How does a Regular 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 a Regular Stress Test ordered?
A regular stress test is considered in the following circumstances:
- Patients with symptoms or signs that are suggestive of coronary artery diseases (CAD).
- Patients with significant risk factors for CAD.
- To evaluate exercise tolerance when patients have unexplained fatigue and shortness of breath.
- To evaluate blood pressure response to exercise in patients with borderline hypertension.
- To look for exercise-induced serious irregular heart beats.
How is a Regular Treadmill 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.
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 minute intervals.
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. The latter may rise above 90 - 100.
Preparing for the Regular Stress Test:
The following recommendations are "generic" for all types of cardiac stress tests:
- Do not eat or drink for three hours prior to the procedure. 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.
- 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 a Regular Treadmill 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 a Regular Stress Test?
If a patient is able to achieve the target heart rate, a regular treadmill stress test is capable of diagnosing important disease in approximately 67% or 2/3 rd of patients with coronary artery disease. The accuracy is lower (about 50%) when patients have narrowing in a single coronary artery or higher (greater than 80%) when all three major arteries are involved. Approximately 10% of patients may have a "false-positive" test (when the result is falsely abnormal in a patient without coronary artery disease).
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