Echocardiography in Coronary Heart Disease 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,210 the application of this technique in ischemic heart disease continues to revolve primarily about the assessment of LV function. In addition, however, ongoing research with contrast agents has shown that echocardiography can be used to assess regional myocardial perfusion. 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 tissue Doppler tracings. 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.211 2D imaging, however, is 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. Therefore, in patients with CAD, standard echocardiographic approaches can be utilized to calculate LV diastolic and systolic volumes as well as ejection fraction. Digital echo analysis and 3D echocardiographic techniques can be used to enhance the accuracy of volume calculations and regional strain patterns.217 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, alterations in the acoustic properties of the myocardium (usually termed tissue characterization), and diminished regional bloodflow (as measured during the LV myocardial phase after intravenous echo contrast injection).213 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 ASE (Fig. 15–97).213 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 AMI214 and chronic coronary artery disease. When LV dysfunction is detected echocardiographically, the specific coronary artery responsible can often be inferred based upon the dyssynergy region(s).215 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.216 | | Figure 15–97. Sixteen-segment format for identification of left ventricular wall segments. Coronary arterial territories are also included. LAX = parasternal long axis; SAX PM = short axis at papillary muscle level; 4C = apical four-chamber; 2C = apical two-chamber; ANT = anterior; SEPT = septal; POST= posterior; LAT = lateral; INF = inferior. (From Segar D, Brown S, Sawada S, et al. Dobutamine stress echocardiography: Correlation with coronary lesion severity as determined by quantitative angiography. J Am Coll Cardiol 1992;19:1197. With permission.) |
Myocardial Infarction and Postinfarction Complications Cardiac ultrasound has achieved an important role in the evaluation of patients with AMI and is frequently used for diagnosis, quantitative functional assessment, risk stratification, and detection of complications214 (Chap. 52). Echocardiography is especially valuable in excluding transmural infarctions, as these are almost always associated with regional akinesis or dyskinesis (Figs. 15–98, 15–99, and 15–100).217,218 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.218 It may also help to select patients for thrombolytic therapy. In addition, patients without contractile abnormalities who ultimately exhibit signs of MI have a low incidence of complications.218 | | Figure 15–98. Diastolic (left) and systolic (right) images (apical two-chamber plane) from a patient with an inferior wall myocardial infarction. The inferobasal segment is dyskinetic (arrows). LV = left ventricle; LA = left atrium. |
| | Figure 15–99. Parasternal long-axis view of a large anteroseptal myocardial infarction, with thinning and dyskinesis of the anteroseptal wall (arrows). LV = left ventricle; LA = left atrium; AO = aorta. |
| | Figure 15–100. Apical four-chamber images of a large apical infarction. Diastole (D) is displayed on the left, systole (S) on the right. During systole, the base of the ventricle contracts, but the apex is dyskinetic (arrows). |
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.219 Similarly, calculation of the wall motion score has identified a cohort of post-MI patients at markedly increased risk for in-hospital complications.218 This prognostic marker appears superior to conventional clinical criteria in predicting events.218 Echocardiography is probably of greatest value in the assessment of complications associated with AMI. 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.220 By definition, postinfarction LV aneurysms are recognized as wide-mouthed, thin-walled myocardial segments that display dyskinetic expansion during systole. Aneurysms are a favored site for development of LV thrombi, which are covered 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. 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 formed221,222 (Fig. 15–101). 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.222 Since the risk of rupture is high, accurate diagnosis and prompt surgical repair of pseudoaneurysms is important. | | Figure 15–101. Modified apical four-chamber view of a large pseudoaneurysm (PAN) communicating with the left ventricle (LV). The rupture site is apparent (arrow); clot (C) is present within the aneurysm. (From Yucel G, Steinberg E, O'Reilly M, Kronzon I. Giant left ventricular pseudoaneurysm. Circulation 1996;94:848. With permission.) |
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.223 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 left and right ventricles (Fig. 15–102). 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. | | Figure 15–102. Modified apical four-chamber image of a distal septal ventricular septal rupture. With 2D imaging (left), the distal septum is incompletely visualized. With color Doppler imaging, however, a high-velocity aliased color jet is seen in the right ventricle (RV). In addition, an area of flow convergence is seen on the left ventricular (LV) side of the rupture (arrow). |
MR is a common sequela of AMI; 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. 15–103).224,225 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.225 | | Figure 15–103. Transverse four-chamber TEE image of a posterolateral infarction causing posterior papillary muscle ischemia and partial rupture. The posterior mitral leaflet (large arrow) is poorly supported (but not actually flail) and prolapses into the left atrium (LA). The basal lateral wall segment (small arrows) of the left ventricle (LV) is dyskinetic. |
In the setting of inferior wall infarction due to occlusion of the proximal right coronary artery, RV MI may occur. The most specific echocardiographic sign of RV infarction is a regional wall motion abnormality, which is usually best visualized in the RV free wall (Fig. 15–104). RV infarction is typically accompanied by RV enlargement and tricuspid regurgitation; associated inferior or posterior LV wall motion abnormalities are virtually always present. | | Figure 15–104. Diastolic (A) and systolic (B) subcostal four-chamber images of right ventricular (RV) myocardial infarction. The RV free wall is dyskinetic (arrows) during systole (B). |
Pericarditis is a common complication of AMI, typically occurring during the acute phase of the illness and much less often in the late phases as part of the 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 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 AMI. Several reports have demonstrated that LV systolic function assessed by 2D imaging improved within 24 h to 10 days of successful thrombolysis.226 More recently, contrast echocardiograms obtained after intravenous or 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.129,130,227 Stress Echocardiography The combination of stress testing and echocardiography (stress echocardiography) has assumed an important role in the diagnosis of CAD228 (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.229,230 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. 15–105). 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.215 | | Figure 15–105. A. Digitized parasternal views during diastole (left) and systole (right) from a normal individual. Upper panels: long-axis plane; lower panels: short-axis plane. B. Digitized apical views during diastole (left) and systole (right) from a normal individual. Upper panels: four-chamber plane; lower panels: two-chamber plane. C. Digitized parasternal long-axis views at peak systole before (left) and immediately after exercise (right). The anteroseptal wall moves normally at rest (arrows) but becomes dyskinetic with exercise. LV = left ventricle; LA = left atrium; AO = aorta. D. Digitized apical four-chamber views at peak systole before (left) and immediately after exercise (right). The apical septal, apical, and apical lateral walls become dyskinetic with exercise, suggesting inducible ischemia in the left anterior descending artery territory. LA = left atrium; LV = left ventricle. E. Digitized parasternal short-axis views (all recorded at peak systole) during dobutamine echocardiography in a patient with three-vessel coronary artery disease. At baseline (upper left panel), the left ventricular systolic function is normal. With low-dose dobutamine (5  g/kg/min, upper right panel), function improves. With 10 g/kg/min, however (lower left panel), function is similar to that at baseline. At 20 g/kg/min (lower right panel), systolic function deteriorates and the left ventricle dilates. This response suggests global ischemia induced by dobutamine infusion. |
The types of stress employed fall into two basic groups, exercise and pharmacologic.215 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).231 Treadmill testing involves a familiar activity, uses equipment that is widely available, and achieves 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 1- to 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.232 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, which increase myocardial oxygen demand and directly produce ischemia.232 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,229–234 and appears superior to PET and thallium scanning for the prediction of improvement in systolic function after coronary bypass surgery.235 The safety and accuracy of stress echocardiography for the diagnosis of myocardial ischemia has been examined in a number of studies.228,233–236 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 MV (SAM) with resultant LV outflow obstruction.228,237,238 In general, stress echocardiography and nuclear scintigraphy yield similar results, although stress echocardiography may be slightly less sensitive and slightly more specific than scintigraphy.229 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.229 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, LVH, or chronic digitalis administration).228,136,239 In this regard, stress echocardiography appears especially useful for detection of ischemia in women,240,241 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. Dobutamine echocardiography may aid in the detection of ischemia in patients with cardiac transplantation and allograft vasculopathy (chronic rejection).242 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. Stress echocardiography can also demonstrate resolution of regional ischemia after successful coronary artery bypass surgery or angioplasty.243 Stress echocardiography can play an important role in determining the prognosis of patients with CAD.244,245 Both exercise and pharmacologic stress echocardiography appear superior to exercise ECG for identification of patients at high risk of recurrent ischemic events after MI. In addition, dobutamine stress echocardiography is useful in predicting perioperative ischemic complications in patients undergoing noncardiac surgery244 and appears to have a very strong negative predictive value. In patients with chronic CHD, dobutamine stress echocardiography can identify hypokinetic yet viable myocardium and predicts improvement in function after successful revascularization.233,234 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.235,246,247 It is likely that this application of echocardiography will continue to evolve over time, particularly for pharmacologic stress testing (Chap. 58). In addition, quantitation of regional myocardial blood flow using intravenous echo contrast agents may enhance the usefulness of stress echocardiography in the detection of both ischemia and viability. There is evidence that exercise echocardiography can provide useful information regarding the hemodynamic status and functional severity of valvular heart disease.248 Specifically, stress echocardiography has been used to assess the degree of obstruction in patients with MS and to quantitate the severity of AS in patients with advanced LV dysfunction.248 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. |
