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|Year : 2010
: 13 | Issue : 2 | Page
|Transesophageal echocardiaography evaluation of thoracic aorta
Hema C Nair
Department of Anaesthesia, Narayana Hrudayalaya Institute of Medical Sciences, Bommassandra Industrial Area, Anekal Taluk, Hosur Road, Bangalore - 560 099, India
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|Date of Submission||31-Dec-2008|
|Date of Acceptance||24-Feb-2009|
|Date of Web Publication||3-May-2010|
| Abstract|| |
Trans-esophageal echocardiaography is a sensitive, minimally invasive, diagnostic tool which gives real time functional image of the aorta. It helps in the diagnosis of pathologies of aorta like atherosclerosis, aneurysm and aortic dissection.
Keywords: Aorta, atherosclerosis, aneurysm, aortic dissection, transesophageal echocardiography
|How to cite this article:|
Nair HC. Transesophageal echocardiaography evaluation of thoracic aorta. Ann Card Anaesth 2010;13:186
| Introduction|| |
Transesophageal echocardiography has been shown to be a very sensitive diagnostic tool in the delineation and management of different aortic disease.  Hence, the ability to effectively interrogate and assess the aorta is an essential skill for an anesthesiologist wishing to practice cardiac anesthesia.
This topic will be discussed under three sections:
- Echocardiography approach to the examination, which includes different views needed to profile the aorta in its entirety
- Normal anatomy of the aorta
- Pathological conditions of the aorta
| Echocardiography Views|| |
The anatomic proximity of the aorta to the esophagus makes it possible to get a good view of the aorta, almost entirely. The proximal portion of the aortic arch lies adjacent to the trachea, which is interposed between the aorta and the esophagus, causing a virtual blind spot in that area. 
Two methods have been devised to circumvent this problem - use of epicardial probe to visualise the distal ascending aorta and adjacent arch, introduction of a fluid-filled balloon into the trachea to obliviate the air-filled space and hence provide a good medium for the ultrasound to pass through.  In routine practice, TOE evaluation of aorta is done by going systematically through six views.
These six imaging planes, defined by the American society of echocardiography (ASE) and Society of Cardiovascular Anesthesiologist (SCA) are-
ME ascending aorta SAX view is obtained at the 0° imaging plane with the probe pulled back about 1-2 cm from the 4 chamber view, approximately 25cm from the lip. A circular SAX view of the aortic root is seen flanked by the left atrium (LA) and right atrium (RA). Rotating the imaging plane to 90° will bring the LAX view showing ascending aorta. The distal ascending aorta is lost behind the air shadow of the trachea. This view is useful for defining wall thickness, aortic blood flow pattern and aortic dimension.
- Mid-esophageal (ME) Ascending Aorta short axis (SAX) view
- Mid-esophageal ascending aorta long axis (LAX) view
- Upper esophageal (UE) aortic arch
- Upper esophageal aortic arch LAX view
- Descending Aorta SAX view
- Descending Aorta LAX view
Next, the descending thoracic aorta is profiled, and for this the probe is positioned to get the ME 4 chamber view on display and then probe shaft is rotated through 180°. The circular cross section of the aorta appears in the upper portion of the view sector. The depth is increased to enlarge the image while also increasing the 2D frequency to improve resolution. Now, push the probe till the image of the aorta disappears, which is usually at the level of the diaphragm. By slowly pulling back, the probe, serial SAX view of the descending aorta can be seen. Rotational adjustment of the shaft should be done to keep the aorta centred. This is because of the changing position of the esophagus in relation to the aorta. The entire length of the descending aorta can be profiled in this fashion by serial SAX views. Rotating the imaging plane to 90° will bring into view the LAX image of the descending aorta.
To view the arch, the descending aortic SAX view is obtained at 0° and the probe pulled back slowly till the aorta elongates into a LAX view. This is the UE LAX view of the aortic arch. The proximal portion of the arch remains obscured behind the tracheal air shadow. The aorta displayed on the right of the screen is the distal portion of the arch, while at the left is the proximal arch. The anterior wall is below and the posterior wall is above and closest to the probe. Multiple SAX views of the arch are obtained at 90° by clockwise rotation of the probe shaft, and the last two branches of the arch, the left subclavian artery and the left common carotid artery can be seen in long axis view taking off from the aorta. Adjacent to the proximal portion of the aortic arch, the main pulmonary artery (MPA) and left pulmonary artery (LPA) can be seen. The right brachiocephalic branch of the arch is not seen by TOE because of the interposition of the trachea. Study of the thoracic aorta should include wall thickness, tissue characteristics, dimensions and blood flow patterns by Doppler assessment.
| Normal Anatomy|| |
The aorta is the largest blood vessel in the body with a diameter of approximately 35mm in adults. The wall of the aorta has three layers, the intima, media and adventitia. The intima is a thin layer of endothelial cells arranged on a basal lamina. Normally it is not very echogenic, but that can change in pathological conditions. The media, which forms 80% of the wall thickness, is formed of smooth muscle fibres arranged in a matrix of elastin and collagen in a spiral fashion. This layer provides the primary structural support and vascular tone to the aorta. The outermost layer is the adventitia which provides the nutrient supply by means of its lymphatics and vasa vasorum.
The aorta starts from the aortic valve, and ends in the abdomen where it divides into the two iliac arteries. In its length it is divided into four parts:
The ascending aorta arises from the left ventricle and the portion distal to the valve is called the aortic root where one can see the sinuses of valsalva and the take off of the two coronary arteries. As it courses superiorly it initially lies posterior and then to the right of the MPA. The distal third of the ascending aorta lies anterior to the trachea and right main stem bronchus.
- Ascending aorta
- Transverse aortic arch
- Descending thoracic aorta
- Abdominal aorta
The arch gives rise to its three branches from its superior aspect from where they course poster superiorly. The innominate and the left common carotid are in close proximity to the anterior aspect of the trachea making their visualisation difficult by TOE. The left subclavian lies to the left of the trachea. The descending aorta begins distal to the left subclavian artery at the ligamentum arteriosum, a remnant of the foetal patent ductus arteriosus (PDA). This area is called the aortic isthmus and is important to visualise in cases of Coarctation of aorta or PDA. This region is also used as a landmark since location of any pathology in the descending aorta is described in relation to the isthmus.
| Pathological Conditions of the Aorta|| |
Three main pathological conditions that TOE can help diagnose are
- Atherosclerotic disease
- Aortic aneurysm
- Aortic dissection
| Atherosclerotic Diseases of the Aorta|| |
With advancing age, generalized atherosclerosis increases and is very commonly seen in the elderly [Figure 1]. Any manipulation or instrumentation of these diseased aortas can lead to systemic embolisation of these atheromatous plaques leading to varying degrees of morbidity, and in severe cases can even lead to death.  The recorded incidence of neurological injury following cardio-pulmonary bypass (CPB) is 1-10%; therefore multisystem involvement could be higher. Generally, surgeons palpate the aorta for atheromatous plaques before making an aortotomy or placing a vascular clamp, but this method can only detect the most severe cases of atherosclerosis. The softer friable atheromas are not palpable, and in some instances the palpation itself can dislodge it and lead to embolic sequele. Severity of atherosclerotic disease can be graded into three:-
Grade I - Plaque < 2mm, insignificant disease
Grade II - Plaque or intimal thickening of 2-5mm, moderate disease
Grade III - Plaque > 5mm or mobile atheroma, severe disease.
In a patient with atheromatous disease, any form of handling the aorta, like cannulation, proximal anastomosis during CABG, clamping, can lead to dislodgement and embolisation. Plaques downstream, from the area of intervention, can also get embolised because of the high flow velocity from the aortic cannula acting as a jet stream.
Clinically, intra-operative atheroma embolisation can manifest in a variety of ways - from cognitive deficits or memory loss in milder cases to acute renal shutdown, bowel ischemia, visual deficits, limb gangrene or CVA in severe cases.
If the surgeon is forewarned to the presence of atherosclerotic disease, the procedure can be modified suitably to significantly reduce the chances of embolisation. In CABG, off pump surgery can be performed using total arterial pedicle grafts, cannulation sites can be chosen so that the atheroma are not disturbed and pump flows can be adjusted so as not to dislodge the plaques by blood flows.
| Aortic Aneurysm|| |
Aortic aneurysm is localized dilatation of the aorta that is at least 1.5 times the diameter of the expected normal value [Figure 2]. Aneurysms can be classified on the basis of their location, morphology and aetiology, and TOE can help in the former two situations. Aneurysm can be described as saccular and fusiform and can occur along the whole length of the aorta. Sixty five percent of all aortic aneurysms occur in the abdominal aorta. The ideal investigative method for this pathology is a CT angiography or MRI, but TOE is an alternative when a rapid diagnosis is necessary especially in the management of suspected aortic dissection.
TOE can help define the size and extent of the aneurysm, and also provide additional information about the presence of intra-luminal thrombus. This helps the surgeon to suitably modify the procedure to prevent embolisation. It can also help diagnose an acute or chronic dissection in these patients. In patients with aneurysm, TOE can help answer questions about the aortic valve and the need to repair/ replace it.
It helps plan management in that aneurysms which can be treated by endovascular stenting can be referred to the intervention cardiologist. In an acute presentation, it helps rule out aortic dissection, which has a bearing on the urgency of the corrective procedure.
| Aortic Dissection|| |
Aortic dissection results from a tear in the intima of the aorta causing blood to flow within the aortic wall forming a false lumen [Figure 3]. It could be localised or spread longitudinally, coursing for varying distances within the distal aorta and re-entering the true lumen through one or more secondary tears. The plane of dissection usually courses along the greater curvature of the ascending aorta and the arch of the aorta, while in the descending aorta it is mainly located lateral to the true lumen but most often spirals along its longitudinal axis following the arrangement of the connective tissue in the media. There are two commonly used classifications depending on the extent of involvement of the aorta. The classification is important in planning surgical repair.
The DeBakey classification:
Type I- Dissection extends from the ascending aorta and arch to the descending aorta.
Type II- Dissection limited to the ascending aorta.
Type III- Dissection begins in the descending aorta extending distally.
Type A, which involves ascending aorta irrespective of origin of tear/extent of dissection
Type B, which involves only descending aorta
Because of the urgent nature of aortic dissection and need for immediate intervention, TOE has a pivotal role in diagnosis. It can be done at the bedside, to give a real time image. Complete examination can be done in 15 to 20 minutes. It can be done in unstable patients, is minimally invasive, and has a proven safety record. Although CT angiography and MRI images are more sensitive and specific, it cannot be used in unstable patients, which is more often the case in this clinical scenario.
TOE gives real time images and is unique in its ability to give functional and hemodynamic information. Flow in both the true and false lumina is understood using Doppler colour flow imaging and pulsed or continuous wave Doppler, allowing for an alternative technique to identify intimal tears, when not directly visualised by two dimensional imaging. The coronary arteries can be observed for possible involvement in the dissection process. It helps in surgical decision making in that it can show the extent of dissection and thus involvement of other branches of aorta like the left sub-clavian artery, and also by demonstrating the entry and exit points.
Additional information obtained by TOE in patients with dissection is:
The pathognomonic echo appearance of dissection is an undulating linear density (the intimal flap) within the aortic lumen separating a true and false lumen, which have different Doppler flow patterns. However, dissection should be suspected whenever the single aortic wall appearance is replaced by two separate echo densities, one representing the intimal wall and the other the outer media and adventitia.
- Assessment of left ventricular function
- Aortic valve examination to look for aortic regurgitation (AR)
- Presence of pericardial or pleural effusion and hemothorax
A CT and MRI are highly sensitive and specific and can provide a lot of information about other branches of the aorta. However, they need to be reserved for a stable patient, require time. TOE is invaluable in unstable patients who need to be subjected to surgery on an urgent basis and also gives additional information pertaining to the function of the heart.
| References|| |
|1.||Kanchugar M, Delphin E. Assessment of surgery in the aorta. In: Savage RM, Aronson S, editors. Comprehensive textbook of intraoperative transesophageal echocardiography Philadelphia: Lippincott Williams and Wilkins; 2005. p. 569-82. |
|2.||Konstadt S. Assessment of thoracic aorta. In: Savange RM, Aronson S, editors. Comprehensive textbook of intraoperative transesophageal echocardiography Philadelphia: Lippincott Williams and Wilkins; 2005. p. 237-48. |
|3.||van Zaane B, Nierich AP, Buhre WF, Brandon Bravo Bruinsma GJ, Moons KG. Resolving the blind spot of transesophageal echocardiography: A new diagnostic device for visualizing the ascending aorta in cardiac surgery. Br J Anaesth 2007;98:434-41. [PUBMED] [FULLTEXT] |
|4.||Almassi GH, Sommers T, Moritz TE, Shroyer AL, London MJ, Henderson WG, et al. Stroke in cardiac surgical patients: Determinants and outcome. Ann Thorac Surg 1999;68:391-7. [PUBMED] [FULLTEXT] |
Hema C Nair
Department of Anaesthesia, Narayana Hrudayalaya Institute of Medical Sciences, Bommassandra Industrial Area, Anekal Taluk, Hosur Road, Bangalore - 560 099
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
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