2054 vessels, Ach evokes a NO-mediated vasodilatory response; however, in patients with endothelial dysfunction, this effect is blunted or paradoxical vasoconstriction may occur. 5 Endothelial function of the coronary microvasculature (resistance vessels) has been assessed by using intracoronary Doppler techniques to measure coronary blood flow in response to pharmacological or physiological stimuli. 6 Although considered by many to be the best assessment of endothelial function, this technique is limited by its invasive nature, expense, and relative inaccessibility.