Epiaortic Imaging of the Ascending Aorta:  CT Anesthesiologists’ and Surgeons’ Interventions and the Impact on Stroke Prevention
Igor O. Zhukov, M.D.  and Kathryn E. Glas, M.D.

​Introduction
Epiaortic ultrasonography (EAU) use has been steadily increasing in the cardiac operating rooms over the last several decades, especially for procedures involving manipulation of the aorta.  This is based on decades of experience that has taught us that one cannot determine in advance those who will or will not have significant atherosclerosis of the ascending aorta based on preoperative examination, short of obtaining a high resolution CT – a not commonplace practice.  We have definitively learned that the surgeon, using only palpation of the ascending aorta, may miss significant atherosclerotic plaques. Epiaortic ultrasonography enables the surgeon to alter his or her approach depending on the size and the location of aortic atheromatous burden, and provides an opportunity for intervention guidance during aortic cannulation, cross clamping and aortotomy [1].
 
Equipment
Since the target of the ultrasound examination is located at the surface of the operative field, resolution, rather than penetration is the primary objective in selecting a suitable ultrasound probe. Frequencies of 8 to 15 MHz provide optimal range of penetration while preserving excellent spatial resolution of the ascending aorta [2]. Ultrasound probe construction employed for epiaortic ultrasonography can be one of 3 types, each with its own array of benefits and tradeoffs [3].

Linear ultrasound probes (Figure 1A) emit a beam with the width equal to the probe itself and are able to image objects through the entire depth of the ultrasound beam. These probes are commonly used in vascular access procedures and regional anesthetics, but can be adapted for EAU as well. The beam width limitation can interfere with adequate visualization of lateral walls of the aorta, thus making the examination technically more challenging and possibly requiring several passes to adequately image the entire aorta.
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Phased array transducers (Figure 1B) emit the scanning ultrasound beam along a fan-shaped sector, capturing an area wider than the probe itself; this may provide sectors as wide as 900 for a more complete lateral wall visualization. These probes are also available in high frequency configurations equal to or greater than 7MHz and are capable of providing excellent spatial resolution. The tradeoff in using the phased-array transducer comes in the inability to image structures immediately adjacent to the footprint of the probe due to the high density of the ultrasound signal in that sector, known as the “near-field artifact”. This can obscure the atheroma on the anterior surface of the aorta if the probe is placed too close to the anterior aortic wall. Using a stand-off device (Figure 2) or bracing the ultrasonographer’s arm such that the probe is elevated approximately 1cm above the aorta mitigates this artifact.

​At our institution, we utilize the connector from the adult breathing bag of the anesthesia circuit as the standoff.  The connector fits snugly over the standard-sized epicardial and epiaortic probes and the probe surface resting 1-2cm below the plane of the standoff allows for optimal imaging of the anterior wall of the aorta.
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With the advent of matrix array transducers (Figure 3), imaging in 2 places simultaneously as well as full-volume sampling for 3D reconstruction became possible.  These transducers can image both long and short axis views during a single passage of the probe and may potentially improve localization of the plaque by triangulating its location on 2 imaging planes simultaneously. The tradeoff of using the matrix array is that the sector width is usually smaller than the phased array transducers, on the order of 600, which may impede lateral wall imaging in a single pass. Also, three-dimensional imaging imposes reduction of spatial and/or temporal resolution that may be detrimental in locating small fast-moving mobile atheromas.  Additionally, there is a cost penalty associated with obtaining matrix array probes as compared to similar size and frequency phased-array units.

​There are no reliable means of sterilizing ultrasound probes, and since these must be placed into the sterile operative field for an EAU examination, a sterile sheath filled with warm saline is used to avoid contamination of the sterile environment (Figure 4).  Ultrasound gel can also be used, but warm saline is much more readily available and is what is routinely used at our institution.

The sterile saline is placed into the sheath by the surgeon making the opening readily accessible to accommodate the probe.  The sheath end is handed to the sonographer and working together, the probe is fed into the entire length of the sheath keeping as much of the sheath sterile as possible.  Although more practical if the surgeon performs the EAU, the probe manipulation can be performed either by the surgeon under ultrasonographer’s guidance or by an ultrasonographer, after appropriate surgical gowning and gloving. The locator marker of the probe by convention is placed towards the patient’s left side and the probe is held such that the imaging plane is orthogonal to the aortic long axis. It can be difficult to visualize the locator marker on the probe when a standoff is used.  In this circumstance, review of the image to ensure the superior vena cava (SVC) is on the left side of the image is recommended. 

​Imaging planes
The ascending aorta is divided into proximal, mid, and distal segments by the relative position of the right pulmonary artery; both short- and long-axis views of each segment are routinely obtained (Figure 5) [3]. Additionally, the proximal aortic arch can usually be seen in short and long axis on an epiaortic examination and it may be the only intraoperative modality available to examine for aortic pathology as transesophageal imaging of this segment is usually impeded by the left main bronchus [4]. Once the SAX views are obtained, turning the probe 900 will allow the LAX views to be evaluated.  In the long axis view of the aortic arch the takeoff of the brachiocephalic and sometimes left carotid and left subclavian arteries can be visualized for a complete aortic arch examination.

Measurement of the aortic lumen diameter is done in the mid-ascending SAX view from anterior wall to posterior wall.  It is measured from inner edge to inner edge.  Due to excellent resolution of the EAU these measurements approximate the measurements obtained on CT or MRI modalities [5]. These measurements should be captured in a true SAX view of the aorta to minimize image distortion.  The SAX view should appear as a circle.  With a little experience, it is easy to obtain a true SAX view of the ascending aorta.  An elliptical cross-section appearance of the aorta should clue an ultrasonographer that the imaging plane has deviated from being orthogonal to the long axis of the vessel. In this instance, a medial-to-lateral measurement can be taken to cross reference the anteroposterior measurement if significant distortion is suspected (Measurement B, Figure 5A).  In order to obtain the proximal, mid and distal epiaortic SAX views, the ultrasonographer must move the probe to the appropriate region, then obtain a circular image so that the intima can be seen at each segment, stopping long enough at each to allow image acquisition.

​Atheroma thickness is also measured in short axis views; it is the distance from the inner aortic lumen-to-atheroma interface with the intimal layer of the aorta (Figure 5A).

In the long axis view of the aorta, the vessel can usually be imaged starting from the aortic valve and sinuses of Valsalva through proximal aortic arch as shown above. This allows further localization and quantification of the atheromatous burden as well as selection of suitable cannulation, cross-clamping and aortotomy sites.

Atherosclerosis assessment and quantification
There are several systems for atheroma grading. The American Society of Echocardiography and the Society of Cardiovascular Anesthesiologists have endorsed the Katz et. al. [6] grading system. This grading scale has also recently been endorsed by the Society of Thoracic Surgeons and European Association of Cardiovascular Imaging [7] and is therefore, the currently recommended atheroma grading methodology.

The walls of the proximal, mid and distal segments of the ascending aorta in short axis are characterized by the maximal thickness of the plaque (Table 1) and plaque location anterior, posterior, right, and/or left (Figure 6).  The diameter of the aorta is usually measured at the mid ascending level. The atheroma grade is assigned based on the maximal thickness of the plaque in each segment, as well as the presence of mobile components, summarized below in (Table 1).  Discovery of grade 3, 4 or 5 disease warrants immediate discussion with the surgeon prior to any aortic manipulation. If the epiaortic examination was performed in the absence of the primary surgeon, (i.e., the cardiac surgery resident doing the initial scan) the probe can be kept on the field until the primary surgeon has the opportunity to exam the ascending aorta.  Furthermore, an attempt should be made to reimage mobile atheroma after the conclusion of surgery.

If at all possible, the examination should be discussed with the surgeon as it is being performed so that treatment strategy for any unusual findings can be formulated. The complete report should be placed in the patient’s chart or EHR as soon as practical.  The latest American Society of Echocardiography recommendation specifies a window of 24 hours. It is highly recommended to use a common template when generating EAU report to ensure complete and consistent evaluations. An example of such template is provided in (Figure 6).

Intervention
Stroke prevention is an important topic in the cardiac operating room as many procedures carry a significant CVA risk (table 2) [8]. CVA after CABG dramatically lowers survival [9] and results in a significant increase in patient morbidity and financial burden on healthcare resources and costs [10]. As seen in (Table 2), CVA risk is higher with combined CABG/valve surgery, all valve surgery and thoracic aortic surgery.  

While epiaortic ultrasound provides minimal CVA protection in instances of negligible aortic plaque, the decrease in morbidity during off-pump CABG as well as alternative anastomotic techniques (11) in patients at high risk for CVA can be significant.  It has been shown that a single clamp and no-touch aortic approach during Coronary Artery Bypass Grafting (CABG) decreases the incidence of post-operative stroke; [11, 12].  EAU and alternative approaches to minimize manipulation of the aorta may achieve the maximal stroke-prevention benefit in patients who are at high risk of CVA complications.  Despite strong controversy in stroke prevention ability of off-pump CABG [13, 14, 15, 16, 17], a 2012 European meta-analysis of almost 9000 patients showed a 30% reduction in perioperative rate of stroke  (1.4 versus 2.1%) when using off-pump bypass surgery compared to traditional on-pump bypass surgery [18].

A significant decline in post-operative stroke incidence was also demonstrated in a recent randomized –controlled trial comparing the outcomes of patients screened with EAU to those that were screened with TEE only. The proposed explanation was the higher rate of using a partial aortic clamp, all arterial grafting, and Y-graft techniques in patients with positive epiaortic ultrasonography findings [19]. The 2011 ACCF/AHA guidelines recommend EAU prior to aortic manipulation during coronary artery bypass grafting. Data continues to accumulate that using EAU scanning of the ascending aorta is warranted whenever manipulation of the ascending aorta is planned to allow for alternative approaches to the aorta if significant atherosclerotic plague is seen [20].

Other uses of Ultrasound in the operative field
Availability of Epiaortic Ultrasound probe in the operating room allows for unconventional application of ultrasound when unexpected surgical complications arise or during uncommon surgical procedures. Aortic dissection during cannulation is an infrequent [21] but serious complication that requires prompt recognition and treatment. Epiaortic ultrasound can supplement transesophageal echocardiographic findings or serve as a sole diagnostic modality [22] that can assist in dissection treatment by guiding the cannulation of the true lumen [23] for cardiopulmonary bypass initiation to allow definitive surgical repair.
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Epicardial examination of the heart may also provide the information needed for a safe execution of heart surgery for patients in whom there is a contraindication to transesophageal echocardiography (TEE). The selection of ultrasound transducers, imaging artifacts, limitations of the transducers, and the handling of the echo probe are similar to the EAU procedure [24]. The views obtained during epicardial ultrasonography are derived from the transthoracic and transesophageal echocardiography standards and bear similar nomenclature as shown in the table below (Table 3).
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As with EAU, probe manipulation can be done by an ultrasonographer. Realistically it is more practical for the surgeon to acquire images with the ultrasonographer’s guidance and input at the ultrasound machine controls.
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Typically, cardiac anesthesiology ultrasonographers and surgeons are less familiar with epicardial examination; however, in select circumstances this intervention may allow the procedure to continue in the absence of a TEE probe and provide a reasonable echocardiographic assessment of the patient’s heart [25].

Summary
Epiaortic ultrasonography of the ascending aorta is an easily acquired skill, which once learned, is easy to perform and provides real-time information that allows interventions at the point of care. Its use continues to gain acceptance by cardiac surgeons and anesthesiologists who understand that advance knowledge of the disease state of the ascending aorta, prior to aortic manipulation, allows the surgeon to change the approach to minimize disruption of plagues and potentially prevent embolic strokes.  The aging cardiac surgery population presents with more co-morbidities and any complication carries high mortality and/or morbidity risks. Palpation of the aorta does not detect all atheromas, EAU of the ascending aorta allows the surgeon to know the location and characteristics of atheroma in the operative field. Armed with data, the surgeon can then decide if an alternative approach should be used. The availability of standardized guidelines for imaging and atheroma quantification will undoubtedly allow for more robust research of EAU usefulness, and ultimately will solidify the usage guidelines for EUA and its role in embolic stroke prevention. The risk/benefit profile clearly favors the use of EAU.

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