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 course98,99 and affords excellent access of the interrogating beam to these structures. Over the past 15 years, 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.100 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 LA, and (3) inferior to the heart (transgastric position) (Fig. 15–43). Figures 15–44, 15–45, 15–46, and 15–47 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.101
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.101,102 TEE is more accurate in detecting vegetations and abscesses than TTE101,103,104 and provides prognostic information as well104 (Fig. 15–48). In addition, TEE imaging may aid in accurate quantification of valvular disease (particularly MR) if TTE is inconclusive105 (Fig. 15–49). TEE is especially useful for Doppler interrogation of the pulmonary veins (Fig. 15–50). Flow patterns in these vessels reflect LA pressure, and systolic reversal of pulmonary venous flow has been identified as an accurate marker of MR.106,107 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 the severity of the regurgitation.108 Multiplane TEE can be used to planimeter the orifice area in AS.109 The technique is also quite helpful in detection of aortic disease, including dissection, aneurysm, congenital malformations, and atherosclerosis.99,110 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 (Fig. 15–51).99,111
Thromboemboli may originate from posterior cardiac structures such as the LA (LA) and appendage, interatrial septum, and aorta112,113; 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 (Fig. 15–52). TEE can also detect spontaneous contrast signals (that appear to represent transient rouleaux formation and predispose to thromboemboli). In addition, TEE has provided unique real-time images of mobile, pedunculated, atherosclerotic "debris" in the thoracic aorta (Fig. 15–53A and B). 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.113–115 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.116 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 thrombosis (Fig. 15–54).
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.117,118 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 patients and also can be used to monitor or guide interventional procedures, such as transseptal catheterization, mitral valvuloplasty, pericardiocentesis, and endomyocardial biopsy. Handheld Echocardiography Recently, advances in electronic technology have led to production of small, relatively lightweight (5 to 6 lb) echocardiography units. These handheld devices can be carried to the clinic exam room or hospital bed, thereby facilitating point-of-care echo evaluation by the physician. Although the quality of images from these scanners has improved steadily, it still does not equal that of state-of-the-art standard ultrasound instruments. In addition, handheld scanners have marginal or nonexistent spectral and color Doppler capabilities (this will likely change in the future). The appropriate use of these scanners is currently controversial, and recommendations will certainly evolve over time. Experts in the field have raised numerous questions about the merits of handheld echocardiography: there are concerns about the sonographic skills required for accurate diagnosis (as physicians—rather than sonographers—will likely perform most studies with these devices), legal ramifications of potential misdiagnosis, added time commitments during clinic visits, and overall quality control (Chap. 12).119 Several studies have shown benefits from handheld scanning in the detection of cardiac and aortic pathology,120 while others have shown a relative lack of utility, especially in critically ill patients. As the technical sophistication of these devices increases, they will likely become more useful in clinical practice. It is clear, however, that the sonographic skills of the person performing the study are critically important. To ensure adequate imaging competency, the ASE currently recommends that individuals performing handheld scanning have level 2 or 3 training in echocardiography.121 Many exams with handheld echo equipment are goal-directed and focused (rather than full, complete cardiac ultrasound studies), and this has spawned research into the arena of "targeted" or "limited" echocardiography.121 A wide spectrum of opinion exists in this area. On one extreme, proponents argue that all echocardiographic studies should be complete and follow a standard and inclusive protocol to avoid missing incidental findings. Conversely, other experts recommend an increased use of limited echo exams, as a proportion of complete echocardiographic studies currently performed may be clinically unnecessary and therefore cost-ineffective. This area is definitely in flux, but at present it may be best to view examinations with handheld echo devices as limited extensions of the stethoscope. Performed by a competent individual, the diagnostic capability of such an examination is at least equal to that of auscultation and may be significantly better (Chap. 12). In the future, examinations with this modality could be performed by various individuals, including physicians and even nursing staff, although medicolegal issues of clinical responsibility will likely play an important role in this evolution.122 |
