The stress test is used to evaluate the heart and vascular system during exercise. It helps answer to two general questions: 1) Is there occult underlying heart disease that only becomes apparent when the heart is stressed by exercise? 2) If there is underlying heart disease, how severe is it?
How is a stress test performed?
The patient is attached to an ECG machine, and a blood pressure cuff is placed on one arm. Sometimes a clothespin-like sensor is attached to the finger to measure the amount of oxygen in the blood. After a baseline ECG is obtained, the patient begins to perform a low level of exercise, either by walking on a treadmill, or pedaling a stationary bicycle. The exercise is "graded" - that is, every three minutes, the level of exercise is increased. At each "stage" of exercise, the pulse, blood pressure and ECG are recorded, along with any symptoms the patient may be experiencing.With a "maximal" stress test, the level of exercise is gradually increased until the patient cannot keep up any longer because of fatigue, or until symptoms (chest pain, shortness of breath, or lightheadedness) prevent further exercise, or until changes on the ECG indicate a cardiac problem. Maximal stress tests should be performed when the goal is to diagnose the presence or absence of coronary artery disease.
With a "submaximal" stress test, the patient exercises only until a pre-determined level of exercise is attained. These tests are used in patients with known coronary artery disease, to measure whether a specific level of exercise can be performed safely.
After the test, the patient remains monitored until any symptoms disappear, and until the pulse, blood pressure and ECG return to baseline.
What kinds of heart disease can the stress test help to evaluate?
The stress test is useful chiefly in the diagnosis of coronary artery disease. Coronary artery disease produces 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 patient exercises, the partially blocked artery may not be able to supply all the blood the heart muscle needs to perform at the high level now 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 placing the stress of exercise on the heart, the stress test can bring out abnormalities caused by partial blockages in the coronary arteries - abnormalities that are often completely unapparent at rest.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 coronary artery disease, 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 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.
