From the Desk of Dr. Accad
Demystifying The Cardiac Stress Test
This article was originally published on March 6, 2015
For most people, the cardiac stress test is the epitome of modern cardiology. For some, the thought of undergoing one may also cause cold sweats…Perhaps a little introduction can help clarify what it is and what it does, and will minimize any misplaced fear about this helpful diagnostic tool.
A not so stressful stress test.
What is the purpose of a cardiac stress test?
The heart is an organ whose work depends exquisitely on the amount of physical activity one performs. Physical activity increases the body’s need for oxygen, which allows muscles—including the heart muscle itself—to “burn” energy stores, such as glycogen and fat. The oxygen is delivered by the blood, and the blood circulates under the action of the heart.
In most forms of heart disease, the heart is still able to deliver sufficient blood to meet the resting needs of the body, including its own needs. Therefore, the resting ECG and ultrasound may look normal.
During exercise, however, the movement of blood in the body and through the coronary arteries of the heart increases several-fold as a result of an increase in cardiac activity.
A condition affecting the heart may therefore not be apparent at rest but may become manifest during exercise by preventing the expected increase in cardiac activity or by causing some detectable change in cardiac function.
The purpose of the stress test, then, is to increase the demands placed on the heart by the body and see if the heart responds as expected. The stress test does not necessarily give the final diagnosis, but it can give a good clue that something is wrong.
How is a cardiac stress test conducted?
There are always two components to a cardiac stress test. The first is to put the heart under stress, and this is usually accomplished by asking the person to exercise. The second is to apply some technology to observe the heart response and detect any abnormalities. This is usually accomplished with the 12-lead ECG. Numerous other high-tech tools can be added, such as an echocardiogram or nuclear scan, but these typically serve niche applications in cardiology. They are not needed for most routine purposes and have limitations of their own.
The stress part:
The typical and time-tested way to put the heart under stress is to ask the person to exercise. Because the person must also be connected to the detecting equipment (the ECG), the most practical way of conducting an exercise stress test is to use a treadmill or a stationary bicycle. The machine is powered automatically by a motor and the subject must “keep up” by walking, running, or pedaling.
The treadmill is by far the most commonly used exercise routine. Because walking or running on a treadmill uses some upper body motion in addition to leg work, and also requires maintenance of the upright posture, the treadmill exercise may place higher demands on the body and therefore may be more sensitive for detecting heart problems than the stationary bicycle. The stationary bicycle, however, is helpful for those who have difficulty walking or running.
A safe and effective way to conduct an exercise stress test is to follow a standard “graded-effort” protocol. This means that the exercise proceeds through stages, very easy at first, and gradually becoming more difficult. This allows the person to warm up, to get used to the equipment and to relax. It also gives a chance for the doctor and the technician to observe the heart response more deliberately.
Because the stress test starts off nice and easy, it provides a good opportunity to chat, exchange jokes, and get to know one another!
The detection part:
An important benefit of the stress test is to simply but objectively observe the person during the effort, and note whether the comfort level matches (more or less), the expected level of fitness. Of course, the basic vital signs—heart rate and blood pressure—are also monitored. On occasion, I find people who struggle more than I would have expected based on their reported activity levels. Conversely, other individuals perform much better than what they judged was normal for their age.
Meanwhile, the person is hooked up to the ECG throughout the test. This serves to monitor the heart rate and detect problems. The two common problems that the ECG is apt to detect are abnormalities of the heart rhythm that can be triggered by exercise, or lack of oxygen going to the heart (“ischemia”), typically caused by coronary artery disease. In this case, the shape of the ECG waveform changes in a characteristic way that the doctor can recognize.
ST segment depression on ECG during stress test. Image attribution: J. Heuser CC:SA 3.0
Throughout the test, the blood pressure is measured at regular intervals. The blood pressure is expected to rise with physical activity. A significant drop or an excessive rise in blood pressure may indicate some cardiovascular problem.
How long does the cardiac stress test take?
The most common protocol used during exercise treadmill tests is called the “Bruce” protocol, named after Dr. Robert Bruce who devised it in the 1950’s. The standard Bruce protocol starts at a slow walking pace, but with the treadmill at a 10% incline. Every 3 minutes, the incline increases and the treadmill speed increases as well. The protocol can be modified in a variety of ways to fit the needs of the patient.
There are typically 3 factors that determine when the exercise must be stopped: 1) the subject reaches their “comfortable” effort tolerance 2) abnormalities are detected and the doctor chooses to stop or 3) the test is stopped at a pre-determined target heart rate.
In general, when a stress test is performed for screening purposes on healthy athletes, it is best to conduct it to the person’s maximal tolerance or to the person’s typical level of effort when he or she engages in sports or exercise activities. The duration of the test, then, depends on the physical condition of the person. Because the Bruce protocol is graded and has an incline as well as a speed component, most people reach that point after 10 or 15 minutes.
There is an important period of recovery after the stress test, lasting a few minutes, and during which we continue to observe the ECG. Not infrequently, important issues are only observed during the recovery period.
Take home messages:
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The stress test is not stressful!
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The stress test is a helpful diagnostic tool, because some heart conditions only are manifest when the person is doing physical activities
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The stress test is easy to perform
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The stress test takes less than 30 minutes from the prep to the end of recovery
From all of us here at Athletic Heart SF, we wish you many years of safe and enjoyable exercising!
Best regards,
– Dr. Accad