Since the introduction of reperfusion therapy in the early 1980s, cardiologists have largely focused on coronary artery flow and diameter, with the goal being to restore normal epicardial artery perfusion and large lumens. For many years, epicardial coronary flow has been simply assessed by the TIMI (thrombolysis in myocardial infarction) flow grade.1 This semiquantitative coronary angiographic tool places a patient's coronary flow into one of four different categories and has been shown to be associated with mortality. The corrected TIMI frame count provides a more objective quantitative index of coronary flow and has been shown to segregate even TIMI grade 3 flow into lower and higher risk subgroups.2-4

It is becoming increasingly clear that tissue perfusion, not just an open artery, is critical to myocardial salvage. For instance, among all patients with “open arteries” (TIMI grade 2 or 3), those with TIMI 2 flow have a higher mortality, probably as a result of impaired microcirculation.5 Myocardial contrast echocardiography demonstrates impaired microvascular flow among TIMI grade 2 patients, and even those with TIMI 3 flow after primary percutaneous transluminal coronary angioplasty (PTCA) have a poor recovery if there is no perfusion by this method.6 Impaired microvascular perfusion in the presence of open epicardial coronary arteries is thought to be caused by downstream microvascular obstruction, α adrenergic neural reflexes, spasm or thrombotic occlusions of microvessels.7