Epiaortic Ultrasonography and Epicardial Echocardiography

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9 Epiaortic Ultrasonography and Epicardial Echocardiography

Kathryn E. Glas, Stanton K. Shernan

Surface Imaging

Probe Orientation

Probe orientation marker is important for surface imaging techniques.

• Conventional imaging planes are based on transthoracic echocardiography (TTE), because orientation is most similar to TTE windows. As with transesophageal echocardiography (TEE), images on the left side of the monitor screen are on the patient’s right and images on the right side are on the patient’s left.
• The probe marker is a raised line, indentation, or mark on the probe (Figure 9-1). It is easily hidden by the standoff in the sheath. Thus, alternative methods to identify orientation are frequently needed.
• Images will appear left to right inverted if probe orientation is incorrect (Figure 9-2). Imaging the left ventricle (LV) in reverse could lead to misdiagnosis of regional wall motion abnormalities. An incorrect diagnosis of dextrocardia could be made if images are inverted.
• Within a given image sector, structures progressively appear from anterior (near-field) to posterior (far-field), as opposed to TEE views that start posterior and progress anterior. For example, on epicardial imaging, the right coronary cusp of the aortic valve will be closest to the transducer instead of farthest away (Figure 9-3).
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Figure 9-1 High-resolution TTE probe. A, Note the applied standoff and the raised mark on the right-hand side of the probe. B and C, Additional examples of standoffs (arrows) are noted.

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Figure 9-2 Probe orientation. A, The images were acquired with the correct orientation. Asc, ascending. B, The probe has been rotated 90 degrees, and the SVC is on the opposite (i.e., incorrectly oriented) side of the screen.

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Figure 9-3 A, An epicardial AV SAX, with the right coronary cusp closest to the transducer (most anterior). L, left; N, noncoronary cusp; R, right. B, An analogous TEE ME AV SAX, with the right coronary cusp farthest from the transducer (most anterior).

Probe Options

Because the probe is close to the structure being imaged, a high-resolution (i.e., high-frequency) transthoracic or surface probe can be used to optimize images.

• Use a probe that provides a transducer frequency of at least 7.5 MHz. Standard TTE probes are no more than 5 MHz, so a dedicated probe is needed. Pediatric TTE probes have higher resolution than equivalent adult probes.
• Linear array probes are typically used for central venous access and work well for epicardial echocardiography (ECE) imaging.
• Image depth is rarely greater than 12 cm for ECE examinations, whereas the optimal depth for epiaortic ultrasound (EAU) is often in the range of 8 cm. Surrounding structures can be used to confirm proper probe orientation.
• Images for all EAU and ECE studies should be acquired at the highest possible resolution. ECE images of the mitral valve (MV), in particular, may not have as high a resolution as the corresponding TEE images.
• A standoff is strongly recommended for EAU examinations with a phased array probe. Most manufacturers do not sell a standoff for phased array, but the hard plastic or rubber component of a disposable airway circuit reservoir bag can be used on some probes.
• Linear array probes do not require a standoff. The probe in the sheath can be placed directly on the ascending aorta because the focal zone is not an issue.

Image Acquisition and Optimization

Two operators are required to complete an ECE or EAU examination. The probe operator does not need experience to obtain the images; however, an expert intraoperative echocardiographer with advanced training should be available to guide image acquisition and interpretation.

• The images are acquired through an open chest; vigilance is necessary to avoid disturbing the surgical field; and sterile procedure skills are mandatory.
• The probe marker for the phased array transducer cannot be seen easily through the sterile sheath. Therefore, the probe should be marked or on-screen imaging can be used to guide orientation.
• Orientation for the linear array probe can be determined by tapping on one end of the probe while simultaneously reviewing the screen to first determine which side is medial or lateral and then reorienting as needed.
• Artifacts may be present on EUA and ECE images similar to TTE and TEE and other surface ultrasound-based images.
• Filling the chest cavity with warm (body temperature) saline can improve imaging and decrease artifacts, especially side lobes. Avoid overly hot or cold saline that can cause arrhythmias or hypertension.
• Linear array probes have a narrow imaging sector that limits simultaneously viewing of the full width and depth of the adult aorta. Therefore, the examination should be performed twice, once with adequate depth and once with adequate width.
• Phased array probes may permit imaging of the entire aorta with a standoff. However, imaging of the anterior left and right walls of the aorta may be limited if a standoff is not used and may require moving the probe from side to side to obtain complete images.

Maintaining a Sterile Field

Use of a TTE for EAU and ECE imaging requires a sterile sheath to cover the probe and maintain sterility of the surgical field.

• The surgical team should inspect and test the sheath for leaks before inserting the probe.
• The sterile sheath should be filled with sterile saline or ultrasonic gel by the scrub technician. The echocardiographer at the head of the bed inserts the probe into the sheath opening. The echocardiographer should ensure that the standoff is at an acceptable depth (~1 cm) before inserting the probe (Figure 9-4).
• Air can be trapped in the standoff and will impair optimal image resolution. The probe operator should invert the apparatus under fluid to remove the air.
• Once the examination is complete and the probe is no longer needed, the probe and sheath are removed from the surgical field together before the sheath is discarded.
• Available probes cannot be sterilized to perform these examinations. Therefore, it is important to strictly follow the maintenance instructions provided by the vendor. Probes should not be submersed for long periods of time and should be allowed to dry thoroughly between examinations.
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Figure 9-4 A, The sterile sheath is filled with saline. B, The echocardiographer inserts the probe within the sterile sheath. C, The operator inverts the probe within the saline to clear air from inside the standoff. D, The operator places the probe on the aorta, angling the probe toward the AV to start imaging.

Epiaortic Ultrasonography

Prevention of Stroke

Atherosclerosis of the ascending aorta and arch has been associated with perioperative stroke in cardiac surgical patients.

Key Points

• TEE examination of the ascending aorta is not as sensitive or specific as EAU imaging for identifying pathology. A significant portion of the ascending aorta cannot be seen with TEE owing to air-filled bronchi between the esophagus and the aorta, thus preventing ultrasound beam transmission.
• High-grade atherosclerosis of the descending aorta seen on TEE is often associated with an increased risk of ascending aortic disease.
• Cardiac surgical patients are at increased risk of perioperative stroke due to direct manipulation of the ascending aorta and subsequent risk of atherosclerotic embolization.
• Many different techniques are available to the surgeon for managing diseased aortas in order to minimize stroke risk, including changing the cannulation or cross-clamp site or minimizing aortic manipulation.

Standard Views and 12-Segment Nomenclature

Key Points

• The ascending aorta is divided into three segments: proximal, mid, and distal. The segment adjacent to the right pulmonary artery (PA) is the mid portion. The proximal segment is from the sinotubular junction (STJ) to the PA. The distal segment is from the PA to the takeoff of the innominate artery (Figure 9-5).
• Each segment of the aorta is then divided into four regions: left, right, anterior, and posterior. The right PA is adjacent to the left side, the superior vena cava (SVC) is adjacent to the right side, the anterior wall is closest to the transducer, and the posterior wall is farthest away.
• Determining the location of atherosclerotic plaques using EAU is critical. The aortic cannula is typically placed in the distal ascending aorta, slightly left of anterior, while the cross-clamp is typically placed at the level of the PA. Thus, the distal left and anterior walls and entire mid segment are key areas of focus during a comprehensive EAU examination.
• Therefore, a comprehensive EAU requires grading of 12 ascending aortic segments: 4 each in the proximal, mid, and distal ascending aorta.
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Figure 9-5 A, LAX epiaortic images of the ascending aorta. RPA, right pulmonary artery. B, Proximal, mid, and distal segmentation of the ascending aorta from a LAX perspective. C, SAX epiaortic image of the ascending aorta. D, Anterior, left, right, and posterior segmentation of the ascending aorta from a SAX perspective.

Performing an Epiaortic Ultrasonography Examination

• The operator places the sterile sheathed probe on the ascending aorta as proximal as possible. The probe usually needs to be angled to the patient’s left and about 20 degrees anterior to direct the ultrasound beam and image toward the AV and through the left ventricular outflow tract (LVOT). AV leaflet motion should not be confused with mobile atheromatous disease.
• The echocardiography probe should be positioned on the aorta perpendicular to blood flow for optimal two-dimensional imaging. The angle is correct if the left-right aortic diameter is equal to the anteroposterior diameter. The probe can be advanced into the proximal aorta past the STJ and a short axis (SAX) image obtained.
• Maintaining proper orientation of the aorta in SAX requires frequent angle orientation changes to the probe. The aorta arises from the AV and changes its orientation in the anterior, lateral, and cephalad planes. Near the mid ascending aorta, less anterior angulation is needed. From the mid segment, slight posterior angulation and rotation toward the midline is needed.
• To obtain the long axis (LAX) view from the mid ascending aortic SAX, the probe should be rotated 90 degrees; with the probe oriented toward the patient’s right shoulder. LAX imaging can frequently permit visualization of the entire ascending aorta in one plane. If the entire aorta is not visualized, the probe can be angled downward toward the AV (left side) to image the proximal aorta in LAX. Small angle manipulations will be needed to maintain the proper image orientation, as with SAX imaging. Reversing the angulation will bring the distal ascending aorta and proximal aortic arch into view.
• LAX views should not be used to measure plaque height because oblique angles can over- or underestimate disease severity.
• At a minimum, digital images should be acquired and stored at each level of the ascending aorta in SAX and LAX and incrementally as needed to demonstrate areas of abnormal thickening and atherosclerosis severity.
• The maximum diameter of the aorta should also be measured from the inner near-field edge to the inner far-field edge during systole (Figure 9-6).
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Figure 9-6 Measuring the ascending aortic diameter. The maximum diameter of the aorta should be measured from the inner near-field edge to the inner far-field edge during systole. Note the RPA coursing from right to posterior along the aorta, indicating this is a mid ascending aortic view.

Atherosclerosis Grading (Figure 9-7)

• Plaque height is measured from the level of the intimal surface to the maximum point of protrusion into the aorta. Each plaque should be measured individually. A grade should be provided for each of the 12 segments noted previously.
• Overall grading should consider the highest level of disease noted on the EAU examination.
• Numerous grading criteria for atherosclerosis have been published. Five-point grading scales are the most common. Each institution should agree on a preferred grading scale to avoid miscommunication about disease severity.
• The absence of significant aortic atherosclerosis (i.e., “normal”) is typically defined as less than 2 mm protrusion into the aortic lumen. The definition of “moderate” is controversial and varies among medical and surgical cohorts owing to the need for aortic manipulation in surgical patients. Evidence seems to suggest that more than 3 mm of plaque height portends increased risk in cardiac surgical patients.
• Severe disease is universally defined as greater than 5 mm protrusion into the aortic lumen (Table 9-1).
• Mobile disease defines any plaque with a mobile component regardless of size.
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Figure 9-7 A suggested algorithm for the use of EAU in the assessment of ascending aortic atherosclerosis. CVA, cerebrovascular accident.

TABLE 9-1 ATHEROSCLEROSIS GRADING SCALE

Grade Severity Description
Grade 1 Normal Normal, no intimal thickening
Grade 2 Mild Intimal thickening ≤ 3 mm
Grade 3 Moderate Sessile atheroma > 3 mm but < 5 mm
Grade 4 Severe Sessile atheroma ≥ 5 mm
Grade 5 Mobile Protruding atheroma with mobile components

Epicardial Echocardiography

When Transesophageal Echocardiography Is Not an Option

ECE predates TEE for surgical diagnosis and evaluation and, currently, is most often used when a TEE probe cannot be placed or is contraindicated. Table 9-2 lists the recommended ECE views and the corresponding TTE views.

TABLE 9-2 EPICARDIAL AND TRANSESOPHAGEAL ECHOCARDIOGRAPHY VIEW NOMENCLATURE

Epicardial Views Corresponding TTE View
AV SAX Parasternal AV SAX
AV LAX Suprasternal AV LAX
LV basal SAX Modified parasternal MV basal SAX
LV mid SAX Parasternal LV mid SAX
LV LAX Parasternal LAX
Two-chamber Modified parasternal LAX
RVOT Parasternal SAX

AV, aortic valve; LAX, long axis; LV, left ventricle; MV, mitral valve; RVOT, right ventricular outflow tract; SAX, short axis.

Key Points

• ECE does not include imaging of structures outside of the immediate surgical field including the descending aorta for diagnosing an aneurysm, Stanford B dissection, or placement of an intra-aortic balloon pump (IABP).
• ECE requires knowledge of TTE imaging planes and anatomy.
• The same probe used for EAU or even a standard TTE probe can also be used for ECE.
• As with all other echocardiographic techniques, a comprehensive ECE is necessary to fully diagnose pathology. Limited examinations, whether owing to time constraints, patient condition, or inability to obtain all images, should be followed with a comprehensive examination as soon as feasible.

Seven Standard Epicardial Echocardiographic Views

• A comprehensive analysis of LV and right ventricle (RV) global and regional function can be assessed from ECE imaging windows.
• The incorporation of Doppler modalities can permit a comprehensive assessment of all four valves for stenosis or regurgitation.
• Machine presets frequently have different Doppler gain and scale settings for TEE and TTE probes. Thus, a thorough review of the settings before obtaining optimal images is necessary to prevent misdiagnosis.

Image Acquisition

Step 1: Aortic Valve Short Axis (see Figure 9-3)

• Place the probe on the ascending aorta with the transducer marker oriented toward the left shoulder. Angle the probe medial and posterior until the AV is in view. In this SAX view, the right coronary cusp is closest to the transducer, the left cusp is to the right, and the noncoronary cusp is to the left on the monitor.

Step 2: Aortic Valve Long Axis (Figure 9-8)

• From the AV SAX image, move the probe until the marker is parallel with the aorta and rotate it until the LVOT and the AV leaflets are seen.
• Measurements of the LVOT, annulus, and aortic root can be made from this view.
• In addition, color and spectral Doppler assessment of gradients or regurgitation severity can be performed because the transducer can be placed parallel to blood flow from this view.
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Figure 9-8 ECE AV LAX. This view allows the proper angle to perform spectral Doppler evaluations in addition to CF Doppler. Note the LVOT can be evaluated from this view for dynamic outflow obstruction, subaortic membrane, or other pathologies. AO, aorta.

Step 3: Left Ventricle Basal Short Axis (Figure 9-9)

• LV epicardial views are obtained through a right ventricular (RV) window. Moving the probe along the length of the RV and changing the angulation and rotation will allow visualization of the MV, subvalvular apparatus, and basilar, mid, and apical LV.
• An epicardial view of the LV will differ from TEE views in that the RV (and tricuspid valve [TV]) are the most anterior structures on the examination.
• When the probe is properly aligned, the MV anterior leaflet will be closest to the transducer and the anterolateral commissure will be on the right.
• Color flow (CF) Doppler evaluation at this level is equivalent to a transgastric (TG) basal TEE view of the MV that demonstrates origin of regurgitant jets and can be used for planimetry and effective regurgitant orifice area calculations.
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Figure 9-9 ECE LV basal SAX. Note the imaging plane is through the RV, with the anterior leaflet (AL) of the MV as the most anterior structure, similar to the surgical anatomic view of the valve. The TV can usually be seen from this view as well. PL, posterior leaflet.

Step 4: Left Ventricle Mid Short Axis (Figure 9-10)

• This view allows visualization of the same structures seen on a TG midpapillary LV SAX view and allows assessment of volume status, global and regional wall motion, and chamber size.
• In order to maintain the proper orientation, move and/or angle the probe inferiorly and to the left from the basilar view to obtain the LV mid SAX view.
• The anterolateral papillary muscle will be on the right and the posteromedial will be on the left of the image.
• The anterior wall of the LV will be at the top of the image and the inferior wall will be on the bottom—opposite to a comparable TEE view.
• Because the medial and lateral orientations are not affected, the left ventricular (LV) septal wall will continue to be on the left side of the image and the lateral wall on the right side.
• Moving the probe laterally will permit visualization of the RV and allow global RV function assessment.
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Figure 9-10 ECE LV mid SAX. The regional wall segments are indicated on this image using the 16-segment nomenclature. A, anterior; I, inferior; P, posterior; S, septal.

Step 5: Left Ventricle Long Axis (Figure 9-11)

• This view very closely approximates the TTE parasternal LAX view and provides significant information about ventricular and valvular function. It most closely resembles the TEE midesophageal (ME) LAX view for anatomic characteristics.
• From the LV mid SAX view, rotate the probe toward the patient’s right shoulder and angle the probe anteriorly until the image approximates the one shown in Figure 9-10.
• The most commonly seen LV walls are the inferolateral and anterolateral, but anatomic variations among patients may generate images that are not perfectly aligned. If you cannot see the interventricular septum, angle the probe until it is in view to ensure you are labeling the region appropriately.
• Images of the MV may not have the same resolution as TEE views, but the presence of leaflet pathology can still be determined, and a CF Doppler evaluation can be performed. Effective regurgitant orifice area determinations are also possible from this view.
• AV regurgitation can be assessed with CF Doppler.
• The LVOT can be measured and evidence of dynamic outflow obstruction can be ascertained from this view as well.
• The interventricular septum can be interrogated for defects.
• Rightward angulation of the transducer will allow visualization of the right atrium (RA) and RV as well as CF and spectral Doppler assessment of the TV.
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Figure 9-11 ECE LV LAX. This view closely approximates the TTE parasternal LAX view (see Table 9-2). This is the best ECE view for mitral subvalvular apparatus assessment. CF Doppler assessment of the AV, MV, and LVOT can be performed from this view.

Step 6: Two-Chamber (Figure 9-12)

• The epicardial two-chamber view closely approximates the TEE ME two-chamber view. The LV, MV, and left atrium (LA) can all be evaluated. The LA appendage can frequently be seen as well, potentially allowing assessment for thrombus or air.
• This view is obtained by rotating the probe clockwise and moving it medially across the RV surface until the LV is directly under the transducer.
• LV regional wall motion of the basal and mid segments of the anterior and inferior walls can be assessed.
• This view can be difficult to obtain. Similar to other LV views, careful review of surrounding structures is necessary to ensure correct identification of the wall in question for assessment of abnormalities.
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Figure 9-12 ECE two-chamber. As with other ECE views, the anterior wall is adjacent to the transducer and the inferior wall is farthest away. In this image, the LV apex is to the left side of the image and the MV and LA are just off the screen to the right. Moving the probe cephalad can provide a view of the MV that is comparable (but inverted) with a TG LAX image. This LV LAX view allows assessment of LV regional wall motion for the anterior and inferior walls. The LV apex is better seen in this ECE view than in TEE so evaluation of a potential LV apical thrombus should be preferentially performed with TTE preoperatively or ECE imaging intraoperatively.

Step 7: Right Ventricular Outflow Tract (Figure 9-13)

• This view is similar to an inverted ME TEE RV inflow-outflow view. In patients with a normal size RV, this ECE view may be difficult to obtain.
• If the ECE examination is performed in the suggested order, beginning with the ECE two-chamber view, the probe now needs to be moved back toward the RV and the marker needs to be reoriented toward the patient’s left shoulder.
• With proper alignment, both the TV and the pulmonic valve (PV) can be assessed with CF and spectral Doppler for the presence of stenosis or regurgitation.
• The main PA can also be seen, allowing evaluation for pulmonary embolus or confirmation of PA catheter placement (see Figure 9-7).
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Figure 9-13 ECE right ventricular outflow tract (RVOT) view. This view approximates the TEE ME RV inflow-outflow view and is the hardest of all the ECE views to obtain. A modified examination, evaluating the pulmonic valve (PV) then adjusting the probe to evaluate the tricuspid, may be easier than imaging all structures simultaneously. CF and spectral Doppler evaluation of both valves should be performed from this view.

Suggested Readings

Epiaortic

1 Glas KE, Swaminathan M, Reeves ST, et al. Council for Intraoperative Echocardiography of the American Society of Echocardiography. Society of Cardiovascular Anesthesiologists. Society of Thoracic Surgeons. Guidelines for the performance of a comprehensive intraoperative epiaortic ultrasonographic examination: recommendations of the American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists; endorsed by the Society of Thoracic Surgeons. Anesth Analg. 2008;106:1376-1378.

2 Davila-Roman VG, Phillips KJ, Daily BB, et al. Intraoperative transesophageal echocardiography and epiaortic ultrasound for assessment of atherosclerosis of the thoracic aorta. J Am Coll Cardiol. 1996;28:942-947.

3 Bucerius J, Gummert JF, Borger MA, et al. Stroke after cardiac surgery: A risk factor analysis of 16,184 consecutive adult patients. Ann Thorac Surg. 2003;75:472-478.

4 van der Linden J, Hadjinikolaou L, Bergman P, Lindblom D. Postoperative stroke in cardiac surgery is related to the location and extent of atherosclerotic disease in the ascending aorta. J Am Coll Cardiol. 2001;38:131-135.

5 Katz ES, Tunick PA, Rusinek H, et al. Protruding aortic atheromas predict stroke in elderly patients undergoing cardiopulmonary bypass: Experience with intraoperative transesophageal echocardiography. J Am Coll Cardiol. 1992;20:70-77.

Epicardial

6 Reeves ST, Glas KE, Eltzschig H, Shernan SK. Guidelines for performing a comprehensive epicardial echocardiography examination: Recommendations for the American Society of Echocardiography Council for Intraoperative Echocardiography and the Society of Cardiovascular Anesthesiologists. Anesth Analg. 2008;105:22-28.

7 Rosenberger P, Shernan SK, Loffler M, et al. The influence of epiaortic ultrasonography on intraoperative surgical management in 6051 cardiac surgical patients. Ann Thorac Surg. 2008;85:548-553.

8 Eltzschig HK, Kallmeyer IJ, Mihaljevic T, et al. A practical approach to a comprehensive epicardial and epiaortic echocardiographic examination. J Cardiothorac Vasc Anesth. 2003;17:422-429.

9 Hilberath JN, Shernan SK, Segal S, et al. The feasibility of epicardial echocardiography for measuring aortic valve area by the continuity equation. Anesth Analg. 2009;108:17-22.

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