In an EST the patient exercises on a treadmill according to a standardized protocol, with progressive increases in the speed and elevation of the treadmill (typically changing at three-minute intervals). During the EST, the patient's electrocardiogram (EKG), heart rate, heart rhythm, and blood pressure are continuously monitored.
If a coronary arterial blockage results in decreased blood flow to a part of the heart during exercise, certain changes may be observed in the EKG (the electrocardiogram), as well as in the response of the heart rate and blood pressure.
The accuracy of the EST in predicting significant coronary artery disease (CAD) depends in part on the "pre-test likelihood" of CAD (also known as Bayes' theorem). In a patient at high risk for CAD (for example, because of advanced age or multiple coronary risk factors), an abnormal EST is quite accurate (over 90% accurate) in predicting the presence of CAD. However, a relatively normal EST may not mean there is an absence of significant coronary artery disease in a patient with the same high risk factors (so-called "false negative EST").
In a patient at low risk for CAD, a normal EST is quite accurate (over 90%) in predicting the absence of significant CAD. And an abnormal EST test may not reflect the true presence of CAD (so-called "false-positive EST").
The EST may miss the presence of significant CAD and so give a false negative result. Or the EST may indicate the presence of significant CAD when, in fact, there is none and so yield a false-positive test result. These false-negative and false-positive results are due to a variety of cardiac circumstances, which may include:
- An abnormal EKG at rest, which may be due to abnormal serum electrolytes, abnormal cardiac electrical conduction, or certain medications, such as digitalis;
- Heart conditions not related to CAD, such as mitral valve prolapse (drooping) or hypertrophy (increased size) of the heart; or
- An inadequate increase in the heart rate and/or blood pressure during exercise.
If the initial EST does not clarify the diagnosis, additional tests are often used to clarify the condition. These further options include radionuclide isotope injection and ultrasound of the heart (stress echocardiography) during the stress test.
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