Stress Echocardiography and Echo in Cardiopulmonary Testing

Exercise Protocols

Either exercise or pharmacologic stress agents can be used to increase myocardial oxygen demand. In general, exercise stress should be preferentially employed in any patient able to exercise given the wealth of prognostic and diagnostic information provided by functional capacity, heart rate response and recovery, blood pressure response, and electrocardiography. Symptom-limited exercise can be performed using a treadmill or cycle ergometer. In general, treadmill exercise is more widely available, allows for the attainment of greater maximal oxygen consumption (VO _2max ), and is more physiologic, but has the disadvantage of allowing for imaging only after exercise, which limits the number of images that can be acquired and fails to record echo parameters at peak exercise when hemodynamics are maximally affected ( Table 27.1 ). The semisupine cycle ergometer with a tilting table permits acquisition of images during each stage of the exercise protocol, including peak exercise. Initial workload and increases in workload are usually adjusted to each patient’s expected functional capacity (10–25 W increase every 2–3 minutes). However, in patients who are not used to the cycle ergometer, VO _2max is expected to be lower than with treadmill exercise. Compared to the cycle ergometer, treadmill tests tend to demonstrate 10%–15% higher VO _2max , 5%–20% higher peak heart rate, and more frequent ST segment changes. The contraindications for performing exercise echocardiography are the same as those for classical exercise testing.

A standard set of echocardiographic images is obtained in the resting state, prior to exercise initiation, and either immediately postexercise (treadmill testing) or at peak exercise (cycle ergometer testing). Standard imaging views include: (1) parasternal long axis view; (2) parasternal short axis at the left ventricle (LV) base; (3) parasternal short axis at the mid-ventricular level; (4) apical 4-chamber; (5) apical 2-chamber; and (6) apical 3-chamber views. For treadmill testing, imaging is performed with the patient in left lateral decubitus position preexercise and immediately postexercise. As with standard exercise testing, achieving a peak heart rate of at least 85% of age-predicted maximal heart rate is considered a diagnostic workload. As ischemia can rapidly resolve following the cessation of exercise, images should be acquired within 60 seconds of exercise termination. With cycle ergometer stress, imaging is typically performed at rest, submaximal exercise (∼25 W), peak exercise, and during recovery.

Pharmacologic Stress Protocols

Although either dobutamine or vasodilator agents can be used with echocardiography, dobutamine is the preferred and most commonly used agent for pharmacologic stress echocardiography. Dobutamine increases myocardial oxygen demand by increasing contractility primarily at lower doses and primarily heart rate at higher doses. In a standard dobutamine stress echocardiogram, dobutamine is infused at 5, 10, 20, 30, and 40 mcg/kg per minute, with the subsequent administration of atropine in 0.25–0.50 mg doses to a total of 2.0 mg to achieve a peak heart rate of 85% age-predicted maximal heart rate. Indications for test termination include (1) achievement of 85% of age-predicted maximal heart rate; (2) new or worsening wall motion abnormalities involving at least two segments; (3) significant arrhythmia; (4) hypotension; (5) severe hypertension; or (6) intolerable symptoms. Although rare, given the potential for serious risks, clinical judgment is essential in selecting patients appropriate for stress testing, as is careful monitoring by appropriately trained staff pre-, during, and posttesting. For dobutamine stress tests in particular, beta-blocking agents (e.g., metoprolol, esmolol) should be available if necessary to treat potential atrial or ventricular tachyarrhythmias, severe hypertension, or angina.

Image Interpretation

Regardless of the stress modality, interpretation of the echocardiographic images is based on assessment of the excursion and thickening of each myocardial segment at rest and with stress, along with changes in left ventricular ejection fraction (LVEF) and LV size with stress ( Table 27.2 ). American Society of Echocardiography (ASE) Guidelines recommend use of the 16-segment model (or 17 segments with inclusion of the apical cap if comparison with other imaging modalities is anticipated) to evaluate segmental motion ( Fig. 27.2 ). Wall motion is classified as: (1) normal [resting] or hyperkinetic [stress]; (2) hypokinetic defined as preservation of thickening and inward systolic endocardial excursion but not to the extent of normal segments; (3) akinetic defined as absence of wall thickening or inward systolic endocardial excursion; and (4) dyskinetic defined as wall thinning and outward motion of the myocardial segment in systole (see Table 27.2 ). The normal response to stress is for all segments to become hyperkinetic. Based on segmental wall motion at rest and with stress, each segment can be classified as normal, ischemic, infarcted, or viable ( Table 27.3 ). An ischemic response is characterized by worsening contractility of at least two contiguous segments ( Fig. 27.3 ). Infarction is characterized by resting dysfunction that fails to improve with stress. Using the 17-segment model, the Wall Motion Score Index (WMSI) is one method to quantify the global ventricular burden of ischemia and/or infarction. Segments are scored as 1 (normal [rest], hyperkinetic [stress]), 2 (hypokinetic), 3 (akinetic), and 4 (dyskinetic) at rest and at stress. The WMSI is calculated as the sum of segmental scores divided by the number of visualized segments. Moderate-severe ischemia is considered present when three or more newly dysfunctional segments are observed with stress. Additional potential etiologies for lack of a hyperkinetic response that must be considered during image interpretation include: (1) low workload including low heart rate secondary to beta-blocker use; (2) prolonged delay in image acquisition following test termination; and (3) severe hypertensive response to stress. Variability in image quality is the main limitation of stress echocardiography. The use of echo contrast to enhance endocardial border delineation in patients with poor acoustic windows can improve the diagnostic performance of the test and should be considered when two or more endocardial segments cannot be visualized at rest.

Changes in global LV function and size are also important for test interpretation. Normally, LVEF should increase and become hyperdynamic with stress. With the treadmill, exercise is normally accompanied by a decrease in LV diastolic and systolic volumes. Increase in LV volume with stress is a high-risk finding in this context, associated with multivessel ischemia ( Fig. 27.4 ). Of note, increase in LV cavity size is not necessarily an abnormal finding with supine cycle ergometry, given the associated preload recruitment.

The sensitivity and specificity of stress echocardiography for the detection of coronary artery disease is approximately 80%. Patients with an intermediate probability of coronary disease will benefit most from stress echocardiography. Diagnostic performance is superior to exercise electrocardiography alone, and similar to nuclear perfusion stress testing. Studies suggest the stress echocardiography has a slightly lower sensitivity but better specificity compared to nuclear perfusion stress testing. As noted previously, the risk of a false positive result is increased in the presence of abnormal septal motion due to left-bundle branch block and a hypertensive response to exercise, while low workload, beta-blocker use, and prolonged delay in poststress image acquisition increase the risk of a false-negative result.

In addition to providing diagnostic information, stress echocardiography also provides important prognostic information. In patients with suspected coronary artery disease across a range of pretest probability, stress echocardiography provides incremental prognostic value beyond clinical, electrocardiographic, and resting echocardiographic variables. A negative stress echocardiogram is associated with a rate of myocardial infarction or cardiac death similar to age-matched controls (<1%/year), suggesting that additional testing and intervention is unnecessary. Similarly, among patients with known coronary artery disease, including prior revascularization, an abnormal stress echocardiogram is independently associated with a twofold greater risk of adverse outcomes. Additional prognostic information is provided by the pharmacologic dose or exercise workload that elicits the ischemic response, the affected coronary territory (left anterior descending vs. left circumflex or right coronary), the presence of multivessel wall motion abnormalities, the peak WMSI, the LVEF and end-systolic volume changes during stress, and the time necessary for recovery of the stress-induced abnormalities.