Much of our knowledge of coronary blood flow regulation is based on global blood flow measurements or from studies of large epicardial coronary arteries. Those studies which have examined the coronary microcirculation, have often considered it as a single homogeneous vascular bed. However, regulation of coronary microvascular resistance is not distributed uniformly, but varies across different segments of the vasculature. Under normal conditions, a major portion of coronary vascular resistance is controlled by the coronary arterioles.

Studies of these vessels in vivo are difficult, due to problems associated with cardiac and ventilatory motion. To eliminate these problems, we have utilized a technique of fluorescent stroboscopic epi-illumination. Dogs (4-12 kg) were anesthetized with sodium pentobarbital, intubated, and ventilated with room air. A 24 gauge catheter was placed in the proximal left circumflex coronary artery for measurements of coronary artery pressure and administration of drugs. Following these procedures, the animal was ventilated on a high frequency jet ventilator synchronized to the cardiac cycle.