Coronary microvascular dysfunction (MVD) has been implicated as an important marker of cardiac risk and has been thought to directly contribute to the pathogenesis of a wide variety of cardiomyopathies. For instance, MVD is believed to cause ischemia (with reduction in coronary flow reserve) in patients with hypertrophic cardiomyopathy (HCM) despite the presence of angiographically normal epicardial coronary arteries. The implication is that MVD in HCM may lead to the ventricular arrhythmias, sudden death, and heart failure. Similarly, patients with idiopathic dilated cardiomyopathy (IDCM) have blunted coronary flow reserve, which appears to be independently associated with poor prognosis.