The stress test is useful in the diagnosis of CAD that is producing blockages in the coronary arteries, the arteries that supply blood to the heart muscle. If a partial blockage is present, the heart muscle supplied by that partial blockage may be getting all the blood it needs in the resting state. But if the person with this partial blockage exercises, the artery may not be able to supply all the blood the heart muscle needs to perform at the high level needed. When a portion of the heart muscle is suddenly not receiving enough blood flow, it becomes oxygen-starved, or ischemic. Ischemic heart muscle often causes chest discomfort (a symptom called “angina”) and characteristic changes on the ECG. It can also cause changes in the heart rhythm, or in the blood pressure. By “stressing” the heart with exercise, the stress test can bring out abnormalities caused by partial blockages in the coronary arteries – abnormalities that are often completely unapparent at rest. It is important to note that the stress test can only help to diagnose CAD that is producing partial blockages – so-called obstructive CAD. CAD often produces plaques in the arteries that are not actually causing obstruction, and these non-obstructive plaques can (and do) rupture, causing acute blood clot formation, which produces an acute obstruction of the artery, often leading to myocardial infarction (heart attack). So it is certainly possible to have a “normal” stress test while still having CAD. Because exercise raises adrenaline levels, stress tests can also be useful in diagnosing certain cardiac arrhythmias that tend to occur at times when adrenaline levels are increased. Stress tests are also useful in measuring the “functional capacity” of patients with heart disease. If a patient has CAD, for instance, the stress test can help assess the significance of partial blockages. If signs of ischemia occur at a low level of exercise, the blockages are likely to be very significant. But if ischemia does not occur or if it occurs only at very high levels of exercise, the blockages are likely to be much less significant. Performing periodic stress tests can also be a useful way of monitoring the progress of patients with congestive heart failure. If the peak level of attainable exercise is worsening over time, either the underlying heart disease may be worsening, or the patient’s medical therapy may need to be re-adjusted.