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Patients with normal appearing coronary arteries sometimes develop symptoms because of episodes of coronary spasm. Spasm refers to temporary constriction of an artery which results in decreased blood flow to the heart. Spasm can occur if the lining of the artery, called the endothelium, is not functioning properly. Acetylcholine is a medicine that temporarily triggers spasms in patients who have abnormal endothelium. It can be given safely in the cardiac catheterization laboratory to help determine if spasm is the cause of a patient's symptoms.