Vasodilator reserve: a functional assessment of coronary health.

Assessment of coronary artery stenosis severity depends on either determination of the anatomic dimensions of the stenosis by angiographic techniques or assessment of the functional significance of the stenosis by measurement of its effect on blood flow. Measurement of myocardial blood flow during maximal pharmacological vasodilation (vasodilator reserve) has been used to examine the functional consequences of a stenosis on perfusion of the dependent region of myocardium. In experimental animals, flow reserve measured with an electromagnetic flowmeter during pharmacological coronary vasodilation corresponds closely to quantitative coronary angiographic measurements of stenosis geometry. Studies using PET imaging with [N]ammonia to measure coronary flow reserve in patients with coronary artery disease also demonstrated an inverse correlation between stenosis severity and flow reserve, but the relationship exhibited a greater degree of scatter than that obtained in animal models. It is not surprising that the correlation between stenosis severity and flow reserve would be less precise in patients with coronary disease, because atherosclerosis introduces potential variability in the behavior of both the epicardial stenotic segment and the coronary resistance vessels. Thus, a coronary stenosis in a patient with atherosclerosis may not produce a fixed degree of anatomic narrowing of the epicardial artery, and the resistance vessels may not predictably undergo maximal vasodilation in response to pharmacological vasodilators. Consequently, interpretation of coronary vasodilator reserve requires consideration of the dynamic characteristics of both the epicardial artery segment and the coronary resistance vessels.