Epicardial echocardiography was first described in 1972. Subsequently, there have been a number of reports documenting its use, particularly during surgery, for congenital heart disease and mitral valve (MV) disease. However, epicardial echocardiography never became widely popular, largely because of the advent of transesophageal echocardiography (TEE) a few years later.
Recently, there has been a resurgence of interest in epicardial echocardiography, and guidelines for performing a comprehensive epicardial echocardiographic examination were published in 2007. Notwithstanding these guidelines, there is limited practical experience with epicardial echocardiography, and its role in clinical practice remains to be defined. Two indications for the use of epicardial echocardiography are clear: (1) to obtain intraoperative echocardiographic images in patients for whom TEE is not possible and (2) to obtain views that are suboptimal or are not able to be attained with TEE imaging.
Imaging of the ascending aorta and aortic arch is often referred to as epiaortic ultrasonography and, in contrast to epiaortic imaging, has been widely incorporated into clinical practice. Several descriptions of epiaortic ultrasonography have been described, and guidelines for performing a comprehensive epiaortic examination, developed by the ASE and the SCA, were published in 2008. The most common application for epiaortic imaging is visualizing the distal ascending aorta and proximal aortic arch, as this part of the aorta is obscured during TEE examination.
The purpose of this chapter is to outline the practical considerations, describe the main views, and review the evidence for the clinical application of epicardial echocardiography and epiaortic ultrasonography.
Practical and technical considerations
Transducers
Ultrasound transducers for epicardial and epiaortic use should operate in the 5- to 12-MHz frequency range. Higher-frequency transducers have the advantage of higher image resolution but, because of the linear relationship between transducer frequency and signal attenuation, may not image more distant structures adequately. For epiaortic scanning, a high-resolution transducer with an imaging frequency of more than 7 MHz and an appropriately small footprint should be used. The choice between a linear and a phased array transducer is determined by preference and availability. Linear array transducers provide superb proximal image resolution but are not suitable for epicardial imaging because of their limited image width and depth. A pediatric or neonatal phased array transducer is suitable for epicardial imaging and, with sufficient “standoff” distance from the anterior surface of the aorta, enables imaging of the entire ascending aorta in long axis within one image plane.
Transducer handling and cleaning
The transducer must be placed in a sterile sheath before its use in the surgical field. This can be accomplished by having the surgeon or scrub nurse hold up a sterile sheath containing ultrasound gel so that the anesthesiologist can feed the nonsterile transducer into the sheath. Once introduced into the sterile field, the surgeon manipulates the transducer while the anesthesiologist adjusts the settings on the machine.
Although some transducers may be submersed for cleaning, most are not watertight and cannot be immersed. Therefore, for intraoperative use, the transducer should be thoroughly wiped with a disinfecting solution before being placed in a sterile cover. In one study, despite one documented break in the sterile sheath, no infections were reported in 287 epicardial echocardiograms in which the transducer and cable were cleaned with a glutaraldehyde solution, “left wrapped in a glutaraldehyde-soaked towel for at least 10 minutes,” and then wiped with sterile saline and introduced into a sterile sheath containing 20 mL of sterile ultrasound gel.
Standoffs
Near-field “clutter” can obscure objects close to the face of the ultrasound transducer. This is mainly a problem with epiaortic scanning when the transducer is placed directly against the aorta. To overcome this problem, a standoff of around 1 cm should be used to move the transducer away from the aorta during epiaortic imaging.
Some transducers have built-in standoffs. For those that do not, one can be improvised. Sterile saline or ultrasound gel can be added to the sterile ultrasound sheath in sufficient quantity to allow the probe to be held away from the surface of the aorta. Another approach is to partially fill a sterile glove or small bag with saline and place it between the ultrasound probe and the aorta. When saline or ultrasound gel is used as the standoff, care should be taken to avoid trapping air bubbles in the sheath, or glove as these can degrade image quality. In addition, agitation of the saline or gel may result in small air bubbles being dissolved in the medium and interfering with ultrasound transmission. Preparing a standoff several minutes before use mitigates this.
An alternative technique is to fill the mediastinum with warm sterile saline. The ultrasound probe can be submerged in the saline and held away from the surface of the aorta.
The views
Transducer and image orientation
Standardized transducer orientation during epicardial image acquisition is important. For short-axis views, correct left–right image orientation is critical for accurate localization of valvular pathology or segmental wall motion abnormalities (SWMAs). Cardiac structures known to be adjacent to the region of interest can be used to aid image orientation (e.g., the right branch pulmonary artery (PA) lies between the aorta and the left atrium).
Orientation markers are present on the ultrasound transducer and on the echocardiography machine display, at the apex of the sector scan. The ultrasound transducer should be positioned with the orientation marker directed toward the patient’s left side for short-axis views and toward the patient’s head for long-axis views. The orientation marker on the display should appear on the right of the sector scan. Left-sided and cranial structures will then be displayed on the right side of the sector scan, and right-sided and caudal structures will be displayed on the left side of the sector scan.
As TEE images the heart from behind and epicardial echocardiography images the heart from the front, anteroposterior image orientation is opposite between the two modalities.
Epicardial views
Epicardial aortic valve short-axis view
The transducer is placed on the most proximal portion of the ascending aorta, with the orientation marker directed toward the patient’s left and turned (usually counterclockwise) until a symmetrical short-axis view of the AV is displayed ( Figure 5-1 ). Compared to a TEE midesophageal AV short-axis view, the positions of right and noncoronary AV cusps are inverted: the right coronary cusp is displayed toward the top of the sector (closer to the transducer), and the noncoronary cusp is displayed toward the bottom of the sector. The orientation of the left coronary cusp is similar to that obtained with TEE imaging.
The epicardial AV short-axis view is useful for assessing AV leaflet morphology and valvular function with 2-D and color flow Doppler imaging.
Epicardial aortic valve long-axis view
The AV long-axis view ( Figure 5-2 ) is developed from the AV short-axis view by turning the transducer approximately 90 degrees counterclockwise so that the long axes of the transducer and the ascending aorta are parallel to each other, with the orientation marker directed cephalad (usually toward the SVC). Tilting the transducer inferiorly displays more of the LVOT in long axis.
The epicardial AV long-axis view is useful for measuring the dimensions of the aortic root with 2-D imaging and for assessing AV function with color flow and spectral Doppler imaging.
Epicardial left ventricular basal short-axis view
From the AV short-axis view, the transducer is moved over the RV free wall toward the apex until a symmetrical short-axis view of the left ventricle and the chordal apparatus of the MV is obtained ( Figure 5-3 ). The orientation marker should be directed toward the patient’s left side. The epicardial left ventricle (LV) short-axis view is analogous to the transgastric LV basal short-axis view obtained with TEE imaging. However, with epicardial imaging, the MV leaflets and coaptation line appear horizontal on the sector scan, while they are obliquely oriented with TEE imaging.
The epicardial short-axis view is useful for assessing MV leaflet function with 2-D and color flow Doppler and for assessing regional LV function at the basal level.
Epicardial left ventricular mid-short-axis view
From the LV basal short-axis view, the transducer is moved farther toward the apex over the RV free wall until a short-axis view of the left and right ventricles at the midpapillary level is obtained ( Figure 5-4 ). Again, the orientation marker should be directed toward the patient’s left. This view is analogous to the transgastric mid-short-axis view obtained with TEE imaging and is used for assessing global and regional LV function, along with LV volume state.
Epicardial left ventricular long-axis view
From the epicardial LV mid-short-axis view, the transducer is turned counterclockwise until the AV (in long axis) and MV are displayed simultaneously ( Figure 5-5 ). The orientation marker should be directed cephalad (and slightly toward the patient’s right). The epicardial LV long-axis view is useful for interrogating the AV and MV with 2D and with color flow Doppler imaging.
Rightward tilt of the transducer may reveal the RV inflow view ( Figure 5-6 ), which is used for assessing the TV with 2D and color flow Doppler imaging.
Epicardial two-chamber view
To obtain the epicardial two-chamber view, the transducer is placed on the anterior surface of the left ventricle, with the orientation marker directed cephalad. Like the analogous transgastric two-chamber view, the epicardial two-chamber view displays the anterior and inferior walls of the left ventricle and the left atrium (LA) appendage. This view may be difficult to obtain.
Epicardial right ventricular outflow tract views
The standard view for assessing the (RVOT) is obtained by positioning the transducer directly over the RVOT, with the orientation marker directed toward the patient’s left ( Figure 5-7 ). Correct alignment of the transducer is evidenced by a (nearly) symmetrical short-axis appearance of the LVOT deep to the RVOT.
