Rapid Assessment and Treatment of Aortic Dissection

Emerg Med 36(10):18-43, 2004

The authors cover topics essential to swift and sure decision-making about patients with aortic dissection, including descriptive classification systems, patient characteristics and risk factors, signs and symptoms, and findings in transesophageal echocardiography and other imaging studies. Current options in medical and surgical management and considerations pertaining to patient follow-up are also discussed.

By David B. Stultz, MD, and Satyendra C. Gupta, MD

Aortic dissection occurs in approximately 1 in 2000 patients. It commonly presents as acute onset of chest or back pain in the emergency department or acute care setting. Clinical acumen and an efficient workup can lead to an early diagnosis and prompt intervention, which are critical to reducing the early mortality associated with this disease. In this article, we will review the etiologies of aortic dissection, classification systems, and patient presentation. We will also discuss management strategies, medication administration, and indications for surgery.
 

TWO THEORIES ON ETIOLOGY

There are two major theories on the etiology of an aortic dissection. The first postulates that the intimal layer of the aorta has been damaged, exposing the medial layer to high blood pressure forces. A jet of blood dissects this layer, causing the medial and intimal layers to separate. A dissection flap forms at the site, creating a false lumen channel.

This process presumes a medial layer that is weakened to begin with. Marfan syndrome is an autosomal dominant genetic disorder that weakens connective tissue by altering elastin and fibrillin synthesis. Patients with Marfan syndrome account for 5% to 9% of all aortic dissections, usually presenting at an early age and with proximal involvement. Ehlers-Danlos syndrome is another inherited connective tissue disorder that has been associated with aortic dissection. There is also an increased incidence of aortic dissection in patients with Noonan's and Turner's syndromes.

There are also less well-defined familial syndromes involving fibrillin-1 gene mutations. Recent gene mapping techniques have led to the identification of certain alleles that may allow for screening in asymptomatic individuals at risk for aortic dissection. Additionally, nonclassic medial degeneration occurs with longstanding hypertension, which is seen in the majority of patients with aortic dissection.

The second etiology of aortic dissection stems from a spontaneous rupture of the vasa vasorum in the medial layer of the aorta. This creates a localized hematoma that is contained within the medial layer. No dissection flap forms, and the false lumen does not communicate with the true lumen.

Trauma is an uncommon cause of aortic dissection; blunt forces typically result in a hematoma or transsection rather than dissection. Iatrogenic causes are infrequent but may result from endovascular procedures such as cardiac catheterization.
 

PATIENT CHARACTERISTICS

The peak incidence of aortic dissection occurs in the sixth and seventh decades of life. It is more common in males by a 2:1 ratio. The box lists common risk factors and predisposing traits for aortic dissection. About 80% of patients have pre-existing hypertension. An association with a bicuspid aortic valve has been noted; about 7% to 14% of patients with aortic dissection have this abnormality. Vasculitis, especially of the giant cell type, has also been implicated in aortic dissection. Cocaine has been cited as a cause of dissection, especially in young men without other risk factors. Patients who have undergone cardiac surgery, especially aortic valve replacement, have an increased risk of dissection.

Pregnancy has been implicated as a risk factor for aortic dissection. In women who are younger than 40 years of age, about 50% of cases of aortic dissection will occur during pregnancy or the postpartum period. The highest incidence is in the third trimester. Women with Marfan syndrome carry the highest risk in this population. Although the association with pregnancy has been noted, the cause is not well defined. The normal increases in blood pressure and cardiac output during pregnancy are not dramatic enough to explain the increased risk of dissection. It has been postulated that the association may simply represent reporting bias.
 

CLASSIFICATION OF AORTIC DISSECTION

Several classification systems are used in describing aortic dissections. The simplest scheme differentiates only proximal from distal anatomic involvement. Proximal dissections involve any portion of the aorta proximal to or involving the origin of the left subclavian artery. Distal dissections involve only that portion distal to the left subclavian artery. This so-called descriptive classification system is easy to use and helps guide treatment.

The Stanford classification is similar to the descriptive system. It divides aortic dissections into types A and B. Type A dissections involve the ascending aorta; type B dissections involve the aorta distal to the left subclavian artery.

The DeBakey system describes three types of dissections. Type I dissections involve both the ascending and descending aorta. Type II involves only the ascending aorta. Type III involves only the descending aorta and is the most common type of aortic dissection.

The figure below illustrates the three classification systems in relation to involvement of the left subclavian artery.

Classification schemes. Several systems of classification have been devised for aortic dissection. The DeBakey system (A) is based on whether the dissection involves the ascending aorta, the descending aorta, or both. The Stanford and descriptive systems (B) are based on the location of the dissection relative to the left subclavian artery.

There is also a temporal component to aortic dissections. Acute dissections are less than two weeks old; chronic dissections are older than that.

back to top

SIGNS AND SYMPTOMS

The patient with aortic dissection typically presents with pain, most often described as "tearing" in quality. It is usually of sudden onset and cannot be relieved with a change in position. Radiation or migration of the pain may occur in up to 20% of patients. Dissections involving the ascending aorta may produce anterior chest pain with radiation to the neck, throat, or jaw. Descending aortic dissection may cause interscapular back pain with radiation to the lower back or abdomen. Nausea, vomiting, and diaphoresis frequently accompany the pain.

Uncommon presentations can include syncope, stroke, or other neurologic complaint, as well as new-onset aortic regurgitation, congestive heart failure, and cardiac tamponade. Pleural effusions may be present.

The table below summarizes common presentations and management of aortic dissection.

BLOOD PRESSURE VARIATIONS

Although longstanding hypertension has been implicated as a cause of dissection, blood pressure on acute presentation may vary. About 70% of patients with distal aortic dissection will have elevated blood pressure; hypotension occurs in less than 5% of this population. With proximal dissections, about one-third of patients will have acute hypertension, while one-quarter will present with hypotension, which is usually a result of acute aortic regurgitation or pericardial tamponade.

Pseudohypotension, a falsely decreased blood pressure, may occur when a proximal dissection involves the brachiocephalic artery. Evaluation of pressures in both arms may diagnose this complication by revealing a significant systolic blood pressure discrepancy. Regardless of the patient's blood pressure, tachycardia is frequently present.

A murmur of aortic regurgitation may be detected in about one-third of proximal dissections, although echocardiographic evidence for aortic regurgitation may be found in up to two-thirds. Neurologic manifestations, such as paresthesias, weakness, or even stroke, can occur in up to one-fifth of patients with any type of dissection.

Dissections may extend from the initial site of compromise to other vascular structures. Proximal dissections can extend into the coronary arteries, typically the right coronary ostium, in about 1% to 2% of cases. This can result in inferior wall myocardial infarction. Distal dissections can extend to involve the renal arteries (5% of cases) or cause mesenteric ischemia or infarction (3% to 5%).
 

INITIAL WORKUP

In addition to a complete history and physical examination, laboratory evaluation is useful in diagnosing aortic dissection. The table below reviews the most common diagnostic tools. Serum markers have not been helpful in establishing the diagnosis, although a novel assay using monoclonal antibodies to smooth muscle myosin heavy chains has shown promise in diagnosing acute dissections. Unfortunately, it is not widely available. An ECG may be useful in evaluating the patient for complications of dissection, such as myocardial infarction. Although there are no diagnostic features on ECG to suggest dissection, the finding of left ventricular hypertrophy may point toward the underlying predisposition of chronic hypertension.

A chest X-ray is commonly obtained as the initial imaging study. Typical findings include a widening of the mediastinum and aortic silhouette, depression of the left mainstem bronchus, and an abnormal aortic contour. Although uncommon, a calcified aortic intima with a 1.0-cm gap from the soft outer tissue border usually indicates dissection. Pleural effusions, especially left-sided, may be found. However, it should be noted that about 1 in 10 patients with aortic dissection have a normal chest X-ray.

Angiography of the aorta was previously the gold standard in evaluating a dissection. It has the advantage of demonstrating an intimal flap and false lumen. Also, it can assess the length of the dissection, evaluate aortic regurgitation, and detect coronary involvement. It cannot diagnose a contained intramural hematoma. However, it is an invasive procedure involving exposure to intravenous (IV) contrast and has largely been replaced with contrast-enhanced computed tomography (CT), magnetic resonance imaging (MRI), and transesophageal echocardiography.

Contrast-enhanced CT allows for rapid assessment, with about 90% sensitivity and specificity. For dissection to be diagnosed, two lumens must be demonstrated. Although noninvasive, this technique does require contrast administration and only rarely will show the site of the intimal tear. Due to its wide availability and prompt results, contrast-enhanced CT is the test of choice in the emergency department when aortic dissection is suspected in a hemodynamically stable patient. Spiral CT offers the advantage of even faster imaging times and possibly even higher sensitivity.

Magnetic resonance imaging is considered the current gold standard for detecting aortic dissection. Its high resolution images offer a superb diagnostic yield. It has about 98% sensitivity and specificity and can identify the intimal tear in almost 90% of cases. Newer motion-capture techniques allow for detection of aortic regurgitation. Although completely noninvasive, MRI does have several potential disadvantages. Access may be limited and results are not available as quickly as with CT. Also, MRI is contraindicated in certain patients with implanted devices such as pacemakers and patients with claustrophobia. Moreover, patient monitoring during the procedure is suboptimal, making this technique inappropriate for an unstable patient.

Echocardiography can discern true and false lumens, with visualization of the intimal flap. A suspected dissection should be seen in several views, and Doppler ultrasound should reveal differential blood flow in the true and false lumens. Echocardiography has the further advantage of detecting complications of aortic dissection, especially aortic regurgitation.

Due to its low sensitivity, transthoracic echocardiography is not sufficient for the diagnosis of aortic dissection. Transesophageal echocardiography is a relatively noninvasive technique that has excellent sensitivity and specificity (95% to 99%). It can also visualize the ostia of the coronary arteries, allowing evaluation for dissection extension. This procedure can be performed at the bedside of a hemodynamically unstable patient, so that the patient can be continuously monitored. The images show common transesophageal echocardiographic findings in aortic dissection.

Advantageous technique. Transesophageal echocardiography has 95% to 99% sensitivity and specificity for aortic dissection. These views of proximal aortic dissection are: (A) a longitudinal view demonstrating large false lumen, intimal flap, and small true lumen; (B) a short axis view of dissection, and (C) a color Doppler showing vigorous blood flow in the true lumen with little flow in the false lumen.

back to top

ACUTE MANAGEMENT STRATEGIES

In general, acute management of aortic dissection is determined by its anatomic location. Dissection proximal to or involving the left subclavian artery typically mandates surgical correction, whereas distal dissections may be managed medically. In the initial evaluation, hemodynamics should be rapidly assessed, and volume resuscitation with a crystalloid is indicated for hypotension. An arterial line may be needed for close blood pressure monitoring.

If the patient's blood pressure does not adequately respond to fluids, norepinephrine or phenylephrine can be used. Dopamine should only be used in low doses because it increases the shear stress on the vessel wall.

The etiology of hypotension should be explored. As noted earlier, pseudohypotension may be caused by dissection involvement of the brachiocephalic artery. Cardiogenic shock due to acute aortic regurgitation should be considered. Cardiac tamponade and aortic rupture are rare causes of hypotension. Patients with tamponade should be transferred promptly to surgical care. Small series have reported detrimental effects with pericardiocentesis in patients with proximal dissection; this procedure should be reserved for patients who have pulseless electrical activity.
 

MEDICATION ADMINISTRATION

In patients who present with hypertension, medications that quickly lower the dP/dt (change in pressure over time) of the aortic wall should be used. Experimental models have demonstrated that high-pressure pulsatile flow is more injurious than smooth laminar flow. Intravenous administration of beta blockers can reduce wall stress. The effect of therapy is titrated clinically by maintaining a heart rate between 60 and 80 beats per minute.

Commonly used beta blockers include propranolol, esmolol, and labetolol. Propranolol is started at 1 mg IV every 3 to 5 minutes up to a maximum total dose of 10 mg, followed by 2 to 6 mg IV every 4 to 6 hours. Esmolol is started as a 500 mcg/kg IV bolus, followed by 50 mcg/kg/min IV, titrating as needed up to 200 mcg/kg/min. Labetolol is started as a 20-mg IV bolus over two minutes, followed by additional doses of 40 to 80 mg every 10 to 15 minutes as needed, up to a maximum total dose of 300 mg. Labetolol may be continued as an infusion at 2 mg/min IV, titrating up to 10 mg/min as needed.

Sodium nitroprusside is a potent vasodilator, but when used alone it can produce a reflex tachycardia, increasing dP/dt. To avoid this complication, beta blockers should always be started prior to sodium nitroprusside. Sodium nitroprusside is started at 20 mcg/min IV and titrated up to a total maximum dose of 800 mcg/min for adequate blood pressure control. Labetalol offers the advantage of having both alpha and beta antagonist properties, obviating the need for nitroprusside.

If beta blockers are contraindicated, IV calcium channel blockers, such as nifedipine, diltiazem, and verapamil, may be used. In distal dissections involving the renal artery, angiotensin-converting-enzyme inhibitors can provide renal protection. Morphine can be used as an adjunct for pain control.
 

URGENT SURGICAL EVALUATION

With proximal dissection, urgent surgical evaluation is indicated. Mortality in the first 48 hours of an unrepaired acute proximal aortic dissection is about 40%. Studies have demonstrated a 30-day survival rate of up to 42% with medical management alone, whereas surgical management yields an 80% 30-day rate.

Surgical intervention is contraindicated, however, in patients with severe comorbid disease that precludes surgery. Stroke is an example. Certain surgical techniques utilize anticoagulant therapy following reconstruction or repair of the aortic arch. Restoration of cerebral perfusion carries the risk of converting an ischemic stroke into a hemorrhagic one.

Distal dissections are typically managed medically, with 30-day survival rates of up to 90%, compared to 75% with surgical treatment. Common indications for surgical intervention of distal dissections include rapid expansion, saccular aneurysm formation, persistent pain, local hemodynamic compromise, blood leakage, or impending rupture.

More recently, less invasive endovascular approaches have been explored. One of these techniques is balloon fenestration of the intimal flap. In cases where the false lumen pressure increases to the point where it partially or completely obstructs the true lumen, this procedure allows for decompression of the false lumen. Fenestration does carry the risk of embolization in patients with prior intraluminal thrombus formation.

Endovascular stenting involves deployment of a covered or uncovered metal stent in either lumen. So far it has been used to ameliorate conditions of compromised target organ blood flow. The stent can be engaged in either the true or false lumen in such a way that it maintains patency of target organ blood flow. This can be combined with fenestration to effectively decompress the flap and restore local hemodynamics.

Major risks of stent use include distal embolization and obstruction of branch vessels, leading to mesenteric or renal ischemia. Only small series have documented the use of these techniques; typically, they have identified high-risk surgical patients with distal dissections. There is very limited experience with their use in proximal dissections. What scant data exist suggest reasonable outcomes in high-risk patients with complications of distal aortic dissections, but long-term data are not yet generally available. Optimal patient selection for these procedures has not yet been described.
 

LONG-TERM FOLLOW-UP

With all aortic dissections, patients who survive the initial phase and are discharged from the hospital have a five-year survival rate of up to 80% and a ten-year survival rate of about 50%. Chronic management of patients who have had a dissection involves treatment of the underlying etiology, if possible. In many cases, this translates to aggressive blood pressure management to diminish the pressures exerted on the weakened intimal layer in other segments of the aorta.

Patients with known aortic dissection or a history of aortic dissection should maintain a systolic blood pressure below 130 mm Hg. Beta blockers are preferred for long-term treatment. Successful blood pressure control can reduce the risk of dissection recurrence by up to one-third.

Aggressive surveillance can facilitate early diagnosis of complications from dissection. There is up to a 25% chance of a dissection forming at a new site, usually within two years of the initial dissection. There is also an approximately 25% risk of a saccular aneurysm developing at the site of an unrepaired distal aortic dissection. One surveillance strategy includes follow-up with CT, MRI, or transesophageal echocardiography at 3, 6, 12, 18, and 24 months after the initial presentation and then every 6 to 12 months.

back to top

Printable version of this article

E-mail this article