Echocardiogram (Treadmill Stress Test)

 

How does an Echocardiogram (Treadmill Stress Test) Work?

Patients with coronary artery blockages may have minimal symptoms and an unremarkable or unchanged EKG while at rest. However, symptoms and signs of heart disease may become unmasked by exposing the heart to the stress of exercise. During exercise, healthy coronary arteries dilate (develop a more open channel) than an artery that has a blockage. This unequal dilation causes more blood to be delivered to heart muscle supplied by the normal artery. In contrast, narrowed arteries end up supplying reduced flow to it's area of distribution. This reduced flow causes the involved muscle to "starve" during exercise. The "starvation" may produce symptoms (like chest discomfort or inappropriate shortness of breath), and the EKG may produce characteristic abnormalities. Most commonly, a motorized treadmill is used for exercise, while a stationary bicycle is used in some exercise laboratories.

When is an Exercise Stress Test ordered?

An exercise stress test is considered in the following circumstances:
  • Patients with significant risk factors for coronary artery disease (CAD), also commonly referred to as coronary heart disease.
  • To evaluate exercise endurance and stress-induced EKG changes when patients have unexplained fatigue and shortness of breath.
  • To evaluate blood pressure response to exercise in patients with borderline hypertension.
  • To determine if there are exercise-induced irregular heart beats (Cardiac arrhythmia).
  • To determine if any symptoms are related to coronary artery disease.

Please remember that the exercise stress test is heavily dependent upon interpretation of EKG changes produced by exercise. Therefore, the accuracy of the test is diminished if there are significant EKG changes at rest (for example in patients with long standing high blood pressure, an artificial cardiac pacemaker, use of medications like digitalis, or presence of a bundle branch block pattern, etc.). In all such cases, the physician will usually order an Echo Stress Test or a Nuclear Stress Test, particularly if he or she is suspecting coronary artery disease. However, an exercise stress test may be sufficient in stable patients or those with a low suspicion of coronary artery disease who are being assessed for exercise tolerance (for example, prior to undergoing a structured exercise or rehab program).

How is an Exercise Stress Test Performed?

The patient is brought to the exercise laboratory where the heart rate and blood pressure are recorded at rest. Sticky electrodes are attached to the chest, shoulders and hips and connected to the EKG portion of the Stress test machine. A 12-lead EKG is recorded on paper. Each lead of the EKG represents a different portion of the heart, with adjacent leads representing a single wall. For example:
  • Leads 2, 3, and aVF = bottom or inferior portion of the heart.
  • Leads V1 and V2 = septum or partition of the heart.
  • Leads V3, V4, V5 and V6 = anterior or front portion of the heart.
  • Leads 1 and aVL = superior or top and outer left portion of the heart.
  • Lead aVR looks at the cavity of the heart and has almost no clinical value in identifying coronary disease.

Three of the EKG leads are also constantly displayed on the treadmill monitor. Each lead representing a different wall. The physician has the option of selecting different combinations of three.

The treadmill is then started at a relatively slow "warm-up" speed. The treadmill speed and it's slope or inclination are increased every three minutes according to a preprogrammed protocol (Bruce is the commonest protocol in the USA, but several other protocols are perfectly acceptable). The protocol dictates the precise speed and slope. Each three minute interval is known as a Stage (Stage 1, Stage 2, Stage 3, etc. Thus a patient completing Stage 3 has exercised for 3 x 3 = 9 minutes). The patient's blood pressure is usually recorded during the second minute of each Stage. However, it may be recorded more frequently if the readings are too high or too low.

As noted earlier, the EKG is constantly displayed on the monitor. It is also recorded on paper at one to three minute intervals. The physician pays particular attention to the heart rate, blood pressure, changes in the EKG pattern, irregular heart rhythm, and the patient's appearance and symptoms. The treadmill is stopped when the patient achieves a target heart rate (this is 85% of the maximal heart rate predicted for the patient's age). However, if the patient is doing extremely well at peak exercise, the treadmill test may be continued further. The test may be stopped prior to achievement of the target heart rate if the patient develops significant chest discomfort, shortness of breath, dizziness, unsteady gait, etc., or if the EKG shows alarming changes or serious irregular heart beats. It may also be stopped if the blood pressure (BP) rises or falls beyond acceptable limits. Please note that the systolic BP (upper number) may normally rise to 200 at peak exercise. At the same time, the diastolic BP (lower number) remains unchanged or falls to a slight degree. In contrast, the BP of patients with hypertension or high BP will show a rise of both systolic and diastolic readings. Abnormally high BP response to exercise can be an indication of higher risk of coronary heart disease and stroke.

Preparing for the Exercise Stress Test

The following recommendations are "generic" for all types of cardiac stress tests:
  • It is recommeded that the patient not eat a heavy meal prior to the procedure. Ideally, he should have no food two to three hours prior to the test. This reduces the likelihood of nausea that may accompany strenuous exercise after a heavy meal. Diabetics, particularly those who use insulin, will need special instructions from the physician's office.
  • However, it is recommended that the patient be well hydrated prior to the test. Drink plenty of water but no coffee. Do not drink any alcoholic beverage on the day of the test.
  • Specific heart medicines may need to be stopped one or two days prior to the test. Such instructions are generally provided when the test is scheduled.
  • Wear comfortable clothing and shoes that are suitable for exercise.
  • An explanation of the test is provided and the patient is asked to sign a consent form.
  • How long does the entire test take? A patient should allow approximately one hour for the entire test, including the preparation.

How safe is an Exercise Stress Test?

The risk of the stress portion of the test is very small and similar to what you would expect from any strenuous form of exercise (jogging in your neighborhood, running up a flight of stairs, etc.). As noted earlier, experienced medical staff is in attendance to manage the rare complications like sustained irregular heart beats, unrelieved chest pain or even a heart attack.

What is the reliability of an Exercise Stress Test?

The treadmill exercise stress test is highly accurate for determining if exercise induces cardiac arrhythmia, for determining abnormal EKG changes such as conduction defect and for BP response to exercise as well as any exercise-induced symptoms.

As far as determination of underlying coronary heart disease is concerned, the test is reasonably accurate. However, sometimes it can be false-negative for coronary heart disease. More advanced stress tests such as a stress test with myocardial perfusion (done with isotope injection in the vein) are more accurate. Additionally, approx. 10% of patients may have a false-positive test (when the result is falsely abnormal in a patient without coronary artery disease).

How quickly will I get the results and what will it mean?

The physician conducting the test will be able to give you the preliminary results before you leave the exercise laboratory. The results of the test may help confirm or rule out a diagnosis of heart disease. In patients with known coronary artery disease (prior heart attack, known coronary blockages, previous treatment with angioplasty, stents or bypass surgery, etc.), the study will help confirm that the patient is in a stable state, or that a new blockage is developing. The results may influence your physician's decision to change your treatment or recommend additional testing such as cardiac catheterization, Echo Stress test, or a nuclear stress test.

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Adapted from A.S.M. Systems, Inc. (HeartSite.com)