Aortic Dissection

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Chapter 30 Aortic Dissection

Asheesh Kumar, MD , Rae M. Allain, MD

1 Define aortic dissection

An aortic dissection is a tearing of the layers within the aortic wall, classically associated with sudden-onset chest or back pain, a pulse deficit, and mediastinal widening on a chest radiograph. Depending on size and degree of aortic involvement, it may result in marked hemodynamic instability and, often, a rapid death. Prompt diagnosis and appropriate treatment are critical to maximize the possibility of survival. Significant dissections are often fatal and rarely survive to clinical attention; the majority of dissections seen in the critical care environment are either subacute, contained, or sparing the major aortic vessels.

2 What is the anatomy of injury in aortic dissection?

The tear usually originates in the intima. It then propagates into the media creating a false channel for blood to flow and hematoma to form. The dissection process may alternatively originate with hemorrhage in the media that secondarily causes disruption of the intima. In approximately 70% of patients, the intimal tear, which is the beginning of the dissection, occurs in the ascending aorta. In 20% of patients it occurs in the descending thoracic aorta, and in 10% of patients it occurs in the aortic arch. Only rarely is an intimal tear identified in the abdominal aorta.

3 Describe the DeBakey and Stanford classifications of aortic dissection

The two classification systems most commonly used both have anatomic as well as management implications.

The DeBakey classification describes three types of dissection (Fig. 30-1):

Type I: extends from aortic root to beyond the ascending aorta

Type II: involves only the ascending aorta

Type III: begins distal to the takeoff of the left subclavian artery and has two subtypes

Type IIIA: limited to the thoracic aorta

Type IIIB: extends below the diaphragm

Figure 30-1 DeBakey classification.

Illustration by Samuel Rodriguez, MD.

The Stanford classification has two types of dissection (Fig. 30-2):

Type A: involves the ascending aorta

Type B: involves the descending aorta, distal to the left subclavian artery

Figure 30-2 Stanford classification.

Illustration by Samuel Rodriguez, MD.

4 What is the epidemiology of dissection, including mortality?

Aortic dissection is a relatively rare but a highly lethal disease. The estimated incidence is 5 to 30 cases per million people per year. Population-based studies suggest that the incidence of acute dissection ranges from 2 to 3.5 cases per 100,000 person-years, which correlates with 6000 to 10,000 cases annually in the United States. It may be that two to three times as many patients die of dissections as of ruptured aortic aneurysms; approximately 75% of patients with ruptured aortic aneurysm will reach an emergency department alive, whereas for aortic dissection 40% die immediately. Furthermore, only 50% to 70% will be alive 5 years after surgery depending on age and underlying cause.

For untreated acute dissection of the ascending aorta the mortality rate is 1% to 2% per hour after onset. For type A dissections treated medically it is approximately 20% within the first 24 hours and 50% by 1 month after presentation. Even with surgical intervention, the mortality rate for type A dissection may be as high as 10% after 24 hours and nearly 20% 1 month after repair.

Although type B dissection is less dangerous than type A, it is still associated with an extremely high mortality. The 30-day mortality rate for an uncomplicated type B dissection approaches 10%. However, patients with type B dissection who have complications such as limb ischemia, renal failure, or visceral ischemia have a 2-day mortality upwards of 20% and may prompt the need for surgical intervention.

5 What are the risk factors and associated conditions for dissection?

The prevalence of aortic dissection appears to be increasing, independent of the aging population, as noted by Olsson and colleagues, who found that the incidence of dissection among Swedish men has increased to 16 per 100,000 yearly. Risk factors include the following:

Hypertension: Present in 70% to 90% of patients with acute dissection.

Advanced age: Mean of 63 years in the International Registry of Acute Aortic Dissection (IRAD).

Male sex: Represented by 65% of patients in the IRAD.

Family history: Recently recognized is a genetic, nonsyndromic familial form of thoracic aortic dissection. Studies of patients referred for repair of thoracic aortic dissections and aneurysms who did not have a known genetic mutation have indicated that between 11% and 19% of these patients have a first-degree relative with thoracic aortic disease.

Trauma (deceleration/torsional injury)

Congenital and inflammatory disorders: present as Marfan syndrome in almost 5% of total patients in the IRAD and half of those patients under age 40 years. Other associated congenital disorders include Ehlers-Danlos syndrome, Loeys-Dietz syndrome, bicuspid aortic valve, aortic coarctation, Turner syndrome, Takayasu and giant-cell aortitis, relapsing polychondritis (Behçet disease, spondyloarthropathies), or confirmed genetic mutations known to predispose to dissections (TGFBR1, TGFBR2, FBN1, ACTA2, or MYH11).

Pregnancy: Associated with 50% of dissections in women under age 40 and most frequently occurring in the third trimester. This might be attributable to elevations in cardiac output during pregnancy that cause increased wall stress.

Circadian and seasonal variations: Producing a higher frequency of dissection in the morning hours and in the winter months.

Iatrogenic: Occurring as a consequence of invasive procedures or surgery, especially when the aorta has been entered or its main branches have been cannulated, such as for cardiopulmonary bypass.

6 Describe the common clinical signs and symptoms of aortic dissection

Pain: The most common presenting symptom is chest pain, occurring in up to 90% of patients with acute dissection. Classically, for type A dissections, sudden onset of severe anterior chest pain with extension to the back occurs that is described as ripping or tearing in nature. However, in the IRAD, pain was more often described as sharp rather than ripping or tearing. The pain is usually of maximal intensity from its inception and is frequently unremitting. It may migrate along the path of the dissection. The pain of aortic dissection may mimic that of myocardial ischemia. Patients with type B dissections are more likely to be seen with back pain (64%) alone.

Syncope: Syncope is a well-recognized clinical feature of dissection, occurring in up to 13% of cases. Impairments of cerebral blood flow can be due to acute hypovolemia, low cardiac output, or dissection-involvement of the cerebral vessels. Patients with a presenting syncope were significantly more likely to die than were those without syncope (34% vs. 23%), likely because of the frequent correlation with associated cardiac tamponade, stroke, decreased consciousness, and spinal cord ischemia.

7 Describe the common clinical findings associated with aortic dissection

A systems-based approach can be used to describe the wide range of clinical findings that can be associated with aortic dissection.

Neurologic symptoms. The reported frequency of neurologic symptoms in pooled data of type A and B dissections approaches 17%; in type A alone, 29% of patients were seen initially with neurologic symptoms, 53% of which represented ischemic stroke. Neurologic complications may result from hypotension, malperfusion, distal thromboembolism, or nerve compression. Acute paraplegia as a result of spinal cord malperfusion has been described as a primary manifestation in 1% to 3% of patients. Up to 50% of neurologic symptoms may be transient.

Cardiovascular manifestations. The heart is the most frequently involved end-organ in acute proximal aortic dissections.

Acute aortic regurgitation may be present in 41% to 76% of patients with proximal dissection and may be caused by widening of the aortic annulus resulting in incomplete valve closure or actual disruption of the aortic valve leaflets from the dissection flap. Clinical manifestations of dissection-related aortic regurgitation span from mere diastolic murmurs without clinical significance to overt congestive heart failure and cardiogenic shock.

Myocardial ischemia or infarction may result from compromised coronary artery flow by an expanding false lumen that compresses the proximal coronary or by extension of the dissection flap into the coronary artery ostium. This occurs in 7% to 19% of patients with proximal aortic dissections. Clinically, these present as electrocardiographic changes consistent with primary myocardial ischemia and/or infarction. Appropriate treatment of myocardial ischemia ought to be initiated without delay and concomitantly with aortic imaging to evaluate for dissection when both diagnoses are suspected.

Cardiac tamponade is diagnosed in 8% to 10% of patients seen with acute type A dissections. It is associated with a high mortality and should prompt consideration for emergent drainage and aortic repair.

Hypertension occurs in greater than 50% of patients with dissection, more commonly with distal disease. Ongoing renal ischemia can produce severe hypertension.

Hypotension/shock may present in up to 20% of patients with dissection. This may be a result of cardiac tamponade from aortic rupture into the pericardium, dissection, or compression of the coronary arteries, acute aortic regurgitation, acute blood loss, true lumen compression by distended false lumen, or an intraabdominal catastrophe. Cardiogenic shock is a relatively uncommon complication, found to occur in approximately 6% of cases. This can be due to acute aortic regurgitation or ongoing myocardial ischemia.

Peripheral vascular complications can manifest as pulse and/or blood pressure differentials or deficits and occur in approximately one third to one half of patients with proximal dissection. Etiology is partial compression, obstruction, thrombosis, or embolism of the aortic branch vessels, resulting in cerebral, renal, visceral, or limb ischemia. Peripheral pulse deficits should alert the clinician to possible ongoing renal or visceral ischemia unable to be detected from physical examination or laboratory values alone.

Pulmonary complications may manifest as pleural effusions, which occur most frequently on the left. Causes include rupture of the dissection into the pleural space or weeping of fluid from the aorta as an inflammatory response to the dissection.

8 What common laboratory abnormalities are associated with aortic dissection?

Laboratory data are usually unrevealing, but anemia from blood loss into the false lumen can occur. A moderate leukocytosis (10,000-14,000 white cells per milliliter) is sometimes seen. Lactic acid dehydrogenase and bilirubin levels may be elevated because of hemolysis within the false lumen. Disseminated intravascular coagulation has been reported. Currently, randomized controlled data do not support the use of D-dimers or experimental serum markers (plasma smooth muscle myosin heavy chain protein, high-sensitivity C-reactive protein).

9 Describe imaging modalities used to diagnose aortic dissection

In 2010, the American Heart Association and American College of Cardiology released guidelines for the diagnosis and management of patients with thoracic aortic disease, which identified high-risk clinical features to assist in the early detection of acute aortic dissection. On the basis of clinical risk factors and conditions, presentation, and associated examination findings, patients are stratified into low-, intermediate-, or high-risk categories. Further work-up is dictated by this pretest probability index. Some patients with acute dissection initially have no high-risk features, creating a diagnostic dilemma. According to most recent guidelines, if a clear alternative diagnosis is not established after the initial evaluation, then obtaining a diagnostic aortic imaging study should be considered.

Although lacking specificity, a chest radiograph should be obtained as part of the initial diagnostic evaluation. A radiograph abnormality is seen in up to 90% of patients with aortic dissection; most frequent is widening of the aorta and mediastinum. Other findings may include a localized hump on the aortic arch, displacement of calcification in the aortic knob, and pleural effusions. However, approximately 40% of radiographs in acute dissection lack a widened mediastinum, and as many as 16% are normal. Thus a negative radiograph must not delay definitive aortic imaging in patients deemed at high risk for aortic dissection by initial screening.

Computed tomography (CT) scanning, magnetic resonance imaging (MRI), and transesophageal echocardiography (TEE) are all highly accurate imaging modalities that may be used to make the diagnosis; all can provide acceptable diagnostic accuracy. Transthoracic echocardiography has limited diagnostic accuracy. Aortography, which was once the test of choice, is no longer used routinely because it is invasive and time-consuming and involves exposure to intravenous contrast dye. The most recent comparative study with nonhelical CT, MRI, and TEE showed 100% sensitivity for all modalities, with better specificity of CT (100%) as compared with TEE or MRI. A recent meta-analysis found that all three imaging techniques provided equally reliable results. Although each imaging modality offers advantages and disadvantages, the choice among CT, MRI, and TEE is probably best based on which is most readily available. It should be noted, however, that the diagnosis of acute aortic dissection can be difficult and occasionally cannot be absolutely excluded by a single imaging study. If a high clinical suspicion exists despite initially negative imaging, then consideration should be given to a second imaging modality. Regardless, prompt surgical consultation should be initiated in any patient with a suspected dissection.

11 Differentiate between the management of Stanford type A and type B dissections

An acute type A dissection is a surgical emergency. However, medical management is critical to halt the progression of the dissection while the diagnostic work-up takes place and while preparations are made to bring the patient to the operating room for definitive treatment. While the diagnosis work-up proceeds and a cardiothoracic surgeon is consulted, the patient’s condition should be carefully monitored and stabilized in an intensive care unit. Pain management and gradual down-titration of blood pressure are critical to prevent extension of the dissection. Sufficient blood products and intravascular access should be available in the event of aortic rupture.

Patients with uncomplicated type B dissection are preferably managed medically with β-blockers and other antihypertensive agents. Surgical intervention has no demonstrable superiority except in cases of failed medical management manifesting as malperfusion, aortic expansion with potential for imminent rupture, or intractable pain. Ongoing advances with less-invasive interventions (endovascular stent grafts and endovascular fenestration procedures) suggest an expanded role for interventional management in the treatment of acute type B dissection, especially in experienced centers.

12 What are the strategies for medical management of dissection and commonly used medications?

The goals of medical therapy are to treat pain, to aggressively control blood pressure, and to determine need for surgical or endovascular intervention. Patients who are seen with hypotension should receive the following:

Prompt but judicious volume resuscitation and hemodynamic support with intravenous vasopressors to maintain a goal mean arterial pressure of 70 mm Hg

Rapid search for underlying etiology (tamponade, myocardial dysfunction, acute hemorrhage)

Emergent surgical consultation for operative management

In those who are seen initially with hypertension, the blood pressure should generally be lowered to a systolic of 100 to 120 mm Hg, to a mean of 60 to 65 mm Hg, or to the lowest level that is compatible with perfusion of the vital organs. The aortic wall stress is affected by the heart rate, blood pressure, and velocity of ventricular contraction (dP/dt).

The ideal antihypertensive regimen must decrease blood pressure without increasing cardiac output through peripheral vasodilatation. This is because an increased cardiac output can increase flow rates producing higher aortic wall stress and thus propagating the dissection. Intravenous β-blockers (commonly esmolol, labetalol, propranolol, or metoprolol) are considered the first-line medical stabilization regimen because they affect all three parameters without increases in cardiac output and aortic wall stress. In patients who are unable to tolerate β- blockade, nondihydropyridine calcium channel antagonists (verapamil, diltiazem) offer an acceptable alternative.

Often, single-drug therapy alone is inadequate to optimize blood pressure management. Adequate pain control is essential not only for patient comfort but also to decrease sympathetic-mediated increases in heart rate and blood pressure. This may be accomplished with intravenous opioid analgesics. If β-blockade and adequate pain control are ineffective to control blood pressure, the addition of a rapidly acting, easily titratable intravenous vasodilator, such as nitroprusside, should be considered. Other agents, such as nicardipine, nitroglycerin, and fenoldopam, are also acceptable. Vasodilator therapy without prior β-blockade may cause reflex tachycardia and increased force of ventricular contraction leading to greater wall stress and potentially causing false lumen propagation; therefore adequate β-blockade must be established first, before the vasodilator is initiated.

13 Describe the surgical approach for repair of Stanford type A dissection

The purpose of surgery is to resect the aortic segment containing the proximal intimal tear, to obliterate the false channel, and to restore aortic continuity with a graft or by reapproximating the transected ends of the aorta. For patients with aortic insufficiency, it may be possible to resuspend the aortic valve, but in some cases replacement of the aortic valve is necessary. In some cases of proximal dissection, reimplantation of the coronary arteries is required. If a DeBakey type II dissection is present, the entire dissected aorta should be replaced. Surgery to repair an aortic dissection generally requires cardiopulmonary bypass and, often, deep hypothermic circulatory arrest.

14 What is a more recent alternative to surgical repair of aortic dissection?

An endovascular technique of stent-grafting and/or balloon fenestration may be used for initial surgical treatment of some dissections. Indications for open or endograft treatment are based on the anatomic features of the lesion, clinical presentation and course, patient comorbidities, and anatomic constraints related to endograft technology. Dissections pose a complex situation because the branches of the aorta may be perfused from either the true or false lumen. Often, both the true and false lumens are patent and some of the visceral, renal, or lower extremity vessels are fed by one channel and the remainder by the other. Consideration must be given to how blood flow reaches vital organs before considering treatment of a dissection with an endovascular stent-graft. For type B dissection, an increasing number of reports show better results with endovascular repair versus open surgical repair. The role of endovascular stent-graft versus optimal medical therapy was recently examined in the literature, but no difference was noted in survival or number of adverse events. However, longer-term (5 year) data are needed to fully assess the potential impact of stent-grafting for acute dissection, including effects on survival, clinical outcomes, and long-term aortic remodeling.

15 What is the future direction for repair of aortic dissections?

The use of fenestrated endografts will likely herald a new era in the treatment of aortic dissections. Unsuitable anatomy is a significant barrier to the use of endovascular stent-grafts for most forms of aortic disease, where the ostia of major vessels would otherwise be partially or completed covered with the deployment of a stent-graft. Using preoperative three-dimensional CT aortic reconstruction, customized stents can be constructed, featuring holes (fenestrations) or side-branches matched to patient-specific anatomy to ensure perfusion to major aortic branch vessels. Current trials are underway in Europe and the United States for their use for complex aneurysmal disease, and expectations are high for similar application to aortic dissection.

Acknowledgment

Drs. Kumar and Allain wish to acknowledge Peter M. Schulman, MD, and Rondall Lane, MD, the authors of this chapter from the previous edition.

Key Points Diagnosis and Treatment of Acute Aortic Dissection

1. Aortic dissection is classically associated with sudden chest or back pain, a pulse deficit, and mediastinal widening on chest radiograph.

2. The imaging modality (CT, MRI, or TEE) that is most readily available should be the one selected to confirm the diagnosis of acute aortic dissection.

3. An acute type A aortic dissection is a surgical emergency. Although type B dissections are usually managed medically, one third of these patients eventually require surgery because of worsening of the dissection, rupture, malperfusion, or intractable pain. In either case, prompt surgical consultation is recommended.

4. When managing acute aortic dissection, adequate β-blockade must be established before the initiation of nitroprusside to prevent propagation of the dissection from a reflex increase in cardiac output.

5. The use of endovascular stent-grafts will likely play a large role in the management of dissections.

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5 Estrera A.L., Miller C.C.III. Safi HJ, et al: Outcomes of medical management of acute type B aortic dissection. Circulation. 2006;114(Suppl 1):I384–I389.

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9 Leurs L.J., Bell R., Degrieck Y., et al. Endovascular treatment of thoracic aortic diseases: combined experience from the EUROSTAR and United Kingdom Thoracic Endograft registries. J Vasc Surg. 2004;40:670–680.

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11 Olsson C., Thelin S., Stahle E., et al. Thoracic aortic aneurysm and dissection: increasing prevalence and improved outcomes reported in a nationwide population-based study of more than 14,000 cases from 1987 to 2002. Circulation. 2006;114:2611–2618.

12 Parsa C.J., Schroder J.N., Daneshmand M.A., et al. Midterm results for endovascular repair of complicated acute and chronic type B aortic dissection. Ann Thorac Surg. 2010;89:97–102.

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15 Zoli S., Etz C.D., Roder F., et al. Long-term survival after open repair of chronic distal aortic dissection. Ann Thorac Surg. 2010;89:1458–1466.

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