Basic Transesophageal

Echocardiography

 

Ian R. Thomson, MD
St. Boniface General Hospital and University of Manitoba


Objectives

  1. To become familiar with the equipment used in transesophageal echocardiography (TEE), and to know the indications and contraindications to its use.
  2. To understand the conventions defining TEE imaging planes, and governing the display of two-dimensional and colour flow TEE images.
  3. To understand the components of, and techniques involved in, a basic TEE examination.
  4. To understand how TEE is used to monitor LV function and detect intraoperative myocardial ischemia.
  5. To review videotaped examples of clinically useful TEE images.

Equipment

A typical TEE probe consists of an ultrasound transducer, mounted at the tip of a flexible endoscope. The transducer contains piezoelectric crystals that convert electrical energy into an ultrasound beam. Conversely, reflected sound waves from the patient, are received by the transducer and transformed into an electrical signal. The probe is connected to a console that converts this electrical signal into a moving, two-dimensional (2D), video image of the heart. The console has controls that vary the characteristics of transmitted ultrasound, and the manner in which the returning signal is processed and displayed. It usually incorporates a video recorder and printer.

The handle of the probe has two knobs that flex the tip anteroposteriorly, or laterally. By advancing, withdrawing, rotating, or flexing the probe, it can be optimally positioned in the patient. There are three main types of TEE probe:
1) monoplane, 2) biplane, and 3) multiplane (omniplane). Monoplane probes have a single transducer that images in the transverse plane, at right angles to the probe (0 plane). Biplane probes incorporate a second transducer that images in the longitudinal plane (90 plane). The operator switches back and forth between these orthogonal imaging planes. Multiplane probes have a single transducer that rotates within a housing at the tip of the probe. This permits imaging in all planes between 0 and 180, although the transverse and longitudinal planes are used predominantly. Multiplane technology is the current standard for adult TEE.

Indications

TEE is useful whenever conventional trans-thoracic imaging is inadequate (e.g. emphysema, prosthetic a-v valves). Structures that are close to the esophagus, such as the left atrium, a-v valves, and descending aorta are particularly well visualized. Thus, atrial septal defects, left atrial tumours and thrombi, prosthetic mitral valve lesions, endocarditis, and aortic dissection are readily diagnosed. Perioperatively, TEE is useful for monitoring myocardial function, detecting ischemia, and in the differential diagnosis of postoperative hemodynamic instability (e.g. hypovolemia, vs. myocardial dysfunction or tamponade). Esophageal disease is the major contraindication to TEE, and a history of dysphagia should be sought prior to probe insertion. Although TEE has proved safe, it should only be undertaken by experienced operators when there is a clear indication for its use. In anesthetized patients the probe can be inserted blindly, or with laryngoscopic visualization. It is helpful to have an assistant support the probe during insertion. A bite-guard should always be used.

Image Display

Echocardiographic images are obtained by electronically sweeping the ultrasound beam through an arc. The resulting pie-shaped images correspond to the sector interrogated by the beam. The apex of the sector corresponds to the position of the transducer, and is conventionally displayed at the top of the video screen. Since the heart is anterior to the esophagus, posterior structures (e.g. left atrium) are usually displayed at the top of the screen. Anterior structures [e.g. apex of the left ventricle (LV)] are at the bottom. With transverse imaging, left-sided structures are displayed on the right side of the screen, as the operator faces it. With longitudinal imaging, cephalad structures are displayed on the right. With transverse imaging I imagine myself looking at transverse slices of a prone patient, while I sit at the head. With longitudinal imaging I imagine myself looking at longitudinal slices of a prone patient whose head is at my right.

Doppler Colour Flow Imaging

The Doppler effect refers to the shift in frequency that occurs when sound is reflected from a moving object (e.g. blood cells). This principle permits measurement of the velocity and direction of blood flow. Colour flow imaging uses this principle to colour code each pixel of a video image within within the heart and great vessels. Conventionally, blood flow away from the transducer is coded as blue, and flow towards the transducer in red. Greater brightness indicates higher velocity flow, with very high velocity or turbulent flow being appearing yellow or green. Colour flow technology is very useful for detecting jets of regurgitant flow from incompetent valves, or revealing shunt flow through an intracardiac defect.

The Basic Exam

TEE examination should be undertaken in a systematic manner, so that all relevant structures are visualized. This involves imaging in the transverse, longitudinal and off-axis planes. In transverse imaging, the principal probe movement is insertion and withdrawal. The "basal short-axis" view is obtained by inserting the TEE probe to the level of the great vessels. A cross-section of the ascending aorta, and aortic valve can be obtained at this level. The left ventricular outflow tract, coronary ostia, left atrial appendage, right ventricular outflow tract, pulmonary artery, and superior vena cava can also be seen with minor changes in probe position. Next, by inserting the probe to mid-atrial level, a "4-chamber" view is obtained. This shows both atria, the inter-atrial septum, the a-v valves, both ventricles, and the interventricular septum. Finally, the "trans-gastric short-axis" view is obtained by advancing the probe into the stomach and flexing it anteriorly. This provides a cross-sectional view in which the left ventricle appears doughnut-shaped. The inferior wall of the LV is closest to the probe, while the anterior wall is farthest away. The lateral and septal walls of the LV appear on the right and left sides of the image. The papillary muscles are seen within the cavity of the LV. The right ventricle is seen as a thin-walled, crescent-shaped structure to the left of the interventricular septum. By rotating the probe to the left, at any level, the descending aorta can be seen in cross-section. By advancing and withdrawing the probe, the entire descending aorta and aortic arch (in long-axis) can be visualized. A distal portion of the ascending aorta, anterior to the right main bronchus, cannot be seen. This is the TEE "blind spot".

In longitudinal imaging, the principal probe movement is rotation. Usually the probe is placed behind the left atrium, and rotated from left to right. On the right, the venae cava can be seen in long-axis, entering the right atrium. The interatrial septum is also seen here. Rotating the probe to the left, an oblique view of the ascending aorta is obtained. The right pulmonary artery is seen in cross-section, posterior to the aorta. Long- and short-axis views of aorta can be with obtained by flexing the probe to the right or left (biplane) or rotating the transducer off-axis (multiplane). On the left, a 2-chamber long-axis view of the left heart is obtained, with the left atrial appendage superior to the base of the LV. On the far left, a long-axis view of the descending thoracic aorta is obtained. Withdrawing the probe permits a cross-sectional view of the aortic arch to be obtained.

Multiplane imaging allows visualization of structures that are not well seen in conventional transverse or longitudinal views. A small motor in the handle of the probe, controlled by two buttons, rotates the multiplane transducer within its housing. A semi-circular display on the video monitor, indicates the current degree of transducer rotation (0-180), to orient the operator or viewer. For example, with multiplane technique, the left ventricular outflow tract, aortic valve, and ascending aorta can be seen in continuity (130), as can the right atrium, right ventricle, pulmonary valve, and pulmonary artery (40). Similarly, perfect cross-sectional images of the aortic valve can be obtained (20-40). Views of the interatrial septum, or left atrial appendage can be optimized.

Intraoperative Monitoring

Intraoperatively TEE may be used to monitor LV function and detect myocardial ischemia. Ejection fraction can be estimated by observing the fractional area change of the left ventricular during systole. Standard echocardiographic software allows the endocardium to be traced at end-diastole and end-systole, but experienced observers can estimate LV ejection fraction accurately without tracing these contours. The transgastric short-axis view at mid-papillary level is most commonly employed for ischemia detection, because myocardium from the territories of all three coronary arteries is represented. Presumably, this optimizes sensitivity for detection of ischemia, and provides insight into pathogenesis. The right coronary artery supplies the right ventricle, the inferior part of the interventricular septum, and the medial part of the inferior wall. The circumflex coronary artery supplies the lateral part of the inferior wall, and the inferior portion of the lateral wall. The left anterior descending coronary artery supplies the anterior wall, and the anterior portions of the septum and lateral wall. Care must be taken to position the probe at midpapillary level. In this view, the papillary muscles are seen in cross-section, just separated from the wall of the LV. If the probe is positioned too close to the base of the heart, chordal structures will be seen. At this level, septal wall motion cannot be accurately judged. If the probe is too close to the apex, the papillary muscles will merge with the wall of the LV, or even disappear. Not infrequently, a high-quality, stable trans-gastric image will not be obtainable. In this situation monitoring the four-chamber view, and switching intermittently to the long-axis, provides a satisfactory alternative.

Regional wall motion is assessed by observing thickening of the left ventricle in systole, and inward excursion of the myocardium. Normally all walls thicken symmetrically, and 30% radial excursion of the endocardium is seen. Abnormal wall motion is graded as hypokinetic, akinetic, or dyskinetic. With dyskinesia, paradoxical outward movement of the endocardium is observed in systole. New wall motion abnormalities, especially akinesis or dyskinesia, suggest ischemia.

Suggested Reading

  1. Bque F, Joffe D, Kleiman S. An introduction to transoesophageal echocardiography: I. Basic Principles. Can Anaesth J 1996;43:252-77
  2. Oxorn D, Edelist G, Smith MS. An introduction to transoesophageal echocardiography: II. Clinical applications. Can Anaesth J 1996;43:278-294

 


Questions

 

  1. The current standard technology for intraoperative adult echocardiographic imaging is:
    1. transthoracic
    2. monoplane
    3. biplane
    4. multiplane

  2. TEE would be least useful for diagnosis of:
    1. left ventricular apical thrombus
    2. left atrial myxoma
    3. type B aortic dissection
    4. prosthetic mitral valve endocarditis

  3. Conventionally, the longitudinal imaging plane is referred to as:
    1. 0 degrees
    2. 90 degrees
    3. 180 degrees
    4. none of the above

  4. With colour flow TEE imaging a regurgitant mitral jet would appear:
    1. blue
    2. pink
    3. purple
    4. red

  5. The conventional view for detection of intraoperative myocardial ischemia is the:
    1. two-chamber long-axis
    2. four-chamber view
    3. transgastric short-axis
    4. basal short-axis

 

 

Answers: 1(d); 2(a); 3(b); 4(d); 5(c)