Coronary Artery Disease

8


Coronary Artery Disease




Evaluation of patients with suspected or documented coronary artery disease is one of the most common indications for echocardiography. Evaluation typically focuses on functional changes due to coronary artery narrowing or occlusion—specifically systolic wall thickening and endocardial motion—rather than on direct imaging of the coronary arteries. Although the proximal left main and right coronary arteries often can be identified, even on transthoracic (TTE) images, ultrasound imaging currently does not provide the detailed knowledge of distal vessel anatomy or the location and severity of coronary artery narrowing that is needed for patient management. Invasive coronary angiography remains the procedure of choice for direct assessment of coronary artery anatomy, because diagnosis often is combined with interventional treatment. Computed tomographic (CT) coronary angiography provides an alternative approach in some clinical situations.


However, echocardiography offers detailed functional assessment of segmental and global left ventricular (LV) systolic function both at rest and with stressors to induce ischemia. Functional assessment provides critical data for patient management. For example, stress echocardiography is a reliable approach for the initial diagnosis of coronary artery disease, especially in patients with a nondiagnostic stress electrocardiogram (ECG). Another example is the use of echocardiography in the emergency department for early diagnosis of acute myocardial infarction in patients with equivocal ECG changes. In addition, the central role of echocardiography in evaluation for complications of acute myocardial infarction has long been recognized. Finally, echocardiography often provides important prognostic data in patients with coronary artery disease.



Basic Principles



Coronary Artery Anatomy


Coronary anatomy varies to some degree from patient to patient, but the overall pattern of coronary artery branching is relatively uniform (Fig. 8-1). The left main coronary artery arises from the superior aspect of the left coronary sinus of Valsalva and divides into (1) the left anterior descending (LAD) artery, which extends via the interventricular groove down the anterior wall to (and sometimes around) the LV apex and (2) the circumflex (Cx) artery, which continues laterally in the atrioventricular groove. The right coronary artery (RCA) arises from the superior aspect of the right coronary sinus of Valsalva and extends inferomedially following the atrioventricular groove. Approximately 80% of patients have a right-dominant coronary circulation; the right coronary artery gives rise to the posterior descending artery (PDA), which lies in the inferior interventricular groove. In about 20% of patients the coronary circulation is left dominant; the circumflex artery gives rise to the posterior descending artery.



Segmental wall motion abnormalities seen by echocardiography correspond closely with the coronary artery blood supply to the myocardium (Fig. 8-2). The left anterior descending artery supplies the anterior portion of the interventricular septum via septal perforating branches and the anterior wall via diagonal branches. The posterior descending artery supplies the inferior aspect of the ventricular septum and the inferior free wall. The lateral wall is supplied by obtuse marginal branches of the circumflex artery. The posterior LV wall may be supplied by extension branches from the right coronary artery or by obtuse marginal branches of the circumflex arteries. There is marked individual variability in the blood supply to the LV apex. In some cases, the left anterior descending artery extends around the apex to supply the apical segment of the inferior wall. In other cases, the posterior descending artery extends around the apex to supply the apical segment of the anterior wall. More commonly, the blood supply to the apex arises from both the left anterior descending and the posterior descending coronary arteries.



Standardized nomenclature for tomographic imaging of the heart allows consistency of terms, correlations between different imaging techniques, and a known relationship of each segment to coronary anatomy. Standard tomographic planes are short-axis, vertical long-axis (equivalent to echocardiographic two-chamber plane), and horizontal long-axis (equivalent to echocardiographic four-chamber plane). In each image plane, the LV is divided into three segments from base to apex—basal, mid-cavity, and apical—to correspond to proximal, middle, and apical lesions of the coronary arteries (Fig. 8-3). In a short-axis view, both at the basal (mitral valve) and mid-cavity (or papillary muscle) levels, the ventricle is divided clockwise, beginning with the interventricular groove, into six segments: the anterior wall, the anterolateral wall, the posterior (or inferolateral) wall, the inferior wall, the inferior septal wall, and the anterior septal wall. The apical region is divided into four segments because of the normal tapering of the ventricle toward the apex: anterior, lateral, inferior, and septal, with an additional segment for the tip of the apex. This results in a total of 17 myocardial segments. The location of wall motion abnormalities can be reported in two-dimensional (2D) or three-dimensional (3D) formats, descriptively using colors to signify wall motion abnormalities or using more quantitative display formats.



The segmental wall motion abnormalities seen with ischemia or infarction correspond to the coronary anatomy as follows:



Depending on the degree to which diagonal branches supply the lateral wall, the anterolateral wall also may be affected. If the left anterior descending artery extends around the apex, the affected area includes apical segments of the inferior and inferolateral walls. The location of the lesion along the length of the coronary artery affects the pattern of wall motion. A lesion in the distal third of the vessel affects only the apex, a lesion in the mid-segment of the vessel affects the midcavity and apical segments, while a proximal lesion affects the entire wall including the basal segments.



Again the extent of segmental wall motion is related to the exact coronary anatomy in an individual patient. Echocardiography is particularly helpful in patients with circumflex disease, because this myocardial region often is “silent” electrocardiographically and is not well seen on a single-plane right anterior oblique LV angiogram.



If the posterior descending artery is a short vessel, the apex will not be affected, while extensive wall motion abnormalities of the apex may be seen if the posterior descending artery extends to supply the ventricular apex.


Other patterns of abnormal wall motion are seen with lesions of the branches of the three major coronary arteries. For example, isolated disease in a diagonal branch of the left anterior descending artery results in a discrete wall motion abnormality in the portion of the anterolateral wall supplied by that vessel. Proximal right coronary artery disease can result in ischemia or infarction of the right ventricular (RV) free wall.


Collateral vessels and previous coronary stenting or bypass surgery also affect the pattern of wall motion. If a myocardial segment has a balanced oxygen demand-to-supply ratio, wall motion will be normal whether blood flow is supplied antegrade by the native vessel, by collateral vessels, or by a bypass graft.



Evaluation of Left Ventricular Wall Motion



Transthoracic Imaging


Regional systolic function for each segment of the LV can be assessed on TTE imaging by combining data from multiple image planes or using 3D echocardiography to obtain multiple simultaneous 2D views or a 3D volumetric LV image set. With 2D imaging, standard views for evaluation of wall motion are (Fig. 8-4):




In the parasternal long-axis view, the basal and mid-ventricular segments of the anterior septum and posterior LV walls are seen. In the parasternal short-axis view, circumferential images of the LV at the base and mid-ventricular levels are obtained. Note that if the transducer is angulated toward the apex from a fixed parasternal position, progressively more apical segments of the posterior wall are imaged while the same segment of the septum is included in the ultrasound image plane. A more parallel alignment between image planes may be obtained by moving the transducer apically to obtain short-axis, mid-cavity, and (sometimes) apical views of the LV (Fig. 8-5) or by using 3D imaging. The apical segments rarely are adequately visualized from the parasternal window or on transesophageal (TEE) images.



From the apical window, evaluation of LV wall motion is performed in four-chamber, two-chamber, and long-axis views. Detailed evaluation of the extent of an abnormal myocardium is possible on 2D imaging by slow rotation of the image plane between the standard views. In the four-chamber view, the inferior septum and anterolateral wall are seen. Anterior angulation to include the aortic valve allows visualization of portions of the anterior septum. In the two-chamber view, the anterior and inferior free walls are seen. Endocardial and epicardial definition of the anterior wall may be difficult because of attenuation by adjacent overlying lung tissue. This problem can be alleviated by careful patient positioning and imaging during held respiration. In the apical long-axis view, the anterior septum and inferolateral (posterior) wall are seen (analogous to the parasternal long-axis view). Care is needed in positioning the transducer at the apex to avoid foreshortening of the ventricle from this approach. Integration of data from parasternal and apical approaches, taking into account image quality in each view, allows assessment of each myocardial segment in at least two views. Evaluation from a subcostal approach also may be helpful. In the subcostal four-chamber view, the inferior septum and anterolateral wall are imaged. In the subcostal short-axis view, the inferior and inferolateral (posterior) walls are nearest the transducer and the anterior and anterolateral walls most distal (Fig. 8-6).



Real-time 3D multiplane or volumetric imaging from an apical approach has the potential to provide more rapid and complete assessment of wall motion (Fig. 8-7). Simultaneous apical views at set angles of rotation or multiple parallel short-axis views can be generated from the 3D volume set, allowing rapid evaluation of wall motion in multiple myocardial segments on the same cardiac cycles. A limitation of apical volumetric scans is that the endocardium is imaged using the lateral, rather than axial, resolution of the ultrasound beam, which may limit identification of endocardial borders for quantitative analysis. When endocardial definition is suboptimal with either 2D or 3D imaging, opacification of the LV with left-sided contrast is recommended for evaluation or regional function (Fig. 8-8).





Transesophageal Imaging


When TTE images are inadequate, or in certain monitoring situations (e.g., intraoperative monitoring of LV function), regional LV function can be evaluated from a TEE approach. From the high left atrial (LA) position a four-chamber view of the LV is obtained (in the 0° plane of the TEE probe) showing the inferior septum and lateral wall. Rotating the image plane to approximately 60° provides a two-chamber view with visualization of the anterior and inferior walls, while further rotation to about 120° results in a long-axis view with imaging of the anterior septum and inferolateral (posterior) wall, although the exact degree of rotation to obtain these views varies slightly from patient to patient. Transducer position and angulation may require adjustment as the image plane is rotated using anatomic landmarks to ensure proper alignment. Even with optimal technique these views may be foreshortened; that is, the apparent apex represents an oblique plane through the anterolateral wall, while the true LV apex is not seen.


From the transgastric position, the transverse image plane provides short-axis views of the LV at the base (mitral valve) and mid-cavity (papillary muscle) levels. Rotation of the image plane at this position provides a two-chamber view, although the apex may be foreshortened. Further advancement of the probe may allow acquisition of an “apical” four-chamber view (in the 0° plane) by flexing the probe tip. Often the true apex is missed in this view because the LV apex does not lie on the diaphragm without intervening lung tissue in a position accessible from the transgastric approach.


TEE imaging of LV wall motion is indicated:



TEE imaging also may be used with stress protocols, although this is not a routine approach.



Sequence of Events in Ischemia


Irreversible myocardial damage (e.g., infarction) results in wall motion abnormalities that are present at rest. With an acute infarction, wall thickness is normal, but systolic wall thickening and endocardial motion are reduced or absent. An old myocardial infarction is characterized by thinning and increased echogenicity of the affected segments due to scarring and fibrosis, in addition to abnormal motion and absent wall thickening.


In contrast, ischemia is a reversible imbalance in the myocardial oxygen demand-to-supply ratio. Even with substantial coronary artery narrowing, blood flow is adequate for myocardial oxygen demands at rest. However, when the narrowing exceeds approximately 70% of the luminal cross-sectional area, blood flow becomes inadequate to meet increased myocardial oxygen demands with exercise, pharmacologic interventions, or mental stress, resulting in ischemia. When oxygen demand returns to baseline, blood flow again is adequate, ischemia resolves, and wall motion returns to normal. Thus, wall motion at rest is normal in patients with coronary artery disease if there has been no prior myocardial infarction.


The sequence of changes as a region of myocardium becomes ischemic is as follows (Fig. 8-9). The first detectable changes associated with heterogeneity of flow to the LV are biochemical followed by a significant perfusion defect (detectable by radionuclide and magnetic resonance techniques). Next, regional myocardial dysfunction, characterized by both abnormal diastolic function and impaired systolic wall thickening, occurs in rapid succession (within a few cardiac cycles). Ischemic ST-segment depression on ECG and clinical angina are relatively late manifestations of ischemia and are not seen consistently. Echocardiography, by detecting abnormal regional wall motion, provides a useful noninvasive method for evaluating ischemia that is more sensitive than electrocardiography given this sequence of events. Echocardiography differs from radionuclide techniques in that the functional consequences of ischemia, rather than the pattern of myocardial perfusion, are assessed.




Evaluation of Global and Regional Ventricular Function


Global LV systolic function can be evaluated either qualitatively or quantitatively in patients with coronary artery disease using the approaches described in Chapter 6. Because the pattern of LV dysfunction typically is not uniform, it is important that qualitative and quantitative evaluation be based on multiple tomographic views or 3D imaging. In patients with coronary artery disease, LV ejection fraction measurement provides essential clinical data because it is a crucial variable in clinical decision making.


Segmental (or regional) LV systolic function most often is evaluated using a semiquantitative scoring system based on 2D or 3D imaging. The endocardial motion for each defined myocardial segment is graded as normal, hypokinetic, akinetic, dyskinetic, or aneurysmal (Table 8-1). Some clinicians prefer to subclassify the degree of hypokinesis as mild, moderate, or severe, but such a subclassification often has significant interobserver and intraobserver variability. Ischemia results both in a decrease in the total amplitude and velocity of endocardial motion and wall thickening and in a delay in the onset of contraction and relaxation. Some centers use a numerical scoring system for wall motion from 1 (normal) to 4 (dyskinetic) for each segment. An overall wall motion score index can be derived by dividing the sum of scores for each segment by the number of segments evaluated:



image


Several more sophisticated approaches to quantitation of wall motion have been proposed based on the total amplitude of endocardial motion, the extent of wall thickening, the velocity of myocardial motion, or the timing of the onset of contraction. Quantitative evaluation of regional function by echocardiography requires:



Approaches that incorporate 3D reconstruction of the LV may improve data acquisition times and may diminish the effects of cardiac motion that potentially result in imaging different regions of the myocardium in systole versus diastole for a given tomographic plane. Although wall thickening or the timing and velocity of motion may be more sensitive methods for the evaluation of regional ventricular function, most current approaches continue to rely on endocardial motion. Other promising approaches include (1) contrast echocardiography to improve endocardial definition or evaluate myocardial perfusion and (2) tissue Doppler or speckle tracking strain and strain rate imaging (Fig. 8-10) (see Suggested Reading and Chapter 4).




Myocardial Ischemia


Because echocardiographic wall motion at rest is normal in a patient with significant coronary artery disease and no prior myocardial infarction, imaging during ischemia is needed for diagnosis. Inducing ischemia during echocardiographic imaging is referred to as stress echocardiography. Ischemia can be induced by increasing myocardial oxygen demand either with exercise or by pharmacologic interventions (Table 8-2).




Basic Principles of Stress Echocardiography


Stress echocardiography is based on the concept that an increased cardiac workload is needed to elicit signs of physiologic dysfunction in many types of cardiac disease. For example, in patients with coronary artery disease, resting myocardial blood flow is adequate so that myocardial function, seen on echocardiography as wall thickening and endocardial motion, is normal at rest. However, when cardiac workload is increased, the increased oxygen demands of the myocardium cannot be balanced by an increase in flow in the coronary artery, resulting in ischemia with impairment of myocardial thickening and endocardial motion (Fig. 8-11). An increase in cardiac workload typically is achieved by having the patient exercise, either on a supine bicycle or an upright treadmill, or by infusion of a pharmacologic agent, such as dobutamine, that increases heart rate (HR) and blood pressure (BP). In addition to echocardiographic imaging, key elements in interpretation of stress test results include:




The basic principles of image acquisition for stress echocardiography are to use standard image planes, ensure that all myocardial segments are visualized in at least one (and preferably two) views, use comparable views at rest and stress, and record images in a digital cine-loop format with side-by-side display of rest and stress images. The cine-loop format is essential because otherwise the change in heart rate between rest and stress makes interpretation of wall motion difficult.


For the evaluation of regional ventricular function, optimal endocardial definition is essential. When endocardial definition remains suboptimal despite careful patient positioning, use of harmonic imaging and other imaging adjustments, contrast echocardiography, or a nonechocardiographic imaging approach should be considered.


The sensitivity of stress echocardiography for the detection of coronary disease depends on acquiring stress images at the maximal cardiac workload. With pharmacologic stress testing, this is rarely an issue, because the stress level can be maintained until image acquisition is complete. However, with exercise stress, the workload declines rapidly on cessation of exercise, so images must be acquired as quickly as possible after exercise. Both the time from stopping exercise and the heart rate at the time of image acquisition compared to maximum heart rate are recorded as indicators of workload. Three-dimensional echocardiographic acquisition systems that allow simultaneous real-time imaging in multiple image planes offer the promise of faster acquisition times at peak stress, with the potential for improved diagnostic sensitivity (Fig. 8-12).


Jul 10, 2018 | Posted by in CARDIOLOGY | Comments Off on Coronary Artery Disease
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