Ian R. Thomson, MD
St. Boniface General Hospital and University of
Manitoba
Objectives
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
Questions
Answers: 1(d); 2(a); 3(b); 4(d); 5(c)