This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles.
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This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles. |
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LESSON 5eCARDIAC STRESS TESTING: EXERCISE TREADMILL TESTNoninvasive cardiac procedures for the diagnosis of coronary artery disease include Electrocardiography, Radiography, Echocardiography, Nuclear Imaging and Positron Emission Tomography. Of all these modalities, the electrocardiogram (ECG) is the most widely used diagnostic modality in cardiology. In addition to providing important diagnostic information on cardiac rhythmic patterns, electrolyte imbalances, and cardiac drug toxicities, the ECG plays a major role in the diagnosis of CAD by differentiating infarction from ischemic processes. Stress Electrocardiography or exercise testing is a useful and noninvasive technique for evaluating cases of suspected or known CAD. The procedure combines electrocardiography with a graded exercise test to detect stress-induced alterations in cardiac electrical patterns that might signify myocardial ischemia. Indications and clinical value. Exercise testing is reasonably safe, easy to perform, and relatively inexpensive. This procedure also may establish the diagnosis of CAD in patients with chest pain when medical history, physical examination, resting ECG, and attempts at relieving pain with nitroglycerin have not confirmed the diagnosis. For patients with known angina pectoris, exercise testing helps assess cardiovascular functional capacity, specifically the ability of coronary arteries to deliver adequate quantities of oxygenated blood to the myocardium under the demands of stress. Such testing can demonstrate the level of physical exertion a patient can safely perform without symptoms or ECG changes. Exercise tests also may help detect cardiac rhythm disturbances induced by exertion, so that appropriate measures can be taken to prevent them. Finally, periodic exercise testing is useful in evaluating an individual's response to conditioning or recuperation programs. For a discussion on an alternative to exercise, advance to pharmacological stress agents. Contraindications and precautions.Based on a patient's past medical history or present complaints, a physician may decide that exercise testing should not be performed.
Special considerations.Certain conditions require special considerations or precautions prior to exercise testing. For example, ECG interpretation may be difficult or even impossible in the presence of conduction disturbances such as bundle branch block. Patients with peripheral vascular, neuromuscular, musculoskeletal, and arthritic disorders may be unable to perform on a treadmill or bicycle as well as those who do not suffer from such impairments. Similarly, patients with severe anemia, marked obesity, or pulmonary, renal, hepatic, or adrenal insufficiency may have great difficulty performing an exercise test. Patients with complete heart block may be unable to accelerate their heart rate adequately in response to exercise. Patients with fixed-rate pacemakers have similar difficulties. Pharmacological stress testing is an ideal method of establishing and/ or confirming CAD in these types of patients. Rationale and procedure. Stress electrocardiography indirectly evaluates the adequacy of coronary blood flow under conditions of maximal oxygen demand. An individual who sustains maximal oxygen demand without demonstrating symptoms or ECG evidence of myocardial hypoxia is assumed to have essentially normal coronary circulation. As myocardial oxygen consumption increases during exercise, a corresponding increase in coronary blood flow should occur. Since the extraction of oxygen by the myocardium is already maximal at rest, the increase in oxygen demand must be met primarily by an increase in coronary blood flow. An ECG recorded during exercise demonstrates the difference between oxygen demand (myocardial oxygen consumption) and oxygen supply (coronary blood flow), as manifested by characteristic ischemic ECG changes, even in the absence of chest pain. Equipment. During stress electrocardiography, the patient may exercise in several ways. European physicians have traditionally used bicycle ergometers, while American cardiologists have favored treadmills.
Preparation. The test is performed at least 2 to 4 hours after eating in order to avoid nausea and ECG changes caused by indigestion. Medications that may affect either the response to or the interpretation of the test may be discontinued from 1 hour to 2 weeks prior to the exercise, at the discretion of the referring physician. The patient is instructed to wear comfortable clothing and low-heeled shoes. Prior to the test, the patient must sign an informed consent form. Monitoring. Exercise testing should be done only under the direct supervision of a trained physician, with continuous ECG monitoring and periodic blood pressure determinations. The 12-lead ECG is routinely recorded in both the supine, sitting and standing positions prior to exercise. A pre-stress tracing with the patient hyperventilating a few seconds is optional. A standard 12-lead may also be recorded, using true arm and leg leads. During stress, one tracing is recorded two minutes into each stage of exercise. The patient is continuously monitored 5 minutes into the Recovery stage, or until the heart rate returns to baseline or to a rate under 100 beats per minute.
MULTISTAGE TREADMILL EXERCISE PROTOCOL (Bruce) Performance. Following the resting ECG, the patient exercises at progressively increasing speeds and grades. For patients undergoing exercise testing, a predicted maximal achievable heart rate (PMHR) is calculated (220 minus the patient's age). The patient's peak achieved heart rate can be expressed as a percentage of the PMHR, giving an assessment of the adequacy of the test in stressing the heart. Studies have shown that if a patient achieves 85% or greater of his or her PMHR without developing signs of ischemia, the likelihood that significant CAD is present is reduced. If a patient fails to reach 85% PMHR at peak exercise, CAD may be present but undetected by the test. The lower the peak rate (relative to the PMHR), the more likely that CAD, if present, will be missed. In the past, many laboratories would stop the exercise test when a patient reached 85% PMHR, to give a margin of safety to the test. Currently, most laboratories perform symptom-limited maximal exercise tests and encourage patients to reach their maximal exercise effort. This way, the symptoms that limit the patient's exercise performance can be determined and the patient can perform his or her maximal work safely. Patients who fail to reach 85% PMHR are still at risk of having significant, but undetected, CAD. The test is terminated when any of the following occur:
Interpretation. A normal ECG during an exercise test demonstrates the absence of ischemic ST-segment abnormalities. The duration of the QRS and ST intervals remains relatively unchanged. Peak exercise levels are usually attained when the patient achieves at least 85% PMHR for his or her age and sex. One way of classifying patients subjected to stress electrocardiography is by the presence or absence of the symptom of chest pain. In the presence of pain, a positive stress ECG suggests, with a great deal of certainty, that the patient has CAD. Approximately 30% to 40% of patients with a positive stress ECG have angina. A negative test, however, does not rule out CAD. Depending on the patient's risk-factor profile, other clinical findings, and the recurrence of pain, further evaluation may be required. In the absence of pain, a negative test is of more diagnostic value than a positive test. A negative test rules out significant CAD with great certainty, whereas a positive test requires further evaluation to reach a definitive diagnosis. A positive exercise ECG is defined as
Other findings that suggest an ischemic response to exercise but must be evaluated according to the patient's clinical condition are:
Premature beats that tend to disappear with exercise may also indicate significant underlying CAD. Although a positive exercise test correlates well with the presence of significant coronary artery obstruction—particularly if two or more vessels are involved, the ischemic ECG response to exercise is not specific for CAD. It may be seen in patients with other types of heart disease, such as left ventricular hypertrophy and/or aortic stenosis, which have decreased tolerance to an oxygen deficit. A positive response may also be seen occasionally in healthy people—notably women—whose arteriograms show normal coronary arteries. Alternatives to treadmill testing Patients who are unable to exercise or who have abnormal baseline electrocardiograms are not good candidates for treadmill testing. The pharmacologic stress test uses medications which alter the coronary blood flow. Such medications include Dobutamine, IV Dipyridamole, Adenoscan, and Regadenoson. next page...
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