Cardiac Stress Testing:

Article Outline

• Abstract
• Indication
• Test Selection
  • Exercise Treadmill Test
  • Exercise Treadmill Test with Nuclear Imaging
  • Pharmacologic Stress Test with Nuclear Imaging
  • Stress Echocardiography
• Special Considerations for Women
• Conclusion
• References
• Copyright

Abstract 

Coronary artery disease (CAD) is the leading cause of death in this country. Appropriate evaluation of chest pain or its equivalent is essential to diagnosis and management of CAD. However, many nurse practitioners have limited knowledge about cardiac stress testing. This article reviews the types of tests available, the information obtained from each, and how to choose the most appropriate test for each patient.

Keywords:  Chest pain , coronary artery disease , electrocardiography , exercise test , nuclear imaging

More than 13 million Americans have coronary artery disease (CAD). Although 6 million patients present annually with classic angina, more complain of atypical chest pain.¹ For even experienced nurse practitioners, the diagnosis and treatment of CAD can be challenging. Cardiac stress testing is a useful tool to determine the presence and, to some degree, the extent of CAD. Health care providers may only think of standard exercise electrocardiography (exercise treadmill testing, or ETT), when in fact several stress modalities and data forms are available. Matching a patient's medical history, physical and exercise capacities (when known), and baseline resting electrocardiogram (ECG) with the appropriate study will allow for the safest and most cost-effective test selection.

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Indication 

Cardiac stress testing is most commonly used to determine whether myocardial ischemia is the cause of chest pain (or suspected equivalent symptoms) in patients at risk for CAD.² Several additional applications, however, are available: evaluation of new or worsening symptoms in patients with known CAD, preoperative risk stratification before noncardiac surgery, assessment for residual ischemia after myocardial infarction, diagnosis of suspected exercise-induced arrhythmias, and determination of functional capacity during evaluation of patients with heart failure being considered for transplantation.³

Patients with chest pain who are evaluated in emergency room settings and identified, using standard algorithms, as low risk for short-term cardiac events are excellent candidates for outpatient stress testing in lieu of hospital admission.⁴ Testing is best used in patients with an intermediate risk of CAD (when discrimination is maximal, 20%-80% pretest probability), rather than patients with low risk (because of a high incidence of false-positive results) or high risk (because of frequent false-negative results).⁵ Stress testing is generally not indicated as a screening test in asymptomatic patients, although it has been made a class IIb recommendation (possibly useful) in men older than 40 years and women older than 50 years who have multiple atherosclerotic risk factors. It may also be appropriate for sedentary patients planning to start vigorous exercise programs and for patients employed at occupations responsible for public safety such as airplane pilots, first responders, and truck drivers.³

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Test Selection 

Exercise Treadmill Test 

Treadmill ECG is a valuable procedure for CAD evaluation, with a mean overall sensitivity and specificity in diagnosing obstructive disease of 67% and 72%, respectively.² The risk of associated myocardial infarction or death is less than 1 in 2500,³ and additional risks include arrhythmias, falling, or injury on the treadmill. It usually can be completed in less than 30 minutes, and results are immediately available. Blood pressure and a 12-lead ECG are recorded at regular intervals, and heart rhythm is monitored continuously while the patient walks on a medical treadmill. The standard Bruce protocol is the most commonly used procedure and increases the treadmill speed and incline every 3 minutes. Other protocols, such as the modified Bruce and Naughton, use lower and more gradual increases in speed and can be used for patients with certain physical limitations.⁶ Criteria for test termination include attainment of at least the target heart rate (THR), which is 85% of maximal predicted heart rate for age [0.85 × (220 – age)]; development of clearly positive ECG findings (horizontal or downsloping ST segment depression >1 mm); hypotension or marked hypertension; serious tachyarrhythmias or bradyarrhythmias; or the perception that the patient is unable to continue the exercise for any reason (fatigue, dyspnea, claudication, back pain). Usually, patients must be able to walk briskly up hill in a comfortable and coordinated manner on a treadmill to successfully complete a maximal ETT.⁷ If a patient is unable to reach THR, the test is considered submaximal and nondiagnostic unless the ECG is clearly positive at the heart rate achieved; an alternative stress modality is usually required for definitive diagnosis in these cases (see below).

Because interpretation of the results of ETT are almost entirely based on ST and T wave changes, patients should have a normal resting ECG to maximize the accuracy of the study. Left bundle-branch block (LBBB), left ventricular hypertrophy with repolarization changes, and significant ST segment deviations at rest decrease test specificity and generally require stress imaging (see below). Certain medications can limit the accuracy of the stress ECG in diagnosis of CAD. Digoxin can produce ST segment depression in patients free of CAD. Nitrates and other antianginal medications can reduce the sensitivity in diagnosing obstructive CAD by exercise testing. β-Blockers, some calcium channel antagonists (verapamil, diltiazem), clonidine,⁸ and parasympathomimetic drugs (drugs that stimulate or mimic the parasympathetic nervous system) can blunt heart rate response to exercise, leaving patients unable to attain THR. β-Blockers should be held for 24 to 48 hours before the test unless the goal of stress testing is to assess the efficacy of an antianginal regimen in patients with established CAD³ or to assess rhythm or rate control in patients with chronic arrhythmias, but this decision should always be individualized according to patient condition. Although not indicated in these conditions, other medical states which can induce myocardial ischemia in the absence of obstructive CAD (aortic stenosis, hypertrophic cardiomyopathy,⁹ uncontrolled hypertension) may yield “false-positive” results (ie, ischemia in the absence of obstructive CAD). Providers should be aware of absolute and relative contraindications before ordering an ETT (Table 1).

Table 1. Contraindications to Exercise Testing

Exercise Treadmill Test with Nuclear Imaging 

Treadmill exercise can be combined with nuclear perfusion imaging to increase test specificity in patients with abnormal baseline ECGs. A radioactive tracer, such as technetium-99 sestamibi, is injected intravenously at rest, and images are obtained. The patient then performs the treadmill exercise protocol. When the patient reaches THR or feels he or she can only continue on for a short period of time, a second higher dose of radioactive tracer is injected, and imaging is repeated. The cardiologist or nuclear radiologist compares the rest and stress images to determine the presence and extent of CAD. Ischemia is diagnosed when the myocardium shows normal isotope uptake at rest but localized (relative) decreased uptake at peak stress. This decreased uptake occurs as areas supplied by partially obstructed arteries cannot increase flow as much as normal coronary vessels, therefore creating variance of isotope uptake. Myocardial infarction or fixed scar is identified as decreased uptake on both rest and stress images in the same locations of the ventricle. The overall specificity and sensitivity of ETT combined with perfusion imaging are 88% and 77%, respectively.³ Planar two-dimensional imaging was the standard acquisition method. It had the advantage of not requiring extensive processing of images; however, if the patient had three-vessel disease, these images could appear normal because of the global decrease in tracer uptake.¹⁰ Newer methods (single-photon emission computerized tomography, or SPECT, and gating) give three-dimensional images and segmental wall motion data which greatly increase specificity of diagnosis and improve accuracy in many patients.³ Other isotopes (eg, thallium-201) are still occasionally used and require a single injection at peak exercise with immediate and delayed (4 or 24 hour) imaging. Treadmill exercise with or without imaging has the additional advantage of providing information on exercise capacity which is useful to determine functional class and prognosis in patients with CAD.⁷

Pharmacologic Stress Test with Nuclear Imaging 

Patients must be able to ambulate to an adequate workload to successfully complete an ETT. If this is unlikely, ETT is not a cost-effective initial test, and pharmacologic stress should be used. The elderly and patients with poor coordination, poor balance, muscle weakness, joint pain or dysfunction, or deconditioning should, in most instances, be considered for drug stress initially. Although obese patients commonly need evaluation for CAD, they can be a difficult group to test because of musculoskeletal disabilities (such as back pain and lower extremity joint pain) or deconditioning that limits their ability to exercise. Imaging can also be technically difficult in obese patients. Nuclear images may show soft tissue attenuation, imitating hypoperfused myocardium.³ Some nuclear scanner systems have weight limits of fewer than 350 pounds. See Table 2, Table 3 for details of test selection criteria.

Table 2. Selection of Stress Modality

NPO indicates nothing by mouth; LBBB, left bundle-branch block; LVH, left ventricular hypertrophy; AV, atrioventricular; SBP, systolic blood pressure; HR, heart rate; MI, myocardial infarction; CHF, congestive heart failure; PE, pulmonary embolism.

Table 3. Selection of Data Method

ECG indicates electrocardiogram; LBBB, left bundle-branch block; LVH, left ventricular hypertrophy.

When patients are physically unable to exercise for any reason or when the ECG findings are obscured by baseline abnormalities (ie, LBBB, left ventricular hypertrophy with repolarization changes), pharmaceuticals are used as an alternative to exercise. Pharmacologic options include vasodilators (adenosine and dipyridamole, the latter working through adenosine receptors) and vasodilating inoptropes (dobutamine). These vasodilators are contraindicated in patients with bronchospastic lung disease, atrioventricular block higher than first degree, severe sinus bradycardia, or hypotension, because these conditions can be exacerbated by the medications. Dobutamine should not be used in poorly controlled hypertension, atrial fibrillation or other chronic persistent tachyarrhythmias, or in pacemaker-dependent patients.⁵ Vasodilators produce differences in isotope uptake by dilating nonobstructed coronary vessels more than atherosclerotic ones, a concept termed coronary steal. The steal occurs because ischemic vessels are already maximally dilated and are incapable of further vasodilation.¹¹ Vasodilators do not require THR response for maximal sensitivity and specificity; therefore, they can be used in patients with chronotropic incompetence and pacemaker dependence. Dobutamine studies, like exercise, require a THR be reached to maximize sensitivity.

Stress Echocardiography 

Stress echocardiography (echo) is similar to nuclear stress testing in that it compares images of the heart before and immediately after exercise to determine the presence of CAD. Stress is produced by exercise or dobutamine. Ischemia is present when a stress-induced wall motion abnormality develops in an area that contracted normally at rest. The overall sensitivity and specificity of stress echocardiography is 76% and 88%, higher with the use of intravenous echo contrast.¹² Advantages of echo imaging are the ability to identify ischemia-induced mitral regurgitation¹³ and that echo contrast, unlike radiopharmaceuticals, can be stocked in the laboratory and do not require advance ordering and delivery.

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Special Considerations for Women 

Choosing an appropriate cardiac stress test for women can be difficult. Noninvasive coronary studies are less sensitive and specific in women than in men. Most studies that examined noninvasive tests for diagnosing CAD did not include, or enrolled a small percentage of, women, resulting in insufficient data in this large subpopulation. Many women have a lower exercise capacity, an average of 2 minutes lower exercise time, and are unable to obtain THR with treadmill exercise.¹⁴ Women are generally older when CAD develops and have higher rates of functional impairment from coexisting medical problems, such as arthritis, at the time diagnostic studies are needed. Women also have a greater likelihood of a false-positive ETT related to baseline and exercise-induced ECG abnormalities. Several reasons have been proposed for these differences, including the digoxin-like effects of estrogen on the ST segment response¹⁵ and possible inappropriate catecholamine response to exercise.¹⁶ Therefore, the addition of imaging is often preferable, although complicated by smaller heart size and breast attenuation. Dobutamine stress echo in women is the only noninvasive test with similar sensitivity and specificity as in men.¹⁷

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Conclusion 

Cardiac stress testing is a useful noninvasive method to assess for CAD. Clinicians need to understand each modality so that the best test is ordered initially. They need to understand what will be expected of the patient with each type of test so that they can provide anticipatory guidance.

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References 

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