of retrograde flow at an average rate of 3.5 ml/min. These results confirm earlier, but disputed, reports that total coronary collateral flow exceeds retrograde flow and are indicative of collateral pathways associated more closely with the microcirculation than with the larger epicardial arteries. Furthermore, the transmural distribution of myocardial blood flow in the ischemic tissue was altered nonuniformly by diversion of retrograde flow: 42K or 86Rb uptake was reduced to 25% of control in subepicardial tissue but only to 70% of control in subendocardial tissue. The transmural distribution of radioactive microspheres was similarly affected by diversion of retrograde flow. The results suggest that collateral pathways differ transmurally and may include significant luminal connections. Retrograde coronary flow is not a sensitive index of collateral flow to acutely ischemic subendocardial tissue.