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EXERCISE STRESS TESTING
The exercise stress test, sometimes referred to as a treadmill test, is essentially an electrocardiogram taken when an individual walks on a treadmill or pedals a stationary bicycle. (See Figure 10.6.) It is used to determine the functional capability of the heart, or in other words, its level of fitness. (See box, "Stress Test.")
As the name "exercise stress test" implies, the patient is exercised in order to create a greater level of work, or stress, for the heart. Exercise testing can reproduce symptoms, such as chest pain (angina pectoris), that a patient may encounter during physical exertion in the course of everyday activities. It allows a physician to determine the amount of exertion under which the patient experiences chest pain, while monitoring specific functions of the heart-primarily the heart rate and blood pressure.
A stress test is usually sustained until pain is provoked, significant changes in the electrocardiogram (ECG) occur, or a target heart rate is achieved. These changes or symptoms will not usually occur during a traditional, resting ECG. The ECG component of the stress test allows for the detection of an abnormality even if pain is not provoked. Electrocardiogram abnormalities are thus a fundamental part of the diagnostic capabilities of the exercise test.
The exercise stress test may reveal the presence of myocardial ischemia (inadequate blood flow to the heart), left ventricular dysfunction (decreased pumping ability), or ventricular ectopic activity (heart rhythm abnormalities originating in the ventricle). It also provides information on the relationships among these findings.
The most common indication for an exercise stress test is the evaluation of chest pain, which may or may not be angina pectoris. Angina occurs when the heart's demand for blood and oxygen exceeds its supply, a condition known as myocardial ischemia. Blockages in the coronary arteries are the main cause of ischemia, but angina usually will not occur at rest unless the blockages are extremely severe. The rate at which the heart's demand for blood and oxygen exceeds the supply during an exercise test generally reveals the severity of the disease. If angina occurs rapidly with little exertion, the blockages are likely to be extensive and the chance of a future heart attack significant.
The stress test is often used to determine the level of heart function and prognosis in a patient with established ischemic heart disease, particularly after he or she has had a heart attack and has been stabilized. It is also a source of clues about the cause of angina that is not easily controlled with medication, and a way of measuring heart function following balloon angioplasty or coronary artery bypass surgery. In nonacute settings, it is widely used to monitor the progress over time of treatments such as angioplasty, bypass surgery, medication, and lifestyle changes.
Less frequently, exercise testing may be part of a physical examination for healthy, middle-aged individuals who do not have symptoms of heart disease. In this case, it is used to establish cardiac fitness for certain occupations (such as piloting commercial air craft), or when such individuals have been sedentary and want to start a program of vigorous exercise, such as jogging.
For reasons not completely understood, stress tests are less accurate in young women without symptoms than in men without symptoms. Because the rate of false positives (an indication that heart disease is present when it is not) is higher in these asymptomatic women, stress tests are generally not recommended unless heart disease is strongly suspected.
In the past few years, exercise stress testing has become an important tool for diagnosing a condition known s "silent" ischemia, which means ischemia without chest pain. During that time, cardiologists have come to realize that the majority of ischemic episodes are silent-as many as 75 percent, according to some studies.
Silent ischemia is often detected in unsuspecting individuals when exercise stress testing is performed as part of a routine physical. However, there is currently much debate over whether the general public should be screened for ischemia via exercise stress testing. Because stress testing is relatively expensive, widespread screening for low-risk populations is not likely to be recommended in the near future. Still, individuals who have a family history of heart disease, or major risk factors, might consult their doctors about exercise stress testing, even if they have no symptoms.
The goal of the stress test is to reproduce symptoms or the appropriate physical state within the first 6 to 15 minutes of physical exertion. This goal is achieved by periodically increasing the speed and incline of the treadmill or the resistance of the pedals on an ergometer (stationary bicycle). A briefer test may not provide enough exertion to reproduce symptoms, while a longer, less rigorous one may tire a patient before symptoms can occur.
The heart's specific level of function is graded using a scale of metabolic equivalents (METs), which represent the workload on the heart during the exercise test. One MET is the amount of energy expended while standing at rest. The patient's score will be determined by the number of METs required to provoke symptoms.
More than a million stress tests are performed each year, with a very low risk of complications. The chance of a nonfatal heart attack occurring during an exercise test is about 1 in 100,000. The risk of complications is presumably highest in patients with severe heart disease.
There is no special preparation for a stress test. Individuals scheduled for this test may be advised to have only a light breakfast or lunch at least two hours before the test, in order to minimize any possibility of nausea that might be brought on by heavy exercise. They are also advised to wear rubber-soled shoes and loose, comfortable clothing, such as shorts or sweat pants and a T-shirt. In order to be sure that the ECG electrodes stay in place, men may need to have small areas of the chest shaved. For the same reason, both men and women are advised not to use body lotion.
The stress test begins and ends with a regular (resting) ECG (see earlier discussion of the electrocardiogram), and blood pressure is taken periodically. The entire test takes about 30 to 40 minutes, with the treadmill or ergometer portion lasting no more than 15 minutes.
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