Transesophageal Doppler Echocardiography

Editorial resolution and has less beam spread. The reason for the slow acceptance of transesophageal imaging in the United States is severalfold. First, higher frequency transducers for transesophaIn this issue of the Proceedings (pages 649 to 680), geal echocardiography have only recently beSeward and associates present a comprehensive come available, and the addition of color Doppler summary of the technique, anatomic correla- enhances this technique further. tions, implementation, and clinical indications Second, cardiologists may feel intimidated by regarding transesophageal echocardiography. the gastroscopic approach, even though patient Although this technique has been used frequently acceptance has been uniformly good. Although for more than 5 years in cardiology practice in Seward and associates routinely use sedation for Germany and for several years as a research tool the technique, our experience and that of most in Japan, it has had a slow integration into of the German cardiologists suggest that intracardiology in the United States. In the past 1 or venous sedation is not routinely necessary. In 2 years, however, many US medical centers have fact, most German centers use no sedation for included transesophageal Doppler echocardiog- the procedure. We have used orally administered raphy as part of a routine procedure offered by diazepam (Valium) (2 to 5 mg) for inpatients and the noninvasive laboratory (along with standard no sedation for outpatients. Also, because no transthoracic Doppler echocardiography, contrast "drying agents" have been used in our laboraechocardiography, and exercise echocardiography). tory, an intravenous line is unnecessary in most The article by Seward and colleagues discusses cases. One word of warning seems appropriate: the initial experience with this technique at the with intravenous use of midazolam hydrochloride Mayo Clinic. (Versed), administration of only 1 to 2 mg can Standard transthoracic ultrasonic examination cause respiratory depression in some patients. of the heart has provided increasingly better Proper training of the cardiologist in endoimages since it was introduced approximately 30 scopic procedures is important. As suggested in years ago. Although today's state-of-the-art equip- the article by Seward and co-workers, initial ment provides fairly good images in most pa- training can probably best be provided by a tients, the standard transthoracic approach is gastroenterologist. A proficient cardiologist can still limited in a few patients. The "far-field" later teach other cardiologists. Under no circumareas—that is, the two atria and atrioventricular stances is this a procedure for nonphysician valves—particularly are visualized suboptimally members of the noninvasive laboratory to underwith the standard apical approach. The low- take alone. Standardization of scan planes and frequency transducers (2.5 and 3.5 MHz) used normal values for two-dimensional measurement for transthoracic examinations have limited reso- of chamber sizes must be established—a task for lution. Also, the transthoracic approach is com- the American Society of Echocardiography. promised by interference from the chest wall and There is little question that transesophageal lungs. echocardiography with the currently available 5In contrast, transesophageal imaging is not MHz transducers provides superior image quality limited by chest wall interference or thoracic in comparison with the standard transthoracic attenuation, and higher frequency transducers approach. Apart from simply obtaining "pretty (typically, 5 MHz) can be used routinely. The pictures," we believe that this approach will be higher transducer frequency will produce better indicated in the following situations: (1) evaluation of prosthetic valves; (2) assessment of possibly infected cardiac valves; (3) evaluation of clots, masses, and tumors in the atria, in the left Address reprint requests to Dr. Ingela Schnittger, Division atrial appendage, or on or around the mitral, of Cardiology, Stanford University Medical Center C-248, Stan- tricuspid, or aortic valves; and (4) diagnosis of ford, CA 94305. aortic dissection.

Transesophageal Doppler Echocardiography

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Transesophageal echocardiography in our experience has been more accurate in the diagnosis of prosthetic valve dysfunction than standard transthoracic Doppler examination (unpublished data). The standard Doppler echocardiographic test may even be falsely negative in the diagnosis of prosthetic valve regurgitation. The reason for this result is probably the "shielding" that normally occurs because of sound attenuation by the prosthetic valve; thus, the chamber beyond the prosthetic valve cannot be well studied. This difficulty is especially evident with mechanical valves.1 Endocarditis has been notoriously difficult to diagnose by standard echocardiography. Transesophageal echocardiography can routinely take advantage of a higher frequency transducer and therefore provide a more precise outline of the cusp edges. Transesophageal echocardiography should improve the sensitivity and specificity for diagnosis of endocarditis by ultrasound. Currently, however, transesophageal echocardiography has limited scan planes, as discussed by Seward and colleagues. Any mass or abnormality in the far field of the standard echocardiogram (the atria and atrioventricular valves) is better examined by transesophageal echocardiography because of closer proximity to the transducer and better resolution. Because the left atrial appendage can be routinely visualized by this approach, the sensitivity and specificity in the detection of intracardiac sources of emboli probably will be substantially enhanced. The left ventricular apex is usually not well visualized by transesophageal echocardiography because it is in the far field of the image. Therefore, left ventricular thrombus usually is not detected better by this technique than by the standard transthoracic approach. The experience of Engberding and associates 2 and of Seward and co-workers suggests that transesophageal echocardiography has superb sensitivity and specificity in making the diagnosis of aortic dissection. It often can be more efficiently performed than computed tomography or magnetic resonance imaging. Also, transesophageal echocardiography necessitates no intravenous contrast agent, an important factor because these patients may have compromised renal perfusion. Therefore, transesophageal echocardiography may emerge as the procedure of choice in diagnosing aortic dissection.

EDITORIAL

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Identification of stenosis of the proximal left and right coronary arteries by using transesophageal Doppler echocardiography seems promising 3 and may be important in a limited number of patients. Transesophageal echocardiography has been recognized as an invaluable tool for monitoring of left ventricular volume and wall motion during anesthesia. In the anesthesia literature, 4 transesophageal echocardiography has been substantiated as a reliable procedure for detection of acute ischemia and is more sensitive than electrocardiography or pulmonary artery wedge pressure readings. Cardiac surgeons may find transesophageal Doppler echocardiography useful in evaluation of mitral valve repair. 5 The epicardial Doppler examination, however, offers several different scan planes and may provide a more comprehensive assessment of mitral valve repair than transesophageal echocardiography. Transesophageal Doppler echocardiography unequivocally provides additional information for the cardiologist, anesthesiologist, and cardiac surgeon and is definitely here to stay. We need to refine the indications for this study, establish standards for training of physicians, and define normal findings further. In the future, improvement in gastroscopic ultrasound design is likely to provide a long-axis scan plane, with new image planes to be defined. Smaller probes may uniformly abolish the need for intravenous sedation, and higher frequency transducers will further help to improve image quality. Ingela Schnittger, M.D. Richard L. Popp, M.D. Division of Cardiology Stanford University School of Medicine Stanford, California

REFERENCES 1. Sprecher DL, Adamick R, Adams D, Kisslo J: In vitro color flow and continuous wave Doppler ultrasound masking of flow by prosthetic valves. J Am Coll Cardiol 9:13061310,1987 2. Engberding R, Bender F, Grosse-Heitmeyer W, Most E, Müller US, Bramann HU, Schneider D: Identification of dissection or aneurysm of the descending thoracic aorta by conventional and transesophageal two-dimensional echocardiography. Am J Cardiol 59:717-719,1987

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3. Schnittger I, Nellesson U, Appleton C, Oppenheim G, Popp RL: Visualization of coronary arteries and detection of coronary blood flow using transesophageal echo/Doppler (abstract). J Am Coll Cardiol 11 (Suppl A):152A, 1988 4. Cahalan MK, Litt L, Botvinick EH, Schiller NB: Advances in noninvasive cardiovascular imaging: implication for the anesthesiologist. Anesthesiology 66:356-372,1987

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5. Bolger A, Czer LS, Friedman A, Uleiman J, DeRobertis M, Chaux A, Maurer G: Intraoperative transesophageal color Doppler imaging: advantages and limitations (abstract). J Am Coll Cardiol 11 (Suppl A):217A, 1988