- Blood vessel (brachial artery reactivity) study to measure the ability of the brachial artery to dilate and increase its blood flow. For this procedure, the subject lies on a table with electrocardiogram leads attached to the chest. A blood pressure cuff is inflated for several minutes and a drop of nasal spray of nitroglycerin is given that may cause a headache. Blood pressure and headache are monitored and treated as needed.