Thoracic Aneurysms and Aortic Dissection
BASIC SCIENCE QUESTIONS
1. A mutation in the fibrillin gene is associated with which of the following?
A. Ehlers-Danlos syndrome
B. Marfan syndrome
C. Loeys-Dietz syndrome
D. Congenital bicuspid aortic valve
Marfan syndrome is an autosomal dominant genetic disorder characterized by a specific connective tissue defect that leads to aneurysm formation. The phenotype of patients with Marfan syndrome typically includes a tall stature, high palate, joint hypermobility, eye lens dislocation, mitral valve prolapse, and aortic aneurysms. The aortic wall is weakened by fragmentation of elastic fibers and deposition of extensive amounts of mucopolysaccharides (a process previously called cystic medial degeneration). Patients with Marfan syndrome have a mutation in the fibrillin gene located on the long arm of chromosome 15.
Vascular type Ehlers-Danlos syndrome is characterized by an autosomal dominant defect in type III collagen synthesis.
Recently described, Loeys-Dietz syndrome is phenotypically distinct from Marfan syndrome. It is characterized as an aneurysmal syndrome with widespread systemic involvement. Loeys-Dietz syndrome is an aggressive, autosomal dominant condition that is distinguished by the triad of arterial tortuosity and aneurysms, hypertelorism (widely spaced eyes), and bifid uvula or cleft palate. It is caused by heterozygous mutations in the genes encoding TGF-β receptors, rather than fibrillin 1.
Bicuspid aortic valve is the most common congenital malformation of the heart or great vessels, affecting up to 2% of Americans. Compared to patients with normal, trileaflet aortic valves, patients with bicuspid aortic valves have an increased incidence of ascending aortic aneurysm formation and, often, a more rapid rate of aortic enlargement. The exact mechanism responsible for aneurysm formation in patients with bicuspid aortic valves remains controversial. The two most popular theories posit that the dilatation is caused by (a) a congenital defect involving the aortic wall matrix that results in progressive degeneration, or (b) ongoing hemodynamic stress caused by turbulent flow through the diseased valve. It is likely that both proposed mechanisms are involved: patients with bicuspid aortic valves may have a congenital connective tissue abnormality that predisposes the aorta to aneurysm formation, especially in the presence of chronic turbulent flow through a deformed valve. (See Schwartz 9th ed., pp 667-668.)
2. Elastin levels are highest in the wall of the
A. Ascending thoracic aorta
B. Aortic arch
C. Descending thoracic aorta
D. Abdominal aorta
The normal aorta derives its elasticity from the medial layer, which contains approximately 45 to 55 lamellae of elastin, collagen, smooth muscle cells, and ground substance. Elastin content is highest within the ascending aorta, as would be expected because of its compliant nature, and decreases distally into the descending and abdominal aorta. Maintenance of the aortic matrix involves complex interactions among smooth muscle cells, macrophages, proteases, and protease inhibitors. Any alteration in this delicate balance can lead to aortic disease. (See Schwartz 9th ed., p 667.)
1. Which of the following is a common cause of mycotic thoracic aortic aneurysms?
A. Candida glabrata
B. Staphylococcus aureus
C. Aspergillus clavatus
D. Stenotrophomonas maltophilia
Primary infection of the aortic wall resulting in aneurysm formation is rare. Although these lesions are termed mycotic aneurysms, the responsible pathogens usually are bacteria rather than fungi. Bacterial invasion of the aortic wall may result from bacterial endocarditis, endothelial trauma caused by an aortic jet lesion, or extension from an infected laminar clot within a pre-existing aneurysm. The most common causative organisms are Staphylococcus aureus, Staphylococcus epidermidis, Salmonella, and Streptococcus. Unlike most other causes of thoracic aortic aneurysms, which generally produce fusiform aneurysms, infection often produces saccular aneurysms located in areas of aortic tissue destroyed by the infectious process. (See Schwartz 9th ed., p 668.)
2. Which of the following is NOT a cause of thoracic aortic aneurysms?
A. Marfan syndrome
B. Osteogenesis imperfecta
C. Ehlers-Danlos syndrome
D. Takayasu’s arteritis
See Table 22-1. Osteogenesis imperfecta is not associated with an increased risk of aortic aneurysm. (See Schwartz 9th ed., p 667.)
TABLE 22-1 Causes of thoracic aortic aneurysms
Nonspecific medial degeneration
Familial aortic aneurysms
Congenital bicuspid aortic valve
Giant cell arteritis
3. Patients with which of the following should undergo elective surgical repair of their condition?
A. 5-cm ascending aortic aneurysm
B. 5-cm descending aortic aneurysm
C. 5-cm ascending aortic aneurysm in a patient with Marfan syndrome
D. Any aneurysm that has grown 0.5 cm in diameter in 1 year
Thoracic aortic aneurysms are repaired to prevent fatal rupture. Therefore, on the basis of the natural history studies discussed earlier, elective operation is recommended when the diameter of an ascending aortic aneurysm is >5.5 cm, when the diameter of a descending thoracic aortic aneurysm is >6.5 cm, or when the rate of dilatation is >1 cm/y. In patients with connective tissue disorders, such as Marfan and Loeys-Dietz syndromes, the threshold for operation is lower with regard to both absolute size (5.0 cm for the ascending aorta and Syncope 6.0 cm for the descending thoracic aorta) and rate of growth. Smaller ascending aortic aneurysms (4.0 to 5.5 cm) also are considered for repair when they are associated with significant aortic valve regurgitation. (See Schwartz 9th ed., p 671.)
4. Which of the following is NOT used to protect perfusion to the spinal cord during open repair of descending thoracic and thoracoabdominal aortic aneurysms?
A. Permissive mild hypothermia (32–34°C)
B. Left heart bypass
C. Reattachment of intercostal arteries
D. Perfusion of intercostal or lumbar arteries with 4°C cystalloid solution
Clamping the descending thoracic aorta causes ischemia of the spinal cord and abdominal viscera. Clinically significant manifestations of hepatic, pancreatic, and bowel ischemia are relatively uncommon. However, both acute renal failure and spinal cord injury resulting in paraplegia or paraparesis remain major causes of morbidity and mortality after these operations. Therefore, several aspects of the operation are devoted to minimizing spinal and renal ischemia (Table 22-2). Our multimodal approach to spinal cord protection includes expeditious repair to minimize aortic clamping time, moderate systemic heparinization (1.0 mg/kg) to prevent small-vessel thrombosis, mild permissive hypothermia [32° to 34°C (89.6° to 93.2°F)] nasopharyngeal temperature], and reattachment of segmental intercostal and lumbar arteries. (See Schwartz 9th ed., p 678.)
Left heart bypass, which provides perfusion of the distal aorta and its branches during the clamping period, is also used during extensive thoracoabdominal aortic repairs. Because left heart bypass unloads the heart, it is also useful in patients with poor cardiac reserve. Balloon perfusion cannulas connected to the left heart bypass circuit can be used to deliver blood directly to the celiac axis and superior mesenteric artery during their reattachment. The potential benefits of reducing hepatic and bowel ischemia include reduced risks of postoperative coagulopathy and bacterial translocation, respectively. Whenever possible, renal protection is achieved by perfusing the kidneys with cold [4°C (39.2°F)] crystalloid. In a randomized clinical trial, reduced kidney temperature was found to be associated with renal protection, and the use of cold crystalloid independently predicted preserved renal function. (See Schwartz 9th ed., p 679.)
TABLE 22-2 Current strategy for spinal cord and visceral protection during repair of distal thoracic aortic aneurysms
• Permissive mild hypothermia [32–34°C (89.6–93.2°F), nasopharyngeal]
• Moderate heparinization (1 mg/kg)
• Aggressive reattachment of segmental arteries, especially between T8 and L1
• Sequential aortic clamping when possible
• Perfusion of renal arteries with 4°C (39.2°F) crystalloid solution when possible
Crawford extent I and II thoracoabdominal repairs
• Cerebrospinal fluid drainage
• Left heart bypass during proximal anastomosis
• Selective perfusion of celiac axis and superior mesenteric artery during intercostal and visceral anastomoses
5. The most common symptom in a patient with an acute aortic dissection is
A. Shortness of breath
The European Society of Cardiology Task Force on Aortic Dissection stated, ‘The main challenge in managing acute aortic dissection is to suspect and thus diagnose the disease as early as possible.’ A high index of suspicion is critical, particularly in younger, atypical patients, who may have connective tissue disorders or other, less common risk factors. Most patients with acute aortic dissection (80 to 90%) experience severe pain in the chest, back, or abdomen. The pain usually occurs suddenly, has a sharp or tearing quality, and often migrates distally as the dissection progresses along the aorta. For classification purposes (acute vs. sub-acute vs. chronic), the onset of pain is generally considered to represent the beginning of the dissection process. Most of the other common symptoms either are nonspecific or are caused by the secondary manifestations of dissection. (See Schwartz 9th ed., p 688.)
6. The acute phase of aortic dissection lasts for
A. 48 hours
B. 7 days
C. 14 days
D. 3 months
Aortic dissection also is categorized according to the time elapsed since the initial tear. Dissection is considered acute within the first 14 days after the initial tear; after 14 days, the dissection is considered chronic. Although arbitrary, the distinction between acute and chronic dissections has important implications not only for decision making about perioperative management strategies and operative techniques, but also for evaluating surgical results. In light of the importance of acuity, Borst and associates have proposed a third phase—termed subacute—to describe the transition between the acute and chronic phases. The sub-acute period encompasses days 15 through 60 after the initial tear. Although this is past the traditional 14-day acute phase, patients with subacute dissection continue to have extremely fragile aortic tissue, which may complicate operative treatment and increase the risks associated with surgery. (See Schwartz 9th ed., p 686.)
For classification purposes (acute vs. subacute vs. chronic), the onset of pain is generally considered to represent the beginning of the dissection process. (See Schwartz 9th ed., p 688.)
7. A patient with a thoracoabdominal aneurysm that involves the entire descending thoracic aorta and extends to the iliac arteries (involving the entire abdominal aorta) is a
A. Crawford Extent I aneurysm
B. Crawford Extent II aneurysm
C. Crawford Extent III aneurysm
D. Crawford Extent IV aneurysm
Thoracoabdominal aneurysms can involve the entire thoracoabdominal aorta, from the origin of the left subclavian artery to the aortic bifurcation, and are categorized according to the Crawford classification scheme (Fig. 22-1). Extent I thoracoabdominal aortic aneurysms involve most of the descending thoracic aorta, usually beginning near the left subclavian artery, and extend down to encompass the aorta at the origins of the celiac axis and superior mesenteric arteries. The renal arteries also may be involved. Extent II aneurysms also arise near the left subclavian artery but extend distally into the infrarenal abdominal aorta, and they often reach the aortic bifurcation. Extent III aneurysms originate in the lower descending thoracic aorta (below the sixth rib) and extend into the abdomen. Extent IV aneurysms begin within the diaphragmatic hiatus and often involve the entire abdominal aorta. (See Schwartz 9th ed., p 678.)
FIG. 22-1. Illustration of the Crawford classification of thoracoabdominal aortic aneurysms based on the extent of aortic involvement. (Reproduced with permission from Coselli JS, LeMaire SA: Descending and Thoracoabdominal Aortic Aneurysms, in Cohn LH (ed): Cardiac Surgery in the Adult, 3rd ed. New York: McGraw–Hill, Inc., 2008, Chap. 54, Fig. 54-5.)
8. The “critical diameter” (i.e., associated with a marked increase in risk of complications) for aneurysms of the descending thoracic aorta is
A. 5 cm
B. 6 cm
C. 7 cm
D. 8 cm
Treatment decisions in cases of thoracic aortic aneurysm are guided by our current understanding of the natural history of these aneurysms, which classically is characterized as progressive aortic dilatation and eventual dissection, rupture, or both. An analysis by Elefteriades of data from 1600 patients with thoracic aortic disease has helped quantify these well-recognized risks. Average expansion rates were 0.07 cm/y in ascending aortic aneurysms and 0.19 cm/y in descending thoracic aortic aneurysms. As expected, aortic diameter was a strong predictor of rupture, dissection, and mortality. For thoracic aortic aneurysms >6 cm in diameter, annual rates of catastrophic complications were 3.6% for rupture, 3.7% for dissection, and 10.8% for death. Critical diameters, at which the incidence of expected complications significantly increased, were 6.0 cm for aneurysms of the ascending aorta and 7.0 cm for aneurysms of the descending thoracic aorta; the corresponding risks of rupture after reaching these diameters were 31 and 43%, respectively. (See Schwartz 9th ed., p 669.)
9. Which of the following is used to protect the spinal cord during endovascular repair of descending thoracic aortic aneurysms?
A. Heparinization (ACT >300 seconds)
C. Spinal fluid drainage
D. Trendelenburg position
To protect patients against spinal cord ischemia during these endovascular repairs, many surgeons use cerebrospinal fluid drainage. Fluid is drained to maintain a cerebrospinal fluid pressure of approximately 12 to 14 mmHg. (See Schwartz 9th ed., p 679.)
10. Which of the following is the most sensitive test for the diagnosis of an acute aortic dissection?
B. Chest x-ray
C. Cardiac enzymes
Several reports have demonstrated that D-dimer is an extremely sensitive indicator of acute aortic dissection; elevated levels are found in approximately 97% of affected patients. Tests that are commonly used to detect acute coronary events—including ECG and tests for serum markers of myocardial injury—deserve special consideration and need to be interpreted carefully. Normal ECGs and serum marker levels in patients with acute chest pain should raise suspicion about the possibility of aortic dissection. It is important to remember that ECG changes and elevated serum marker levels associated with myocardial infarction do not exclude the diagnosis of aortic dissection, because dissection can cause coronary malperfusion. Ultimately, although the issue has not been well studied, ECGs seem to have little utility for detecting or ruling out dissection. Similarly, although chest x-rays (CXRs) may show a widened mediastinum or abnormal aortic contour, up to 16% of patients with dissection have a normal appearing CXR. The value of the CXR for detecting aortic dissection is limited, with a sensitivity of 67% and a specificity of 86%. (See Schwartz 9th ed., pp 688-689.)
11. Which of the following in an indication for delayed (rather than emergent) repair of an acute ascending thoracic dissection?
A. Recent (3 weeks) cardiac surgery
B. Evidence of acute myocardial infarction
C. Mesenteric ischemia
D. Marfan syndrome
In most patients with acute ascending aortic dissection, the risk of a fatal complication, such as aortic rupture, during medical management outweighs the risk associated with early operation. Therefore, acute ascending aortic dissection has traditionally been considered an absolute indication for emergency surgical repair. However, specific patient groups may benefit from nonoperative management or delayed operation. Delayed repair should be considered for patients who (a) present with acute stroke or mesenteric ischemia, (b) are elderly and have substantial comorbidity, (c) are in stable condition and may benefit from transfer to specialized centers, or (d) have undergone a cardiac operation in the remote past. Regarding the last group, it is important that the previous operation not be too recent; dissections that occur during the first 3 weeks after cardiac surgery pose a high risk of rupture and tamponade, and such dissections warrant early operation. (See Schwartz 9th ed., p 690.)
12. Which of the following is the imaging modality of choice for the diagnosis of thoracic aortic aneurysms?
A. Plain radiograph of the chest
C. CT scan
Computed tomographic (CT) scanning is widely available and provides visualization of the entire thoracic and abdominal aorta. Consequently, CT is the most common—and arguably the most useful—imaging modality for evaluating thoracic aortic aneurysm. Systems capable of constructing multiplanar images and performing three- dimensional aortic reconstructions are widely available. In addition to establishing the diagnosis, CT provides information about an aneurysm’s location, extent, anatomic anomalies, and relationship to major branch vessels. CT is particularly useful in determining the absolute diameter of the aorta, especially in the presence of laminated clot. Contrast-enhanced CT provides information about the aortic lumen and can detect mural thrombus, aortic dissection, inflammatory periaortic fibrosis, and mediastinal or retroperitoneal hematoma due to contained aortic rupture.
Chest radiographs (CXRs) often appear normal in patients with thoracic aortic disease and thus cannot exclude the diagnosis of aortic aneurysm.
Although useful in evaluating infrarenal abdominal aortic aneurysms, standard transabdominal ultrasonography does not allow visualization of the thoracic aorta.
Although diagnostic aortography was, until recently, considered the gold standard for evaluating thoracic aortic disease, CT and MRA have largely replaced this modality. Technologic improvements have enabled CT and MRA to provide excellent aortic imaging while causing less morbidity than catheter-based studies do, so CT and MRA should now be considered the gold standard. Therefore, the role of diagnostic angiography in patients with thoracic aortic disease is currently limited. In selected cases, aortography is used to gain important information when other types of studies are contraindicated or have not provided satisfactory results. For example, information about obstructive lesions of the brachiocephalic, visceral, renal, or iliac arteries is useful when surgical treatment is being planned; if other imaging studies have not provided adequate detail, aortograms can be obtained in patients with suspected branch vessel occlusive disease. (See Schwartz 9th ed., pp 669-671.)
13. Which of the following is NOT a typical complication of an acute ascending aortic dissection?
A. Myocardial infarction
B. Pleural effusion
C. Aortic valve regurgitation
D. Pericardial effusion
Ascending aortic dissection can directly injure the aortic valve, causing regurgitation. The severity of the regurgitation varies with the degree of commissural disruption, which ranges from partial separation of only one commissure, producing mild valvular regurgitation, to full separation of all three commissures and complete prolapse of the valve into the left ventricle, producing severe acute heart failure.
Ascending dissections also can extend into the coronary arteries or shear the coronary ostia off of the true lumen, causing acute coronary occlusion; when this occurs, it most often involves the right coronary artery. The sudden disruption of coronary blood flow can cause a myocardial infarction.
The thin and inflamed outer wall of a dissected ascending aorta often produces a serosanguineous pericardial effusion that can accumulate and cause tamponade. Suggestive signs include jugular venous distention, muffled heart tones, pulsus paradoxus, and low voltage electrocardiogram (ECG) tracings. Free rupture into the pericardial space produces rapid tamponade and is generally fatal.
As the dissection progresses, any branch vessel from the aorta can become involved, which results in compromised blood flow and ischemic complications (i.e., malperfusion). Therefore, depending on which arteries are involved, the dissection can produce acute stroke, paraplegia, hepatic failure, bowel infarction, renal failure, or a threatened ischemic limb. (See Schwartz 9th ed., p 688, and Table 22-3.)
TABLE 22-3 Anatomic complications of aortic dissection and their associated symptoms and signs
14. Which of the following is indicated in the initial treatment of a patient with an acute aortic dissection?
C. Emergency catheterization and stenting of the aorta
D. Emergency surgery to repair the aorta
The initial management strategy, commonly described as antihypertensive therapy or blood pressure control, focuses on reducing aortic wall stress, the force of left ventricular ejection, chronotropy, and the rate of change in blood pressure (dP/dT). Reductions in dP/dT are achieved by lowering both cardiac contractility and blood pressure. The drugs initially used to accomplish these goals include IV beta-adrenergic blockers, direct vasodilators, calcium channel blockers, and angiotensin-converting enzyme inhibitors. These agents are used to achieve a heart rate between 60 and 80 bpm, a systolic blood pressure between 100 and 110 mmHg, and a mean arterial blood pressure between 60 and 75 mmHg. These hemodynamic targets are maintained as long as urine output remains adequate and neurologic function is not impaired. Achieving adequate pain control with IV opiates, such as morphine and fentanyl, is important for maintaining acceptable blood pressure control.
Beta antagonists are administered to all patients with acute aortic dissections unless there are strong contraindications, such as severe heart failure, bradyarrhythmia, high-grade atrioventricular conduction block, or bronchospastic disease. Esmolol can be useful in patients with bronchospastic disease because it is a cardioselective, ultra-fast-acting agent with a short half-life. Labetalol, which causes both nonselective beta blockade and postsynaptic alpha1-blockade, reduces systemic vascular resistance without impairing cardiac output. Doses of beta antagonists are titrated to achieve a heart rate of 60 to 80 bpm. In patients who cannot receive beta antagonists, calcium channel blockers such as diltiazem are an effective alternative. Nitroprusside, a direct vasodilator, can be administered once beta blockade is adequate. When used alone, however, nitroprusside can cause reflex increases in heart rate and contractility, elevated dP/dT, and progression of aortic dissection. Enalapril and other angiotensin-converting enzyme inhibitors are useful in patients with renal malperfusion. These drugs inhibit renin release, which may improve renal blood flow. (See Schwartz 9th ed., p 690.)
15. The imaging study of choice in a hemodynamically stable patient with a suspected acute thoracic aortic dissection is
A. Transesophageal echocardiography
B. CT scan
C. Chest x-ray
Contrast-enhanced CT has a sensitivity of 98% and a specificity of 87% for diagnosis of aortic dissection. Although MRA is now considered the gold standard, with both a sensitivity and a specificity of 98%, CT scanning is the preferred imaging modality in the emergency department, mainly because of its swift image acquisition. In appropriate hands, TEE has a demonstrated sensitivity and specificity as high as 98 and 95%, respectively. Furthermore, TEE offers important information about ventricular function and aortic valve competency. Finally, TEE is the diagnostic modality of choice for hemodynamically unstable patients in whom the diagnosis of ascending dissection is suspected; ideally, these patients should be taken to the operating room, where the TEE can be performed and, if the TEE is confirmatory, surgery can be started immediately. (See Schwartz 9th ed., p 689.)
16. An aortic dissection that extends from the left subclavian artery to the aortic bifurcation is a
A. DeBakey Type I dissection
B. DeBakey Type II dissection
C. DeBakey Type IIIa dissection
D. DeBakey Type IIIb dissection
Dissections are categorized according to their anatomic location and extent to guide treatment. The two traditional classification schemes that remain in common use are the DeBakey and the Stanford classification systems (Fig. 22-2). In their current forms, both of these schemes describe the segments of aorta that are involved in the dissection, rather than the site of the initial intimal tear. The main drawback of the Stanford classification system is that it does not distinguish between patients with isolated ascending aortic dissection and patients with dissection involving the entire aorta. (See Schwartz 9th ed., pp 684, 686.)
FIG. 22-2. Illustration of the classification schemes for aortic dissection based on which portions of the aorta are involved. Dissection can be confined to the ascending aorta (left) or descending aorta (middle), or it can involve the entire aorta (right). (Reproduced with permission from Creager MA, Dzau VS, Loscalzo J (eds): Vascular Medicine. Philadelphia: WB Saunders, 2006. Copyright © Saunders/Elsevier, 2006, Fig. 35-2.)
17. The best treatment for a patient with an uncomplicated descending thoracic dissection is
A. Nonoperative management
B. Catheterization with placement of an aortic stent
C. Replacement of the involved aorta with a graft (single stage repair)
D. Elephant trunk procedure (staged repair)
Nonoperative, pharmacologic management of acute descending aortic dissection results in lower morbidity and mortality rates than surgical treatment does. The most common causes of death during nonoperative treatment are aortic rupture and endorgan malperfusion. Therefore, patients are continually reassessed for new complications. At least two serial CT scans—usually obtained on day 2 or 3 and on day 8 or 9 of treatment—are compared with the initial scan to rule out significant aortic expansion. (See Schwartz 9th ed., p 690-692.)
Many patients can be discharged after their blood pressure is well controlled with oral agents and after serial CT scans confirm the absence of aortic expansion. Surgery is typically reserved for patients who experience complications. In general terms, surgical intervention for acute descending aortic dissection is intended to prevent or repair ruptures and relieve ischemic manifestations.
During the acute phase of a dissection, the specific indications for operative intervention include aortic rupture, increasing periaortic or pleural fluid volume, rapidly expanding aortic diameter, uncontrolled hypertension, and persistent pain despite adequate medical therapy. Acute dissection superimposed on a pre-existing aneurysm is considered a life-threatening condition and is therefore another indication for operation. Finally, patients who have a history of noncompliance with medical therapy may ultimately benefit more from surgical treatment if they are otherwise reasonable operative candidates. (See Schwartz 9th ed., p 692.)