Shunting of microspheres across the canine coronary circulation

Coronary shunting of 9 +/-1 micrometer and 25 +/- 5 micrometer radiolabeled microspheres was examined in anesthetized, open-chest dogs, whose left common coronary arteries were perfused at controlled pressures. Shunting was estimated from the difference in radioactivity between perfusion line and coronary sinus blood samples during selective elevations of coronary perfusion pressure (CPP), left ventricular afterload, and inspired oxygen. A linear relationship was found between coronary shunting of 9-micrometer microspheres and CPP over the range 100-200 mmHg. According to regression analysis, percent shunt flow was 4.0% at control CPP (100 mmHg) and 10.0% at CPP of 200 mmHg. No shunting of 25-micrometer microspheres occurred at any CPP. Raising afterload did not affect shunting at control CPP but attenuated the increase in shunting at elevated CPP. Changing inspired gas from room air to 100% oxygen did not influence shunting at control or elevated CPP. Raising CPP to 150 and 200 mmHg also released 2.5% and 5.9% of pretrapped 9-micrometer microspheres, respectively. This study demonstrates that vessels permitting passage of microspheres across coronary circulation are sensitive to elevated perfusion pressure.