<qa>  What is a drug used routinely in patients to relax heart blood vessels?We are studying the affect of caffeine on the sensitivity of detecting coronary artery disease blockages in the blood flow to the heart with adenosine tracer scans. Adenosine is a drug used routinely in patients to relax heart blood vessels in order to assess for the presence of coronary artery disease. Often, if patients have had caffeine, the adenosine scan is not used because of the belief that caffeine may reduce the ability to detect coronary artery disease. We would like to test whether caffeine affects our ability to detect coronary artery disease with adenosine tracer scanning. We will perform an imaging study of the heart with adenosine after you have received caffeine. 