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Cardiac stress testing can be useful in identifying partial blockages in your coronary arteries (the arteries within your heart muscle). Blockages in these arteries can lead to heart attacks.
Many times, the presence of coronary artery disease (CAD) is easily missed when a person is at rest because there may be no sign of a problem either on physical examination or on the electrocardiogram (a recording of the heart's electrical activity). 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 two general questions: 1) Is CAD present that only becomes apparent when the heart is stressed by exercise? 2) If there is underlying heart disease, how severe is it likely to be?
How is a stress test performed?
First, you will have leads (wires) to an ECG machine attached to your chest and a blood pressure cuff is placed on your arm. A clothespin-like sensor may be placed on your finger to measure the amount of oxygen in your blood. After a baseline ECG is obtained you will be asked to begin performing a low level of exercise by walking on a treadmill. The exercise is "graded" - that is, every three minutes, the level of exercise is increased. At each "stage" of exercise, your pulse, blood pressure and ECG are recorded, along with any symptoms you may be experiencing.
Maximal vs Submaximal
With a "maximal" stress test, the level of exercise is gradually increased until you cannot keep up any longer because of fatigue or until your symptoms (chest pain, shortness of breath, or lightheadedness) prevent further exercise or until changes on your ECG indicate a cardiac problem. Maximal stress tests should be performed when the goal is to diagnose the presence or absence of CAD.
With a "submaximal" stress test, you will exercise only until a pre-determined level of exercise is attained. Submaximal tests are used in patients with known CAD, in order to measure whether a specific level of exercise can be performed safely.
After the test, you will be monitored until any symptoms disappear, and until your 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 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.
What are the limitations of the stress test?
In some patients, ECG changes suggestive of ischemia can occur even in the absence of CAD. (In other words, "false positive" stress tests are not uncommon.) In other patients, no significant ECG changes are seen even in the presence of CAD. (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 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.
At Bridgewater Primary Care and Cardiology we perform two kinds of Cardiac Stress Tests: Nuclear Stress Tests and Stress Echos
Digirad provides equipment, staffing, quality control, nuclear licensing and radiopharmaceuticals to allow patients to be imaged in the comfort and convenience of our office under Dr.Terzian's direct supervision.
The Cardiolite scans (Cardiolite is the trade name for sestamibi) are tests that show how well blood is flowing to various portions of the heart muscle. These tests, so-called "nuclear perfusion studies," are generally used in conjunction with stress tests to non-invasively detect the presence of coronary artery disease (CAD) that is producing partial obstructions in the coronary arteries.
What is Cardiolite?
Cardiolite is a radioactive substance. When injected into the bloodstream, this substance collects in the portions of heart muscle that have good blood flow. If one of the coronary arteries (the arteries that supply blood to the heart muscle) is blocked or partially blocked, relatively little Cardiolite accumulates in the muscle supplied by that blocked artery.
How are nuclear perfusion studies performed?
During your stress test, Cardiolite will be injected into your vein when you have attained your maximum level of exercise. The radioactive substance will distribute itself throughout your heart muscle in proportion to the blood flow received by that muscle. Cardiac muscle receiving normal blood flow accumulates a larger amount of Cardiolite than cardiac muscle that is supplied by diseased coronary arteries.
An image of your heart will then be made by a special camera that can "see" the Cardiolite. From these pictures, any portions of the heart that are not receiving normal blood flow (because of blockage in the coronary arteries) can be identified.
What are nuclear perfusion studies good for?
Using Cardiolite perfusion imaging greatly increases the accuracy of the stress test in diagnosing obstructive CAD. A normal Cardiolite test is an excellent indication that you have no significant blockages in your coronary arteries. On the other hand, patients with abnormal perfusion scans are highly likely to have significant blockages.
What are the risks of nuclear perfusion scans?
These noninvasive studies are very safe. Their only drawback is that radiation is used. The level of radiation the patient receives is felt to produce only a very small risk of harm, and for appropriately selected patients the potential for benefit far outweighs this small risk.
Exercise Stress Echocardiograms
Echocardiograms are sometimes used in conjunction with stress tests. An echocardiogram is performed at rest and then is repeated during exercise, to look for changes in the function of the heart muscle when exercise is performed. Deterioration in muscle function during exercise can indicate coronary artery disease.
For more information on this test please read about the two components separately: Exercise Stress Test (Link) and Echocardiogram (Link)