A heart stress test / cardiac stress testing is a common diagnostic technique to assess a person’s cardiac and pulmonary function as well as exercise capacity.
The typical cardiac stress test utilizes a treadmill that begins at a low speed and begins flat or at a slight incline. In intervals of two or three minutes, the treadmill speed is increased and the slope of the treadmill is raised. The test terminates at the discretion of the supervising personnel. This typically happens when the patient is fatigued, a specific heart rate goal is achieved, or with symptoms of significant shortness of breath or chest pain occur.
During the test, the electrical signals of the heart are continuously monitored and blood pressure measurements are acquired. A physician will then interpret the heart tracings. The stress test may be accompanied by heart imaging studies using echocardiography or nuclear perfusion testing (see below) to enhance the diagnostic capabilities of the test. On occasion, bicycle stress tests are utilized rather than the treadmill.
We ask your cooperation in the following areas to make your test safe and of the highest diagnostic quality possible.
Nuclear Studies Considerations — Premenopausal women must do an in home pregnancy test 24 hours prior to having a nuclear test. You will be required to sign a release at the time of the nuclear test indicating you have done the pregnancy test and the results are negative.
Patients currently using Viagra, Levitra or Cialis — If you are currently taking Viagra, Levitra or Cialis you must discontinue use for 48 hours prior to your treadmill appointments.
The most common reason to do a stress test is to evaluate symptoms of chest discomfort and determine if a heart artery narrowing is the cause. The electrocardiograms obtained during the study are inspected for changes that often reflect ischemia, or inadequate blood flow to working heart muscle.
This is seen in changes to the portion of the cardiac electrical signal called the “ST segment”. Characteristic deviation of this segment upward or downward are deemed positive for ischemia. A small percentage of patients will have abnormal ST segment responses without ischemia and further cardiac testing is required.
Additional important data is gathered from the stress test. The exercise capacity of the patient can be judged, and importantly, what symptoms limit a patient from achieving the expected exercise duration. Chest pain elicited from exercise may indicate ischemia and increases the likelihood that significant coronary artery narrowing is present. Heart rate, rhythm, and blood pressure responses can also be abnormal in certain situations. This means multiple responses to the stress tests must be considered and integrated into the physician’s interpretation of the stress test.
Some patients are physically incapable of exercising on a treadmill. To perform stress testing, pharmacologic agents have been utilized to simulate some of the aspects of exercise.
The two medications used by South Denver Cardiology Associates are Dobutamine and Regadenoson (Lexiscan®). By themselves, these medications usually do not cause ST segment changes and symptoms. Heart rate and blood pressure changes are not as meaningful as with exercise. Therefore, these medications are always used with imaging tests.
Dobutamine is infused in increasing doses through an intravenous catheter during the exam. This drug increases the heart rate and pumping strength of the heart muscle. This requires an increase in blood flow to the heart, but if inadequate blood flow is present due to a blocked artery, changes are seen in the imaging pictures. Lexiscan® dilates heart arteries, but diseased arteries are unable to dilate. This produces abnormalities in the nuclear perfusion pictures. It less commonly produces abnormalities in echocardiogram pictures, and is rarely used in this country with echocardiography.
Exercise testing alone can miss some patients with coronary artery blockages, typically when they are relatively milder. Some patients will also have abnormal ST segment responses on the electrocardiogram that do not reflect coronary artery disease. To deal with these limitations, heart imaging studies have been developed to enhance the diagnostic capability of exercise or pharmacologic stress testing. These tests are also more proficient in identifying abnormalities within specific arteries, and thereby can be helpful in monitoring changes in patients with established coronary artery disease.
Nuclear tracers are radioactive elements that are injected into the bloodstream with an intravenous catheter, and enter heart cells in proportion to the blood flow reaching those cells. Thus, with more blood flow to the heart, greater amounts of radioactive tracer enter the heart cells. These tracers have not been found to have side effects in clinically used dosages.
The tracers give off radioactive particles called photons that can be detected and measured by a special camera. The camera reconstructs an image of the heart based on the number of photons received. The tracers are injected initially in the resting state and imaging starts. The patient then undergoes stress testing, either with exercise or pharmacologic stress. During the procedure, a second tracer injection is performed and another set of images is taken. The two sets of images are analyzed and compared by a physician trained in the technique.
Each portion of the left ventricle of the heart is evaluated. The cardiac blood supply is deemed to be normal if the heart muscle takes up maximum amounts of tracer. A reduced amount of tracer may be taken up in a specific heart segment on the stress images but be normal on the rest images. This situation indicates a significant heart artery blockage. In the resting state, blood flow may be adequate to meet the needs of the heart even with a partial blockage, but in the stress situation that artery is incapable of increasing blood flow. Thus, relatively less tracer is delivered to the heart muscle supplied by that artery, compared to heart segments with less blood flow. This is reflected as “defect” in the heart pictures. Lastly, a defect may be present on both the rest and stress images. This suggests a “myocardial infarction” or heart attack, whereby heart muscle is irreversibly damaged and the dead cells cannot take up the tracer.
South Denver Cardiology currently uses a One-Day Rest/Stress imaging protocol using Technetium 99m Sestamibi. This protocol is used to reduce patient wait time compared to previous protocols.
Echocardiography combined with stress testing utilizes sound waves to create pictures of the working heart muscle. The motion of the muscle can be described for all the segments of the left ventricle. The heart is imaged at rest, and then post exercise. Dobutamine is used in place of exercise for those who are unable to walk on a treadmill or ride a bike.
Additional images are obtained at the immediate end of exercise (or during dobutamine infusion). A normal study shows all segments moving or contracting well both at rest and stress. In the presence of a tight heart artery blockage, the function of the muscle worsens during stress, reflecting inadequate blood flow to that segment. This is termed a “regional wall motion abnormality”. In muscle that has suffered a heart attack, the motion of the segment is reduced or absent both on the rest and stress images.