Dipyridamole cardiac imaging is a useful alternative technique to exercise stress testing in the evaluation of patients with
ischemic heart disease. Intravenous
dipyridamole is still in the investigational phase, while oral
dipyridamole is widely available. The hemodynamic effects of
dipyridamole include an increase in coronary blood flow (due to coronary vasodilation) which is in excess of the increase in myocardial oxygen consumption and cardiac output. The disparity in the increase in coronary blood flow relative to the cardiac output results in an increase in myocardial
thallium activity and an increase in the myocardial/background activity ratio. The quality of the
thallium images is better or similar to that of exercise
thallium images. The optimal dose of intravenous
dipyridamole is 0.56 mg/kg, and of the oral dose it is 300 to 400 mg, although higher doses may be necessary in some patients. Analysis of the
thallium images has been to a large extent based on visual inspection of the planar images. Delayed images are helpful to establish the nature of the perfusion abnormalities (transient or fixed). The process of redistribution is based on disparate rates of washout from the normal and abnormal zones. The sensitivity and specificity of
dipyridamole thallium imaging, whether intravenous or oral, have been shown in a number of studies to be quite adequate and comparable to that achieved during exercise
thallium imaging.
Dipyridamole two-dimensional echocardiography has also been used in the detection of
coronary artery disease; transient (new or worsening of preexisting) wall motion abnormalities have been found to be a specific marker of
coronary artery disease. Transmural as well as regional coronary steal phenomena have been postulated as the mechanism for
dipyridamole-induced regional wall motion abnormalities. Compared to exercise two-dimensional echocardiography,
dipyridamole echocardiography provides high-quality studies and in higher proportions of patients. The results of
dipyridamole thallium imaging have also been extremely important in identifying high-risk patients after acute
myocardial infarction or patients with
peripheral vascular disease undergoing elective
vascular surgery; the presence of a
dipyridamole-induced perfusion abnormality identifies patients at high risk for future
cardiac events. Thus,
dipyridamole cardiac imaging is helpful in the diagnosis of
coronary artery disease and in risk stratification.