Chapter 13: THE ECHOCARDIOGRAM TRANSESOPHAGEAL ECHOCARDIOGRAPHY
Transthoracic echocardiography (TTE) usually defines cardiac anatomy and function satisfactorily, often obviating the need for further cardiac imaging. Occasionally, however, TTE does not provide complete or adequately detailed information. This is especially true in the evaluation of posterior cardiac structures (e.g., the LA, the left atrial appendage, the interatrial septum, the aorta distal to the root), in the assessment of prosthetic cardiac valves, and in the delineation of cardiac structures less than 3 mm in size (e.g., small vegetations or thrombi). Ultrasonic imaging from the esophagus is uniquely suited to these situations, as the esophagus is adjacent to the LA and the thoracic aorta for much of its course138,139 and affords excellent access of the interrogating beam to these structures.
Over the past decade, a number of technologic advances have occurred in the field of transesophageal echocardiography (TEE), and flexible transesophageal ultrasound probes capable of multiplanar imaging of the heart are now widely available.140-142 The current generation of probes also provide full pulsed-wave, CW, and CFD capabilities.
Although images can be recorded from a variety of probe positions most authorities recommend three basic positions: (1) posterior to the base of the heart, (2) posterior to the left atrium, and (3) inferior to the heart (transgastric position; Fig. 13-43). Figures 13-44 through 13-47 show TEE images obtained in various planes through the heart. It must be emphasized that, with the transducer in the esophagus, posterior structures appear at the top of the image. With the transducer in the stomach, a short-axis view is standardly obtained, with long-axis and apical views available to a variable degree. Upon withdrawing the transducer to the esophagus, one usually obtains apical-equivalent four-chamber and long-axis views, with multiple intermediate projections. Further withdrawal of the probe to the base yields excellent views of the atria, great vessels and semilunar valves, and pulmonary veins. Of particular value are views that delineate the LA appendage, all three leaflets of the aortic valve in short axis, and the transverse and descending aorta.!43
TEE has become an important imaging modality for the diagnosis and management of infective endocarditis and its complications, including valvular vegetations, chordal rupture, fistulas, perivalvular abscesses, and mycotic aneurysms.143-148 TEE is more accurate in detecting vegetations and abscesses than TTE143,149,150 and provides prognostic information as well150 (0+;B; Fig. 13-48). In addition, TEE imaging may aid in accurate quantification of valvular disease (particularly mitral regurgitation) if TTE is inconclusive!^! Fig. 13-49, Plate 57).
TEE is especially useful for Doppler interrogation of the pulmonary veins (B+;0í Fig. 13-50, Plate 58). Flow patterns in these vessels reflect LA pressure, and systolic reversal of pulmonary venous flow has been identified as an accurate marker of mitral regurgitation.152,153 Although mitral regurgitant color jets are easier to see with TEE than TTE, they are usually larger, and care must be exercised not to overestimate severity of the regurgitation.154 Multiplane TEE can be used to planimeter the orifice area in AS.155,155a The technique is also quite helpful in detection of aortic disease, including dissection, aneurysm, congenital malformations, and atherosclerosis.139,156,157 Because of its portability, accuracy, and short preparation and procedural times, TEE is now recommended as the preferred diagnostic study in many cases of suspected aortic dissection (B-hB; Fig. 13-51, Plate 59).139458
Thromboemboli may originate from posterior cardiac structures such as the left atrium (LA) and appendage, interatrial septum, and aorta159-168; therefore, TEE has received wide application in the evaluation of possible cardiogenic embolization. Since the most common site of LA thrombi is the appendage, the ability of TEE to visualize this structure is of particular value (B+-0- Fig. 1352). TEE can also detect spontaneous contrast signals (that appear to represent transient rouleaux formation and predispose to thromboemboli).169 In addition, TEE has provided unique real-time images of mobile, pedunculated, atherosclerotic "debris" in the thoracic aorta (BnBi Fig. 13-53). Although the optimal therapy for this disorder is currently unknown, warfarin may be helpful and mobile or protruding aortic atheromas appear to be significant risk factors for embolic events.167,168,170,171171a-171b The optimal role for TEE in the detection of intracardiac sources of emboli is controversial, and clinical trials are ongoing to evaluate the effect of treatment after discovery of potential embolic sources.
One of the proven applications of TEE is the evaluation of prosthetic valve dysfunction, particularly mechanical valves in the mitral position.172-174 Since the materials used in artificial valves are strong reflectors and often cause ultrasonic shadowing, the areas behind prosthetic valves are usually hidden from view when transthoracic imaging is used. Because of its unique window on the heart, TEE is clearly superior to TTE imaging for detection of prosthetic regurgitation, infection, tissue ingrowth, and thrombosis172'174 (B-hE; Fig. 13-54).
TEE has also become an important intraoperative tool for the detection of cardiac ischemia, the evaluation of valve function after repair or replacement, and the delineation of congenital heart disease.175-183 Cardiac surgeons often request intraoperative TEE for evaluation of cardiac anatomy and confirmation of a success of surgical repair before closing the chest. In this regard, TEE has almost completely replaced epicardial echocardiography. When TEE images are inadequate, TEE is helpful in managing critically ill patients184-187 and also can be used to monitor or guide interventional procedures, such as transseptal catheterization,185-190 mitral valvuloplasty, pericardiocentesis, and endomyocardial biopsy.191
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.