The value of performing transesophageal echocardiography (TEE) in the intensive care unit (ICU) is well established. Although transthoracic echocardiography (TTE) is a useful diagnostic tool in the ICU, TEE has superior diagnostic accuracy and therapeutic impact in several clinical situations, particularly for patients in shock states.1–3 Several authors have demonstrated that TEE findings lead to major therapeutic decisions between 43% and 68% of the time.1,4–6 TEE produces good image quality due to the position of the probe proximate to the heart, allowing for the use of higher frequency ultrasound with superior resolution of cardiac structures than with TTE. Although improvements in imaging, software, and portable systems have reduced the rates of inadequate image quality seen with TTE, there remain a significant percentage of patients in the ICU whose image quality with TTE is inadequate. Many factors account for this including inadequate patient positioning, lung hyperinflation, obesity, edema, and the presence of chest devices, wounds, and dressings. TTE results in adequate image quality in approximately 55% of mechanically ventilated ICU patients, with the remaining 23% and 22% of studies being of suboptimal and poor quality, respectively.3 In addition to overcoming poor image quality of TTE, TEE is often necessary for the evaluation of specific diagnoses in the ICU such as endocarditis, identifying an embolic source, intracardiac shunt, aortic dissection, and loculated pericardial effusion. For hemodynamic assessment, TEE is the only method to assess superior vena cava (SVC) variation, a predictor of volume responsiveness.7 When compared with helical computed tomography (CT), TEE has good sensitivity and specificity for central pulmonary embolism (PE) associated with right ventricular dilatation.8,9
Critical care TEE differs from standard cardiology TEE in several ways. Typically, TEE performed by a cardiologist is an elective procedure, which has specific indications such as the evaluation for left atrial appendage thrombus, diagnosis of congenital heart disease, identification of valvular abnormalities such as endocarditis, and assessment of prosthetic valve function. Most often, the patient is not on ventilatory support and the procedure is performed outside of the ICU. In comparison, critical care TEE emphasizes hemodynamic evaluation of cardiopulmonary failure. It is a useful diagnostic tool allowing differentiation between shock subtypes in the critically ill while replacing invasive procedures such as the insertion of a pulmonary artery catheter. Because this chapter is written for the intensivist, our emphasis will be on utilization of TEE in the critical care setting rather than focusing on a cardiology type TEE examination.
Critical care TEE is routinely performed in large ICUs in Europe; for example, in Hospital Ambroise-Pare in Boulogne and the Erasme University Hospital in Brussels, where it is considered a routine procedure for the assessment of the shock state. In France, there is a well-defined training sequence for attendings and fellows developed by Societe de Reanimation de Langue Francaise that includes training in critical care TEE. In Australia, TEE is also is a common tool for the assessment of hemodynamic failure in the ICU. By comparison, critical care TEE is not widely used by intensivists in North America with the exception of cardiac anesthesiologists who have responsibility within an ICU. One reason for this dichotomy is that in the Europe, critical care specialists come from a variety of training backgrounds including cardiology. As a result, some intensivists have formal cardiology training in echocardiography before entering critical care subspecialty training. In the United States, formal training for critical care fellows in TEE is still very uncommon.
Indications for TEE
The most common indications for critical care TEE are as follows:
Assessment of hemodynamic failure, if TTE views are inadequate.
Echocardiography is a primary tool for the evaluation of shock in the ICU. There is logic to the consideration that early echocardiography should be a mandatory component of initial and continuing assessment of all patients with hemodynamic failure.10 This being the case, TEE is indicated when TTE views are inadequate due to factors such as obesity, edema, heavy musculature, dressings, wounds, or hyperinflation. Critical care TEE is particularly useful in certain circumstances for the evaluation of hemodynamic failure where even good quality TTE images may not answer clinical questions such as:
Identification of preload sensitivity. There are a variety of validated methods for the assessment of preload sensitivity using TTE (see Chapter 9). On occasion these may yield equivocal results. Using TEE, the evaluation of size variation in the SVC during mechanical ventilation cycling is easy to perform and may allow the clinician to clarify an ambiguous TTE result in order to identify preload sensitivity.
Identification of PE. Central PE can be visualized with TEE, whereas TTE is not as effective for this application.
Unexplained hypoxemia. Intracardiac shunt (e.g., patent foramen ovale with high right-sided pressures) may be assessed using agitated saline injection. TTE imaging is frequently inadequate, while TEE has excellent image quality for this application.
Identification of aortic dissection. TTE offers only a limited view of the ascending aorta, while TEE allows imaging of the ascending and descending aorta.
Cardiac arrest. TEE can be used to assess the etiology and adequacy of resuscitation efforts during cardiac arrest on a continuous basis, whereas TTE is limited to a brief subcostal view during pulse checks.
Other questions that cannot be definitively answered with TTE regardless of image quality. These include an examination for intracardiac thrombus, subtle valvular abnormalities (e.g. vegetation), and detailed Doppler analysis of pulmonary venous inflow. Examination for this type of abnormality represents an overlap with the skill set that is typical for cardiology TEE, but that can be mastered by the intensivist.
Training for Critical Care TEE
Similar to other aspects of critical care ultrasonography, training in critical care TEE includes the mastery of image acquisition, image interpretation, and the cognitive elements of the field (see Chapter 4). Competence in TTE is helpful in training for TEE, as the learner is already familiar with the echocardiographic anatomy. As with all forms of critical care, skill in image acquisition is gained only through practice. Charron et al. report that trainees developed competence in critical care TEE following the performance of approximately 31 supervised studies. In this study, the authors describe a competency-based examination that is a model for fellowship programs to follow in order to assure that training has been successful.11,12 Sophisticated TEE simulators are available to aid in the acquisition of transducer manipulation skills prior to scanning an actual patient.
Patient Selection and Preparation
One of the distinguishing points between cardiology and critical care TEE is that intensivists do not generally perform TEE unless the patient is on mechanical ventilatory support. Therefore, the discussion on patient preparation will be limited to the patient who is intubated and on ventilatory support.
TEE is a minimally invasive and safe procedure with a few definite risks.13 Because, by definition, the patient has an endotracheal tube in place, the risk of airway complication in the mechanically ventilated patient in the ICU is low when compared with the performance of cardiology TEE. Other complications are rare, and include esophageal abrasion, perforation, and bleeding. These can be avoided by appropriate patient selection and by minimizing the rotational movement of the endoscope tip while under flexion (see section Transducer Manipulation).
TEE is contraindicated in the setting of esophageal disease such as esophageal varices, strictures, bleeding, recent surgery, tumor, or diverticula. It is important that an accurate history is obtained that addresses the risk of esophageal injury during TEE. If such conditions are present, a barium swallow or endoscopic evaluation of the esophagus is recommended prior to the procedure.13 Coagulopathy or thrombocytopenia is a relative contraindication to TEE. If the clinical situation permits, significant abnormalities should be corrected.
Endotracheally Intubated Patients
Since mechanically ventilated patients have a secure airway that facilitates intubation, they require minimal airway preparation. Our approach is to augment intravenous sedation so that the patient is deeply sedated during the TEE examination. The patient is monitored with an electrocardiogram (ECG), arterial blood pressure, and oxygen saturation. An ECG signal should be displayed on the TEE ultrasound screen at all times. We recommend that the patient not be spontaneously breathing during the procedure, which may require transient use of a neuromuscular blocking agent. This is important when assessing preload sensitivity with TEE.
In most patients, the probe can be inserted blindly. The well-lubricated probe is introduced through a mouthpiece in the midline with the transducer surface facing the tongue. It is advanced with gentle forward force and with gradual anteflexion of the device. Flexion of the neck may facilitate insertion into the esophagus.
On occasion, blind insertion is unsuccessful, with the operator encountering significant resistance to forward movement of the probe. In this case, we recommend use of an intubating laryngoscope to expose the esophageal opening. A video laryngoscope is particularly useful in this situation. The TEE probe is then introduced under direct visualization. If the patient has a gastric tube in place, this may need to be removed in order to improve image quality.
The modern TEE probe utilizes a multicrystal, phased-array transducer placed at the tip of a flexible endoscope. The probe can be advanced within the esophagus and positioned directly posterior to the heart, with excellent resolution of cardiac structures.
Historically, TEE probes were equipped with a monoplane transducer that could provide only a single-plane view of the heart that was transverse in orientation. The development of multiplane probes allowed the transducer to be rotated to any position between 0° and 180°. This provides multiple views of the heart when combined with movements of the endoscope such as advancement, turning, or flexion.
An advantage of the proximity of the TEE probe to the heart is that it enables the use of ultrasound frequencies between 5 and 7.5 MHz. This results in superior resolution of posterior cardiac structures when compared with TTE. However, anterior cardiac structures may require reduction in the ultrasound frequency for better penetration, at the expense of reduced resolution. The frequency of the transducer is adjusted according to the distance to the imaging target.
It is important to standardize the description of operator manipulation of the probe, particularly for communication when two operators are involved with scanning. Each TEE view requires a specific position and orientation of the transducer with respect to the heart. The American Society of Echocardiography (ASE) recommends that the following terms be used to describe transducer movement.
Advancement/withdrawal of probe: This is accomplished by moving the probe in and out of the esophagus (the depth of insertion is noted on the shaft of the endoscope).
Flexion of probe from the neutral position in four directions: This is accomplished by rotation of the control knobs on the shaft of the endoscope. Rotation of the large knob results in anteflexion of the probe face (moving the face of the probe in an anterior/superior-directed view) or retroflexion of the probe face. Rotation of the small knob results in right or left flexion of the probe face.
Turning of the probe to the right or left side: This is accomplished by twisting the shaft in a counterclockwise (to look towards the left side of patient) or clockwise motion (to look towards the right side of patient).
Rotation: This is accomplished by changing the plane of orientation of the crystal within the probe. If the operator is standing to the left of the patient and looking down at the patient, the face of the transducer will be facing anteriorly, that is, toward the operator. Rotation of the transducer beam plane occurs in a counterclockwise fashion via an electronic switch that allows for 1° incremental changes. The exact orientation of the transducer is represented by the angle indicator on the screen with values between 0° and 180°.
Orientation of TEE Views
Knowledge of the standard TEE view orientation relies on two main principles:
The ultrasound beam originates from behind the heart. The top or “apex” of the screen displays structures closest to the esophagus, that is, the atria and great vessels when the endoscope is in a neutral position within the esophagus; and the inferior wall of the heart, when it is anteflexed from within the stomach. Structures in the far field of the screen represent anterior cardiac structures. The one exception to this convention is the deep gastric apical four-chamber view, which results in the apex of the heart being projected at the top of the screen.
The orientation is described by the degree rotation of the ultrasound beam plane. For example, 0° pertains to the transverse plane, with the leftmost part of the screen pertaining to the rightmost part of the patient (similar to the orientation of a chest x-ray). Increasing the degree rotation corresponds to a counterclockwise rotation of the scan plane; thus, a 90° view results in a longitudinal view, with superior structures to the right of the screen and inferior structures to the left of the screen.
The operators should be familiar with the degree of orientation of the four primary TEE views: (1) 0°: transverse plane, (2) 45°: short-axis view of aortic valve (AV), (3) 90°: oblique, long-axis view, and (4) 135°: “true” long-axis view. These four primary views, when combined with turning and flexion, can produce all of the standard ASE views.
The number and sequence of views required for a critical care TEE examination have not been standardized. In its simplest iteration, the examination can be as limited as that of the goal-directed TTE examination. Benjamin et al. reported on the use of a monoplane pediatric probe by intensivists.4 Their protocol required four views with three quantitative assessments. Examinations were completed in 12 minutes, and resulted in therapeutic changes in 52% of cases. Intensivists easily developed skill at this type TEE examination. Along these lines, there is now available a miniaturized disposable TEE probe developed for limited assessment of hemodynamic function. Due to its small diameter and flexibility, it may be left indwelling for up to 72 hours, which allows for regular reassessment of hemodynamic status. Vieillard-Baron et al. have demonstrated its safety and feasibility in the ICU,14 and it has a short learning curve relative to standard TEE. The device is designed to rapidly and repeatedly obtain three basic views for hemodynamic assessment: the transverse SVC view, the four-chamber view, and the gastric short-axis view. Vieillard-Baron et al. described three views with four qualitative assessments.15 These qualitative assessments were similar to results obtained quantitatively in the same patient. These studies support the concept of goal-directed echocardiography.