Norfolk County Cardiologist Association
The Stress test
Some forms of cardiac disease are easily missed when the patient is at rest, because at rest the patient's physical examination and ECG are often entirely normal. In these cases, cardiac abnormalities may become apparent only when the heart is asked to perform at increased workloads.
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 ECC 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.
What are some of the variations used with stress tests?
The accuracy of the stress test in diagnosing coronary artery disease is greatly increased by performing a nuclear perfusion study in conjunction with the stress test. A radioactive substance called thallium (or a similar substance called sestamibi or Cardiolite) is injected into a vein during exercise. The thallium collects in the parts of the heart that have good blood flow. Pictures of the heart are taken with a special camera that can image the radioactivity of the thallium. From these pictures, portions of the heart that are not receiving good blood flow (because of blockage in the coronary arteries) can be identified. The thallium study greatly increases the accuracy of the exercise study in diagnosing coronary artery disease. The amount of the patient receives from thallium is less than that from a chest x-ray.
Echocardiograms are sometimes used in conjunction with stress tests. An echo test is made at rest, and then with exercise, looking for changes in the function of the heart muscle during exercise. Deterioration in muscle function during exercise can indicate coronary artery disease.
Sometimes patients are not able to perform exercise because of physical limitations. The drugs persantine and dobutamine can be used in these cases to simulate the effects of exercise on the heart.
What are the limitations of the stress test?
In some patients, ECG changes suggestive of ischemia can occur even in the absence of coronary artery disease. (In other words, "false positive" stress tests are not uncommon.) In other patients, no significant ECG changes are seen even in the presence of coronary artery disease. (So "false negative" stress tests can be seen.) False positive and false negative studies can significantly limit the usefulness of the stress test in many patients. By adding a nuclear perfusion study to the stress test, this limitation is minimized, and the diagnostic capacity of the stress test is greatly improved.
What are the risks of having a stress test?
The stress test has proven to be remarkably safe. It poses about the same level of risk as taking a brisk walk or walking up a hill. While it is possible that the ischemia provoked by such stress can lead to a myocardial infarction (heart attack) or to serious heart rhythm disturbances, in practice this event is rare. Further, when these serious events do occur during a stress test, they occur in the presence of trained medical personnel who can deal with them immediately.