Cardiovascular Anesthesiology: Echo Didactics
A 62-year-old man recovered for 1 hour in the post-anesthesia care unit following a colectomy. He became acutely hypotensive and hypoxic and did not respond to initial fluid bolus or epinephrine. He has a history of esophageal strictures and dysphagia, precluding the use of transesophageal echocardiography (TEE). A transthoracic echocardiogram (TTE) was performed to evaluate the cause of his hemodynamic instability.
TTE and TEE are 2 common perioperative cardiac imaging modalities. Understanding the similarities, differences, and benefits of TTE and TEE is useful for the perioperative echocardiographer. Familiarity with TTE views provides an excellent knowledge base for recognition of the related TEE views. For instance, anatomical structures present in the TTE apical 4-chamber view are easily recognized in the TEE midesophageal 4-chamber view.
TTE and TEE are complementary in their ability to provide better resolution of specific structures. TTE offers superior resolution of the anterior structures, such as the right ventricle, right ventricular outflow tract, pulmonic valve, and anterior pericardium. TTE apical views provide superior imaging of the pericardium and left ventricular apex. Imaging of the posterior structures, such as the left atrium, mitral valve and subvalvular apparatus, interatrial septum and left atrial appendage, is best achieved with TEE. The following paragraphs will compare 5 of the basic TTE views and their corresponding TEE views, and identify their advantages and disadvantages. A discussion of proximal aortic imaging with TTE and TEE is also included.
The TEE probe is a flexible ultrasound gastroscope that is advanced along the esophagus and into the stomach to image the heart and great vessels. Because of the close proximity to the heart, TEE requires less depth penetration and uses higher frequency transducers (5–7 MHz) enabling superior spatial resolution of the posterior cardiac structures when compared with TTE. TEE remains the imaging modality of choice for intraoperative cardiovascular assessment, especially during cardiac surgery.1,2
TTE uses lower frequency (3-5 MHz) ultrasound to allow greater depth penetration at the expense of spatial resolution. The TTE transducer is placed directly on the patient’s chest in 4 windows: parasternal, apical, subcostal, and suprasternal. In contrast to TEE, TTE images anatomic structures from anterior to posterior.
Chamber quantification is more readily accomplished by TTE, where the distance between the probe allows for complete capture of the ventricular and atrial borders.6 TTE can be performed rapidly, noninvasively, and provide accurate hemodynamic assessments during cardiovascular emergencies such as cardiac tamponade and circulatory shock.4,5 TTE generally provides more reliable hemodynamic data as it affords more flexibility of transducer position. However, TTE may be limited by factors such as supine patient positioning, inability to access the chest without contaminating the surgical field, and presence of mediastinal tubes. Mediastinal air, tubes, and surgical dressings can impede ultrasound transmission and limit the utility of TTE imaging.
COMPARABLE TTE AND TEE VIEWS
In Table 1, 5 basic TTE views along with corresponding TEE views are listed with the structures that are imaged.
TTE and TEE Left Ventricle Long-Axis Views
In TTE, the parasternal long-axis view is obtained by placing the probe at the 3rd and 4th intercostal space directly left of the sternum with the index mark pointing toward the right shoulder as demonstrated in composite Video 1 (see Supplemental Digital Content 1, http://links.lww.com/AA/A528). This tomographic plane images the anteroseptal and inferolateral left ventricle walls, as well as the anterior and posterior mitral valve leaflets and the motion of the aortic valve leaflets. These structures may be imaged with TEE in the midesophageal long-axis view obtained at approximately 120° rotation.
Left-sided valve lesions can be viewed in the parasternal long-axis view and the midesophageal long-axis view with color Doppler. The midesophageal long-axis view offers superior spatial resolution of the mitral and aortic valve leaflets compared with the parasternal long-axis view. However, one distinct advantage of the parasternal long-axis view over the midesophageal long-axis view is the ability to visualize aortic regurgitation that may be obscured by dropout below a prosthetic mitral valve or mitral annular calcification. These 2 comparison views can be seen in Figure 1 and composite Video 1 (see Supplemental Digital Content 1, http://links.lww.com/AA/A528).
TTE and TEE Right Ventricle Inflow–Outflow Views
The right ventricle, tricuspid valve, right ventricular outflow tract, pulmonic, and aortic valves can be evaluated by TTE in the parasternal short-axis view. With the transducer in the parasternal position and the index mark pointed toward the left shoulder, transducer tilt (the orientation of the transducer along a plane parallel to the ultrasound scan sector) is adjusted to demonstrate the aortic valve in short axis as demonstrated in composite Video 2 (see Supplemental Digital Content 2, http://links.lww.com/AA/A529). These structures may be imaged with TEE in the midesophageal right ventricular inflow–outflow view obtained at approximately 65° rotation. Color Doppler analysis of the tricuspid, pulmonic, and aortic valves is possible in these views. Figure 2 and composite Video 2 (see Supplemental Digital Content 2, http://links.lww.com/AA/A529) demonstrate these TTE and TEE views.
TTE and TEE Left Ventricle Short-Axis Views
Left ventricular structure and function are best examined in short-axis views, where the myocardial distribution of all 3 coronary arteries (left anterior descending, circumflex, and right coronary) can be viewed. With the TTE transducer in the parasternal position and the index mark pointed toward the left shoulder, short-axis views of the left ventricle may be obtained as demonstrated in composite Video 3 (see Supplemental Digital Content 3, http://links.lww.com/AA/A530). Subtle adjustments in transducer tilt allow imaging of the basal, mid, and apical left ventricle.
In TEE, the corresponding views may be obtained in the transgastric window with 0° rotation. TEE affords excellent wall motion and endocardial border visualization of the left ventricle in short axis. Adjusting anteflexion/retroflexion and/or probe advancement allow imaging of the basal, mid, and apical left ventricle in short axis. The TTE and TEE short-axis views are inverted with respect to one another. Therefore, the anterior wall of the left ventricle is at the top of the TTE image and the bottom of the TEE image.
TTE views of the left ventricle in short axis can be easily obtained on the unstable cardiac patient, to exclude etiology such as cardiac tamponade caused by a circumferential pericardial effusion or cardiogenic shock.4,5 It is important to note that other views, including the 4-chamber, 2-chamber, and long-axis views, are required to exclude focal fluid collections around the atria or right ventricle that may also cause tamponade especially after chest surgery. The parasternal short axis (midpapillary position) and transgastric short axis can be seen in Figure 3 and composite Video 3 (see Supplemental Digital Content 3, http://links.lww.com/AA/A530).
TTE and TEE 4-Chamber View
A 4-chamber view in TTE may be obtained with the transducer in the apical position and the index pointed toward the floor as demonstrated in composite Video 4 (see Supplemental Digital Content 4, http://links.lww.com/AA/A531). Both atria, tricuspid valve, mitral valve, the right ventricular free wall, the inferoseptal and anterolateral left ventricular walls are visualized. In TEE, the corresponding view is obtained from the midesophageal acoustic window with transducer rotation at approximately 10°. During the intraoperative period, the midesophageal 4-chamber view is useful and complementary to the transgastric short-axis view to monitor biventricular filling and function. For both TTE and TEE, care must be taken to eliminate foreshortening that occurs when the ultrasound plane does not transect the left ventricular apex. Foreshortening may lead to erroneous left ventricular volume estimation and overestimation of left ventricular wall thickening. Color and spectral Doppler analysis of the mitral valve, tricuspid valve, and pulmonary veins, as well as tissue Doppler analysis of the mitral annulus are possible in these views. The midesophageal 4-chamber view offers superior axial and lateral resolution of the mitral valve leaflets owing to a higher frequency TEE transducer and its close proximity to the mitral valve. It is important to note that comprehensive imaging of the mitral valve and subvalvular apparatus requires multiple views and is beyond the scope of this paper. Figure 4 and composite Video 4 (see Supplemental Digital Content 4, http://links.lww.com/AA/A531) demonstrate TTE and TEE 4-chamber views.
TTE and TEE 2-Chamber Views
A 2-chamber view in TTE may be obtained with the transducer in the apical position and the index pointed toward the patient’s head as demonstrated in composite Video 5 (see Supplemental Digital Content 5, http://links.lww.com/AA/A532). The inferior and anterior left ventricular walls, mitral leaflets, left atrium, and left atrial appendage are imaged in a 2-chamber view. In TEE, a 2-chamber view is obtained with the transducer in the midesophageal position and approximately 90° rotation. Figure 5 and composite Video 5 (see Supplemental Digital Content 5, http://links.lww.com/AA/A532) demonstrate these views. Due to its proximity to the esophagus, TEE in the midesophageal 2-chamber view is superior to TTE apical 2-chamber view in examination of the mitral leaflets and left atrial appendage for presence of thrombus.7
TTE and TEE Imaging Considerations of the Ascending Aorta and Aortic Root
TEE and TTE may be used to image the aortic root and ascending aorta to assess for aortic plaques, hematomas, or dissections; however, both have limitations.
TEE is able to visualize the aortic root and ascending aorta with superior resolution compared with TTE; however, TEE is unable to visualize the distal ascending aorta because of air in the interpositioned left mainstem bronchus. Thus, it is possible to miss an aortic dissection in the distal ascending/proximal aortic arch with TEE. Despite this limitation, the sensitivity of TEE in the diagnosis of thoracic aortic dissection is quite high.8
The parasternal and midesophageal long-axis views allow visualization of the proximal aortic root as demonstrated in Figure 1 and composite Video 1 (see Supplemental Digital Content 1, http://links.lww.com/AA/A528). However, multiple views are required to address the diagnosis of thoracic aortic dissection that are beyond the scope of this paper. Alternative imaging of the aortic root and ascending aorta may be accomplished preoperatively with computerized tomography, magnetic resonance imaging, or epiaortic ultrasonography during the intraoperative period.E
Name: Sasha K. Shillcutt, MD, FASE.
Contribution: This author helped write the manuscript.
Attestation: Sasha K. Shillcutt approved the final manuscript.
Conflicts of Interest: This author has no conflicts of interest to declare.
Name: Julian S. Bick, MD.
Contribution: This author helped write the manuscript.
Attestation: Julian S. Bick approved the final manuscript.
Conflicts of Interest: Julian S. Bick discloses honoraria from Imacor, Inc.
This manuscript was handled by: Martin J. London, MD.
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