ACS With Angiographically Normal Coronary Arteries: Recommendation Class IIb 1. If coronary angiography reveals normal coronary arteries and endothelial dysfunction is suspected, invasive physiological assessment such as coronary flow reserve measurement may be considered.629,773 776 (Level of Evidence: B) ACS associated with angiographically normal or nonob- structive (<50% stenosis) coronary arteries (also referred to as syndrome X) may be related to coronary endothelial ; plaque rupture that may be evident only with ; coronary vasospasm779 ; and coro- nary artery dissection.780 Myocarditis may present with elec- trocardiographic and biomarker findings similar to ACS and can be distinguished by magnetic resonance imaging.781 783 Intracoronary ultrasound and/or optical coherence tomog- raphy to assess the extent of atherosclerosis and exclude obstructive lesions may be considered in patients with possi- ble ACS and angiographically normal coronary arteries.778 If ECGs during chest pain are not available and coronary spasm cannot be ruled out, coronary angiography and provocative testing with acetylcholine, adenosine, or methacholine and 24-hour ambulatory ECG may be undertaken after a period of stabilization. Endothelial dysfunction is more common in women than in men,679,777,784 786 and chest pain is typical In the absence of a culprit coronary lesion, prognosis of coronary endothelial dysfunction and/or occult plaque rupture is favorable.765,787 Risk factor reduction and medical therapy with nitrates, beta blockers, and CCBs alone or in combination are consid- ered for endothelial dysfunction.788 790 High doses of arginine have also been given.791 Imipramine or aminophylline have been used in patients with endothelial dysfunction for contin- ued pain despite optimal medical therapy. In postmenopausal women, estrogen reverses acetylcholine-induced coronary arterial vasoconstriction, presumably by improving endothe- lium-dependent coronary vasomotion, and reduces frequency of chest pain.792 However, estrogen is not recommended because of its demonstrated increase in cardiovascular and Spontaneous coronary artery dissection affects a young, predominantly female population. 