Myocardial 'no-reflow'--diagnosis, pathophysiology and treatment

In acute ST-segment elevation myocardial infarction (STEMI), improvement in reperfusion strategies has contributed to improvement in mortality. Nonetheless up to 40-50% of patients who achieve satisfactory epicardial patency do not necessarily achieve patency at the coronary microvascular level, a condition referred to as the 'no-reflow' phenomenon. The 'no-reflow' phenomenon is associated with a worse prognosis at follow up. The pathogenic mechanisms underlying the 'no-reflow' phenomenon is complex and dynamic. This includes a variable combination of mechanisms including distal atherothrombotic embolisation, ischaemic injury, reperfusion injury and heightened susceptibility of coronary microcirculation to injury. Accurate detection of 'no-reflow' is crucial because it is independently associated with adverse ventricular remodelling and patient prognosis. The diagnosis of 'no-reflow' can be made using angiography, electrocardiography, nuclear scintigraphy, myocardial contrast echocardiography or cardiovascular magnetic resonance (CMR). Despite our improved understanding on the pathogenesis and diagnosis of 'no-reflow', the treatment of 'no-reflow' remains the 'Achilles heel' in the treatment of patients with acute myocardial infarction. Several therapeutic strategies have been tested for the prevention and treatment of 'no-reflow', however none have been associated with improvement in clinical outcomes. Therefore there exists a need for 'in-lab' tools that will be able to aid early identification of patients at increased risk of 'no-reflow'. This may enable patients at heightened risk of 'no-reflow' to be treated with the most appropriate individualised treatment early. We review the pathogenic mechanisms and diagnostic techniques of the 'no-reflow' phenomenon as well as the prevention and treatment strategies of the candidate mechanisms.