By Julian Booker, MD
Unless otherwise stated, any stress test referenced on a MyHeart related blog post will refer specifically to cardiac stress tests. Other heart related stress tests like cardiopulmonary exercise tests can be discussed at a later date.
In generic terms, a stress test is a procedure to indirectly determine whether there are significant blockages in your coronary arteries.
Your heart arteries will not be visualized in an isolated stress test. The goal of stress test is to identify regions of heart muscle that are starving for adequate blood flow, also called ischemia. Your physician will have several different ways to achieve this goal but all commonly used stress methods can be categorized under to broad categories. The end result of these two broad categories, increased blood flow to your hearts arteries (or sometimes lack thereof), is the key to diagnosis.
The heart is the powerhouse of your body. With stress tests that increase the heart workload, the heart is forced to pump more blood each minute than at rest. The same way that your body is fueled by food, your heart is fueled by blood (queue the obligatory True Blood reference). Blood supplies nutrients such as oxygen, glucose and fatty acids to the heart. If there is a blockage in one of your heart’s arteries then any portion of your powerhouse supplied by that artery does not have access to adequate energy to perform. This results in electrical and mechanical changes that we can detect. We will discuss some of these methods in future posts.
There are two main ways that we increase your heart’s workload. The first is via exercise. The most popular method of exercise stress in most places is via the treadmill.
Many physicians love exercise stress testing either alone or in conjunction with imaging as a stress modality of choice because it serves not only to stress the heart but also provides powerful additional prognostic information. Some places prefer to use a bicycle ergometer but the same principle applies.
For those that are unable to exercise adequately a pharmacologic or medical stress can be used instead. Intravenous infusions can simulate the effect of exercise on your heart. Dobutamine is a catecholamine that works a bit like adrenalin. It increases your heart rate and can be used in lieu of exercise.
These methods of stress are versatile. As mentioned above, when the heart rate increases sufficiency, regions of heart muscle without sufficient blood flow will struggle. The temporary changes have a formal name called ischemia. Ischemia is similar to the process that happens during angina but it takes place under safe, controlled circumstances. Active ischemia can be detected by electrocardiogram (ECG a.k.a. EKG), echocardiography, nuclear perfusion imaging, MRI perfusion imaging and CT perfusion imaging.
The second broad category of stressing can be considered vasodilator testing. Vasodilators are another type pharmacologic stress agent that work differently than dobutamine. Stress vasodilators are based around adenosine, another one of the body’s miracle substances. These agents may increase heart rate to a degree but their main action is to dilate the coronary arteries. Normal coronary arteries can drastically increase blood flow when dilated whereas diseased vessels are much less capable.
Perfusion imaging uses these differences in blood flow with vasodilation to its advantage. Blood flow in the heart muscle is then evaluated either by direct quantification or by comparison to surrounding muscle. When perfusion imaging is used in conjunction with vasodilator pharmacologic agents, your doctor will be able to identify muscle that is supplied by a severely diseased coronary artery.
Considerations before taking your stress test
You should discuss with your health care provider what medicines should be withheld prior to your stress test. Some agents like beta-blockers and certain calcium channel blockers may prevent your heart rate from increasing adequately and can make workload-type stress tests difficult. If you are having a vasodilator stress test you should withhold caffeine for several hours before hand. If you have active asthma or COPD then these medications may incite breathing difficulties. If you think you are having a heart attack or are having active chest pain, stress testing in any form is probably not the best idea.