Caffeine and nicotine are vasoconstrictors. These stimulants will cause the blood vessels to constrict or become smaller. The reason why the doctor wants you to not have them before the stress test is because they can effect the results of the test. A stress echo is done to determine if there are any blockages in the coronary arteries that are causing reduced blood flow to the heart muscle (this is called ischemia). If you consume caffeine or nicotine before the exam it can cause the test to be inaccurate because the stimulants can restrict the blood vessels making it appear that you have a blockage when you really may not. It s also possible for nicotine or caffeine to make a possible heart condition worse. (Example if you already have a blockage, consuming caffeine and undergoing a stress test could lead to a possible complications.) These stimulants can also cause periodic arrhythmia s which can also effect the results of the test. There may also be a drug interaction with the caffeine and nicotine if the doctor does a drug induced stress test instead of an exercise stress test. Please follow your doctor s orders before the stress test. They are given to keep you safe and prevent complications. If you have any concerns talk to your cardiologist.