Myocardial perfusion imaging has been shown to be of value in identifying areas affected by coronary artery disease.

Adequate coronary vasodilation is essential to myocardial perfusion imaging. Coronary vasodilation increases coronary blood flow. This increase creates differences in the distribution of the imaging agent great enough to identify regions supplied by stenosed coronary vessels and to distinguish ischemic from nonischemic areas.

Exercise stress testing often is employed for dilating coronary vessels, hence increasing coronary blood flow. However, maximal exercise levels are required for sufficient vasodilation, and exercise capacity varies greatly among patients. Therefore, exercise may not always be an option. Peripheral vascular disease, medications, poor patient motivation, and a variety of coexisting conditions all may preclude certain patients from exercising at maximal levels (Table 1).

Table 1. Potential Limitations to Maximal Exercise Testing

Peripheral vascular disease Poor physical condition Poor patient motivation Lower limb amputation Arthritis/rheumatism/orthopedic conditions Other conditions that limit physical exertion

Intravenous Adenoscan is a pharmacologic stress agent indicated as an adjunct to thallium-201 myocardial perfusion scintigraphy in patients unable to exercise adequately. Adenoscan has shown consistency in producing maximal vasodilation of coronary arteries relative to intracoronary papaverine. The short half-life of Adenoscan is responsible for its short-acting pharmacologic effects, making it useful for diagnostic evaluation and risk stratification in coronary artery disease (Table 2).

Table 2. Clinical Uses for Myocardial Perfusion Imaging

Diagnosis of coronary artery disease1 Functional significance of coronary disease1 Surgical intervention assistance1 Assessment of therapeutic effects of interventions, including medical therapy and revascularization1 Assessment of myocardial viability1 Risk stratification of patients1 Evaluation of acute chest pain2

Chemical Description

Adenosine is an endogenous nucleoside found in small amounts throughout the body. Its chemical formula is C10H13N5O4, or 6-amino-9-beta-D-ribofuranosyl-9-H-purine, and its molecular weight is 267 (Figure 1). Adenosine is a white crystalline powder, soluble in water and practically insoluble in alcohol.

FIGURE 1. Chemical structure of adenosine.