Number 29, 2006
Bypass surgery for coronary artery disease: a vanishing treatment?

Value of perioperative transesophageal echocardiography

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Otto Kamp
Department of Cardiology, Institute for Cardiovascular Research VU, VU University Medical Center, Amsterdam, The Netherlands
Correspondence: Dr Otto Kamp, Department of Cardiology, Institute for Cardiovascular Research VU, VU University Medical Center, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands.
Tel: +31 20 444 2244; fax: + 31 20 444 2446; e-mail: o.kamp@vumc.nl

Introduction
Transesophageal echocardiography (TEE) has evolved rapidly since its initial use in the 1970s [1,2] until, today, it is increasingly gaining popularity for the anatomical and hemodynamic evaluation of patients in the perioperative setting. Common indications for the technique are summarized in Table I. Stress TEE using atrial pacing, pharmacologic stress, or volume loading is potentially available for the patient treated with a respirator, for detection of myocardial ischemia and for evaluation of dynamic mitral regurgitation. Recent advances in ultrasound instrumentation and computer technology have led to dynamic 3-dimensional echocardiography, thus introducing a new era in cardiovascular imaging. Furthermore, evolution in the field of contrast echocardiography, using contrast agents capable of pulmonary passage, can further enhance diagnostic imaging and might bring myocardial contrast echocardiography to the operating room. Developments in quantification techniques, together with miniaturization of the probes, will allow continuous monitoring of the patient in the perioperative setting. Selection of the particular TEE examination should therefore be tailored to clinical circumstances.

Table I. Common indications for transesophageal echocardiography in the perioperative setting.

Perioperative monitoring of myocardial ischemia
Perioperative applications of TEE enable continuous monitoring of surgical procedures without disturbing the sterile field of work, thus allowing for the noninvasive evaluation of regional and global left ventricular function (Figure 1). In addition, the technique can accurately identify new regional wall motion abnormalities as markers of myocardial hypoperfusion before and immediately after bypass.

Assessment of native valve function and repair
Transesophageal echocardiography provides information of greatest value in mitral reconstructive surgery. The pathomorphology of the diseased valve and subvalvular apparatus can be clearly defined by TEE, which may have an important impact on the feasibility of surgery and the surgical strategy during operation. This is of importance in surgical correction of mitral regurgitation, because valve repair has been shown to be more beneficial than valve replacement, and to improve long-term survival.
Decisions based on TEE findings made at the time of operation may affect both early and late survival, and the need for reoperation or replacement of the valve during operation. In this respect, 3-dimensional TEE may become more and more important. 3-Dimensional echocardiography displays a bird's-eye view of the mitral valve, either looking down from the left atrium or looking up from the left ventricle. Thus 3-dimensional reconstruction allows spatial perception of all integrated components of the mitral valvular complex, such as the atrioventricular orifice and leaflets (Figure 2). In patients with mitral valve regurgitation, 3-dimensional echocardiography allows detailed visualization of mobility, the coaptation zone, the extent of prolapse, perforation, erosion, retraction, and restriction of valve leaflets [4–6].

Evaluation of endocarditis
Transesophageal echocardiography is abnormal in endocarditis, with the demonstration of either a vegetation-like mass (Figure 3) or a lesion that frequently or almost always results from the endocarditis process such as an abscess (echolucent space), leaflet perforation (Figure 4), fistula, prosthetic valve dehiscence, and paravalvular regurgitation. TEE is the most sensitive technique for the diagnosis of endocarditis, with a sensitivity of approximately 90%. Further diagnostic improvements might be obtained using 3-dimensional echocardiography, revealing complications of endocarditis by views in any plane, including the surgeon's view. The immense diagnostic potential of TEE, especially in the setting of infective endocarditis, has led to a revision of the diagnostic criteria to include echocardiographic findings as a major diagnostic criterion.

Investigation of cardiac masses
Transesophageal echocardiography has been shown to be superior to transthoracic echocardiography for visualization of left atrial thrombi, aortic atheroma and other cardiac thrombi, tumors (Figure 5), or vegetations, as possible sources of cardiac embolism. In this respect, the value of 3-dimensional echocardiography has yet to be determined, but is promising [7].

Stress transesophageal echocardiography
In patients in the perioperative setting, stress TEE using atrial pacing, dobutamine, or volume loading offers a valuable extension of its diagnostic ultrasound capabilities. In this regard, we evaluated the capability of simultaneous TEE and atrial pacing [8]. The technique had a high accuracy and diagnostic yield in assessing myocardial ischemia, multivessel disease, and ischemic mitral regurgitation [9]. Alternatively, dobutamine and dipyridamole may be used in TEE [8–11], see also Table II.

Table II. Sensitivity and specificity of stress transesophageal echocardiography for detection of coronary artery disease (ischemia) or myocardial viability.

Contrast-enhanced transesophageal echocardiography
The appearance of a myocardial ‘blush’ after contrast injections into the coronary arteries triggered the development of myocardial contrast echocardiography as a means of noninvasive assessment of myocardial perfusion [12]. Importantly, simultaneous assessment of regional anatomy and perfusion and of function, together with the ability to perform serial measurements with a high degree of spatial and temporal resolution became possible (Figure 6). With the advent of new and more stable (second-generation) transpulmonary contrast agents, in addition to important improvements in imaging techniques (such as second-harmonic imaging), visually detectable myocardial contrast can be produced via the intravenous route. Second-harmonic TEE probes are now available and are important for the assessment of myocardial contrast. Alternatively, intra-aortic (or intracoronary) contrast injections using a pigtail catheter in the operating room or critical care unit and second-harmonic TEE might potentially be useful to diagnose perfusion of specific myocardial areas in the setting of patients treated acutely with a respirator [13]. Absence of perfusion, defined as reduced opacification of myocardial segment(s) after a contrast injection, predicts morbidity (heart failure) and absence of viability in that area. In a perioperative setting, myocardial contrast echocardiography has been found to be helpful in determining the adequacy of cardioplegia solution; it also identified a group of patients with significantly worse postoperative outcome in terms of need for inotropic support [14].

Monitoring global left ventricular function
Newly commercially available automatic border detection systems can be used in combination with TEE monitoring. Acoustic Quantification (Philips, Andover, Massachusetts, USA) uses the difference in backscatter between blood and myocardium, thus facilitating quantitative description of the endocardial boundaries [15]. During continuous monitoring, within a selected region of interest, several cross-sectional areas (using video frames: 30 frames per second) from a short-axis cross-sectional view of the left ventricle are provided, in addition to plots of derived variables such as the fractional area change and rate of change of area versus time (Figure 1). Limitations of this automatic border detection system are the facts that papillary muscles are presented outside the cross-sectional areas and that no epicardial border detection can be performed, and the technique is thus not helpful in estimating left ventricular wall thickness. However, a load-independent measure of contractility in real-time from TEE monitoring and peripheral pressure can be obtained [15]. Because Acoustic Quantification is a pixel-classification technique rather than a true contour detection, only overall blood area can be calculated, not regional wall motion. Using minimum-cost contour tracking with cardiac management systems (ECHO-CMS) (MEDIS Medical Imaging Systems, Leiden, The Netherlands), continuous, connective, smooth contours allow calculations of regional wall motion, volume estimations, and user corrections. However, at present, this Quantification technique needs an off-line analysis workstation and user interaction [16].
Both Acoustic Quantification and ECHO-CMS are operator dependent and require much practice. The use of intravenous contrast might provide more consistent and reproducible results in the future, enabling appropriate treatment to be initiated without delay.

Conclusion
Developments that are in progress will further influence the perioperative application of TEE. Stress TEE is a realistic option today, 3-dimensional TEE is already commercially available, and second-harmonic TEE has been introduced recently. Despite new developments in computed tomography and magnetic resonance imaging technologies, giving excellent spatial and temporal resolution, the relative low cost, speed, portability, and on-line results of TEE are much in favor of this technique in the management of patients in the perioperative setting. At present, TEE has a major impact on diagnosis in acute and subacute disease states in patients who are hemodynamically compromised. Further developments in automated border detection systems will allow continuous monitoring of the critical care patient, to guide the clinician to appropriate and more timely medical and surgical therapy.

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