Epidemiology

Aortic dissection is longitudinal cleavage of the aortic wall by blood, which creates a false lumen that may propagate. The disease was first described in detail by Morgagni in 1761. In 1955, with the advent of cardiopulmonary bypass, DeBakey successfully repaired a dissected descending aorta, thereby providing the first cure. The proximal aorta was first repaired and medical management was established in the 1960s, thus setting the stage for the modern approach to this disease.

A patient in the emergency department (ED) whose proximal aorta has dissected has about a 2% chance of dying every hour during the first 12 hours and almost a 50% chance of dying within 48 hours without surgical treatment.^1,2 Unfortunately, this disease is often difficult to diagnose. A study involving three academic EDs found that the diagnosis was suspected during the initial encounter in only 43% of cases.³

The incidence is 3 per 100,000 people per year.^4,5 A typical large urban ED sees several cases per year.³ About 1 in 350 patients evaluated in an ED for chest pain has aortic dissection.^4–6

Sixty percent of aortic dissections involve the ascending aorta, either alone or with other parts of the aorta; they are referred to as type A in the now standard Stanford classification. The 40% that do not involve the ascending aorta are Stanford type B (Fig. 65.1). This distinction has important prognostic and therapeutic implications.

Fig. 65.1 Classification of aortic dissection.

Types A and type B represent the two subtypes of dissection under the Stanford classification system. Subtypes of the older DeBakey classification system are types I, II, and III.

(From Zipes DP, Libby P, Bonow RO, et al, editors. Braunwald’s heart disease: a textbook of cardiovascular medicine. 7th ed. Philadelphia: Saunders; 2005.)

The Young Patient

About 7% of patients with aortic dissection are young (<40 years) and usually have no history of hypertension or other known medical problems.⁷ These patients nearly always have occult structural cardiovascular disease.

The most important structural vascular disease is Marfan syndrome, which has a prevalence of 1 in 10,000 and occurs in all races. Marfan syndrome accounts for half of aortic dissections in patients younger than 40 years.⁷ It is caused by an autosomal dominant mutation in the gene for a type of fibrillin, a protein that makes up part of the elastic fibers in connective tissue of the aorta, lens, and periosteum. Patients are typically tall with long digits, scoliosis, pectus excavatum, and visual problems because of lens dislocation (Fig. 65.2, A to C). Most important, however, thoracic aneurysms invariably develop early in life, usually in the ascending aorta. If their aneurysms are not repaired, most patients will die of aortic dissection or rupture.

Fig. 65.2 These patients with Marfan syndrome have long extremities and digits, tall stature, and pectus carinatum.

(Photographs courtesy National Marfan Foundation.)

Other young patients without a history of severe hypertension who are nevertheless at risk for dissection are those with bicuspid aortic valves and those experiencing acute insults such as cocaine use and trauma. A family history of aortic aneurysms and dissection, beyond syndromes usually associated with aortic pathology, is a newly recognized risk factor seen in 10% to 20% of patients with dissection.⁸

Pathophysiology

The aorta dissects by two possible mechanisms (Fig. 65.3). The classic mechanism is an intimal tear, which is generally transverse and extends through the very thin tunica intima into the tunica media. Under pulsatile force, blood enters a layer of the media and dissects longitudinally and usually in a distal direction. The other major mechanism is rupture of the vasa vasorum, usually of a penetrating branch within the tunica media, with consequent bleeding into the media. Progression then occurs in the same fashion, either with or without secondary tearing through the intima into the aortic lumen. The aorta generally tears near tethering points, where the vessel undergoes the greatest flexion stress during cardiac contractions. Thus the most common location for initiation of dissection is the first few centimeters of the ascending aorta, the next most common being the origin of the descending aorta just distal to the left subclavian artery.

Fig. 65.3 Two mechanisms of aortic dissection.

A, Intimal tear. B, Rupture of the vasa vasorum. A, Adventitia; I, intima; M, media.

(From Zipes DP, Libby P, Bonow RO, et al, editors. Braunwald’s heart disease: a textbook of cardiovascular medicine. 7th ed. Philadelphia: Saunders; 2005.)

Major complications of aortic dissection, in order of risk for mortality, are (1) rupture through the thin remaining outer wall; (2) proximal propagation, which can cause coronary occlusion, acute aortic regurgitation, or cardiac tamponade (or any combination of such complications); and (3) occlusion or dissection of branch arteries.

Those who suffer dissection are predisposed to it by a weakened aorta, specifically, degeneration of the tunica media, or suffer a hemodynamic or traumatic insult, or both (Table 65.1).^7–9 Medial degeneration can be secondary to chronic hypertension, hereditary diseases of elastin (Marfan syndrome) or collagen (Ehlers-Danlos syndrome), hereditary structural abnormalities (bicuspid aortic valve and aortic coarctation), chronic inflammation, and aneurysm of any cause, which increases wall tension according to Laplace’s law.

Table 65.1 Risk Factors for Aortic Dissection

Data from references 7–9.

Hemodynamic stress on the aorta is produced by hypertension and shear force (dP/dt)*, or the force of blood ejected from the left ventricle. A hypercontractile, tachycardic heart produces the greatest shear force. These factors may contribute to the initiation of dissection and strongly determine whether a dissection propagates. Exacerbations of hypertension, cocaine intoxication, and third-trimester pregnancy are examples of states that can cause hemodynamic stress on the aorta and result in dissection.

*

Traumatic causes include medical procedures, such as aortic catheterization and cardiac surgery, and deceleration events (e.g., falls, motor vehicle collisions), although in blunt trauma, aortic rupture is far more common than dissection.

Presenting Signs and Symptoms

Dissection of the aorta is usually extremely painful, so acute pain is a chief complaint in nearly 95% of cases (Table 65.2).^7,10–14 Aortic pain is sudden and maximal at onset, with its intensity often proportional to the length of dissection. The location of pain is midline and, classically, correlates with the location of dissection: dissection of the ascending aorta results in chest pain, dissection of the arch results in neck or jaw pain, and dissection of the descending aorta results in back and sometimes abdominal pain. Thus the pain may migrate as the dissection propagates. However, there is considerable variability in symptoms and major overlapping of symptoms in type A and type B dissections.^3,9

Table 65.2 Symptoms, Signs, and Findings in Patients with Aortic Dissection

Data from references 7, 10–14.

The typical patient is 50 to 70 years old with a history of poorly controlled hypertension. He (2 : 1 male-to-female ratio) has intense chest or back pain (or both) and is likely to describe it as sharp rather than tearing. The pain does not respond to nitroglycerin. The patient is not dyspneic unless suffering from one of the cardiac complications of dissection.

The patient appears to be in obvious pain and often has elevated blood pressure (see Table 65.2). On cardiac examination the emergency physician (EP) may hear a fourth heart sound (S₄) because of left ventricular hypertrophy. The patient must be examined for two life-threatening complications of type A dissection with retrograde propagation: aortic regurgitation and cardiac tamponade. Aortic regurgitation occurs audibly in a third of cases and is manifested as a decrescendo early diastolic murmur, best heard at the left lower sternal border with the patient sitting up, leaning forward, and holding breath at end-expiration; peripheral pulses can be bounding if the pulse pressure is wide. Cardiac tamponade may be manifested as distant heart sounds, but more often tachycardia and jugular venous distention occur and are followed by hypotension; the pulsus paradoxus should be 10 mm Hg higher, and bedside ultrasound will reveal a pericardial effusion.

Peripheral pulses should also be examined. If the false lumen is occluding one of the subclavian arteries, a focal, palpably weak pulse is usually present, a finding specific for aortic dissection.¹¹ Blood pressure should be measured in both arms—a difference of 20 mm Hg or greater between extremities should alert the EP to the possibility of subclavian occlusion. It must be remembered, though, that nearly 20% of hypertensive patients have a blood pressure differential of at least 10 mm Hg between extremities.¹⁵ Lower extremity pulses should also be evaluated because frank ischemia of the lower extremities occurs in nearly 10% of cases of aortic dissection.¹³

A thorough neurologic examination should be performed. Fifteen percent of patients with aortic dissection have a focal neurologic deficit, and in the setting of severe acute chest or abdominal pain, this finding is also highly specific for the disease.¹¹ Potential neurologic deficits in patients with aortic dissection include stroke secondary to carotid occlusion, spinal cord syndromes as a result of spinal or intercostal arterial occlusion, and peripheral neuropathy caused by neuronal ischemia or compression of a peripheral nerve by the false lumen.

Differential Diagnosis and Medical Decision Making

Aortic dissection is often manifested as chest pain (in up to 75% of cases in recent large studies). In any patient with chest pain, the EP must think of three difficult diagnoses that are life-threatening: acute coronary syndrome, pulmonary embolism, and aortic dissection. Acute coronary syndrome often causes a pressurelike pain that is more crescendo than sudden; electrocardiography (ECG) frequently shows ischemic changes, and in the absence of complete coronary occlusion, the pain is usually responsive to nitroglycerin. In fact, one distinguishing feature of both pulmonary embolism and aortic dissection is that in the first several hours of either, the pain typically does not resolve. With pulmonary embolism, the pain is generally pleuritic and associated with significant pulmonary symptoms or signs. Other diagnoses to consider are pericarditis or cardiac tamponade, spontaneous pneumothorax, and esophageal rupture.

In one prospective study, the absence of aortic pain (sudden and severe), a pulse or blood pressure differential, and mediastinal or aortic widening on chest radiography had a sensitivity of 96% for ruling out the diagnosis of aortic dissection. Conversely, the presence of a pulse deficit or a focal neurologic deficit, with positive likelihood ratios of 47 and 33, respectively, markedly raised the probability of aortic dissection.¹¹

Chest Radiography

The chest radiograph classically shows a wide mediastinum, predominantly on the right in patients with dissection of the ascending aorta and on the left with dissection of the descending aorta (Fig. 65.4). The physician’s subjective impression, rather than formal measurements, is the best indicator of whether the mediastinum is wide. A widened mediastinum is not a sensitive indicator of aortic dissection. It probably occurs in about 60% of cases, but some reports note a widened mediastinum in less than 20%.^3,10 Other abnormalities of the aorta, such as inward displacement of intimal calcification and a double aortic shadow, may be observed. Pleural effusion may also occur, usually on the left.

Fig. 65.4 Chest radiographs of aortic dissection.

A, Chest radiograph from 3 years previously showing that the patient’s cardiac structures are normal. B, Current chest radiograph of the same patient showing interval enlargement of the aortic knob (arrow).

(From Zipes DP, Libby P, Bonow RO, et al, editors. Braunwald’s heart disease: a textbook of cardiovascular medicine. 7th ed. Philadelphia: Saunders; 2005.)

D-Dimer

After the history, physical examination, ECG, and chest radiography, the clinician must decide whether aortic dissection is a reasonable possibility (Fig. 65.5). If so, additional testing is required.

Fig. 65.5 Algorithm for the diagnosis and treatment of aortic dissection.

BP, Blood pressure; CT, computed tomography; CXR, chest radiograph; ECG, electrocardiography; ED, emergency department; HTN, hypertension; labs, laboratory tests; TEE, transesophageal echocardiography.

Numerous small studies have demonstrated that the serum biomarker D-dimer is useful for excluding the diagnosis. A meta-analysis that included seven studies with a total of 300 patients with dissection found that D-dimer—at the 500-ng/mL cutoff value commonly used for excluding pulmonary embolism—is 97% sensitive for acute aortic dissection.¹⁶ D-dimer is probably inadequately sensitive in patients with subacute manifestations and those with pure intramural hematoma. In acute manifestations in which the pretest probability of dissection is low, it is reasonable to use D-dimer to exclude the diagnosis and forego further testing.

If the pretest probability is not low, the manifestation is subacute, or D-dimer levels are elevated, an advanced imaging study is required (Table 65.3). All of the modalities described are highly sensitive (98% to 100%) and specific (95% to 98%) and thus in most cases will definitively rule in or rule out dissection.¹⁷

Table 65.3 Advanced Imaging Studies for Aortic Dissection

Computed Tomography

The diagnostic study of choice is computed tomographic (CT) angiography of the aorta (Fig. 65.6). It is immediately available in most EDs and can be interpreted quickly by a radiologist. It is highly sensitive for dissection, especially as CT technology advances—in fact, a 2005 study reported that multidetector CT scanning was 100% accurate in patients with suspected acute aortic disease.¹⁸ Moreover, when its results are negative for dissection, CT scanning frequently shows alternative serious disease to explain the patient’s symptoms. In the ED, CT scanning is more readily available than transesophageal echocardiography (TEE) or magnetic resonance imaging (MRI), and most patients (even if somewhat clinically unstable) are able to undergo CT scanning to enable the diagnosis to be made.

Fig. 65.6 Computed tomography scans of aortic dissection.

The first axial scan (A) shows aortic dissection, demonstrated as serpiginous radiolucencies in the ascending and descending aorta. The second scan (B) shows dissection of the aortic arch.

(Courtesy Leslie Quint, University of Michigan.)

Transesophageal Echocardiography

Generally performed by a cardiologist, TEE is an alternative technique in patients with renal failure, which is a relative contraindication to CT scanning (Fig. 65.7). TEE is as sensitive as CT scanning for type A aortic dissection but may be slightly less sensitive for type B lesions¹⁹; TEE has the advantage of assessing the aortic valve and pericardial space for complications of type A dissection. It may also detect focal ventricular wall motion abnormalities suggesting myocardial ischemia as the cause of the patient’s symptoms. Finally, unlike CT scanning, TEE usually identifies the exact location of the intimal tear and assesses overall cardiac function, which aids in operative planning and risk assessment. The downside of TEE, especially during off hours, is the difficulty performing TEE on an emergency basis. Nonetheless, cardiothoracic surgeons may request the study even when dissection has already been diagnosed.

Fig. 65.7 Transesophageal echocardiograms of the proximal ascending aorta in long-axis view in a patient with proximal aortic dissection.

A, The left atrium (LA) is closest to the transducer. The aortic valve (AV) is seen on the left in this view, with the ascending aorta extending to the right. Within the proximal aorta is an intimal flap (I) that originates just at the level of the sinotubular junction above the right sinus of Valsalva. The true lumen (T) and the false lumen (F) are separated by the intimal flap. B, The addition of color flow Doppler imaging in the same view confirms the presence of two distinct lumens. The true lumen (T) fills completely with brisk blood flow (bright blue), whereas minimal retrograde flow (dark orange) is seen in the false lumen (F).

(From Zipes DP, Libby P, Bonow RO, et al, editors. Braunwald’s heart disease: a textbook of cardiovascular medicine. 7th ed. Philadelphia: Saunders; 2005.)

Treatment

The main goal of ED care is to identify the disease and then manage the patient’s blood pressure and heart rate to minimize aortic shear force. The goal for systolic blood pressure is 100 to 120 mm Hg,²⁰ and the goal for the heart rate is a rate lower than 60 beats/min.²¹ Note that the blood pressure goal is independent of the patient’s baseline blood pressure, unlike the approach to most hypertensive emergencies. Therefore, the clinician must watch for signs of iatrogenic end-organ malperfusion. Very short-acting antihypertensives should be used and administered as drips to precisely titrate the dose to effect.

Table 65.4 summarizes the intravenous hypertensive agents used for treatment of aortic dissection.^20,22,23 Beta-blockers are the first-line antihypertensive agents in patients with aortic dissection because they most effectively reduce both blood pressure and shear force (in patients with significant asthma, diltiazem can be used instead). The ideal beta-blocker is esmolol because of its short half-life and thus its titratability. If blood pressure control with beta-blockers is limited by bradycardia or a plateau effect, a second antihypertensive agent may be needed, invariably a vasodilator.

The standard second-line agent is nitroprusside, which is extremely short-acting and universally effective. Fenoldopam, a highly selective dopamine agonist, is another option. To avoid reflex tachycardia (which increases shear force), vasodilators should be started only after the heart rate has been controlled pharmacologically.

A minimum of two peripheral intravenous lines will be needed, along with an arterial line for continuous blood pressure monitoring. If possible, the arterial line should be placed in the right radial artery so that it is less likely to be affected by the dissection occluding the upstream artery.

The EP must remember that because aortic pain is very severe, adequate analgesia with intravenous opioids should be provided. Opioids help reduce blood pressure.

For any thoracic aortic dissection the clinician should consult a cardiothoracic surgeon immediately. Cardiology consultation may be needed as well, either for admission of a patient with type B dissection or for preoperative TEE, if needed, in a patient with type A dissection.

Type A aortic dissection generally requires emergency thoracic surgery because the mortality rate in patients who do not undergo surgery increases hourly and is 50% to 80% at 1 month. Typically, the surgeon excises the segment of aorta containing the intimal tears and replaces it with a graft, often with some dissected aorta left in place. The aortic valve frequently needs to be replaced as well. With surgery, survival of patients with type A dissection is improved remarkably, to greater than 95% at 1 year.²⁴ Recently, some experts have advocated nonsurgical management for patients with subacute manifestations or a history of aortic valve replacement (both confer a better prognosis) and for patients with acute stroke or severe comorbid conditions (both confer a worse surgical prognosis).²⁰

Type B dissection is usually treated medically, with a 90% survival rate at 1 month. Type B dissection that is complicated by persistent pain, uncontrolled hypertension, branch artery obstruction, or aneurysm has traditionally required surgery. However, endovascular stent placement and balloon fenestration have become therapeutic options; malperfusion syndromes can be reversed in the vast majority of cases, and the false lumen can often be obliterated, thus reducing the chance of aneurysm development and other future complications.²⁵ Mortality appears to be substantially lower (10% to 15%) with stenting than with surgery (25% to 35%) for complicated type B dissection.²⁶

Follow-Up, Next Steps in Care, and Patient Education

All patients with newly diagnosed aortic dissection should be admitted to the intensive care unit. Patients with type A dissection may go to the operating room first. In the ED, these patients need intensive care unit–level management and should be in a resuscitation bay with a 1 : 1 nurse-to-patient ratio to ensure adequate monitoring of blood pressure and response to therapy (see the Red Flags box).