CHAPTER 94 Stress Echocardiography

Although one of every three deaths (more than half a million deaths a year) in the United States is caused by coronary artery disease (CAD), which is the leading cause of death in both genders, we are doing a better job at keeping these patients alive. Consequently, primary care clinicians will continue to manage an increasing number of patients with known CAD. Not only is stress echocardiography (“stress echo”) a safe, cost-effective, and noninvasive method for diagnosing CAD, but it is also helpful for managing CAD, including determining the prognosis. Some clinicians also use stress echo to screen high-risk asymptomatic individuals or to stratify risk in preoperative evaluations. With medical management of CAD more successful than ever, primary care clinicians’ skills in diagnosing and managing CAD have become more important. Performing stress echo is one method of maximizing these skills.

With improved ultrasound technology and image quality, adequate imaging can now be produced from 85% to 90% of patients undergoing stress echo. Improved digital imaging recording capabilities are also more likely to provide adequate pre- and postexercise stress echo images for comparison, side by side. The cost of echo equipment has also decreased significantly as the technology has expanded and improved. Consequently, echo has seen some of the most rapid growth among procedures performed by primary care clinicians.

Adding a cardiac imaging test to an exercise electrocardiography (ECG) test (EET) or a pharmacologic stress test is indicated when there is need to define the location, overall extent, or functional significance of CAD. Differing from an EET, which cannot predict which obstructed coronary artery is causing a positive test, stress echo is able to determine which coronary artery is obstructed by noting which ventricular wall becomes ischemic (Table 94-1). Although coronary angiography may not be the best method for determining the functional significance of CAD, it can be used as the gold standard when comparing the accuracy of nuclear myocardial perfusion imaging with stress echo. In the revascularized patient, several meta-analyses compared the accuracy of these two tests for detection of restenosis. Based on results showing very similar sensitivities, specificities, and accuracies for diagnosis of CAD, the choice of imaging for CAD after revascularization will likely depend upon clinician experience and other local factors, such as availability and expertise. Regarding prognosis for patients with CAD, several other meta-analyses compared stress echo with myocardial perfusion imaging and demonstrated comparable results. If echo images are excellent, stress echo may be more accurate than myocardial perfusion imaging.

TABLE 94-1 Left Ventricular Wall Segments and Corresponding Coronary Artery Supply

Echocardiographic Segment	Coronary Artery
Basal, mid-, and apical anterior	LAD
Basal, mid-, and apical anterior septum	LAD
Apical lateral	LAD
Basal and mid-anterolateral	LAD/LCA
Basal and mid-inferior	RCA
Basal and mid-inferior septum	RCA
Apical inferior	RCA/LAD
Basal and mid-inferolateral	LCA

LAD, left anterior descending artery; LCA, left circumflex artery; RCA, right coronary artery.

Further, when evaluating patients for cardiac problems, stress echo can diagnose other possible causes such as hypertrophic cardiomyopathy, aortic dissection, valvular heart disease, diastolic dysfunction, and pericardial effusion, which sets it apart from myocardial perfusion imaging. Stress echo is also faster to complete, is less expensive, and does not require irradiation.

In addition to greater convenience to the patient, benefits for primary care clinicians performing stress echo (especially in the office setting) include having test results immediately available, improving communication and referral patterns to cardiologists, and improving their echo reading skills. Clinicians performing stress echo also naturally improve their understanding of CAD pathophysiology as well as exercise physiology. With immediately available results, patient satisfaction is usually improved and liability for the clinician from failure to diagnose should be decreased.

Physiology of Stress Echocardiography

Normally, as the heart rate increases with exercise, the walls of the left ventricle increase contractility, increase endocardial excursion (>5 mm), become hyperdynamic, and thicken in systole. Depending upon the exercise protocol utilized, the ejection fraction will also normally increase. Consequently, the end-systolic volume, which is the actual size of the left ventricle at end-systole, decreases. In the patient with obstructive CAD, with increasing exercise, a threshold is eventually reached at which the heart’s demand for oxygen exceeds the supply. At this threshold, the heart becomes ischemic, first regionally or in segments, and then often globally. Although it is best to obtain echo images within the first minute of discontinuing exercise, this blunting of wall motion will typically persist for 3 to 5 minutes depending on the severity and duration of the preceding ischemia. Interpretation of a stress echo test consists of quantifying these transient regional wall motion abnormalities (Fig. 94-1) as well as the global function (i.e., ejection fraction/end-systolic volume). Transient ischemia is assessed and quantified by comparing pre- and postexercise echo images. (Depending on the protocol, images are also sometimes obtained at maximal exertion.)

Figure 94-1 Stress echocardiography test score form. CFX, circumflex coronary artery; DOB, date of birth; LAD, left anterior descending coronary artery; MVA’, mitral valve annulus velocity; MVE’, mitral inflow velocity; RCA, right coronary artery.

From an ECG perspective, with subendocardial ischemia, the patient usually demonstrates ST segment depression (i.e., a positive exercise EET; see Chapter 93, Exercise Electrocardiography [Stress] Testing) in several leads. Usually this depression is followed by chest discomfort (i.e., angina) as the “ischemic cascade” progresses. The term “ischemic cascade” describes the predictable sequence of events that occur after the onset of ischemia. With stress echo, earlier aspects of this ischemic cascade can be noted. Soon after the metabolic abnormalities are produced from ischemia, diastolic abnormalities appear, and these can be seen with stress echo. The appearance of these abnormalities is rapidly followed by myocardial perfusion defects and, in turn, by wall motion abnormalities again seen on stress echo. In other words, ischemic changes can be seen on perfusion imaging and stress echo prior to the development of ECG changes on EET. Eventually chest pain may develop, the exception being patients who experience so-called “silent angina” (typically patients >70 years of age, possibly earlier in diabetics). In patients with silent angina, dyspnea or dyspnea with exertion is a common presentation of an anginal equivalent.

Because exercise capacity is an important, separate predictor of outcome from a cardiovascular perspective, for those patients able to exercise, exercise echo is recommended over pharmacologic stress echo. For those able to walk on a treadmill, it is probably the preferred mode of exercise testing because the workload achieved as well as the maximal heart rate is usually higher than with a bicycle. The most common forms of exercise in the United States are walking and jogging, so patients are often more comfortable with a treadmill and it is possibly less effort- or motivation-dependent. However, the maximal blood pressure achieved is usually higher on a bicycle than with a treadmill, and images can be obtained during exercise and at peak workload with use of a supine bicycle. A bicycle is also often the preferred form of exercise for patients with orthopedic problems.

Indications

AUTHOR’S NOTE: The American College of Cardiology (ACC)/American Heart Association (AHA) suggest that for the patient capable of exercise with no baseline ECG abnormalities affecting the ability to monitor or interpret an EET (e.g, left bundle branch block [LBBB], digitalis effect, Wolff-Parkinson-White [WPW] syndrome, left ventricular hypertrophy [LVH], >1 mm ST segment depression), EET is the preferred method of evaluation to exclude CAD. Not every patient needs to be evaluated with imaging. Even in the patient with an intermediate pretest probability of disease, the ACC/AHA recommend a stepwise strategy for diagnosing CAD; in these patients, an EET is the preferred initial test. That said, there are plenty of indications for stress echo:

• Detection of CAD.

• Dyspnea possibly due to CAD.

• Equivocal, uninterpretable, or suspected false-positive EET.

• High probability of false-positive EET (e.g., women, mitral valve prolapse).

• Baseline ECG abnormalities that would affect the ability to monitor or interpret an EET (e.g., LBBB, digitalis effect, WPW syndrome, LVH, >1 mm ST segment depression).

• New-onset atrial fibrillation or heart failure, especially if there is an intermediate pretest probability of CAD.

• Need to define extent, location, or functional significance of CAD.

• Risk stratification/prognostic assessment in patient with known CAD (preoperative, post–myocardial infarction).

AUTHOR’S NOTE: Dobutamine echo is probably the preferred method of evaluating a patient preoperatively for high-risk or vascular surgery. See Box 94-1 for shortcuts to determine indications for noninvasive testing prior to noncardiac surgery. (See also Chapter 230, Preoperative Evaluation.)

• Risk stratification after coronary revascularization (percutaneous coronary intervention or coronary artery bypass graft) procedures.

• Viability assessment in candidate for revascularization.

• Radionuclide perfusion imaging not available.

• High probability of false-positive radionuclide perfusion imaging (e.g., correction software package not available for perfusion imaging when likely to have attenuation artifact due to nearby adipose tissue such as in women or large-chested men).

• Asymptomatic patients with multiple risk factors for CAD, or high-risk coronary calcium score from electron beam computed tomography scan (EBCT) or on ambulatory ECG (Holter or event) monitoring (see Chapter 87, Ambulatory Electrocardiography: Holter and Event Monitoring).

• Incomplete EET (failure to achieve 85% of predicted maximal heart rate; pharmacologic testing may be indicated).

• A pharmacologic agent may be indicated with stress echo (dobutamine, arbutamine preferred for stress echo over vasodilator infusion; vasodilator infusion usually preferred for myocardial perfusion imaging) in patients unable to exercise (e.g., peripheral artery disease, orthopedic problems, low exercise tolerance, contraindications to exercise [acute coronary syndrome with negative serial cardiac markers, equivocal aortic stenosis with low cardiac output]). Vasodilator (e.g., adenosine, dipyridamole, regadenoson) infusion produces only mild to moderate increases in heart rate and a mild decrease in blood pressure.

• Contrast study may be indicated with stress echo if two or more segments are not seen on noncontrast images.

• Permanent or transesophageal pacing may be indicated with stress echo (the pacing rate can be increased until the desired heart rate is achieved; however, these techniques are beyond the scope of this chapter).

• Mitral and aortic valve stenosis and regurgitation can be assessed, usually involving color Doppler (these techniques are beyond the scope of this chapter).

• Careful assessment of aortic stenosis by noting rapid change in pre– and post–aortic valve peak pressures.

Box 94-1 Shortcuts to Determine Indicators for Noninvasive Testing before Noncardiac Surgery

From ACC/AHA: 2007 Guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery. Circulation 116:1971–1996, 2007.

Conversely, if vascular surgery is planned and patient has three or more clinical risk factors (CAD, CHF, ASCVD, diabetes, or renal insufficiency), noninvasive testing is reasonable if the results will change the management.

Also, if the answer is Yes to both of the following two questions, noninvasive testing is reasonable *:

1 Poor functional/exercise capacity by questionnaire or specific questioning? (<4 METs or inability to climb one flight of stairs)

2 Clinical risk factor present? (CAD, CHF, ASCVD, diabetes, or renal insufficiency)

ASCVD, atherosclerotic cardiovascular disease; CABG, coronary artery bypass graft; CAD, coronary artery disease; CHF, congestive heart failure; METs, metabolic equivalents; PCI, percutaneous coronary intervention.

^* For intermediate- or high-risk surgery with only one or two clinical predictors present, there is insufficient evidence to determine the best management—β-blockade therapy versus noninvasive testing.

Results of a comprehensive, expert survey are available regarding the appropriate use of stress echo. The most common and appropriate indications for primary care clinicians include diagnosing CAD (especially when EET is uninterpretable or equivocal or there is a high risk of a false-positive EET or myocardial perfusion imaging test) and managing CAD (especially when it is necessary to define the extent, location, or functional significance of CAD such as in the revascularized patient). These guidelines also cover special situations in which stress echo is useful (e.g., dyspnea, pulmonary hypertension, valvular stenosis, new-onset atrial fibrillation, or heart failure), especially if combined with intermediate probability of CAD. The use of stress echo often helps sort out whether the symptom or problem is coming from CAD or another source. As previously discussed, clinicians should recall that by the age of 70 (possibly earlier for diabetics), the most common symptom for presentation of an acute myocardial infarction (MI) is dyspnea as opposed to chest pain. This is the so-called “silent angina” or anginal equivalent that patients often develop resulting in dyspnea with exertion.

Indications for Diabetics

Because of a disproportionate burden of CAD in diabetic patients, the American Diabetes Association has suggested indications for cardiac testing in diabetic patients. Although these indications have varied over the years, two indications have persisted:

1 Typical or atypical cardiac symptoms

2 Abnormal resting ECG (especially if suggestive of ischemia or myocardial infarction)

Many diabetic patients are unable to undergo an EET due to peripheral artery disease or neuropathy. Such patients are generally at higher risk for cardiovascular events than those able to undergo an EET. Dobutamine stress echo has been shown to provide independent prognostic information in this group.

Determination of Pretest Probability

Determining pretest probability may help the clinician decide whether stress echo is the indicated and proper diagnostic procedure. Stress echo is probably not indicated in patients with a low pretest probability of CAD (<10%) if they are able to exercise and have an interpretable electrocardiograph (i.e., absence of baseline ECG abnormalities such as LBBB, digitalis effect, WPW syndrome, LVH, >1 mm ST segment depression).

Pretest probability can be estimated by a description of the chest pain and the patient’s gender and age. Typical angina is described as substernal, exertional, and relieved by rest or nitroglycerine. Chest discomfort with two of these three characteristics is atypical angina. Chest discomfort with only one of these characteristics is considered nonanginal chest pain. Using these three descriptions, pretest probability tables and their corresponding graphs (see Table 93-1 and Fig. 93-4 in Chapter 93, Exercise Electrocardiography [Stress] Testing) can be readily applied to determine pretest probability. Determining pretest probability may also be helpful when assessing prognosis.

Contraindications

• Unable to obtain adequate resting echo images

• Very recent acute MI (within 2 days) or other acute cardiac event (submaximal testing may be indicated unless patient has undergone angioplasty or stent placement)

• High-risk unstable angina

• Severe symptomatic left ventricular dysfunction or uncontrolled symptomatic heart failure

• Potentially life-threatening or uncontrolled ventricular arrhythmias causing symptoms or hemodynamic compromise

• Third-degree atrioventricular block or second-degree Mobitz type II block without pacemaker

• Acute pericarditis, myocarditis, or endocarditis

• Severe aortic stenosis, hypertrophic cardiomyopathy, or other form of outflow obstruction (can be excluded by performing echo prior to test)

• Suspected dissecting aneurysm

• Severe arterial hypertension (resting >200 mm Hg systolic or >115 mm Hg diastolic)

• Electrolyte abnormality

• Acute pulmonary edema, embolus, or infarction

• Acute thrombophlebitis, deep vein thrombosis, or intracardiac thrombi

• Acute or serious general illness or infection

• Neuromuscular, musculoskeletal, or arthritic condition that precludes exercise (pharmacologic testing may be an option)

• Uncontrolled metabolic disease, such as diabetes, thyrotoxicosis, or myxedema

• Medication intoxication from drugs such as digoxin, sedatives, or psychotropic agents

• Advanced or complicated pregnancy

• Patient inability or lack of desire or motivation to perform the test, including severe emotional distress

• Nonavailability of advanced cardiac life support (ACLS) equipment or of an individual certified to perform ACLS

NOTE: Some of these contraindications are relative. In selected cases, a skilled cardiologist may perform testing for patients with these diagnoses (generally in a referral center). All are contraindications to testing in the office.

Equipment

• For best images, an ultrasound machine with several probes of varying frequency should be available. Among these probes, a low frequency (e.g., 2.5 MHz) probe with two-dimensional (2D), M-mode, and cardiac Doppler capabilities is necessary. An ultrasound machine with harmonic imaging ensures high-quality images. A method of recording the examination (e.g., server, DVD, VCR) is also needed for documentation.

• Ultrasonic jelly and towels to clean up after scanning.

• Patient gown.

• A treadmill with adjustable speed and grade (see Fig. 93-11 in Chapter 93, Exercise Electrocardiography [Stress] Testing)—this is the most common equipment used for stress echo. Advantages include the ability to test most patients under the actual physiologic conditions of exercise. Disadvantages include the fact that the treadmill may be difficult to use for patients with lower-extremity or lower-back problems or for patients who are very obese. The equipment is also more expensive, causes more motion artifact, and is noisier than a bicycle ergometer.

• Bicycle ergometers (see Fig. 93-8 in Chapter 93, Exercise Electrocardiography [Stress] Testing) use adjustable resistance and pedal frequency to exert the patient. Advantages with the bicycle ergometer include the capability to terminate the test instantly. If a bicycle ergometer is utilized, images can be obtained at peak workload, especially if a supine bicycle ergometer is utilized. These images are especially useful for evaluating diastolic and valvular dysfunction, especially with use of color Doppler. Many patients feel more secure sitting on the bicycle. This method is also associated with less artifact, and blood pressure (BP) measurements are easier to obtain. Unfortunately, in the United States leg fatigue is common because most patients do not bike. In fact, as a result of leg fatigue the procedure often fails to determine VO_{2 max}. Bicycle ergometry is also dependent on motivation throughout its duration. As a result, in the United States if the patient can tolerate the treadmill, most clinicians prefer to use it.

• Arm ergometer (see Fig. 93-9 in Chapter 93, Exercise Electrocardiography [Stress] Testing) enables patients with severe orthopedic problems to be tested. However, muscle fatigue often occurs before the maximum heart rate is achieved.

• ECG machine (see Fig. 93-10 in Chapter 93, Exercise Electrocardiography [Stress] Testing)—A continuous monitor is needed (a three-channel model with continuous tracing and a screen-freeze option is desirable) as well as a 12-lead ECG recorder. With modern equipment, the recorder also runs the treadmill. Most equipment is now digital, allowing data to be filtered to provide a smooth baseline.

• Electrodes, cables, and belt—A disposable or washable belt is preferred because patients usually sweat.

• Sphygmomanometer, including various cuff sizes—A gauge manometer is adequate because the most important readings are those relative to resting pressures (not the absolute pressures).

• Stethoscope.

• Razor, rubbing alcohol.

• Intravenous (IV) setup and medications for pharmacologic testing, if planned, including atropine.

• Emergency equipment (see Fig. 220-4 in Chapter 220, Anaphylaxis) includes a monitor/defibrillator; oxygen; airways, intubation, and suction equipment; and an emergency drug kit containing IV fluids and tubing (available drugs should be able to support ACLS protocols).

All emergency equipment should be checked daily, and medications should be checked weekly to monthly, depending on their use. The exercise testing equipment should be inspected and calibrated periodically, based on manufacturer recommendations. ACLS certification cards should be kept on file along with any other information or written protocols.

NOTE: It is also helpful to have a trained technician assisting (see Fig. 93-11 in Chapter 93, Exercise Electrocardiography [Stress] Testing). Technician certification for exercise testing is available through the American College of Sports Medicine. In many centers the technician prepares the patient; monitors the electrocardiograph and the patient’s response to exercise, his or her heart rate, and BP; obtains the pre- and postexercise echo images; and prepares the results for interpretation. For low-risk patients, the technician may actually perform the entire study without a physician being present. Otherwise, the clinician should examine the patient before, during, and after the procedure; confirm which protocol to use; and terminate the study. The clinician should also monitor the ECG tracing when the technician is taking BP readings, and the clinician should interpret the final results.