Chapter 20. Epicardial Echocardiography and Epiaortic Ultrasonography

Despite its overwhelming popularity and favorable influence on perioperative clinical decision making and outcome, the transesophageal echocardiographic (TEE) approach to a comprehensive echocardiographic examination may be limited by impaired imaging of the distal ascending aorta and aortic arch, difficulty in advancing the probe within the esophagus in some patients, and contraindications for probe placement in those with gastroesophageal pathology. Furthermore, TEE may be rarely associated with perioperative morbidity from oropharyngeal and gastroesophageal injury.1,2 In recognition of these potential limitations, the Society of Cardiovascular Anesthesiologists (SCA), American Society of Anesthesiologists (ASA), and American Society of Echocardiography (ASE) currently recommend that advanced intraoperative ultrasonographers also become familiar with epicardial echocardiography and epiaortic ultrasound in addition to TEE.3,4 The ASE and SCA have subsequently published guidelines specifically focused on acquisition techniques and indications for both epicardial echocardiography and epiaortic ultrasonography.5,6 Thus, while TEE remains the most frequently used intraoperative tool for imaging cardiac and intrathoracic vascular structures, it is imperative for an experienced intraoperative ultrasonogapher to also be familiar with other imaging modalities including epicardial echocardiographic and epiaortic ultrasound techniques in order to conduct a comprehensive perioperative echocardiographic examination.

Epicardial and epiaortic imaging are performed by placing the ultrasound transducer on the surface of the heart or aorta, respectively, to acquire two-dimensional (2D), and color-flow and spectral Doppler images in multiple planes. Due to the proximity of the probe to the heart, these techniques typically use higher frequency probes (5 to 12 MHz). Epicardial and epiaortic imaging require adherence to strict sterile technique while manipulating the probe within the operative field. Consequently, these images may only be obtained by an operator who is wearing a sterile gown and gloves. The probe is placed in a sterile sheath along with sterile acoustic gel or saline in order to optimize acoustic transmission. Warm sterile saline can be poured into the mediastinal cavity to further enhance acoustic transmission from the probe to the cardiac or aortic surface. Additional manipulation of depth, transmit focus, gain, and transducer frequency may be required to optimize the image.

The ASE/SCA guidelines currently recommend that the following seven epicardial echocardiographic imaging planes be obtained to perform a comprehensive 2D and Doppler echocardiographic evaluation.5 However, the guidelines also recognize that individual patient characteristics, anatomic variations, or time constraints may limit the ability to obtain every component of the recommended comprehensive epicardial echocardiographic examination. Furthermore, modification of the recommended views may be required to obtain a more detailed interrogation of specific anatomy or pathology.

Epicardial Aortic Valve Short-Axis View

The ultrasound transducer is placed on the aortic root above the aortic valve (AV) annulus, with the ultrasound beam directed towards the AV in a short-axis (SAX) orientation to obtain the epicardial AV SAX view (Figure 20–1). Appropriate transducer alignment requires up to 30° of clockwise rotation with the orientation marker (indentation) on the transducer directed toward the patient's left.

Epicardial Aortic Valve Long-Axis View

The epicardial aortic valve long-axis (LAX) view is obtained from the epicardial AV SAX view by positioning the probe upward along the right-side surface of the aortic root with the orientation marker slightly rotated clockwise and directed toward the patient's left. The ultrasound beam is directed posteriorly to visualize the left ventricular outflow tract (LVOT) and AV (Figure 20–2).

Epicardial Left Ventricle Basal SAX View

The epicardial left ventricular (LV) basal SAX view is obtained from the epicardial AV SAX position by moving the probe towards the apex along the right ventricle (RV) with the transducer orientation marker again directed towards the patient's left (Figure 20–3). In this view, the RV is on top in the near field, while the LV is below in the far field of the ultrasound beam sector. The mitral valve (MV) is visualized including both leaflets forming the classic “fish mouth” appearance, with the anterior leaflet on the top of the screen and the posterior leaflet underneath. The anterolateral commissure lies on the right, and the posteromedial commissure on the left of the screen.

Epicardial Left Ventricle Mid SAX View

Angulating the probe inferiorly and to the left from the epicardial LV basal SAX view in an apical direction along the RV myocardial surface allows visualization of the RV and LV in SAX at the level of the papillary muscles (Figure 20–4). When the transducer orientation marker faces the patient's left, the anterolateral papillary muscle will be on the right side of the display and the posteriomedial papillary muscle will be on the left side. The septal wall of the LV is displayed on the left followed by the anterior, lateral, and inferior walls, respectively, in a clockwise rotation. The RV can be evaluated similarly by moving the transducer further towards the patient's right.

Epicardial Left Ventricle Long-Axis View

From the epicardial LV mid SAX view, the ultrasound beam can be angled superiorly and rotated towards the patient's right shoulder to generate the epicardial LV LAX view (Figure 20–5).

Epicardial Two-Chamber View

From the epicardial LV LAX view, movement of the probe toward the anterior surface of the LV and further clockwise rotation will develop the epicardial two-chamber view, permitting evaluation of the LA, left atrial appendage, MV, and LV (Figure 20–6). Basilar and mid segments of the anterior and inferior walls of the LV can be assessed in this view.

Epicardial Right Ventricular Outflow Tract View

The epicardial right ventricular outflow tract (RVOT) view is developed by moving the transducer over the RVOT and directing the ultrasound beam towards the patient's left shoulder (Figure 20–7). The RVOT, pulmonic valve (PV), and proximal main pulmonary artery (PA) can be visualized.

The ASE/SCA recommended comprehensive epiaortic ultrasound examination includes a minimum of five views for the evaluation of the ascending aorta from the sinotubular junction to the origin of the innominate artery, and the aortic arch.6 The ascending aorta should be assessed in SAX in each of the proximal, mid, and distal segments. A LAX view of the ascending aorta including visualization of the proximal, mid, and distal segments should also be acquired. A LAX epiaortic ultrasound examination of the arch includes visualization of the proximal arch, and ideally all three arch vessel origins.

The ASE/SCA guidelines recommend that the ascending aorta be divided into 12 areas including the anterior, posterior, and left and right lateral walls within the proximal, mid, and distal ascending aorta segments.6 The proximal ascending aorta is defined as the region from the sinotubular junction to the proximal intersection of the right pulmonary artery (RPA). The mid ascending aorta includes that portion of the aorta that is adjacent to the RPA. The distal ascending aorta extends from the distal intersection of the RPA to the origin of the innominate artery. The diameter of each aortic segment should be measured as the maximum diameter in the SAX orientation, from the near-field inner edge to the far-field inner edge (internal diameter).6

While epiaortic scanning may be used to evaluate ascending aortic aneurysms and dissection, this technique is most commonly utilized to evaluate the extent of aortic atherosclerotic burden to guide the surgical approach towards cannulation for cardiopulmonary bypass with the intention of avoiding the generation of emboli and excessive aortic trauma. While several grading systems have been recommended,7–10 a comprehensive epiaortic ultrasonographic examination should include an evaluation of each of the following measurements for each of the three ascending aortic short-axis segments and for the aortic arch: (1) maximal plaque height/ thickness, (2) location of the maximal plaque within the ascending aorta, and (3) presence of mobile components. Additional measurements indicating the extent of atheroma burden, such as circumferential area of maximal plaque obtained by planimetry, may also be acquired. When plaque area is measured, aortic diameter should also be noted to quantify atheroma burden as a ratio of plaque area to aortic area.11 Measurements may be repeated as necessary for multiple plaques. Due to the preponderance of data demonstrating an increased risk of adverse neurological outcomes associated with plaques that are greater than 5 mm in thickness, or those that possess a mobile component,7–10 the presence and location of these plaques should be discussed with the surgeon prior to aortic manipulation.

SAX Examination

The ultrasound probe is positioned on the ascending aorta as proximal to the aortic valve as possible, with the orientation marker directed toward the patient's left shoulder to obtain an imaging window that is perpendicular to the LAX of the aorta (Figures 20–8 and 20–9). After identifying the proximal ascending aorta and AV, slowly advancing the probe distally in a cephalad direction along the aorta permits visualization of the mid ascending aorta, and the distal ascending aorta towards the aortic arch at the origin of the innominate artery. Advancing the probe slightly further permits examination of the proximal aortic arch.

LAX Examination

The LAX orientation is achieved by rotating the probe 90° from the SAX orientation (Figure 20–10). Proximally, the sinus of Valsalva, sinotubular junction, and aortic valve can be visualized. The probe is then advanced distally in a cephalad direction along the aorta, changing the rotation and angulation accordingly to keep the aorta in a longitudinal LAX view (Figure 20–11). Imaging of the ascending aorta should extend towards the aortic arch with visualization of the innominate, left common carotid, and left subclavian artery origins (Figure 20–12).

Compared to TEE, epicardial echocardiography and epiaortic ultrasonography are unique in requiring a collaborative effort with the cardiac surgeon to either allow the echocardiographer to guide them in obtaining images, or alternatively permit the echocardiographer to have direct access to the epicardial or epiaortic surface within the operative field. In addition, experience in epicardial echocardiography is considered an important component of advanced, rather than basic, perioperative echocardiographic training. Current guidelines published by the ASE and SCA recommend that epicardial echocardiographic and epiaortic ultrasonographic training should include the study of 25 examinations, of which five are personally directed under the supervision of an advanced echocardiographer, before a trainee should pursue independent interpretation and application of the information to perioperative clinical decision making.5,6

A comprehensive epicardial echocardiographic and epiaortic ultrasonographic examination can be performed efficiently and safely,12 and may be the most practical intraoperative imaging technique when a TEE probe cannot be inserted or when probe placement is contraindicated. In addition, these techniques may offer better windows for imaging anterior cardiac structures including the aorta, AV, pulmonic valve, and pulmonary arteries, and therefore they may have a favorable influence on perioperative surgical decision making.13–15 Rosenberger et al analyzed the medical records of 6051 consecutive cardiac surgical patients who underwent epiaortic ultrasonography to determine a potential impact on intraoperative surgical decision making.16 The overall impact of epiaortic ultrasonography on surgical decision making was 4.1%, and included a change in the technique for inducing cardiac arrest during cardiopulmonary bypass in 1.8%, aortic atherectomy or replacement surgery in 0.8%, requirement for off-pump coronary artery bypass grafting in 0.6%, avoidance of aortic cross-clamping and use of ventricular fibrillatory arrest in 0.5%, change in arterial cannulation site in 0.2%, or avoidance of aortic cannulation in 0.2%. In addition, the authors noted that the overall stroke rate was lower in patients in whom intraoperative epiaortic ultrasonography was performed, compared with all patients undergoing cardiac surgical procedures. Nonetheless, epicardial and epiaortic imaging do have certain limitations, including a requirement for a sternotomy to permit direct access to the anterior surface of the heart and aorta, the inability to perform continuous monitoring, and the requirement for at least a brief interruption of the surgical procedure. Despite these limitations, a fundamental understanding of the skills required to obtain and interpret epicardial echocardiographic and epiaortic ultrasound images is an advantageous adjunct to intraoperative TEE in performing a comprehensive intraoperative echocardiographic examination.