It is important to realise that even with severe coronary stenoses, myocardial perfusion at rest is normal. This is because coronary flow is regulated by the small resistance arterioles, which are too small to be seen on angiography. As in the upper panel above (click image to enlarge), at rest these vessels are the flow limiting resistance in the coronary system and the main pressure drop (P) is across these vessels. On exercise, the resistance vessels dilate and there is an even pressure gradient down the epicardial vessel. In the lower panel, a severe stenosis is present. To maintain normal resting perfusion, the resistance vessels dilate and the pressure drop in the system is across the lesion. On exercise, the fixed stenosis of the lesion limits flow. GTN dilates the epicardial vessels but not the resistance vessels. Dipyridamole (Persantin) and adenosine dilate the resistance vessels (and can be used as a substitute for physical or dobutamine stress in perfusion and other functional imaging).