New insights into the pathophysiology, classification, and diagnosis of coronary microvascular dysfunction.

The coronary microvasculature plays a key role in determining and modulating coronary blood flow across the spectrum of myocardial demand. Our understanding of this complex system has been limited partly due to our inability to visualize the anatomy of an extensive microvascular bed and its complicated functional pathways. Nonetheless, research has led to the current belief that coronary microvascular dysfunction (CMVD) is a clinical entity that is an independent predictor of poor long-term outcomes in patients across a broad spectrum of cardiac diseases. CMVD exists in many clinical forms, in the presence and absence of epicardial coronary artery disease and structural heart disease. Both invasive and noninvasive tools have been used to assess the functional aspects of CMVD and both come with limitations. To date, invasive testing to assess coronary blood flow and microvascular resistance in response to provocative and hyperemic stimuli remains the gold standard. A recent clinical classification has been put forth to correctly categorize patients with CMVD. Despite the adverse outcomes associated with CMVD, proven targeted therapies remain elusive. Symptom relief and cardiovascular risk factor modification are the goals of current recommendations. There is a strong need for adequately powered trials to test specific management strategies and their effect on outcomes among patients with CMVD.