Thoracic Aortic Aneurysm

Background

Definitions

Aneurysmal degeneration can occur anywhere in the human aorta. By definition, an aneurysm is a localized or diffuse dilation of an artery with a diameter at least 50% greater than the normal size of the artery.

Most aortic aneurysms (AAs) occur in the abdominal aorta; these are termed abdominal aortic aneurysms (AAAs). Although most AAAs are asymptomatic at the time of diagnosis, the most common complication remains life-threatening rupture with hemorrhage. Aneurysmal degeneration that occurs in the thoracic aorta is termed a thoracic aortic aneurysm (TAA). Aneurysms that coexist in both segments of the aorta (thoracic and abdominal) are termed thoracoabdominal aneurysms (TAAAs). TAAs and TAAAs are also at risk for rupture. One population-based study suggests an increasing prevalence of TAAs.

TAAs are subdivided into the following three groups depending on location:

• Ascending aortic aneurysms
• Aortic arch aneurysms
• Descending thoracic aneurysms or thoracoabdominal aneurysms

Aneurysms that involve the ascending aorta may extend as far proximally as the aortic annulus and as far distally as the innominate artery, whereas descending thoracic aneurysms begin beyond the left subclavian artery. Arch aneurysms are as the name implies.

Dissection is another condition that may affect the thoracic aorta. An intimal tear causes separation of the walls of the aorta. A false passage for blood develops between the layers of the aorta. This false lumen may extend into branches of the aorta in the chest or abdomen, causing malperfusion, ischemia, or occlusion with resultant complications. The dissection can also progress proximally, to involve the aortic sinus, aortic valve, and coronary arteries.

Dissection can lead to aneurysmal change and early or late rupture. A chronic dissection is one that is diagnosed more than 2 weeks after the onset of symptoms. Dissection should not be termed dissecting aneurysm, because it can occur with or without aneurysmal enlargement of the aorta.

The shape of an aortic aneurysm is either saccular or fusiform. A fusiform (or true) aneurysm has a uniform shape with a symmetrical dilatation that involves the entire circumference of the aortic wall. A saccular aneurysm is a localized outpouching of the aortic wall, and it is the shape of a pseudoaneurysm.

For patient education resources, see Aortic Aneurysm.

Treatment

Treatment of AAAs, TAAAs, and TAAs involves surgical repair in good-risk patients with aneurysms that have reached a size sufficient to warrant repair. Surgical repair may involve endovascular stent grafting (in suitable candidates) or traditional open surgical repair.

The development of treatment modalities for thoracic aneurysms followed successful treatment of abdominal aortic aneurysms. Estes' 1950 report revealed that the 3-year survival rate for patients with untreated AAAs was only 50%, with two thirds of deaths resulting from aneurysmal rupture.^[1] From that tiem forward, increased attempts were made to devise methods of durable repair.

Most of these initial successful repairs involved the use of preserved aortic allografts, thus triggering the establishment of numerous aortic allograft banks. Simultaneously, Gross et al successfully used allografts to treat complex thoracic aortic coarctations, including those with aneurysmal involvement.^[2]

In 1951, Lam and Aram reported the resection of a descending thoracic aneurysm with allograft replacement.^[3] Ascending aortic replacement required the development of cardiopulmonary bypass and was first performed in 1956 by Cooley and DeBakey.^[4] They successfully replaced the ascending aorta with an aortic allograft. Successful replacement of the aortic arch, with its inherent risk of cerebral ischemia, was understandably more challenging and was not reported until 1957 by DeBakey et al.^[5]

Although the use of aortic allografts as aortic replacement was widely accepted in the early 1950s, the search for synthetic substitutes was well underway. Dacron was introduced by DeBakey. By 1955, Deterling and Bhonslay believed that Dacron was the best material for aortic substitution.^[6] Numerous types of intricately woven hemostatic grafts have since been developed and are now used much more extensively than their allograft counterparts. Such Dacron grafts are used to replace ascending, arch, thoracic, and thoracoabdominal aortic segments.

However, some patients required replacement of the aortic root, as well. Subsequently, combined operations that replaced the ascending aneurysm in conjunction with replacement of the aortic valve and reimplantation of the coronary arteries were performed by Bentall and De Bono in 1968, using a mechanical valve with a Dacron conduit.^[7] Ross, in 1962, and Barratt-Boyes, in 1964, successfully implanted the aortic homograft in the orthotopic position.^{[8, 9]} In 1985, Sievers reported the use of stentless porcine aortic roots.^[10]

Subsequently, less invasive therapies for descending TAA were developed. Dake et al reported the first endovascular thoracic aortic repair in 1994.^[11] In March 2005, the US Food and Drug Administration (FDA) approved the first thoracic aortic stent graft, the GORE TAG graft (W.L. Gore and Associates; Flagstaff, AZ). Since 2005, two other devices have gained FDA approval: the Talent Thoracic endograft (Medtronic; Santa Rosa, CA) and the Cook TX2 endograft (Cook; Bloomington, IN). Several successive next-generation reiterations of all of these devices have also gained approval.

Given the relative acceptance of the indications for thoracic endografts as an alternative to open procedures in the treatment of uncomplicated diseases of the descending thoracic aorta, experienced users of the devices now use them "off-label" in increasingly more complex indications, including use via "hybrid procedures" in the ascending aorta and aortic arch. However, little long-term data are available at this time to support use in this fashion.

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Anatomy

A blood vessel has the following three layers:

• Intima (inner layer made of endothelial cells)
• Media (containing muscular elastic fibers)
• Adventitia (outer connective tissue)

Aneurysms are either true or false. The wall of a true aneurysm involves all three layers, and the aneurysm is contained inside the endothelium. The wall of a false or pseudoaneurysm only involves the outer layer and is contained by the adventitia. An aortic dissection is formed by an intimal tear and is contained by the media; hence, it has a true lumen and a false lumen.

Ascending aortic aneurysms occur as proximally as the aortic annulus and as distally as the innominate artery. They may compress or erode into the sternum and ribs, causing pain or fistula. They also may compress the superior vena cava or airway. When symptomatic by rupture or dissection, they may involve the pericardium, aortic valve, or coronary arteries. They may rupture into the pericardium, causing tamponade. They may dissect into the aortic valve, causing aortic insufficiency, or into the coronary arteries, causing myocardial infarction.

Aortic arch aneurysms involve the aorta where the innominate artery, left carotid, and left subclavian originate. They may compress the innominate vein or airway. They may stretch the left recurrent laryngeal nerve, causing hoarseness.

Descending thoracic aneurysms originate beyond the left subclavian artery and may extend into the abdomen. Thoracoabdominal aneurysms are stratified according to the Crawford classification, as follows:

• Type I involves the descending thoracic aorta from the left subclavian artery down to the abdominal aorta above the renal arteries
• Type II extends from the left subclavian artery to the renal arteries and may continue distally to the aortic bifurcation
• Type III begins at the mid-to-distal descending thoracic aorta and involves most of the abdominal aorta as far distal as the aortic bifurcation
• Type IV extends from the upper abdominal aorta and all or none of the infrarenal aorta

Descending thoracic aneurysms and thoracoabdominal aneurysms may compress or erode into surrounding structures, including the trachea, bronchus, esophagus, vertebral body, and spinal column.

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Pathophysiology

Aneurysms are usually defined as a localized dilation of an arterial segment greater that 50% its normal diameter. Most aortic aneurysms occur in the infrarenal segment (95%). The average size for an infrarenal aorta is 2 cm; therefore, AAAs are usually defined by diameters greater than 3 cm. The normal size for the thoracic and thoracoabdominal aorta is larger than that of the infrarenal aorta, and aneurysmal degeneration in these areas is defined accordingly. The average diameter of the mid-descending thoracic aorta is 26-28 mm, compared with 20-23 mm at the level of the celiac axis.

The occurrence and expansion of an aneurysm in a given segment of the arterial tree probably involves local hemodynamic factors and factors intrinsic to the arterial segment itself.

The medial layer of the aorta is responsible for much of its tensile strength and elasticity. Multiple structural proteins make up the normal medial layer of the human aorta. Of these, collagen and elastin are probably the most important. The elastin content of the ascending aorta is high and diminishes progressively in the descending thoracic and abdominal aorta. The infrarenal aorta has a relative paucity of elastin fibers in relation to collagen and compared with the thoracic aorta, possibly accounting for the increased frequency of aneurysms in this area.

In addition, the activity and amount of specific enzymes is increased, which leads to the degradation of these structural proteins. Elastic fiber fragmentation and loss with degeneration of the media result in weakening of the aortic wall, loss of elasticity, and consequent dilation.

Hemodynamic factors probably play a role in the formation of aortic aneurysms. The human aorta is a relatively low-resistance circuit for circulating blood. The lower extremities have higher arterial resistance, and the repeated trauma of a reflected arterial wave on the distal aorta may injure a weakened aortic wall and contribute to aneurysmal degeneration. Systemic hypertension compounds the injury, accelerates the expansion of known aneurysms, and may contribute to their formation.

Hemodynamically, the coupling of aneurysmal dilation and increased wall stress is defined by the law of Laplace. Specifically, the law of Laplace states that the (arterial) wall tension is proportional to the pressure times the radius of the arterial conduit (T = P × R). As diameter increases, wall tension increases, which contributes to increasing diameter. As tension increases, risk of rupture increases. Increased pressure (systemic hypertension) and increased aneurysm size aggravate wall tension and therefore increase the risk of rupture.

Aneurysm formation is probably the result of multiple factors affecting that arterial segment and its local environment.

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Etiology

Aneurysmal degeneration occurs more commonly in the aging population. Aging results in changes in collagen and elastin, which lead to weakening of the aortic wall and aneurysmal dilation. According to the law of Laplace, luminal dilation results in increased wall tension and the vicious cycle of progressive dilation and greater wall stress. Pathologic sequelae of the aging aorta include elastic fiber fragmentation and cystic medial necrosis. Arteriosclerotic (degenerative) disease is the most common cause of thoracic aneurysms.

A previous aortic dissection with a persistent false channel may produce aneurysmal dilation; such aneurysms are the second most common type. False aneurysms are more common in the descending aorta and arise from the extravasation of blood into a tenuous pocket contained by the aortic adventitia. Because of increasing wall stress, false aneurysms tend to enlarge over time.

Authorities strongly agree that genetics play a role in the formation of aortic aneurysms. Of first-degree relatives of patients with aortic aneurysms, 15% have an aneurysm. This appears especially true in first-degree relatives of female patients with aortic aneurysms. Thus, inherited disorders of connective tissue appear to contribute to the formation of aortic aneurysms.

Marfan syndrome is a potentially lethal connective-tissue disease characterized by skeletal, heart valve, and ocular abnormalities. Individuals with this disease are at risk for aneurysmal degeneration, especially in the thoracic aorta. Marfan syndrome is an autosomal dominant genetic condition that results in abnormal fibrillin, a structural protein found in the human aorta. Patients with Marfan syndrome may develop annuloaortic ectasia of the sinuses of Valsalva, commonly associated with aortic valvular insufficiency and aneurysmal dilation of the ascending aorta.

Type IV Ehlers-Danlos syndrome results in a deficiency in the production of type III collagen, and individuals with this disease may develop aneurysms in any portion of the aorta. Imbalances in the synthesis and degradation of structural proteins of the aorta have also been discovered, which may be inherited or spontaneous mutations.

Atherosclerosis may play a role. Whether atherosclerosis contributes to the formation of an aneurysm or whether they occur concomitantly is not established. Other causes of aortic aneurysms are infection (ie, bacterial [mycotic or syphilitic]), arteritis (ie, giant cell, Takayasu, Kawasaki, Behçet), and trauma. Aortitis due to granulomatous disease is rare, but it can lead to the formation of aortic and, on occasion, pulmonary artery aneurysms. Aortitis caused by syphilis may cause destruction of the aortic media followed by aneurysmal dilation.

Traumatic dissection is a result of shearing from deceleration injury due to high speed motor vehicle accidents (MVA) or a fall from heights. The dissection occurs at a point of fixation, usually at the aortic isthmus (ie, at the ligamentum arteriosum, distal to the origin of the left subclavian artery), the ascending aorta, the aortic root, and the diaphragmatic hiatus.

The true etiology of aortic aneurysms is probably multifactorial, and the condition occurs in individuals with multiple risk factors. Risk factors include smoking, chronic obstructive pulmonary disease (COPD), hypertension, atherosclerosis, male gender, older age, high body mass index (BMI), bicuspid or unicuspid aortic valves, genetic disorders, and family history.

In late 2018, the FDA issued a warning that fluoroquinolone use can increase the risk of AA and urged healthcare providers to avoid prescribing these antibiotics to patients with or at risk for an AA, such as those with peripheral atherosclerotic vascular disease, hypertension, or certain genetic conditions (eg, Marfan syndrome and Ehlers-Danlos syndrome), as well as the elderly.^[12]

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Epidemiology

Although findings from autopsy series vary widely, the prevalence of aortic aneurysms probably exceeds 3-4% in individuals older than 65 years. Aortic aneurysms are more common in men than in women and are more common in persons with COPD than in those without lung disease.

Death from aneurysmal rupture is one of the 15 leading causes of death in most series. The estimated incidence of TAAs is 6 cases per 100,000 person-years. In addition, the overall prevalence of AAs has increased significantly in the past 30 years. This is partly due to an increase in diagnosis based on the widespread use of imaging techniques. However, the prevalence of fatal and nonfatal rupture has also increased, suggesting a true increase in prevalence.

Population-based studies suggest an incidence of acute aortic dissection of 3.5 per 100,000 persons; an incidence of thoracic aortic rupture of 3.5 per 100,000 persons; and an incidence of abdominal aortic rupture of 9 per 100,000 persons. An aging population probably plays a significant role.

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Prognosis

According to Culliford et al from 1982,^[13] Cabrol et al from 1988,^[14] and Donaldson and Ross from 1982,^[15] the early hospital mortality following repair of ascending aneurysms is 4-10%. Contemporary surgical series demonstrated a continued wide range in operative mortality (2-17%). Stroke occurs in 2-5% of patients.

As would be expected, the early mortality after repair of arch aneurysms is considerably higher, approaching 25% in series by Crawford and Saleh from 1981,^[16] by Crawford et al from 1979,^[17] by Columbi et al from 1983,^[18] by Ergin et al from 1982,^[19] and by Galloway et al from 1989.^[20] More contemporary results from Coselli and Ueda demonstrate an operative mortality of 6-12%. Stroke rate varied from 3-22%. Renal failure that necessitated dialysis occurred in 7% of patients.

The mortality after repair of descending thoracic aneurysms is lower, approximately 5-15% according to Crawford et al from 1981,^[16] to Donahoo et al from 1977,^[21] to Livesay et al from 1985,^[22] and to Pressler and McNamara from 1985.^[2] Contemporary results are unchanged, with 12-15% mortality.

As a group, including all repairs, according to Crawford et al from 1978,^[23] Crawford et al from 1981,^[16] and Kitamura et al from 1983,^[24] survival rates after surgery for chronic aortic aneurysms are approximately 60% at 5 years and 30-40% at 10 years.

The longest follow-up data for a multicenter trial comparing endovascular and open techniques for management of TAAs are the results of a phase II multicenter trial for the GORE-TAG thoracic endovascular stent. A 1.5% 30-day mortality for endovascular repairs was demonstrated, temporary or permanent spinal cord paraplegia occurred in 3% of patients and stroke in 4% of patients.^[25] At 2 years, aneurysm survival was 97% and overall survival 75%.^[25] For the Medtronic Talent device, the incidence of paraplegia in the stent group was 0-9%; stroke, 3.7-8.1%; 30-day mortality, 2.9-9.7%; and procedural success, more than 95%.^[26]

When endovascular stent grafting was compared with open surgery for the GORE-TAG device, the rate of paraplegia was 3% in the stent group vs 14% in the open group^[27] ; operative mortality was 1% vs 6%, and early death was 2% vs 10%.^[28] The patients in the stent group had shorter ICU and hospital stays, a quicker recovery time, and a lower incidence of major adverse events (except for vascular complications). Complications at 2 years included 4% proximal stent migration, 6% migration of the graft components, and 15% of patients had an endoleak.

Overall, survival rates were equivalent between the endovascular and open groups at both 2-years and 5-years, 80% and 70% respectively, but aneurysm-related survival significantly favored endovascular repairs at 5 years (97% vs 88%).^[29] However, more contemporary "real world" experienced application has not been as supportive of this discrepancy, as noted by Greenberg et al, who discerned no significant differences in mortality or paraplegia in their population at 30 days (5.7 vs 8.3%) or at 1 year (15.6% vs 15.9%).^[30]

Midterm results comparing open descending thoracic aneurysm repair with endovascular stent grafting demonstrate less early operative mortality with endovascular repair (10% for stent grafting vs 15% for open repair) but similar late survival (actuarial survival rate at 48 months of 54% for stent grafting vs 64% for open repair).

Success with the results of endovascular repair of contained, degenerative TAAs of the descending aorta have created an environment to use endografts for treatment of arch aneurysms as well as acute catastrophes of both the arch and descending aortas.

Data from a multicenter, nonrandomized, prospective study of the use of endografts in emergency pathologies of the descending aorta have been published.^[31] In situations that have reported mortality as high as 90%, the authors found that in the management of acute type B dissections, traumatic aortic tears, or ruptured aortic aneurysms, endovascular management compared to open resulted in a 14% vs 30% 30-day composite mortality/paraplegia rate.

Although freedom from aortic-related events was 84.5% at 1 year for the endovascular cohort, survival was only 66% with the subset of ruptured aneurysms have the worst survival (37%). Another multicenter trial evaluating use in ruptured aneurysms confirmed the perioperative mortality but also noted considerable neurologic complications (8%), procedure-related complications such as endoleak (18%), and ongoing aneurysm-related death (25% at 4 years).^[32]

Others have used endografts for arch pathologies, which usually necessitates a "hybrid" approach, a combination of endovascular and open techniques. Small (N < 30) single-institution series with limited follow-up have reported perioperative mortality, stroke, and paraplegia rates of 0-25%, 0-25%, and 0-4%, respectively, questioning the durability and futility of the repairs.^{[33, 34]} However, a series from a single tertiary care medical center highlighted the results of 400 consecutive patients, demonstrating a 6.5% and 53% 30-day and 4-year mortality, respectively, and a paraplegia and stroke rate of 4.5% and 3%, respectively.^[35]

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History and Physical Examination

Most patients with aortic aneurysms are asymptomatic at the time of discovery. Thoracic aortic aneurysms (TAAs) are usually found incidentally after chest radiographs or other imaging studies. Abdominal aortic aneurysms (AAAs) may be discovered incidentally during imaging studies or a routine physical examination as a pulsatile abdominal mass.

The most common complication of AAAs is rupture with life-threatening hemorrhage manifesting as pain and hypotension. The triad of abdominal pain, hypotension, and a pulsatile abdominal mass is diagnostic of a ruptured AAA, and emergency operation is warranted without delay for imaging studies.

Patients with a variant of AAA may present with fever and a painful aneurysm with or without an obstructive uropathy. These patients may have an inflammatory aneurysm that can be treated with surgical repair.

Other presentations of AAA include lower-extremity ischemia, duodenal obstruction, ureteral obstruction, erosion into adjacent vertebral bodies, aortoenteric fistula (ie, gastrointestinal [GI] bleeding), and aortocaval fistula (caused by spontaneous rupture of aneurysm into the adjacent inferior vena cava [IVC]). Patients with aortocaval fistula present with abdominal pain, venous hypertension (ie, leg edema), hematuria, and high-output cardiac failure.

Patients with TAAs are often asymptomatic. Most patients are hypertensive but remain relatively asymptomatic until the aneurysm expands. Their most common presenting symptom is pain. Pain may be acute, implying impending rupture or dissection, or chronic, from compression or distention. The location of pain may indicate the area of aortic involvement, but this is not always the case. Ascending aortic aneurysms tend to cause anterior chest pain, whereas arch aneurysms more likely cause pain radiating to the neck. Descending thoracic aneurysms more likely cause back pain localized between the scapulae. When located at the level of the diaphragmatic hiatus, the pain occurs in the mid back and epigastric region.

Large ascending aortic aneurysms may cause superior vena cava obstruction manifesting as distended neck veins. Ascending aortic aneurysms also may develop aortic insufficiency, with widened pulse pressure or a diastolic murmur, and heart failure. Arch aneurysms may cause hoarseness, which results from stretching of the recurrent laryngeal nerves. Descending thoracic aneurysms and thoracoabdominal aneurysms may compress the trachea or bronchus and cause dyspnea, stridor, wheezing, or cough. Compression of the esophagus results in dysphagia.

Erosion into surrounding structures may result in hemoptysis, hematemesis, or GI bleeding. Erosion into the spine may cause back pain or instability. Spinal cord compression or thrombosis of spinal arteries may result in neurologic symptoms of paraparesis or paraplegia. Descending thoracic aneurysms may thrombose or embolize clot and atheromatous debris distally to visceral, renal, or lower extremities.

Patients who present with ecchymoses and petechiae may be particularly challenging because these signs probably indicate disseminated intravascular coagulation (DIC). The risk of significant perioperative bleeding is extremely high, and large amounts of blood and blood products must be available for resuscitative transfusion.

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Complications

The most common complications of TAAs are acute rupture and dissection. Some patients present with tender or painful nonruptured aneurysms. Although debate continues, these patients are thought to be at increased risk for rupture and should undergo surgical repair on an emergency basis.

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Laboratory Studies

Laboratry studies to be consiered in this setting include the following:

• Complete blood count (CBC)
• Electrolyte evaluation and blood urea nitrogen (BUN)/creatinine value - Determining renal function is important for stratifying morbidity.
• Prothrombin time (PT), international normalized ratio (INR), and activated partial thromboplastin time (aPTT)
• Blood type and crossmatch
• Liver function tests and amylase lactate values - These tests are indicated for patients with acute dissection or risk of distal embolization

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Imaging Studies

Chest radiography

In the case of ascending aortic aneurysms, chest x-rays may reveal a widened mediastinum (see the image below), a shadow to the right of the cardiac silhouette, and convexity of the right superior mediastinum. Lateral films demonstrate loss of the retrosternal air space. However, the aneurysms may also be completely obscured by the heart, and the chest x-ray appear normal.

View Image

Chest radiograph showing widening of superior mediastinum.

Plain chest radiographs may show a shadow anteriorly and slightly to the left for arch aneurysms and posteriorly and to the left for descending thoracic aneurysms. Aortic calcification may outline the borders of the aneurysm in the anterior, posterior, and lateral views in both the chest and abdomen.

Echocardiography

Transthoracic echocardiography (TTE) demonstrates the aortic valve and proximal aortic root. It may help detect aortic insufficiency and aneurysms of the sinus of Valsalva, but it is less sensitive and specific than transesophageal echocardiography (TEE).

TEE images show the aortic valve, ascending aorta, and descending thoracic aorta, but they are limited in the area of the distal ascending aorta, transverse aortic arch, and upper abdominal aorta. TEE can help accurately differentiate aneurysm and dissection, but the images must be obtained and interpreted by skilled personnel.

Ischemia may be evaluated using dipyridamole-thallium or dobutamine echocardiography scans.

Ultrasonography

Infrarenal abdominal aortic aneurysms (AAAs) may be visualized by means of ultrasonography, but these images do not help define the extent of thoracoabdominal aortic aneurysms (TAAAs).

Carotid ultrasonography may be needed for patients with carotid bruits, peripheral vascular disease, a history of transient ischemic attacks, or cerebrovascular accidents to evaluate for carotid disease.

Intraoperative intravascular ultrasonography (IVUS) can also be used to provide additional anatomic information and guidance during placement of endovascular stents.

Intraoperative epiaortic ultrasonography can be performed to scan the aorta for atherosclerotic disease or thrombus.

For more information, see Bedside Ultrasonography, Abdominal Aortic Aneurysm.

Aortography

Aortography (see the image below) can delineate the aortic lumen, and it can help define the extent of the aneurysm, any branch vessel involvement, and the stenosis of branch vessels. It describes the takeoff of the coronary ostia.

View Image

Ascending aortogram showing ascending aortic aneurysm. Patient also underwent computed tomography (CT).

For patients older than 40 years or those with a history suggestive of coronary artery disease, aortography helps evaluate coronary anatomy, ventricular function by ventriculography, and aortic insufficiency. It does not help in defining the size of the aneurysm, because the outer diameter is not measured, which may miss dissections.

Disadvantages include the use of nephrotoxic contrast and radiation. The risk of aortography includes embolization from laminated thrombus and carries a 1% stroke risk.

Computed tomography

Computed tomography (CT) with contrast has become the most widely used diagnostic tool. Contrast CT scans rapidly and precisely evaluate the thoracic and abdominal aorta to determine the location and extent of the aneurysm and the relation of the aneurysm to major branch vessels and surrounding structures. They can help accurately determine the size of the aneurysm and assesses dissection, mural thrombus, intramural hematoma, free rupture, and contained rupture with hematoma. (See the image below.)

View Image

Computed tomography (CT) scan depicting descending thoracic aortic aneurysm with mural thrombus at level of left atrium.

Sagittal, coronary, and axial images may be obtained with three-dimensional (3D) reconstruction. Stent graft planning for endovascular descending thoracic aneurysm repairs requires fine-cut images from the neck through the pelvis to the level of the femoral heads. The takeoff of the arch vessels is critical to determine the adequacy of the proximal landing zone, as is assessing the patency of the vertebral arteries, if the left subclavian artery should be covered by the stent graft. Assessment of the common femoral artery access is essential to determine the feasibility of large-bore sheath access. A spiral CT scan with 1-mm cuts and 3D reconstruction with the ability to make centerline measurements is crucial to stent graft planning.

Aortic size on imaging is widely used to guide clinical decision making in regards to patients who have thoracic aortic aneurysms (TAAs). It has been found that the double-oblique plane yields improved agreement with planimetry and differed from the axial plane in proportion to aortic geometric obliquity; therefore, the double-oblique measurement is recommended.^[36]

CT angiography (CTA) may create multiplanar reconstructions and cines. This requires nephrotoxic contrast and radiation, but the procedure is noninvasive.

Magnetic resonance imaging

Compared with contrast CT, magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) have the advantage of avoiding nephrotoxic contrast and ionizing radiation.

MRI and MRA can also help accurately demonstrate the location, extent, and size of the aneurysm and its relation to branch vessels and surrounding organs. These studies also precisely reveal aortic composition. However, they are more time-consuming, less readily available, and more expensive than CT is.

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Other Tests

Baseline electrocardiography (ECG) should be performed. Transthoracic echocardiograms noninvasively screen for valvular abnormalities and cardiac function.

Patients with a smoking history and chronic oibstructive pulmonary disease (COPD) should be evaluated by using pulmonary function tests with spirometry and room-air arterial blood gas determinations.

Patients with a history of coronary artery disease or those older than 40 years should undergo cardiac catheterization.

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Histologic Findings

Histologic findings may include elastic fiber fragmentation, loss of elastic fibers, loss of smooth muscle cells, cystic medial necrosis, intraluminal thrombus, and atherosclerotic plaque and ulceration.

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Approach Considerations

Indications and contraindications

Indications for surgical treatment of thoracic aortic aneurysms (TAAs) are based on size or growth rate and symptoms. Because the risk of rupture is proportional to the diameter of the aneurysm, aneurysmal size is the criterion for elective surgical repair.

Elefteriades published the natural history of TAAs and recommended elective repair of ascending aneurysms at 5.5 cm and descending aneurysms at 6.5 cm for patients without any familial disorders such as Marfan syndrome.^{[37, 38]} These recommendations are based on the finding that the incidence of complications (rupture and dissection) exponentially increased when the size of the ascending aorta reached 6.0 cm (31% risk of complications) or when the size of the descending aorta reached 7.0 cm (43% risk).^{[39, 38]} Patients with Marfan syndrome or familial aneurysms should undergo earlier repair, when the ascending aorta grows to 5.0 cm or the descending aorta grows to 6.0 cm.

In addition, relative aortic aneurysm size in relation to body surface area may be more important than absolute aortic size in predicting complications.^[40] Using the aortic size index (ASI) of aortic diameter (cm) divided by body surface area (m²), one can stratify patients into the following three groups^{[40, 41]} :

• ASI < 2.75 cm/m² are at low risk for rupture (4%/year)
• ASI 2.75-4.25 cm/m² are at moderate risk (8%/year)
• ASI >4.25 cm/m² are at high risk (20-25%/year)

Rapid expansion is also a surgical indication. Growth rates average 0.07 cm/year in the ascending aorta and 0.19 cm/year in the descending aorta.^[38] A growth rate of 1 cm/year or faster is an indication for elective surgical repair.

Symptomatic patients should undergo aneurysm resection, regardless of aneurysm size. Acutely symptomatic patients require emergency operation. Emergency operation is indicated in the setting of acute rupture. Rupture of the ascending aorta may occur into the pericardium, resulting in acute tamponade. Rupture of the descending thoracic aorta may cause a left hemothorax.

Patients with acute aortic dissection of the ascending aorta require emergency operation. They may present with rupture, tamponade, acute aortic insufficiency, myocardial infarction, or end-organ ischemia. Acute dissection of the descending aorta does not necessitate surgical intervention, unless complicated by rupture, malperfusion (eg, visceral, renal, neurologic, leg ischemia), progressive dissection, persistent recurrent pain, or failure of medical management.

Patients who undergo surgery for symptomatic aortic insufficiency or stenosis with an associated enlarged aneurysmal aorta should have concomitant aortic replacement if the aorta reaches 5 cm in diameter. Concomitant aortic replacement should be consider for patients with bicuspid aortic valves with an aorta more than 4.5 cm in diameter.

As one may imagine, the quality of data and level of evidence supporting these recommendations widely vary. Given these variations and the importance of the subject matter, in 2010, a joint task force spearheaded by the American College of Cardiology Foundation and the American Heart Association, and composed of members of many professional societies specializing in treatment of diseases of the thoracic aorta, produced an Executive Summary detailing guidelines for diagnosis and management of this disease, which will be annually updated.^[42]

Indications for surgical treatment may be summarized as follows:

• Aortic size - Ascending aortic diameter ≥5.5 cm or twice the diameter of the normal contiguous aorta; descending aortic diameter ≥6.5 cm; subtract 0.5 cm from the cutoff measurement in the presence of Marfan syndrome, family history of aneurysm or connective tissue disorder, bicuspid aortic valve, aortic stenosis, dissection, patient undergoing another cardiac operation; growth rate ≥1 cm/year
• Symptomatic aneurysm
• Traumatic aortic rupture
• Acute type B aortic dissection with associated rupture, leak, distal ischemia
• Pseudoaneurysm
• Large saccular aneurysm
• Mycotic aneurysm
• Aortic coarctation
• Bronchial compression by aneurysm
• Aortobronchial or aortoesophageal fistula

Aneurysm surgery has no strict contraindications. The relative contraindications are individualized, based on the patient's ability to undergo extensive surgery (ie, the risk-to-benefit ratio). Patients at higher risk for morbidity and mortality include elderly persons and individuals with end-stage renal disease, respiratory insufficiency, cirrhosis, or other comorbid conditions.

For descending thoracic aneurysms, endovascular stent grafting is less invasive and is an ideal alternative (with appropriate anatomic considerations) to open repair for patients at high risk for complications of open repair. Stent grafts are also a reasonable alternative (with the appropriate anatomy) to open repair in patients who are not at high risk for complications. Patients must understand that life-long follow-up is required and that long-term durability is unknown.

Future and controversies

Early morbidity and mortality are related to bleeding, neurologic injury (eg, stroke), cardiac failure, and pulmonary failure (eg, acute respiratory distress syndrome [ARDS]). Risk factors include emergency operation, older age, dissection, congestive heart failure (CHF), prolonged cardiopulmonary bypass time, arch replacement, previous cardiac surgery, need for concomitant coronary revascularization, and reoperation for bleeding. Late mortality is usually related to cardiac disease or distal aortic disease.

Bleeding is a potential complication for all aneurysm repairs. It is minimized by the use of antifibrinolytics, felt strips, and factors, including fresh frozen plasma and platelets. For patients who undergo hypothermic circulatory arrest, the use of aprotinin is controversial, but most groups routinely use aminocaproic acid. Coagulopathy and bleeding in severe cases may warrant the use of recombinant factor VII.

Aprotinin, an antifibrinolytic agent used to reduce operative blood loss in patients undergoing open heart surgery, is now only available via a limited-access protocol. Fergusson et al reported an increased risk for death compared with tranexamic acid or aminocaproic acid in high-risk cardiac surgery.^[43]

Stroke is a major cause of morbidity and mortality and typically results from embolization of atherosclerotic debris or clot. Transesophageal echocardiography (TEE) and epiaortic ultrasonography may be beneficial in localizing appropriate areas to clamp. Patients undergoing arch repairs are at the highest risk of permanent and transient neurologic injury. Retrograde cerebral perfusion is beneficial for flushing out embolic debris, but it may be detrimental, with increased intracranial pressure and cerebral edema. Antegrade cerebral perfusion is beneficial for reducing neurologic injury during hypothermic circulatory arrest. Stroke incidence for open surgical repair versus endovascular repair of descending TAAs is equivalent.

Myocardial infarction may occur with technical problems with coronary ostia implantation during root replacement for ascending aortic aneurysms and may require reoperation. Pulmonary dysfunction and renal dysfunction are other potentially morbid complications.

Paraparesis and paraplegia, either acute or delayed, are the most devastating complications of descending thoracic aneurysm and thoracoabdominal aneurysm repairs. Despite cerebrospinal drainage, reimplantation of intercostal arteries, evoked potential monitoring, mild hypothermia, and atrial femoral bypass, spinal cord injury still occurs. Endovascular stent grafting has not eliminated spinal cord paraplegia; the incidence varies widely, with an overall incidence of 2.7%.^{[27, 44, 45, 46]}

Complications specific to endovascular stenting include endoleaks, stent fractures, stent graft migration or thrombosis, iliac artery rupture, retrograde dissection, microembolization, aortoesophageal fistula, and complications at the site of delivery (eg, groin infection, lymphocele, seroma).

• References

Medical Therapy

All aneurysms must be treated with risk-factor reduction. Systemic hypertension probably contributes to the formation of aneurysms and certainly contributes to expansion and rupture. This is especially true of TAAs. Strict control of hypertension is implemented in all patients, regardless of aortic aneurysm size.

Tobacco use contributes to aneurysm formation, though the exact pathophysiology is not well understood. Cessation of smoking is recommended. Control of other risk factors for peripheral arterial obstructive disease may be beneficial.

For acute aortic dissections, the first-line treatment of hypertension is a short-acting beta blocker (eg, esmolol). Beta blockade decreases the force of contraction, thus decreasing the dP/dt and shear force exerted on the dissection by minimizing the rate of rise of the aortic pressure. It also decreases the heart rate and the inotropic state of the myocardium, and reduces the likelihood of propagation of the dissection. A second agent added is a vasodilator (eg, nitroprusside), which reduces the systolic blood pressure to, in turn, decrease the aortic wall stress and the possibility of rupture.

• References

Surgical Options

Most aneurysm repairs involve aortic replacement with a Dacron tube graft. Dacron grafts allow ingrowth in the interstices to form a pseudoendothelial layer to minimize the risk of embolization. They may be knitted or woven. Knitted grafts are more porous and incorporate tissue well; however, they are prone to more bleeding. Woven grafts are more impervious and therefore are the most commonly used for aortic replacement. Grafts are typically impregnated with collagen to avoid preclotting the graft and to promote optimal healing.

Endovascular repair of a thoracoabdominal aortic aneurysm (TAAA) is shown in the video below.

View Video

Endovascular repair of thoracoabdominal aortic aneurysm (TAAA). Procedure performed by Inkyong Kim, MD, and Rajeev Dayal, MD, ColumbiaDoctors, New York, NY. Video courtesy of ColumbiaDoctors (https://www.columbiadoctors.org).

Choice of procedure

Ascending aortic aneurysms

Surgical treatment of ascending aortic aneurysms depends on the extent of the aneurysm both proximally (eg, involvement of the aortic valve, annulus, sinuses of Valsalva, sinotubular junction, coronary orifices) and distally (eg, involvement to the level of the innominate artery). The choice of operation also depends on the underlying pathology of the disease, the patient's life expectancy, the desired anticoagulation status, and the surgeon's experience and preference.^[47]

Ascending aortic aneurysms with normal aortic valve leaflets, annulus, and sinuses of Valsalva are typically replaced with a simple supracoronary Dacron tube graft from the sinotubular junction to the origin of the innominate artery, with the patient under cardiopulmonary bypass (CPB).

If the aortic valve is diseased but the aortic sinuses and annulus are normal, the aortic valve is replaced separately and the ascending aortic aneurysm is replaced with a supracoronary synthetic graft, leaving the coronary arteries intact (ie, Wheat procedure).

Sinus of Valsalva aneurysms with normal aortic valve leaflets and aortic insufficiency due to dilated sinuses may be repaired with a valve-sparing aortic root replacement. Two valve-sparing procedures have been developed: the remodeling method and the reimplantation method. The remodeling method involves resecting the aneurysmal sinus tissue while maintaining the tissue along the valve leaflets and scalloping the Dacron graft to form new sinuses to remodel the root. The reimplantation method involves reimplanting the scalloped native valve into the Dacron graft. Both require reimplantation of the coronary ostia into the Dacron graft.^[48]

Patients with an abnormal aortic valve and aortic root require aortic root replacement (ARR). In nonelderly patients who can undergo anticoagulation with reasonable safety, the aortic root may be replaced with a composite valve-graft consisting of a mechanical valve inserted into a Dacron graft coronary artery reimplantation (eg, classic or modified Bentall procedure or Cabrol procedure).^{[49, 50]}

For elderly patients, young active patients who do not desire anticoagulation, women of childbearing age, and patients with contraindications to warfarin, the options include stentless porcine roots,^[51] aortic homografts, and pulmonary autografts (ie, Ross procedure).^[52] For elderly patients who cannot undergo a complex operation, another option is reduction aortoplasty (ie, wrapping of the ascending aorta with a prosthetic graft).

Patients with Marfan syndrome have abnormal aortas and cannot undergo tube graft replacement alone. They must have either a valve-sparing ARR or a complete ARR.

ARR with a homograft is ideal in the setting of aortic root abscess from endocarditis.

Aortic arch aneurysms

Arch aneurysms pose a formidable technical challenge. Deep hypothermic circulatory arrest (DHCA) with or without antegrade or retrograde cerebral perfusion is usually used to facilitate reanastomosis of the arch vessels. Aortic arch reconstruction techniques vary, depending on the arch pathology.

In patients with proximal arch involvement extending from the ascending aorta, a hemiarch replacement may be performed. The ascending aorta is replaced with a Dacron graft beveled as a tongue along the undersurface of the arch. In patients whose conditions mandate replacement of the entire arch, the distal anastomosis is the Dacron graft to the descending thoracic aorta. The head vessels are reimplanted individually or as an island. Grafts have been developed with a trifurcated head-vessel attachment and with a fourth attachment for the cannula. In this case, the head vessels are attached individually to the trifurcated branches.

For patients in whom the arch replacement is part of a staged procedure, preceding the delayed repair of a concomitant descending thoracic aneurysm, an "elephant trunk" is used. That is, the Dacron graft used to reconstruct the transverse arch ends distally in an extended sleeve that is telescoped into the descending thoracic aorta, facilitating later replacement of the descending thoracic/abdominal aneurysm (two-stage procedure).

The higher morbidity and mortality associated with ascending and arch aortic surgery combined with the increasing experience using thoracic endografts in the descending aorta has evolved into the use of endografts in the ascending and arch aortas.

So-called "hybrid" procedures represent a combination of open and endovascular procedures. Although potentially still requiring a median sternotomy and, often, revascularization of some or all of the arch vessels, they offer the advantage of potentially less invasiveness through one-stage or single-incision procedures, avoidance of aortic cross-clamping or hypothermic circulatory arrest, or less total revascularization, especially if the ascending or descending aorta is also involved.

Further evolution of the hybrid procedure is demonstrated by the "frozen elephant trunk" technique, which involves standard ascending and arch repair as in the elephant trunk procedure, followed by antegrade reconstruction of the descending thoracic aorta, through the opened transverse arch. These potential advantages theoretically translate into decreased morbidity and mortality. Limited experience with these hybrid procedures has been reported; however, the long-term results and durability have yet to be defined.

Descending thoracic aortic aneurysms and thoracoabdominal aortic aneurysms

Descending TAAs may be repaired with open surgery or, if appropriate, with endovascular stent grafting techniques.^{[27, 44, 26, 45, 41, 46]} Stent graft repair of descending TAAs should be performed if the predicted operative risk is lower than that of an open repair. Patient age, comorbidities, symptoms, life expectancy, aortic diameter, characteristics and extent of the aneurysm, and landing zones, should also be taken into consideration.

Surgically, descending TAAs may be repaired with or without the use of a bypass circuit from the left atrium to the femoral artery or femoral vein–femoral artery cardiopulmonary bypass, depending on the length of the anticipated ischemic cross-clamping and the experience of the surgeon. Discrete aneurysms with an anticipated clamp time of less than 30 minutes may be repaired without left-heart bypass or CPB (ie, "clamp and go" technique). More complex or larger aneurysms are probably safer to repair with the aid of either left-heart, partial, or full CPB with hypothermic circulatory arrest. The use of left-heart bypass or CPB is favored to reduce hemodynamic instability and the risk of spinal cord paraplegia.

Descending TAAs with the appropriate anatomy may be repaired by using endovascular stent grafts. The GORE TAG nitinol-based stent graft, approved by the US Food and Drug Administration (FDA) in March 2005, was designed for descending TAA repair. Subsequently, the Zenith TX2 endovascular graft (Cook Medical Inc, Bloomington, IN) was approved in March 2008, followed by the Talent Thoracic Stent Graft (Medtronic Inc, Minneapolis, MN) in June 2008. The Valiant Thoracic Stent Graft (Medtronic Inc, Minneapolis, MN) is approved for use outside the United States.

With the GORE TAG, an appropriate proximal neck of 2 cm prior to the aneurysm is required. Ideally, the proximal landing zone is beyond the left subclavian artery, though in some circumstances the stent may be placed proximal to the left subclavian artery. Distally, a sufficient landing zone of 2 cm prior to the celiac artery is required. The aortic inner neck diameters in the proximal and distal landing zones must fall within 23-37 mm. In addition, appropriately sized femoral and iliac arteries (typically >8 mm in diameter) that lack tortuosity and calcium are required for implantation.

TAAAs, accounting for approximately 10% of thoracic aneurysms, may be repaired with the use of a partial bypass of the left atrium to the femoral artery.

Crawford type I TAAAs involve Dacron graft replacement of the aorta from the left subclavian artery to the visceral and renal arteries as a beveled distal anastomosis, using sequential cross-clamping of the aorta. Crawford type II TAAA repair requires a Dacron graft from the left subclavian to the aortic bifurcation with reattachment of the intercostal arteries, visceral arteries, and renal arteries. Crawford type III or IV TAAA repairs, which begin lower along the thoracic aorta or upper abdominal aorta, may use either the partial bypass of the left atrial artery to the femoral artery or a modified atriovisceral and/or renal bypass.

Prevention of paraplegia is one of the principal concerns in the repair of descending and thoracoabdominal aneurysms.

Previous investigational trials by Chuter at the University of California at San Francisco Medical Center and Greenberg at the Cleveland Clinic treated TAAAs with custom-built fenestrated and branched stent grafts. Such devices required precise anatomic tailoring of the grafts to the specific patient's anatomy for placement of the scallops (for visceral flow) or branches (for direct stenting into the visceral vessels) and resulted in prolonged operative delays.

Subsequent data and improvement in devices demonstrated that standardized multibranched endografts were applicable to approximately 90% of the patient population, thereby eliminating manufacturing delays and expanding the applicability of these devices in TAAAs.^[53]

• References

Operative Details

Preparation for surgery

Ascending aortic aneurysm

Preoperative assessment of coronary artery disease (CAD) is essential to determine the need for concomitant coronary artery bypass grafting (CABG). Transesophageal echocardiography (TEE) is crucial preoperatively to examine the need for aortic valve replacement. Patients with aortic stenosis or aortic insufficiency in whom the valve leaflets are anatomically abnormal require replacement, whereas patients with aortic insufficiency and normal aortic valve leaflets may be candidates for valve-sparing procedures. TEE is valuable for accurate delineation of the aortic root at the sinuses of Valsalva and sinotubular junction.

Aortic arch aneurysm

The major morbidities from aortic arch aneurysm repair are neurologic, cardiac, and pulmonary in nature. All patients require preoperative assessment of cardiac function and evaluation for CAD. In the operating room, TEE is used to monitor ventricular function and to assess for atherosclerosis of the aorta.

A major concern in arch surgery is neurologic injury, both transient neurologic dysfunction and permanent neurologic injury. Patients with a higher risk of stroke undergo preoperative noninvasive carotid ultrasonography, and those with a history of stroke undergo computed tomography (CT) of the brain. In the operating room, steroids are often given at the onset of the procedure if hypothermic circulatory arrest is anticipated. Evidence suggests that steroids given preoperatively several hours before the operation may have benefit. Some institutions monitor electroencephalographic (EEG) silence to assess for adequate duration and temperature of cerebral cooling for hypothermic circulatory arrest.

Descending thoracic aneurysms and thoracoabdominal aneurysms

A devastating complication of descending TAA and TAAA repair is spinal cord injury with paraparesis or paraplegia. Preoperatively, some groups perform spinal arteriograms to attempt to localize the artery of Adamkiewicz for reimplantation during surgery. Neurologic monitoring with somatosensory evoked potentials or motor evoked potentials is used by some to assess spinal cord ischemia and identify critical segmental arteries for spinal cord perfusion. Finally, preoperative placement of catheters for cerebrospinal fluid (CSF) drainage is performed to increase spinal cord perfusion pressure during aortic cross-clamping.

Spinal cord injury is less prevalent with endovascular stent grafting than with open repair but exists with both types of surgical treatment.^{[27, 44, 45, 46]}  For endovascular stent grafting, CSF drainage and avoidance of hypotension are the primary mechanisms used to prevent paraplegia. The use of CSF drainage is selective among most centers. For some discrete aneurysms, stent graft coverage may allow preservation of spinal arteries. Others require coverage of the entire descending thoracic aorta. Indications for use of CSF drains include the following:

• Anticipated endograft coverage of T9-T12
• Coverage of the long segment of the thoracic aorta
• Compromised collateral pathways from prior infrarenal abdominal aortic aneurysm (AAA) repair
• Symptomatic spinal ischemia

Although not recommend as primary therapy, a report of an induced endoleak to allow spinal cord perfusion for persistent cord ischemia following endovascular repair, despite CSF drainage, proved successful and may represent a "bail-out" technique to be considered in exceptional circumstances.^[54]

Spinal cord ischemia is an uncommon complication following thoracic endovascular aortic repair, but its development can be identified by a preoperative renal insufficiency. Blood pressure augmentation alone, or in combination with CSF drainage, serves as an effective early detection process for most patients, the majority of whom enjoy a complete and long-term neurologic recovery.^[55]

Brain protection

Methods used for brain protection during deep hypothermic circulatory arrest (DHCA) include intraoperative EEG monitoring, evoked somatosensory potential monitoring, hypothermia (to temperatures < 20^o C), packing the patient's head in ice, Trendelenburg positioning (ie, head down), mannitol, carbon dioxide flooding, thiopental, steroids, and antegrade and retrograde cerebral perfusion.

General monitoring and anesthesia

Venous access is obtained with two large-bore peripheral IVs and a central line. Filling pressures and cardiac output monitoring are performed with a pulmonary artery catheter. Continuous blood pressure monitoring is performed with a radial arterial line. Nasopharyngeal and bladder probes monitor systemic temperature. Intraoperative TEE is used to assess myocardial and valvular function.

Procedures

Ascending aortic replacement

CPB is established, and the aorta is cross-clamped just below the innominate artery. The heart is arrested with cardioplegia. The aorta is transected at the sinotubular junction and sized for the appropriate Dacron tube graft. The tube graft is sutured to the proximal aorta with running 4-0 polypropylene with a strip of felt. The tube graft is measured to length distally and sutured to the distal aorta using running 4-0 polypropylene with a strip of felt.

Valve-sparing aortic root replacement

Once the aorta is transected at the sinotubular junction, the valve is inspected for normal anatomy. If sparing is feasible, the appropriate size tube graft is chosen to allow coaptation of the aortic valve leaflets without aortic insufficiency. In the remodeling technique, the tube graft is tailored to form aortic sinuses. The sinuses of Valsalva of the native aorta are removed, and the coronary ostia are mobilized. The neosinuses of the tube graft are sutured to the scalloped aortic valve with running 4-0 polypropylene and a strip of felt.

In the reimplantation technique, Tycron sutures are placed along the subannular horizontal plane and passed through the tube graft. The scalloped aortic valve is placed within the tube graft, and the proximal suture line is secured. The scalloped aortic valve is positioned in the graft to achieve valve competence, and the subcoronary suture line along the scalloped valve is performed with running 4-0 polypropylene. The valve is examined for competence within the graft. The coronary ostia are reimplanted in the graft. The graft is measured to length distally and sutured to the distal aorta.

In the reimplantation technique, Tycron sutures are placed along the subannular horizontal plane and passed through the tube graft. The scalloped aortic valve is placed within the tube graft, and the proximal suture line is secured. The scalloped aortic valve is positioned in the graft to achieve valve competence, and the subcoronary suture line along the scalloped valve is performed with running 4-0 polypropylene. The valve is examined for competence within the graft. The coronary ostia are reimplanted in the graft. The graft is measured to length distally and sutured to the distal aorta.

Aortic root replacement

The aorta is transected, and the aortic valve is removed. The annulus is sized, and the appropriate valved conduit, stentless root, mechanical composite, or homograft is brought to the field. The coronary ostia are mobilized. Annular sutures are placed and are passed through the valve conduit. The proximal suture is thus secured. The coronary ostia are reimplanted. The distal suture line is performed for the mechanical valve composite, but an additional Dacron graft extension may be required for the stentless roots or homografts, depending on their length.

In the modified Bentall procedure ("buttons"), the right and left coronary arteries are dissected as a button, which is then reimplanted into the Dacron composite graft as an aortic button.

The Cabrol procedure, though rarely performed, may be used when the aortic tear or dissection extends into the coronary ostia. It may also be used when adequate mobilization of the coronary ostia is not possible (i.e., from scarring in a reoperation), or when the coronary ostia are too low. The coronary buttons are dissected and anastomosed to a separate 6- or 8-mm Dacron interposition graft; this graft is then anastomosed into the Dacron composite graft.

This technique results in a tension-free anastomosis of the coronary buttons and also allows easier access for hemostasis. However, it is subject to twisting and kinking with resultant myocardial ischemia and, thus, is not as reproducible as the modified Bentall.

Open distal anastomosis

Deep hypothermic circulatory arrest with or without antegrade or retrograde cerebral perfusion is used. When cooled to 18°C (64.4°F), the pump is turned off and the arterial line is clamped. The patient is placed in the Trendelenburg position, and the aortic cross-clamp is removed. The distal anastomosis is performed open with running 4-0 polypropylene and a strip of felt. The distal anastomosis may be at the level of the innominate artery or, in the case of hemiarch replacement, along the undersurface of the arch to the level of the left subclavian artery.

Once the anastomosis has been completed, the pump is restarted with blood flow antegrade into the new graft and open proximal tube graft to flush out air and debris. The graft is then clamped; the proximal aortic reconstruction is performed during rewarming.

Hypothermia decreases oxygen consumption. For each drop in temperature by 1º C, the oxygen consumption by the tissues is reduced by 10%.

Air (ie, nitrogen) is poorly soluble in blood. The risk of air embolism is reduced by flooding the surgical field with carbon dioxide. Carbon dioxide is denser than air and displaces air. It is rapidly soluble in blood and causes less risk of embolization. Any carbon dioxide absorbed in the blood is removed by increasing the sweep speed of cardiopulmonary bypass.

Aortic arch aneurysm repairs

Cannulation for arch repairs varies among groups. They include the femoral artery, right axillary artery, and ascending aorta. Hypothermic circulatory arrest is required for arch repairs; the safe period of arrest to avoid neurologic injury is 30-45 minutes at 18°C (64.4°F), but some advocate a shorter period of 25 minutes. Antegrade cerebral perfusion to minimize neurologic injury is thus advocated. Others advocate cooling to 11-14°C (51.8-57.2°F).

Once the patient is cooled to the desired temperature, the circuit is turned off. For retrograde cerebral perfusion, flow is established through the superior vena cava as the arch reconstruction is performed. For antegrade cerebral perfusion, flow is continued through the axillary artery with the innominate artery clamped or individual perfusion catheters are placed into the innominate artery, left carotid artery, and left subclavian arteries.

The arch reconstructions are also varied. They basically involve performing the distal anastomosis to the aorta beyond the left subclavian artery as an open distal procedure with or without an elephant trunk. The three head vessels may be reanastomosed individually or as an island. They may be reimplanted directly to the graft or anastomosed to a separate graft, which is then attached to arch graft.

Descriptions of different hybrid procedures have been standardized according to the location of the most proximal placement of the endograft in relation to the arch vessels, under the Criado classification, as follows^[56] :

• Zone 0 extends distally from the ascending aorta to the innominate artery
• Zone 1 extends from distal to the innominate artery origin to the left common carotid artery (CCA)
• Zone 2 extends from distal to the left CCA to the left subclavian artery (LSA)
• Zone 3 extends from distal to the LSA to the proximal descending thoracic aorta

Zone 0 pathology by definition involves all aortic arch vessels and requires revascularization of at least the innominate artery and left CCA and possibly revascularization of the LSA in the case of symptoms of left-arm ischemia, functional left internal mammary artery (LIMA) bypass graft, or dominant left vertebral artery circulation. Revascularization is usually accomplished via a median sternotomy and the use of a bifurcated or trifurcated graft from the ascending aorta to the arch vessels. Following revascularization and during the concomitant operation, a stent-graft is then implanted either in an antegrade or retrograde fashion.

Zone 1 placement, commonly avoids a median sternotomy, via revascularization of the left CCA by a right CCA to left CCA bypass, prior to endograft placement. Depending on the quality of angiographic resources in the operating room, this procedure may be performed in a single or staged procedure to allow use of a dedicated angiographic suite.

A Zone 2 landing requires partial or complete coverage of the LSA. In general, this is well tolerated, however, several reports have detailed higher incidences of neurologic complications with LSA coverage and, therefore, a thorough assessment of the carotid, vertebral and circle of Willis circulations should be preoperatively performed.^[57]

Descending thoracic aneurysm and thoracoabdominal aneurysm repairs

Measures to reduce spinal cord injury include CSF drainage, reimplantation of intercostal arteries, partial bypass, and mild hypothermia. A left thoracotomy or a thoracoabdominal incision is performed. The aorta is cross-clamped either just beyond the left subclavian or between the left carotid and left subclavian for Crawford types I and II. The cross clamp is placed more distally for Crawford types III and IV.

Atrial femoral bypass is established with a Bio-Medicus circuit, and the patient is cooled to 32-34°C (89.6-93.2°F). Distal cross-clamping is performed at T4-T7 to allow continued spinal cord, visceral, and renal perfusion. The proximal anastomosis is performed with running 4-0 polypropylene and a strip of felt. When complete, the proximal clamp is released and reapplied more distally on the tube graft. The distal cross-clamp is moved sequentially down, if feasible, to allow visceral and renal perfusion. The intercostal arteries may be reimplanted, if desired, or oversewn. If sequential cross-clamping is not feasible, direct catheters may be placed in the visceral and renal vessels to allow continuous perfusion.

If the distal aneurysm extends to the renals, then the distal anastomosis may be beveled to incorporate the visceral and renal vessels and distal aorta. If the distal aneurysm extends to the bifurcation, the visceral and renal vessels are reattached to the tube graft. The left renal artery typically requires a separate anastomosis, but the celiac, superior mesenteric, and right renal arteries are often incorporated as a single island. The patient is rewarmed, and the partial bypass is discontinued as the tube graft perfuses the intercostals and abdominal vessels. The distal anastomosis at the bifurcation is performed as an open distal procedure.

For appropriate descending TAAs, endovascular stent grafting is a good alternative. Depending on the size of the patient's femoral or iliac arteries and size of the stent graft required, femoral or iliac artery exposure is performed under general or local anesthesia plus sedation. A sheath is placed and a wire guided under fluoroscopy into the arch. When in proper position, the floppy wire is exchanged with a soft catheter and rewired to a stiffer wire for device placement. The sheath is exchanged for the appropriate device sheath. The contralateral groin is used for angiocatheter placement.

After angiography and determination of stent placement, the device is loaded and, under fluoroscopic guidance, is positioned and deployed. More than one stent may be used, with as much overlap as is feasible, for stability. The proximal and distal landing zones are ballooned to seal the endograft to the aorta. The overlapping stent-graft segments are also ballooned. Angiography is performed to check for endoleaks. Endoleaks may require additional stents.

TAAAs may involve arteries supplying the abdominal viscera. In this case, for a completely endovascular repair, aortic stent grafts with fenestrations or branches oriented towards the intended covered arteries have been devised. These grafts previously have been individualized to the specific anatomy of the patient, although recent data have demonstrated that noncustomized branch grafts may work for most patients.^[53]

Initial placement of the aortic stent graft ensues, carefully aligning the fenestrations or branches to the abdominal viscera. The abdominal visceral arteries are then cannulated with separate guide wires in a retrograde fashion for cranially oriented arteries, or through the brachial artery for in an antegrade fashion for caudally oriented arteries. A bridging covered stent is then deployed to create a visceral seal zone.

Ross procedure (pulmonary autograft)

The aortic root and proximal ascending aorta are replaced with a pulmonary autograft.^[52]  The pulmonary valve is then replaced with a pulmonary homograft. Most commonly performed in children with congenital disease, the Ross operation may be used for active young adults with aneurysmal disease (excluding those with connective tissue disorders), women of childbearing age who desire pregnancy, or patients with contraindications to anticoagulation.

• References

Postoperative Care

Patients who have undergone ascending aneurysm repairs are observed for signs of coronary ischemia, particularly if the coronary ostia were reimplanted, and for signs of aortic insufficiency when the aortic valve is repaired. After repair of arch aneurysms, particular attention must be given to neurologic status, and patients who have had the elephant trunk repair must be observed for signs of paraplegia because the telescoped sleeve in the descending aorta may obstruct critical spinal vessels.

Paraplegia is the main concern in patients who have had repair of the descending and thoracoabdominal aorta. CSF drainage may be continued for up to 72 hours postoperatively if necessary, along with motor evoked potential monitoring. Paraplegia and paraparesis may be acute or delayed postoperatively. If they are delayed, increased mean arterial pressure with pressors and reinstitution of CSF drainage may augment spinal cord perfusion to reverse this complication. Paraplegia due to occlusion of critical spinal arteries that were not reimplanted cannot be reversed by these maneuvers.

Acute postoperative renal dysfunction may be due to extended periods of ischemic cross-clamping or to hypothermic circulatory arrest.

Patients undergoing endovascular stenting are often extubated early postoperatively with a decreased length of stay in the intensive care unit (ICU).

• References

Complications

Early morbidity and mortality are related to bleeding, neurologic injury (eg, stroke), cardiac failure, and pulmonary failure (eg, acute respiratory distress syndrome [ARDS]). Risk factors include emergency operation, older age, dissection, congestive heart failure (CHF), prolonged CPB time, arch replacement, previous cardiac surgery, need for concomitant coronary revascularization, and reoperation for bleeding. Late mortality is usually related to cardiac disease or distal aortic disease.

Bleeding is a potential complication for all aneurysm repairs. It is minimized by the use of antifibrinolytics, felt strips, and factors, including fresh frozen plasma and platelets. For patients who undergo hypothermic circulatory arrest, the use of aprotinin is controversial, but most groups routinely use aminocaproic acid. Coagulopathy and bleeding in severe cases may warrant the use of recombinant factor VII.

Aprotinin, an antifibrinolytic agent used to reduce operative blood loss in patients undergoing open heart surgery, is now only available via a limited-access protocol. Fergusson et al reported an increased risk for death compared with tranexamic acid or aminocaproic acid in high-risk cardiac surgery.^[43]

Stroke is a major cause of morbidity and mortality and typically results from embolization of atherosclerotic debris or clot. TEE and epiaortic ultrasonography may be beneficial in localizing appropriate areas to clamp. Patients undergoing arch repairs are at the highest risk of permanent and transient neurologic injury. Retrograde cerebral perfusion is beneficial for flushing out embolic debris, but it may be detrimental, with increased intracranial pressure and cerebral edema. Antegrade cerebral perfusion is beneficial for reducing neurologic injury during hypothermic circulatory arrest. Stroke incidence for open surgical repair versus endovascular repair of descending TAAs is equivalent.

Myocardial infarction may occur with technical problems with coronary ostia implantation during root replacement for ascending aortic aneurysms and may require reoperation. Pulmonary dysfunction and renal dysfunction are other potentially morbid complications.

Paraparesis and paraplegia, either acute or delayed, are the most devastating complications of descending TAA and TAAA repairs. Despite CSF drainage, reimplantation of intercostal arteries, evoked potential monitoring, mild hypothermia, and atrial femoral bypass, spinal cord injury still occurs. Endovascular stent grafting has not eliminated spinal cord paraplegia; the incidence varies widely, with an overall incidence of 2.7%.^{[27, 44, 45, 46]}

Complications specific to endovascular stenting include endoleaks, stent fractures, stent graft migration or thrombosis, iliac artery rupture, retrograde dissection, microembolization, aortoesophageal fistula, and complications at the site of delivery (eg, groin infection, lymphocele, seroma).

• References

Long-Term Monitoring

Development of another aneurysm postoperatively is not uncommon in these patients. For this reason, serial evaluations (ie, CT or magnetic resonance imaging [MRI] for ascending, arch, or descending aneurysms; echocardiography for ascending aneurysms) may be performed every 3-6 months during the first postoperative year and every 6 months thereafter.

In a study that evaluated the differences between male and female patients undergoing thoracic endovascular aneurysm repair in an FDA-approved trial, female patients had higher rates of periprocedural complications, required more blood transfusions, had longer hospital stays, and experienced more major adverse events after 30 days.^[58] However, female patients also more often had successful aneurysm treatment at 1-year follow-up.

• References