Dipyridamole and adenosine

Flow through moderately severe coronary artery stenosis is commonly normal at rest but becomes inadequate for the increased metabolic requirements and blood flow during stress. Coronary blood flow normally increases to four times resting baseline flow rates after coronary artery vasodilators such as dipyridamole and adenosine. A stenosis restricts maximal blood flow capacity compared to normal coronary arteries, thereby causing a disparity in regional perfusion of areas supplied by a stenotic artery compared to normal coronary arteries. This disparity manifests as a relative perfusion defect during stress, corresponding to the ischemic myocardial territory supplied by a stenotic artery. Furthermore, the quantitative severity of the relative perfusion defect is proportional to the severity of the stenosis under conditions of maximal coronary flow after dipyridamole or adenosine stress [24]. 

The principle of dipyridamole and adenosine thallium imaging is related to their coronary arteriolar vasodilator properties. Dipyridamole inhibits adenosine cellnlar renptake, resulting in increased concentrations of adenosine in the blood and tissues. Adenosine is a potent coronary artery vasodilator and can increase perfusion four to five times over baseline. Areas distal to a coronary artery obstruction will show a relative hypoperfnsion compared with normal coronary arteries because there is reduced perfusion pressure owing to preferential perfusion of normal segments over stenotic segments. Acutely, these areas will appear as cold spots, but on the redistribution scans, the defects will fill, indicating viable bnt jeopardized myocardium. 

Taillefer R, Amyot R, Turpin S, Lambert R. Comparison between dipyridamole and adenosine as pharmacologic coronary vasodilators in detection of coronary artery disease with thallium 201 imaging. J Nucl Cardiol 1996 3 204-211. 

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