The examination of the thoracic aorta is an essential component of both routine and emergent echocardiographic examinations. The aorta and its major branches can be visualized with echocardiography using a variety of imaging fields, as well as methods of ultrasound. When a transesophageal echocardiography (TEE) examination is combined with epiaortic ultrasound (see Chapter 5 ), detailed information on a range of surgical pathologies is provided.
In addition to the information provided in this chapter, readers are referred to the comprehensive review on thoracic aortic pathology Hiratzka et al.
Anatomy
The thoracic aorta is divided into the aortic root, ascending aorta, aortic arch, and descending thoracic aorta. The aortic root includes the (AV) annulus, cusps, and sinuses of Valsalva. The ascending aorta begins at the sinotubular junction, includes the tubular portion of the ascending aorta, and extends toward the right and around the main (PA), crossing the right PA anteriorly. The ascending aorta is typically 4 to 5 cm in length and ends at the origin of the brachiocephalic (innominate) artery. The aortic arch starts at the origin of the brachiocephalic artery and extends to the origin of the left subclavian artery. The aortic arch lies anterior to the trachea, superior to the PA, posterior to the innominate vein, and to the left of the esophagus; it is 4 to 5 cm in length. The descending thoracic aorta arises at the origin of the left subclavian artery and extends to the level of the diaphragm from which it continues as the abdominal aorta. The descending thoracic aorta starts to the left of the vertebral column and moves to the right as it descends.
The normal dimensions of the ascending and descending aorta are provided in Appendix 3 .
Echocardiographic classification
The thoracic aorta can be divided into six zones, which correspond to regions of potential manipulation during cardiac surgery. The ascending aorta is divided into zones 1 to 3 (the proximal, middle, and distal ascending aorta), the arch is split into zones 4 and 5 (proximal and distal halves), and the entire descending thoracic aorta forms zone 6. Zone 1 is the site of aortotomy for AV replacement; zone 2 is the site of the proximal anastomoses of coronary grafts; zone 3 is the site of aortic cross-clamping and antegrade cardioplegia cannulation; zone 4 is the site of aortic cannulation; zone 5 is not manipulated directly but can be affected by the jet stream of CPB flow; and the proximal part of zone 6 is the site for a correctly positioned IABP tip. Only zone 1, part of zone 2, and zones 5 and 6 can be reliably imaged with TEE.
Echocardiographic examination
It is important to use a systematic approach to the examination of the thoracic aorta; the image sequence should follow the outline given in Chapter 3 . When imaging the arch and descending aorta, it is helpful to reduce the sector depth to 5 to 6 cm, increase the probe frequency, and reduce the gain setting. These adjustments provide a more detailed image of the aorta and help to differentiate the intima (gray) from the media (white).
Ascending aortic views
A useful starting point is the midesophageal AV short-axis view (see Figures 3-4 and 10-2 ). Pathology involving the ascending aorta is frequently associated with abnormalities of the AV and aortic root. In particular, the presence of a bicuspid AV (which is associated with dilatation of the ascending aorta and aortic coarctation), a dilated aortic root, and aortic regurgitation should be noted. In the midesophageal AV long-axis view (see Figure 3-5 ) the dimensions of the aortic annulus and sinotubular junction can be measured (see Figure 10-5 ). This view is also useful for assessing aortic regurgitation and identifying aortic root pathology such as a sinus of Valsalva aneurysm and aortic dissection.
The midesophageal ascending aortic short- and long-axis views (see Figures 3-6 and 3-7 , respectively) are useful for the identifying aortic atheroma and dissection and for measuring the diameter of the proximal ascending aorta. As a rough guide, the aorta should be about the same diameter as the PA.
Aortic arch and descending aortic views
The upper esophageal aortic arch long- and short-axis views allow visualization of the distal aortic arch (see Figure 3-23 ). In the short-axis view (60 to 90 degrees), the origin of the left subclavian artery can usually be identified. This is an important landmark that has implications in a number of clinical situations. The tip of an IABP should be just distal to the left subclavian artery. The origin of a patent ductus arteriosus lies opposite the origin of the subclavian artery, and this is the most common site for aortic coarctation and traumatic aortic injury. Finally, whether the origin of a descending thoracic aneurysm occurs above or below the left subclavian artery determines the CPB technique used to facilitate surgical repair.
The descending thoracic aorta is usually well seen in the descending aortic short- and long-axis views. In short axis, it is frequently necessary to turn the shaft of the probe to the left or right to keep the aorta centered on the screen. The short-axis image may become noncircular (“sausage shaped”) in the presence of an ectatic or unfolded aorta. The view of the aorta is usually lost or becomes poor as the stomach is entered. The descending thoracic aortic views are useful for the identification of aortic atheroma, aortic dissection, and left pleural effusion.
Limitations of TEE for assessing the thoracic aorta
Imaging of the distal ascending aorta (zone 3) and proximal arch (zone 4) is limited by interposition of the trachea and left main bronchus between the esophagus and the aorta. Consequently, these regions cannot be visualized with TEE in most patients. Imaging of the proximal ascending aorta (zone 2) also suffers because it is an anterior structure and lies in the far lateral position (on the right) of the sector scan. In addition, posterior aortic calcification causes ultrasound dropout, further degrading the quality of the anterior wall image. For these reasons, epiaortic ultrasound is used for assessing the distal ascending aorta and proximal aortic arch. TEE examination of the thoracic aorta is affected by reverberation artifacts, which can occur from calcification in the aortic wall or from vascular catheters. Reverberation artifacts can appear as a mobile linear echodensity within the aortic lumen and be misdiagnosed as an aortic dissection. A curved artifact in the ascending aorta is often seen from a PA catheter; this too can be misdiagnosed as a dissection. In the descending aorta, a reverberation artifact of the whole aorta is often seen ( Figure 11-1 ) and may resemble an aneurysm or a dissection. Disconcertingly, color Doppler is often seen in the reverberation artifact, although with less intensity than in the true image. Reverberation artifacts should be twice or half the distance from the probe to the true image. Occasionally, side-lobe artifacts can also give the appearances of a dissection flap within the aorta.
Inaccuracies in the measurements of aortic dimensions can arise because of the effects of oblique or off-axis imaging (see Figure 21-7 ). In short axis, the aorta may appear oval due to the effects of oblique imaging, leading to overestimation of the aortic diameter. In long-axis imaging, the aortic diameter may be underestimated by off-axis imaging, which fails to cut through the center of the aorta. An oblique cut, most apparent in the aortic arch long-axis view, can make the aortic wall appear falsely thickened. The curved shape of the aortic arch can give the erroneous impression of atheromatous thickening in the lateral parts of the sector scan. These artifacts are all less likely with epiaortic ultrasound.
The aorta expands slightly in systole, causing a small increase in its diameter. To eliminate this effect, all measurements should be made at the same time during the cardiac cycle (the aortic dimensions provided in Appendix 3 are from end diastole), measured from inner edge to inner edge.
Aortic atheroma
Aortic atheroma is a significant risk factor for stroke in both medical and surgical populations. Dislodgement and fragmentation of atheroma during aortic manipulation (e.g., cannulation and clamping) have been identified as important causes of perioperative stroke and probably contribute to neurocognitive dysfunction following cardiac surgery.
A direct relationship between the amount of aortic manipulation during cardiac surgery and stroke has been demonstrated.
The atheroma burden in the cardiac surgical population increases with distance along the aorta from the aortic root so that the ascending aorta has the lowest frequency of atheroma and the descending aorta has the highest frequency. Older patients are more affected than younger patients, with nearly one third of those older than 70 years in one series having moderate or severe atheroma in the ascending aorta and proximal arch. In addition, those patients with coronary artery disease, peripheral vascular disease, cerebral vascular disease, renal impairment, calcific aortic stenosis, and mitral annular calcification all have a high incidence of aortic atheroma.
There are two key steps to reducing neurologic morbidity from aortic atheroma: (1) detecting the atheroma accurately and (2) avoiding dislodging it from the aortic wall during surgical manipulations.
Detection of atheroma in the thoracic aorta
Epiaortic ultrasound
Epiaortic ultrasound is the most accurate method for detecting atheroma in the ascending aorta and proximal aortic arch. Refer to Chapter 5 for a detailed discussion of the epiaortic ultrasound examination, and the measurement and grading of atheroma.
Direct manual palpation
Many surgeons use manual palpation to assess aortic atheroma. This method is inferior to epiaortic ultrasound, and when compared with epiaortic ultrasound, at best about 50% of atheroma is detected. Soft, noncalcified atheroma is more likely to embolize than calcified atheroma and is poorly detected by manual palpation.
Transesophageal echocardiography
TEE is accurate for detecting atheroma ( Figures 11-2 and 11-3 ), and examination of the entire aorta that can be visualized with TEE is an important part of a routine perioperative examination. However, evaluation of the distal ascending aorta and proximal arch is limited by the interposition of the trachea and bronchi between the esophagus and the aorta. These are the most important areas to visualize because they are where most surgical manipulation of the aorta occurs. Despite the limitations of TEE, there is some evidence that mobile atheroma detected in the descending thoracic aorta is predictive of postoperative stroke. Furthermore, in two small studies, the absence of moderate-to-severe atheroma in the parts of the aorta that were visible with TEE was associated with a very low risk of perioperative stroke.
However, moderate-to-severe atheroma is far more common in the descending thoracic aorta than in the ascending aorta or aortic arch. Therefore, atheroma in the descending aorta is a poor predictor of atheroma in the ascending aorta or arch, and TEE is able to identify fewer than 30% of patients shown with epiaortic scanning to have moderate or severe atheroma in the ascending aorta or arch.
TEE findings of severe atheroma in the descending aorta may rule out the use an IABP and the use of femoral artery cannulation for CPB.
To provide visualization of the ascending aorta and thus exclude the presence of significant atheroma with TEE, the use of a saline-filled balloon in the distal trachea and left bronchus has been described.
A suggested strategy for detecting atheroma
Current intraoperative practice with respect to screening the aorta for atheroma is variable. Epiaortic ultrasound is the method of choice for reliably determining atheroma at sites where surgical manipulation of the aorta occurs. There are currently no large studies to guide practice, but we suggest that, as a minimum, epiaortic ultrasound be used on high-risk patients (e.g., patients over 70 years of age and patients with a history of cerebrovascular or peripheral vascular disease), as well as on those in whom significant atheroma (e.g., >3 mm or a mobile component irrespective of size) is detected with TEE in the descending aorta.
Therapeutic options when significant atheroma is detected
Screening for atheroma is of little value unless coupled with proactive interventions to avoid displacement of atheroma during surgical manipulation. Several possible alterations in surgical technique can be considered. However, there are currently no prospective, randomized trials demonstrating improved outcome or a reduced rate of complications based on altering the surgical procedure because of the intraoperative assessment of ascending aorta or aortic arch atheroma burden. The simplest technique is to select sites for aortic manipulation that are distant from the sites of atheroma, but it may not be technically possible to do this. For standard CABG surgery with CPB and aortic cross-clamping, a single aortic cross-clamp (rather than a complete aortic cross-clamp which is subsequently exchanged for a partial or ‘side-biting’ clamp to complete the proximal graft anastomoses) has been advocated. The arterial cannulation site may be moved to avoid cannulating the ascending aorta or aortic arch. Suitable alternative sites may inculde the axillary or femoral artery. Off-pump CABG surgery can be considered, although some aortic manipulation is required unless a technique that doesn’t require partial aortic manipulation is required unless a technique that doesn’t require partial aortic occlusion for proximal anastomoses is used. An exclusive pedicle Y graft technique or a technique using a clampless anastomotic device have been used to achieve this. Deep hypothermic circulatory arrest can be employed to arrest the heart without clamping the ascending aorta.
Aortic aneurysm
An aneurysm is a localized or diffuse dilatation of the aorta to 50% or more above normal size ( Figure 11-4 ). Some of the more common causes of thoracic aortic aneurysms are outlined in Table 11-1 . The most common location for aneurysms to occur is the ascending aorta and the aortic root. Aneurysms of the descending aorta are less common, and arch involvement is least common.
|
* Formerly known as cystic medial necrosis. There is destruction and loss of elastic fibers, increased deposition of proteoglycans, and loss of smooth muscle cells in the media.
† Idiopathic destruction of the elastic fibers of the media affecting the aortic annulus and proximal ascending aorta.
Patients with aneurysms are at increased risk of: (1) aneurysm rupture, (2) aortic dissection, (3) compression of adjacent structures (e.g., left recurrent laryngeal nerve, trachea, left main bronchus, esophagus, and SVC), and (4) a thrombus formation within the aneurysm, with or without embolization.
Indications for intervention depend on the cause of the aneurysm, the location of the aneurysm within the aorta, the rate of aneurysmal growth, and the presence and severity of associated aortic regurgitation. Symptom onset, especially pain, is an indication for urgent intervention, irrespective of aneurysm size.
Beyond establishing the presence of an enlarged aorta, echocardiography may reveal associated cardiac pathology that suggests the underlying etiology of the aortic disease (e.g., bicuspid AV) and its effects (e.g., aortic regurgitation and a dilated left ventricle). Caution should be exercised when diagnosing an aneurysm with TEE if the image quality is poor or if the aortic dimensions are at the upper limits of normal. If there is any doubt, or in the case of an unexpected finding in the operating room, epiaortic ultrasound should be used for more accurate measurement.
Aortic root and ascending aortic aneurysms
Aneurysms of the aortic root and ascending aorta are repaired with open surgical techniques. Indications for repair typically include an ascending aorta or aortic sinus diameter of 5.5 cm or greater. To avoid acute dissection or rupture, patients with Marfan syndrome or other genetic disorders (e.g., Turner syndrome or bicuspid AV) usually undergo elective operation at smaller diameters (4.0 to 5.0 cm, depending on the condition). Current recommendations are that patients undergoing AV repair or replacement who have an ascending aorta or aortic root diameter of greater than 4.5 cm should be considered for concomitant repair of the aortic root or replacement of the ascending aorta.
The operative technique depends on a number of factors, including whether or not the aortic root and valve are involved, and may involve a combination of graft deployment, coronary reimplantation, and AV-sparing procedures or valve replacement.
Dilatation of the aortic root may be primary or secondary (e.g., distal to a stenotic bicuspid AV). Aortic dilatation that primarily involves the sinotubular junction is a common cause of aortic regurgitation as a result of loss of coaptation of normal AV cusps ( Figure 11-5 ). It is important to ascertain the mechanism of aortic regurgitation and to differentiate between normal and abnormal aortic leaflet morphology on the pre-(CPB) TEE examination, as these findings have implications for the surgical approach to the AV (valve sparing or replacement). In particular, the presence of leaflet thickening or calcification, commissural fusion, and leaflet prolapse (diastolic doming) ( Figure 11-6 ) should be identified. Suitable 2-D TEE views are the midesophageal AV short- and long-axis views, the transgastric long-axis view, and the deep transgastric long-axis view. The direction of the regurgitant jet should also be ascertained using color flow Doppler imaging. Predictors of the need for AV replacement include commissural thickening, cusp calcification and central aortic regurgitation (suggestive of rheumatic disease or annular dilation).
Following repair, TEE is used to assess AV, (LV), and (RV) function and to rule out new (SWMAs), which are suggestive of coronary insufficiency. For operations involving reimplantation of coronary arteries, it is useful to identify flow in the coronary ostia with either TEE or epicardial imaging. Abnormal flow (such as occurs with a kinked artery) is of high velocity and associated with a mosaic pattern on color flow Doppler imaging. The coronary arteries are best imaged by withdrawing the probe slightly from the midesophageal AV short-axis view until the origin of the left (see Figure 10-3 ) and, variably, the right coronary arteries are seen.
Descending thoracic and thoracoabdominal aortic aneurysms
Intervention for descending thoracic and thoracoabdominal aneurysms is a rapidly evolving area and may involve open surgical repair or endovascular stent grafting. In patients with extensive disease, limited physiologic reserve, or complex aneurysmal anatomy, a combined open and endovascular (“hybrid”) approach may be undertaken. Current management recommendations are as follows: for patients with chronic dissection, particularly if associated with a connective tissue disorder but without significant comorbid disease, and a descending thoracic aortic diameter exceeding 5.5 cm, open repair is recommended. For patients with degenerative or traumatic aneurysms of the descending thoracic aorta exceeding 5.5 cm, saccular aneurysms, or postoperative pseudoaneurysms, endovascular stent grafting should be strongly considered when feasible. Finally, for patients with thoracoabdominal aneurysms, in whom endovascular stent graft options are limited and surgical morbidity is elevated, elective surgery is recommended if the aortic diameter exceeds 6.0 cm or less if a connective tissue disorder such as Marfan syndrome is present.
TEE is a useful monitor during surgical repair of descending aortic aneurysms. With femoral–femoral bypass, TEE can be used to verify that the position of the venous cannula is optimal. With a multihole cannula, the tip should be in the (SVC) just above the (RA–SVC) junction to ensure drainage from the SVC, right atrium, and (IVC). This is best visualized in the midesophageal bicaval view. With a left thoracotomy, the heart cannot be directly inspected during CPB, and TEE may be invaluable in detecting unexpected ventricular dilatation secondary to aortic regurgitation. If partial (left heart) bypass—in which blood is drained from the left atrium and returned to the femoral artery or descending aorta—is used, then TEE can be used to confirm the LA cannula is in the left atrium, not through the MV, and to monitor LV volume and RV function. When a clamp-and-sew technique is used, the afterload imposed by aortic cross-clamping may precipitate acute LV distension, myocardial ischemia, or both, which can be detected as new SWMAs. Worsening of pre-existing aortic and mitral regurgitation or global LV impairment can also occur with aortic cross-clamping. When the cross clamp is removed, TEE may be used to assess LV volume.
Endovascular approaches most commonly involve stent graft deployment from the femoral artery. Once the graft has been deployed, the proximal and distal ends of the stent graft are “sealed” to the aortic wall by endoluminal balloon inflation. TEE is useful for confirming the expected aortic pathology, confirming adequate proximal (and sometimes distal) landing zones for stent graft seal (e.g., distal to the left subclavian artery, which can be imaged in the upper esophageal aortic arch views), and confirming the absence of thrombus or calcification at the landing zone, which might impair endograft apposition to the aortic wall. TEE is also useful to exclude other aortic pathology, particularly atheromatous plaques, which may not have been well seen on prior computed tomography (CT) imaging. Significant atheroma at the site of the stent graft landing zone may result in embolization of atheromatous lesions following stent graft deployment. Compared with angiography, TEE may provide more exact vessel and lesion sizing, as well as localization. The stent graft system can be clearly visualized in the descending aorta on TEE at all stages of the procedure, from guidewire insertion to balloon inflation and stent expansion. Accurate placement of the stent graft, which is essential to ensure exclusion of the aneurysmal sac from aortic flow, can be assisted by TEE ( Figure 11-7 ). In Stanford type B aortic dissections, TEE can be helpful in guiding the placement of the guidewire into the true lumen (see the following section). The TEE probe can also serve as a useful marker of aortic level on fluoroscopy.
