|Year : 2007 | Volume
| Issue : 4 | Page : 324
Transesophageal echocardiography and anaesthesiologist
Thomas Koshy1, Bhupesh Kumar2, Prabhat Kumar Sinha3
1 MD, PDCC, Additional Professor, Department of Anaesthesiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
2 MD, DM Cardiac Anaesthesia Resident, Department of Anaesthesiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
3 MD, Associate Professor, Department of Anaesthesiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India
|Date of Web Publication||20-Mar-2010|
Sree Chitra Tirunal Institute for Medical Sciences & Technology, Trivandrum-695011, Kerala
Source of Support: None, Conflict of Interest: None
Keywords: Transesophageal echocardiography, Anesthesia
|How to cite this article:|
Koshy T, Kumar B, Sinha PK. Transesophageal echocardiography and anaesthesiologist. Indian J Anaesth 2007;51:324
| Introduction|| |
Echocardiography - the use of ultrasound to examine the heart- is a safe, powerful, non-invasive and painless technique. The introduction of transesophageal echocardiography (TEE) has provided a new acoustic window to the heart and mediastinum. Superior quality images of most cardiovascular structures can be obtained readily with an ultrasound probe in the esophagus. TEE has revolutionized cardiac anaesthesia and has also become increasingly popular in the management of patients in the intensive care unit (ICU) and during certain noncardiac surgical procedures as well.
Although clinical application of TEE is recent, transesophageal application of ultrasonography to assess cardiac function was first reported by Side and Gosling in 1971.  The early transesophageal probes did not attract clinicians because of the problems associated with swallowing of the probe in conscious patients and also due to the logistical difficulties of introducing a rigid endoscope. Transducers were subsequently mounted on flexible endoscopes, and DiMagno et al first reported the use of a linear phased array flexible endoscope in 1980.  In 1982, Souquet and co-workers, first produced a clinically usable flexible endoscope with a sector phased array two-dimensional echocardiographic transducer having a frequency of 2.25 MHz - a frequency equal to that of the standard precordial transducer.  This represented the definitive breakthrough for the transesophageal approach. Application of these probes took different directions in the United States and Europe. The cardiologists in Europe began using TEE to supplement the diagnosis of different cardiac pathologies. In the United States on the other hand, increasing number of anaesthesiologists started using TEE probes for monitoring patients in the operating rooms (OR). As the practice expanded it became clear that very few cardiologists were able to spend long period in the OR since cardiology has become much more invasive speciality in last 20 yrs. Also the cardiovascular physiology of anaesthetized patients is not similar to physiology of awake patients and anaesthesiologists with knowledge and understanding of this altered physiology is in better position to interpret the information obtained from intraoperative TEE. In current practice in the UK, 90 % of the TEEs are performed not by cardiologist but, by cardiac anaesthesiologist.  As we view the field of echocardiography in the 2000s, it is almost impossible to conceive of a state-of-the-art cardiac OR without TEE machines.
Several authors have described the usefulness of TEE on cardiac surgical practice. , Kneeshaw and coworkers had showed that in a group of 309 patients who underwent intraoperative TEE, management changed in 26% patients.  In 6% of patients the changes were in inotrope use and fluid management. However, in 20% of patients, TEE resulted in a change in the procedure carried out. Among noncardiac surgical procedures it has been found to be useful in management of neurosurgical patients for detection of air embolism and other major surgical areas where haemodynamic instability may occur. The role of TEE in ICU is also steadily increasing. It was found to be of particular value in managing haemodynamic instability in post cardiopulmonary bypass (CPB) period. Now it is being also used as rapid imaging modality in the general ICU as an adjunct to other monitoring modality to provide data in haemodynamically unstable patients. To summarize, the usefulness of TEE includes, augmentation of preoperative cardiac evaluation, planning of surgery, evaluation of surgical results and differential diagnosis of haemodynamic instability. TEE is associated with a long learning curve, because the complexities of transducer positioning, imaging sector alignment and three dimensional cardiac anatomies are not familiar to most beginners.
| Basic concepts|| |
Physics of ultrasound
In TEE, the heart and great vessels are insonated with ultrasound from probe inside the esophagus, which is sound above the human audible range (frequency above 20,000 Hz). Frequency within the range of human hearing (20-20,000 Hz) are referred to as audio frequencies, while those above this range are referred to as ultrasonic frequencies. Echocardiography machines commonly operate in the frequency range of 2-10 million cycles per second (2-10MHz). The ultrasound is partially reflected by the cardiovascular structures. From these reflections, distance, velocity and density of objects within the chest are derived.
Ultrasound waves are characterized by their wavelength, frequency, and velocity. Wavelength is the distance between the two nearest points of equal pressure or density in an ultrasound beam, and velocity is the speed at which the waves propagate through a medium. As the waves travel past any fixed point in an ultrasound beam, the pressure cycles regularly and continuously between a high and a low value. The number of cycles per second (Hertz) is called the frequency of the wave. Frequency (ƒ), wavelength (?) and velocity (v) are related by the following equation
The velocity of sound changes with the properties of the medium through which it travels. For soft tissues and blood, this approximates 1540 m/sec. Because the frequency of an ultrasound beam is determined by the properties of the emitting transducer and the velocity is a function of the tissues through which the sound waves travels, wavelengths vary according to the equation expressed above.
| Forming the image|| |
Interaction of ultrasound waves with tissues
The formation of an ultrasonic image is dependent upon wave reflections occurring at the interfaces between different media. The strength of reflection at an interface depends upon the difference in acoustic impedance between the two media. For example, a blood-fat interface produces a stronger reflection than a blood-muscle interface, because there is a greater difference in density between blood and fat than between blood and muscle.
When an ultrasound wave is propagated through a living tissue, it is partly absorbed, partly reflected, and partly scattered. Attenuation refers to the loss of ultrasound power as it traverses tissue. Ultrasound reflection, scattering and absorption are responsible for tissue attenuation. A high percentage of ultrasound is absorbed by the tissues and is converted to heat, the higher the frequency the greater the absorption. The greater the ultrasound reflection and scattering, the less ultrasound energy is available for penetration and resolution of deeper structures. Water, blood and muscle have low tissue impedance resulting in low ultrasound attenuation, whereas air and bone have very high tissue ultrasound impedance, limiting the ability of ultrasound to traverses these structures. Because sound waves travel through soft tissue at a constant velocity, the length of time for the ultrasound beam to be returned back to the transducer can be used to calculate the precise distance between the transducer and the object being interrogated.
| Ultrasonic transducers|| |
A transducer is a device that converts energy from one form into another. Ultrasonic transducers use piezoelectric crystals to emit and receive high-frequency sound waves. These transducers convert electrical energy from the ultrasound instrument into acoustic energy when transmitting, and they convert acoustic energy reflected from the tissues into electrical energy, that is used by the instrument to form the image.
A transducer with a mechanism to sweep the ultrasound beam automatically in a fan-like fashion is called a mechanical sector scanner. In a phased array scanner, the ultrasonic beam is formed and steered by firing an array of small closely spaced transducer elements in a sequence. A TEE probe typically has 64 elements.
| Imaging techniques|| |
This is the most basic form of ultrasound imaging. In this mode, the density and position of all tissues in the path of a narrow ultrasound beam (i.e., along a single line) are displayed [Figure 1]. M-mode is not currently used as a primary imaging mode because only a very limited part of the heart is being observed. However this mode is useful for the precise timing of events within the cardiac cycle. Because M-mode images are updated 1000 times per second, they provide greater temporal resolution than 2D echo, thus, more subtle changes in motion or dimension can be appreciated. The ultrasound signal should be aligned perpendicularly to the structure being examined. Finer analysis of valve motion or thickness and motion of cardiac chambers are best done with this mode.
Two-dimensional echo (2D Echo)
The acquired image which resembles an anatomic section of the heart is easily interpreted [Figure 2]. This 2D echo image ("live" real image) is obtained by rapid, repetitive scanning along many different radii within an area in the shape of a fan (sector). Information on structures and motion in the plane of a 2D scan is updated 30 to 60 times per second.
The principle of Doppler echo is based on the reflection of ultrasound by moving red blood cells. The reflected ultrasound has a frequency shift relative to the transmitted ultrasound, determined by the velocity and direction of blood flow. The Doppler effect is defined as the apparent change in the frequency of waves occurring when the source and observer are in motion relative to each other, with frequency increasing when the source and observer approach each other and decreasing when they move apart. To obtain sufficient signal strength and penetration for a good Doppler signal, higher ultrasonic intensity levels and lower frequencies are used than for 2-D imaging.
In all modern echo machines Doppler capabilities are coupled with 2D imaging facilities. After the desired view of the heart has been obtained by 2D echo, the Doppler beam, represented by a cursor, is superimposed on the 2D image. The echocardiographer positions the cursor as parallel as possible to the assumed direction of blood flow. In clinical practice, a deviation from parallel of up to 20 degrees can be allowed, because this only results in an error of 6% or less.
Doppler echo is used to quantify valvular stenosis, valvular regurgitation and it can also demonstrate intracardiac shunts. Doppler technology is usually used in three different ways to measure blood velocities: pulse wave, continuous wave and color flow.
Pulsed wave doppler
This facility allows a flow disturbance to be localized precisely or blood velocity from a small region to be measured like the mitral inflow into the left ventricle [Figure 3]. Echo machine uses a single crystal to transmit an ultrasound signal and then to receive after a pre-set time delay. There is a limit to the maximum velocity that can be accurately detected, before a phenomenon known as 'aliasing' occurs usually at velocities in excess of 2 m/s
Continuous wave doppler
In this mode two crystals are used-one transmits while other receives signal continuously. This technique is useful for measuring high velocities across restrictive shunts like ventricular septal defects or stenotic valves but its ability to localize a flow signal precisely is limited since the signal can originate at any point along the length or width of the ultrasound beam [Figure 4].
| Color flow Doppler|| |
Color flow Doppler uses pulsed-wave technology to measure blood flow velocity at multiple sites. Here real time blood flow is displayed within the heart as colors, while also showing 2D images in black and white. The velocities and directions of blood flow are color encoded. Color flow Doppler velocities away from the transducer are in blue, those towards it in red. Areas of high turbulence or regions of high flow acceleration (e.g. mitral regurgitation) have green color added to either red or blue [Figure 5].
| Equipment|| |
Transthoracic echocardiographic views are particularly difficult to obtain in patients with obesity, emphysema, or abnormal chest wall anatomy. This is because bone, fat and air containing lung interfere with ultrasonic penetration. TEE transducers were developed to avoid these problems. They are mounted on modified probe similar to those used for upper gastrointestinal endoscopy. Sound waves emitted from an esophageal transducer only have to pass through the esophageal wall and the pericardium to reach the heart, improving image quality and increasing the number of echocardiographic windows. Other advantages of TEE include the stability of the transducer position and the possibility of obtaining continuous recordings of cardiac activity for extended periods of time during surgery. Majority of TEE probes use ultrasound between 3.5 and 7 MHz. Two types of probes are available in the market [Figure 6]. The adult probe has a shaft length of 100 cm and a diameter of around 12 mm. This can be introduced in patients up to 20-25 Kg. The paediatric probe measures approximately 7 mm in diameter and the company recommends its use in patients weighing more than 4 Kg. All TEE probes have similar specifications including standard endoscope without optics or suction, ultrasound transducer, four way movable tip, pulsed, colored and continuous wave Doppler capabilities and multiplane transesophageal echocardiographic capabilities.
| Movements of the probe|| |
The TEE probe produces a 90 0 imaging sector which can be directed by a variety of maneuvers [Figure 7]. The shaft of the probe may be advanced into or withdrawn from the esophagus and turned to the right (clockwise) or to the left (anticlockwise). The tip of the probe may be anteflexed (anteriorly) or retroflexed (posteriorly) by rotating the large control wheel on the handle of the probe. Rotating the small control wheel flexes the tip of the probe to the left or to the right (lateral flexion). This facility (lateral flexion) may not be available in paediatric probe because of size limitation. All modern TEE probes are multiplane as compared to older biplane probes and the scanning plane can be rotated from 00 to 180 0 . At 00 the sector scan lies in the transverse image plane and runs perpendicular to the shaft of the probe. At 90 0 the sector scan lies in the longitudinal or vertical plane and runs parallel to the shaft of the probe. A transducer icon which indicates the degrees of imaging sector rotation is located at one corner of the image display. It allows tracking the degrees of forward or backward multiplane angle rotation.
| Preparation for TEE examination|| |
Anaesthesiologists may need to insert the TEE probe in awake or anaesthetized patients. In both scenarios, the probe needs to be well lubricated. One need to be very careful in patients with history of dysphagia, hematemesis, operations on GIT and cervical spine disease. Introduction of TEE probe into the esophagus in intubated patients under general anaesthesia may be difficult at times and alternative maneuvers are described in literature. An awake patient must be fasting for at least 4-6 hours before the procedure. Blood pressure and heart rate are measured. Dentures and oral prostheses should be removed. Airway, oxygen delivery system, bite guard, suction, standard crash cart should be immediately available. An intravenous access is generally established before TEE examination.
| Premedication|| |
Awake patients are usually premedicated for the following reasons:
Antibiotics: help prevent infective endocarditis in selected high-risk patients. The issue of endocarditis prophylaxis during TEE remains controversial. Since the procedure is similar to that of endoscopic examinations, there may be some merit to administering bacterial endocarditis prophylaxis. 
- Topical anaesthesia: of oropharynx and hard and soft palates diminishes gag reflex. It can be produced by an aerosol local anaesthetic like lidocaine solution or viscous lidocaine.
- Sedation: is carried out intravenously to decrease anxiety and discomfort, with administration of a sedative belonging to the benzodiazepines group (e.g. diazepam or midazolam).
- Drying agents: lessen salivary and gastrointestinal secretions reducing the risk of aspiration (e.g. glycopyrrolate)
| Technique of introduction|| |
The pharynx is anaesthetized with a topical anaesthetic spray. The patient is placed in the left lateral position and the neck slightly flexed to allow better oropharyngeal entry. Introduction of the probe can also be performed with the patient in the supine position and if necessary in the upright sitting position. A bite guard is essential to allow manipulation and protection of the TEE probe. Distal portion of the transducer is coated with lubricating jelly. The echographer passes the probe tip through the bite guard and over the tongue maintaining it in the midline. The tip is advanced until resistance is encountered, then the patient is asked to swallow and with gentle forward pressure the transducer is advanced into position behind the heart. When TEE procedure is over, the precautions that should be taken by the patient include not to drink any hot liquid until oropharyngeal anaesthesia has worn off (1-2 hours), not to eat until gag reflex returns (1-4 hours) and not to drive for 12 hours (if a sedative was given).
| Care of the TEE probe|| |
The TEE probe should be inspected for defects with the transducer tip in the neutral position and all flexed directions. These defects cause trauma or expose the patient to infective, caustic or electrical complications. After each procedure, flexible shaft of the TEE probe and the bite guard should be cleaned and disinfected. They are first washed with an enzymatic solution to remove saliva and secretions, then they are rinsed thoroughly with tap water and placed in a glutaraldehyde disinfectant solution such as cidex for twenty minutes - a period proved to be sufficient to destroy any viral or bacterial contaminants. Then they are rinsed thoroughly with tap water and allowed to dry for twenty minutes before use on another patient to allow any residual adherent glutaraldehyde to evaporate. 
| Standard image display|| |
It is imperative that an examiner be comfortable with imaging sector orientation and the resulting image display. These are key concepts. Mastering them will allow to predict the images which will result from the various probe manipulations and to display a desired cross-section.
The apex of the sector scan is shown at the top of the echo screen, which displays posterior cardiac structures (parts closer to the probe in the esophagus). In the transverse imaging plane (transducer at 0 0 ), the left of the image is towards patient's right, and the right of the image is towards the patient's left [Figure 8]. In the vertical image plane (90 0 ), the left side of the image is inferior and points towards the patient's feet and right side of the image is anterior and points towards the patient's head [Figure 9].
| Centering the image|| |
Once we centre a cardiac structure in one image plane, it will continue to remain there as the transducer is rotated from 0 0 to 180 0 facilitating the three-dimensional assessment of that particular structure. To centre a structure in the transverse imaging plane (0 0 rotation), the shaft of the probe should be turned to the left or to the right so that the structure of interest is aligned in the middle of the display. If the probe is in the vertical image plane (90 0 rotation) advancing or withdrawing the probe will achieve the same result.
| Standard views and systematic examination|| |
Patient details are usually entered and the machine controls are adjusted for optimal resolution before starting the examination. Images are collected at four depths. The depth of placement of TEE probe for adult patients are:
The American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists had published guidelines for comprehensive intraoperative multiplane TEE examination.  They recommended 20 standard images [Figure 10]. The details of each view is given in standard textbooks of cardiac anaesthesia.
- Upper esophageal (20-30cm)
- Mid esophageal (30-40cm)
- Transgastric (40-45cm)
- Deep transgastric (45-50cm)
The great majority of images are obtained at mid esophageal and transgastric levels. The goal is not to get all 20 views in all patients. The goal is to elucidate the structure and function of the heart and great vessels. The transducer is first moved into the desired depth, and then the probe is manipulated to orient the imaging plane to obtain the desired cross-sectional image. This is achieved by watching the image developed as the probe is manipulated, rather than by relying on the depth markers on the probe or the multiplane angle icon.
Mid esophageal views
The mid-esophageal (ME) views fall into two groups: ME aortic views and ME ventricular views.
Mid-esophageal aortic views
These views image the aortic valve and proximal ascending aorta. Six views are obtained at this level: ME aortic valve short axis (SAX), ME aortic valve long axis (LAX), ME right ventricle inflow-outflow view, ME bicaval view, ME ascending aorta SAX and ME ascending aorta LAX. The aortic root, aortic valve cusps, inter atrial septum, right ventricle, tricuspid valve, and the vena cavae are assessed with these views.
Mid-esophageal ventricular views
These views (four-chamber, two-chamber, commissural, and LAX) are important in the assessment of mitral valve, left ventricle, inter ventricular septum and both atrium. Progressive rotation of the transducer from the four-chamber view (0 0 ) to long-axis view (130 0 ) allows visualization of all segments of anterior and posterior mitral leaflets and a complete evaluation of left ventricular wall motion.
From the mid-esophageal position the TEE probe is further advanced into the stomach and anteflexed (to keep it apposed to the diaphragmatic surface of the stomach) to develop the transgastric (TG) views. The five views (TG mid SAX,TG two chamber, TG basal SAX, TG LAX, TG RV inflow) obtained at this level are useful in the assessment of the mitral valve and left and right ventricles. In particular the transgastric mid SAX view is very commonly used by the anaesthesiologists in the assessment of LV function, ejection fraction and volume status.
To obtain deep TG LAX view, the probe is advanced further into the stomach and then slowly withdrawn with tip sharply anteflexed until it contacts the diaphragmatic surface of stomach wall. This view shows all four cardiac chambers, aortic valve and the left ventricular out flow tract. Since the ultrasound beam is parallel to the blood flow through the aortic valve, this image is ideal for estimation of velocity through the AV and cardiac output.
Descending thoracic aorta, Aortic arch views
TEE can image both aortic arch and descending aorta with four standard views (Upper esophageal aortic arch SAX and LAX, Descending aortic SAX and LAX)
| Indications|| |
The American Society of Anesthesiologists (ASA) and the Societyof Cardiovascular Anesthesiologists (SCA) jointly published the practice guidelines for perioperative transesophageal echocardiography in 1996.  Indications for perioperative TEE were divided into three categories (I-III) based on the strength of evidence or expert opinion supporting a clinical benefit [Table 1].
Category I indication: supported by strongest evidence or expert opinion; TEE is frequently useful in improving clinical outcome.
Category II indication: supported by weaker evidence or expert opinion; TEE may be useful in improving clinical outcome.
Category III indication: little current scientific or expert support; TEE is infrequently useful in improving outcome.
| Emerging indications|| |
Many studies published after the task force report in 1996, emphasize the inclusion of many more indications for perioperative TEE. Should the task force revise the guidelines, the number of category I indications would almost certainly be increased.
- Assessment of ventricular function in high risk patients undergoing coronary artery surgery and patients with coronary artery disease or ventricular dysfunction undergoing non-cardiac surgery in general and major vascular surgery in particular. ,,
- During placement of left ventricular assist device as well as during weaning from temporary LV assistance. 
- Deployment of intravascular devices such as endoluminal stenting of thoracic aortic aneurysm and trans catheter closer of atrial septal defect. 
- Positioning of cannulae: facilitates correct placement of coronary sinus cannula, femoral venous cannula tip in the right atrium and with standard bicaval cannula, its relation with IVC and hepatic vein.
- As a substitute for the pulmonary artery catheter (PAC): TEE has been demonstrated to be superior to PAC for monitoring global and regional systolic LV function and for guiding fluid therapy. , In emergency, TEE probe can be inserted faster than PAC. Although TEE has certainly reduced the requirement of PAC, it is unlikely to replace it entirely. Some haemodynamic parameters are more easily made with a PAC (e.g. low systemic vascular resistance). In certain situations like reduced LV compliance, each of these monitors contributes information in making diagnosis. Furthermore, the PAC is more suitable for continuous monitoring especially in the ICU and also in awake patients.
| Safety and contraindications|| |
Although safe when properly conducted, in rare circumstances, TEE can cause serious and even fatal complications. ,,,, The complications from TEE can be related either to the probe itself or to the procedure. Probe related complications include thermal pressure injuries, compression of the structures adjacent to esophagus, or direct mechanical injuries (soft tissue damage in esophagus, hypopharynx or stomach). Procedure related complications include circulatory complications (hypertension, hypotension, arrhythmias) and pulmonary complications (laryngospasm, bronchospasm, hypoxia).
The following recommendations are made to ensure the continued safety of TEE. Prior to each insertion, the probe should be inspected for cleanliness and structural integrity. It should be inserted gently and if resistance is met, the procedure is aborted. The echo machine is kept in freeze (standby) mode to ultrasonic transmission when not in use. Finally, when not imaging the probe should be left in neutral, unlocked position to avoid prolonged pressure on the esophageal mucosa.
There are not many contraindications to the routine use of TEE. Absolute contraindications to TEE include perforated viscus, active upper gastrointestinal bleeding; esophageal stricture, tumors, diverticula, varices or scleroderma and recent upper gastrointestinal surgery.  Relative contraindications include atlantoaxial disease, prior irradiation to the chest and hiatal hernia.
| Assessment of haemodynamics|| |
One of the common indications for TEE in the ICU or OR is the assessment of LV or RV function in a patient with unexplained hypotension. It provides critical bedside information regarding the cause of hypotension. The usual causes of hypotension that may be diagnosed by TEE include LV or RV systolic dysfunction, hypovolemia, pericardial compression, valvular disorders, dynamic LVOT obstruction and ventricular septal rupture.
TEE allows the anaesthesiologist to obtain multiple haemodynamic parameters to guide in the care of the patient. Doppler effect allows the velocity of blood to be estimated, and the dimensions of cardiac structures can be obtained with 2D imaging. From these information volumes (end-diastolic or end-systolic volume) and flows (cardiac output) can be derived. The pressure gradient across a restrictive orifice (e.g. mitral stenosis) can also be estimated from velocity. It is also possible to estimate an unknown pressure in one chamber (like right ventricular systolic pressure) of the heart from a known pressure on the connecting chamber (right atrial pressure or central venous pressure) using continuous wave or pulse wave Doppler. Similarly pulmonary artery diastolic pressure, left atrial pressure, and left ventricular end diastolic pressure can also be assessed.
| Conclusion|| |
TEE has become the cornerstone in the non-invasive diagnostic evaluation and monitoring of patients with suspected cardiovascular diseases or haemodynamic instability. Further TEE is an image based technology and is best learned through printed materials, video loops and regular hands-on training.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]