An Exercise Tolerance Test (ETT) is a medical test that evaluates a patients heart function during exertion and detects the presence of coronary artery disease and arrhythmias. During the test, a patient exercises on either a treadmill or a stationary bike, and the intensity of the exercise is gradually increased until the patient becomes fatigued. Blood pressure, breathing, and heart rate are monitored throughout the test, which is administered by a clinical exercise physiologist and a physician. Oxygen saturation may also be monitored in some cases. The ETT is commonly used to detect coronary artery disease or to determine safe levels of exercise following a heart attack or heart surgery. The test is also used to evaluate the adequacy of blood supply to the heart during exercise, especially in patients without known heart disease who are having chest pain. The ETT can be used to investigate coronary heart disease, which is due to narrowing of the coronary arteries and can cause chest pains (angina) and other problems. The test is usually conducted in a closely supervised situation, and there is a very small risk of complications.