Although originally of greatest value in valvular heart disease and cardiomyopathy, echocardiography has now become one of the most important techniques for the detection and quantitative assessment of myocardial ischemia and infarction. Cardiac ultrasound-because it is rapid, portable, noninvasive, and inexpensive-is especially well suited to the evaluation of ischemic heart disease. Although visualization of coronary artery structure and flow has been achieved by echocardiography,417-420 the application of this technique in ischemic heart disease continues to revolve primarily about the assessment of LV function.
Currently, the primary application of echocardiography in patients with coronary heart disease is based upon the detection of the effects of myocardial ischemia and/or infarction upon LV structure and function. Interruption of coronary flow or imposition of an oxygen demand that exceeds oxygen supply quickly leads to impaired systolic thickening and excursion of the affected myocardium. If flow is not restored and transmural infarction occurs, the affected myocardium may become akinetic or dyskinetic and eventually thinned and fibrotic. In addition, myocardial ischemia produces diastolic dysfunction, which may be detected by analysis of transmitral Doppler flow recordings or endocardial expansion profiles. These changes in the structure, contraction, and relaxation of myocardium are often readily detected by echocardiography.
The echocardiographic detection of myocardial ischemia was initially described using M-mode echocardiography, and this modality remains useful because of its excellent sensitivity and temporal resolution.421 2D imaging, however, has now become the primary technique for the examination of LV size, wall thickness, myocardial thickening, and regional wall motion, since it enables visualization of all LV wall segments. Thereby, in patients with CAD, standard echocardiographic approaches can be utilized to calculate LV diastolic and systolic volumes as well as ejection fraction.
The echocardiographic manifestations of CAD consist of one or more of the following: reduction in systolic thickening, abnormal segmental wall motion during systole or diastole, and alterations in the acoustic properties of the myocardium (usually termed tissue characterization).422 These abnormalities may be expressed as a disturbance in global LV size and function, an increase in LV volume, and a decrease in LVEF calculated by standard approaches. In addition, using the standard tomographic planes, the LV can be divided into 16 wall segments according to the format recommended by the American Society of Echocardiography Fig. 13-97) 423 By grading the contraction of each of the 16 segments as hyperkinetic, normal, hypokinetic, akinetic, or dyskinetic (and assigning a numerical value to each grade), a semiquantitative wall motion score can be calculated as the mean numerical value for all segments. Wall motion scores of this kind have been used to assess prognosis in both acute myocardial infarction424 and chronic coronary artery disease.425 When LV dysfunction is detected echocardiographically, the specific coronary artery responsible can often be inferred based upon the dyssynergy region(s).426,427 The echocardiographic findings of akinesis with segmental myocardial thinning can also be used to distinguish CAD from dilated cardiomyopathy, which typically manifests global hypokinesis and decreased wall thickness. There is overlap in the echocardiographic findings between these two groups, however, as severe ischemic disease may cause global hypokinesis and nonischemic cardiomyopathy may sometimes cause heterogeneous dysfunction.428 Myocardial Infarction and Postinfarction Complications
Cardiac ultrasound has achieved an important role in the evaluation of patients with acute myocardial infarction (MI) and is frequently used for diagnosis, quantitative functional assessment, risk stratification, and detection of complications424429-432 (see also Chap. 47). Echocardiography is especially valuable in excluding transmural infarctions, as these are almost always associated with regional akinesis or dyskinesis (&H0; Figs. 13-98, 13-99 and 13-100).433,434 Non-Q-wave infarctions are more difficult to diagnose with certainty, however, as the echocardiogram may show subtle regional hypokinesis or even normal wall motion in some cases. Thus, echocardiography has been used to evaluate chest pain in the emergency department and appears to have a reasonable sensitivity and specificity in the diagnosis of MI.433,434 It may also help select patients for thrombolytic therapy.435 In addition, patients without contractile abnormalities who ultimately exhibit signs of MI have a low incidence of complications.434
Echocardiography is now the most commonly utilized approach to assess the effects of MI upon LV function. Ultrasound imaging studies of LV remodeling have demonstrated that infarct expansion occurs commonly with anterior infarctions, often beginning within the first 10 days, and conveys an adverse prognosis.436,437 Similarly, calculation of the wall motion score has identified a cohort of post-MI patients at markedly increased risk for in-hospital complications.434 This prognostic marker appears superior to conventional clinical criteria in predicting events.434
Echocardiography is probably of greatest value in the assessment of complications associated with acute MI. Most such complications are quickly detected by echocardiography, and the fact that it is portable, rapid, and noninvasive render the technique extremely valuable in these circumstances. As indicated above, severe LV dysfunction resulting in advanced heart failure or shock can be readily identified by echocardiography. In addition, aneurysm formation is usually quite apparent in ultrasonic images.438 By definition, postinfarction LV aneurysms are recognized as wide-mouthed, thinned-walled myocardial segments that display dyskinetic expansion during systole. Aneurysms are a favored site for development of LV thrombi, which are discussed in detail in the discussion of cardiac masses, below. A less frequent complication is rupture of the LV free wall, which is usually rapidly fatal and therefore rarely imaged by echocardiography.439 However, the presence of significant pericardial effusion on echocardiography in patients with hemodynamic compromise in the postinfarction period should suggest this condition. If a free wall rupture is sealed off by clot and pericardial inflammation, a pseudoaneurysm is formed440,441 Fig. 13-101). This lesion is distinguished from a true aneurysm by its highly localized nature and the presence of a narrow neck connecting it with the ventricle. Pseudoaneurysms frequently have multilayered thrombi within them and exhibit characteristic Doppler flow signals at the junction with the ventricle.441 Since the risk of rupture is high, accurate diagnosis and prompt surgical repair of pseudoaneurysms is important.
Although postinfarction free wall rupture does not lend itself well to echocardiographic detection, acquired defects of the interventricular septum are more commonly delineated by cardiac ultrasound.442,443 Acquired ventricular septal defects often consist of a latticework of tissue rather than a discrete orifice, but nevertheless echocardiographic images can depict absence of myocardium and distinct flow jets communicating between the LV and RVs Fig. 13-102,
Plate 68L444 These color jets are typically high-velocity and aliased, coursing from the septum into the RV. The echocardiographic location of the defect and jet correlate well with the location by cineangiography, surgery, or autopsy, and an apical location is most amenable to surgical correction.444
MR is a common sequela of acute MI; if severe, it may result in profound congestive heart failure and shock. Several mechanisms may be responsible for the occurrence of postinfarction MR including dilation of the LV cavity and mitral annulus, papillary muscle dysfunction, and partial or complete rupture of a papillary muscle Fig. 13-103).445 447 MR from papillary dysfunction may lead to eccentric color jets within the LA. In general, the recognition and quantitation of MR occurring in the postinfarction period is no different from that of any other type of MR. Acute ischemic MR, however may cause a smaller flow disturbance by color Doppler than comparable grades of chronic MR, particularly with transthoracic imaging. Therefore, TEE may play an important role in the identification and quantitative assessment of this complication, as well as in ensuring adequate operative repair.447
In the setting of inferior wall infarction due to occlusion of the proximal right coronary artery, right ventricular MI may occur. The most specific echocardiographic sign of right ventricular infarction is a regional wall motion abnormality, which is usually best visualized in the RV free wall Fig. 13-104).448 RV infarction is typically accompanied by RV enlargement and tricuspid regurgitation; associated inferior or posterior left ventricular wall motion abnormalities are virtually always present.
Pericarditis is a common complication of acute MI, typically occurring during the acute phase of the illness and much less often in the late phases as part of Dressler syndrome. Postinfarction pericarditis, however, is not typically associated with marked echocardiographic abnormalities. If a pericardial effusion is present at all, the amount of fluid is usually quite small. Therefore, the absence of pericardial fluid on ECG cannot rule out pericarditis, and the presence of a large effusion with tamponade should raise the suspicion of a LV free wall rupture.
TEE has recently assumed a central role in the evaluation of patients with significant hemodynamic abnormalities in the postinfarction period. When TTE is technically suboptimal, transesophageal images can rapidly identify LV dyssynergy, valvular dysfunction, and other abnormalities associated with infarction. TEE may enable direct visualization of acquired ventricular septal defects when the lesion is not obvious or seen only as a disturbed flow stream in the RV with transthoracic imaging. Perhaps of greatest significance, TEE can provide definitive identification of a ruptured papillary muscle and a quantitative assessment of postinfarction mitral regurgitation.
Echocardiography has been used to evaluate the extent of reperfusion after thrombolytic or interventional therapy for acute MI. Several reports have demonstrated that LV systolic function assessed by 2D imaging improved within 24 h to 10 days of successful thrombolysis.449,450 More recently, contrast echocardiograms obtained by direct intracoronary injection have shown that reperfusion of the infarct-related epicardial coronary artery by angiography is not necessarily accompanied by evidence of normal flow in the downstream microcirculation. In addition, this "no-reflow" phenomenon on echocardiography heralds a poor prognosis, including failure of improvement of LV performance as well as increased late complications.208-210,4503
Recently, the combination of stress testing and echocardiography (stress echocardiography) has found an important role in the diagnosis of CAD451-453 (see also Chap. 42). The utility of this technique improved dramatically when technologic advances permitted side-by-side viewing of rest and stress images together in a cine-loop format.454 The application of stress echocardiography is based upon the concept that a stress-induced imbalance in the myocardial supply/demand ratio will produce regional ischemia and resultant abnormalities of regional contraction, which can be readily identified by echocardiography Fig. 13-105). The location of wall motion abnormalities may be used to predict the stenosed coronary vessel(s), while the ratio of dyssynergic to normal myocardium can provide a quantitative assessment of LV ischemia.426,427 Although the digital techniques currently employed limit the number of views available and restrict the examination to eight frames during systole, this process does not seem to impair the ability to identify contractile dysfunction.455,456
The types of stress employed fall into two basic groups, exercise and pharmacologic.426,427 Other forms, such as mental stress and atrial pacing, are not widely used. Exercise testing can be performed either on a treadmill or a stationary bicycle (either upright or supine).457 Treadmill testing involves a familiar activity, uses equipment that is widely available, and achieves a greater oxygen consumption than bicycle ergometry. Echo imaging usually can be accomplished only before and after treadmill exercise, however, whereas bicycle exertion facilitates the acquisition of images during the exercise protocol. Thus far, treadmill has been the preferred exercise modality. Of importance, all postexertional images should be obtained within a 2-min window following exercise to avoid recording normal contractile function after recovery from ischemia.
Pharmacologic stress has the advantages of reducing the motion artifact of exercise, enabling continuous imaging throughout the protocol, and assessing myocardial viability.458-469 Pharmacologic stress echocardiography can employ vasodilator agents such as dipyridamole or adenosine, which induce a heterogeneity of myocardial perfusion in ischemic heart disease, or inotropic agents such as dobutamine and arbutamine, which increase myocardial oxygen demand and directly produce ischemia.458-469 As with exercise stress, diagnostic criteria include induction of regional wall motion abnormalities and LV dilatation. It is important to recognize that the normal response to exercise is hyperkinesis, and wall motion abnormalities may take the form of a lesser degree of hyperkinesis of a given segment in comparison with the rest of the LV myocardium. Dobutamine stress echocardiography appears to be of particular value in detecting myocardial viability.455-463,466-469,469a,469b
The safety and accuracy of stress echocardiography for the diagnosis of myocardial ischemia has been examined in a number of studies.453,469-472 Both exercise and pharmacologic stress carry an extremely low risk of arrhythmia or infarction, although dobutamine can result in hypotension or systolic anterior motion of the mitral valve (SAM) with resultant LV outflow obstruction.453,473,474 In general, stress echocardiography and nuclear scintigraphy yield similar results, although stress echocardiography may be slightly less sensitive and slightly more specific than scintigraphy.455,464,475 In a study performed in an institution with high volumes and expertise in both ultrasound and radionuclide stress imaging, the two techniques were found to be comparable in their accuracy of detecting coronary artery disease.455
The most common clinical application of stress echocardiography is in the diagnosis of CAD, and it appears especially useful in cases where exercise electrocardiography (ECG) may be inaccurate or falsely positive (e.g., abnormal baseline ECG, LV hypertrophy, or chronic digitalis administration).453,472,476,477 In this regard, stress echocardiography appears especially useful for detection of ischemia in women,478,478a,478b in whom stress ECG yields a high incidence of false-positive results. Stress echocardiography also adds independent prognostic information to exercise ECG, even in multivessel CAD.479 Dobutamine echocardiography may aid in the detection of ischemia in patients with cardiac transplantation and allograft vasculopathy (chronic rejection).480 In patients with known CAD, exercise echocardiography may facilitate localization and quantitation of ischemia, guide revascularization procedures, and assess the functional severity of coronary artery stenoses.481 Stress echocardiography can also demonstrate resolution of regional ischemia after successful coronary artery bypass surgery or angioplasty.482-485
Stress echocardiography can play an important role in determining the prognosis of patients with CAD.486-494'494a-494b-494c Both exercise and pharmacologic stress echocardiography appear superior to exercise ECG for identification of patients at high risk of recurrent ischemic events after MI.486-491 In addition, dobutamine stress echocardiography is useful in predicting perioperative ischemic complications in patients undergoing noncardiac surgery and appears to have a very strong negative predictive value.492-494
In patients with chronic CHD, dobutamine stress echocardiography can identify hypokinetic yet viable myocardium and predicts improvement in function after successful revascularization.457-463,467,469a,469b,470 Functional improvement in a hypokinetic segment with low-dose dobutamine infusion which then progresses to hypokinesis or akinesis with higher dobutamine dose (the so-called biphasic response) correlates well with the presence of ischemic yet viable ("hibernating") myocardium. Studies have suggested that dobutamine stress echocardiography compares well with positron emission tomography and thallium single-photon emission computed tomography (SPECT) imaging in this regard.466-468,495-505,501a,501b It is likely that this application of echocardiography will continue to evolve over time, particularly for pharmacologic stress testing (Chap. 48).
There is evidence that exercise echocardiography can provide useful information regarding the hemodynamic status and functional severity of valvular heart disease.502-505 Specifically, stress echocardiography has been used to assess the degree of obstruction in patients with MS503 and to quantitate the severity of AS in patients with advanced LV dysfunction.505 These data may help guide the timing of surgical valve repair or replacement.
As is true of all diagnostic modalities, stress echocardiography has certain limitations. High-quality ultrasound images may be difficult to acquire in some patients-a situation that may be exacerbated by exertion and the time constraints inherent to exercise stress testing. In addition, considerable expertise is required to interpret stress echocardiographic images accurately, and this learning curve precludes the use of stress echocardiography by all but experienced echocardiographers. Nevertheless, stress echocardiography has many advantages over alternate diagnostic approaches such as radionuclide scintigraphy and coronary angiography, including its noninvasive and relatively inexpensive nature, rapid acquisition and interpretation times, and freedom from ionizing radiation. Harmonic imaging (both with and without intravenous echocardiographic contrast) has also enhanced endocardial border definition, facilitating stress echo studies in many patients with suboptimal fundamental (nonharmonic) echo images.Therefore it is anticipated that the use of stress echocardiography will continue to increase in the foreseeable future.
PREVIOUS | NEXT
Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20
Copyright ©2001-2002 The McGraw-Hill Companies. All rights reserved.
For further information about this site contact tech [email protected].
Last modified: August 10, 2002 .
Was this article helpful?
When it comes to pieces of aerobic equipment to buy, treadmills continue to be the most popular and the most valuable. And why not? They are simple to use and naturally intuitive. They burn calories effectively and offer a wide range of exercise options, whether walking, climbing, or jogging.