Aortic dissection is the most common catastrophe of the aorta, 2-3 times more common than rupture of the abdominal aorta. When left untreated, about 33% of patients die within the first 24 hours, and 50% die within 48 hours. The 2-week mortality rate approaches 75% in patients with undiagnosed ascending aortic dissection.
The establishment of the International Registry of Acute Aortic Dissection in 1996, which gathers information from 24 centers in 11 countries, has helped in the development of an understanding of the complexity of aortic dissection.
Dissections of the thoracic aorta have been classified anatomically by 2 different methods. The more commonly used system is the Stanford classification, which is based on involvement of the ascending aorta and simplifies the DeBakey classification.
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The Stanford classification divides dissections into 2 types, type A and type B. Type A involves the ascending aorta (DeBakey types I and II); type B does not (DeBakey type III).
This system helps to delineate treatment. Usually, type A dissections require surgery, while type B dissections may be managed medically under most conditions.
The DeBakey classification divides dissections into 3 types, as follows:
· Type I involves the ascending aorta, aortic arch, and descending aorta
· Type II is confined to the ascending aorta
· Type III is confined to the descending aorta distal to the left subclavian artery
Type III dissections are further divided into IIIa and IIIb. Type IIIa refers to dissections that originate distal to the left subclavian artery but extend proximally and distally, mostly above the diaphragm.
Type IIIb refers to dissections that originate distal to the left subclavian artery, extend only distally, and may extend below the diaphragm.
Thoracic aortic dissections should be distinguished from aneurysms (ie, localized abnormal dilation of the aorta) and transections, which are caused most commonly by high-energy trauma.
Establishing the diagnosis in the field is usually difficult or impossible, but certain salient features of aortic dissection may be observed. It is life threatening if not quickly recognized and treated.
Radio communication with the receiving hospital permits the medical control physician to direct care and select a capable destination hospital, while permitting the emergency department (ED) to mobilize appropriate resources.
In the rare event that the diagnosis can be made based on prehospital information, the physician directing prehospital care should request transport to a facility capable of operative treatment of an aortic dissection.
Emergency Department Care
The mortality rate of patients with aortic dissection is 1-2% per hour for the first 24-48 hours. Initial therapy should begin when the diagnosis is suspected. This includes 2 large-bore intravenous lines (IVs), oxygen, respiratory monitoring, and monitoring of cardiac rhythm, blood pressure, and urine output.
Clinically, the patient must be assessed frequently for hemodynamic compromise, mental status changes, neurologic or peripheral vascular changes, and development or progression of carotid, brachial, and femoral bruits.
Aggressive management of heart rate and blood pressure should be initiated.
Beta blockers should be given initially to reduce the rate of change of blood pressure (dP/dt) and the shear forces on the aortic wall.
The target heart rate should be 60-80 beats per minute.
The target systolic blood pressure should be 100-120 mm Hg.
End organ perfusion should be evaluated. Balancing the risks of dP/dt on the aortic wall versus the benefits of acceptable end organ perfusion may be a difficult clinical decision.
Retrograde cerebral perfusion may increase the protection of the central nervous system during the arrest period.
The mortality rate from aortic arch dissections is about 10-15%, with significant neurologic complications occurring in another 10% of patients. The mortality rate is influenced by the patient's clinical condition.
The American College of Radiology has established ACR Appropriateness Criteria for the diagnosis and treatment of suspected aortic dissection.
Type A dissections
Urgent surgical intervention is required in type A dissections.
The area of the aorta with the intimal tear usually is resected and replaced with a Dacron graft.
The operative mortality rate is usually less than 10%, and serious complications are rare with ascending aortic dissections.
The development of more impermeable grafts, such as woven Dacron, collagen-impregnated Hemashield (Meadox Medicals, Oakland, NJ), aortic grafts, and gel-coated Carbo-Seal Ascending Aortic Prothesis (Sulzer CarboMedics, Austin, Tex), has greatly enhanced the surgical repair of thoracic aortic dissections.
With the introduction of profound hypothermic circulatory arrest and retrograde cerebral perfusion, the morbidity and mortality rates associated with this highly invasive surgery have decreased.
Dissections involving the arch are more complicated that those involving only the ascending aorta, because the innominate, carotid, and subclavian vessels branch from the arch. Deep hypothermic arrest usually is required. If the arrest time is less than 45 minutes, the incidence of central nervous system complications is less than 10%.
Aortic stent grafting is a challenging technique. It may prove feasible and has offered good results in a small series of patients. It may be a reasonable alternative in high-risk patients in the near future.
Type B dissections
The definitive treatment for type B dissections is less clear.
Uncomplicated distal dissections may be treated medically to control blood pressure. Distal dissections treated medically have a mortality rate that is the same as or lower than the mortality rate in patients who are treated surgically.
Surgery is reserved for distal dissections that are leaking, ruptured, or compromising blood flow to a vital organ.
Acute distal dissections in patients with Marfan syndrome usually are treated surgically.
Inability to control hypertension with medication is also an indication for surgery in patients with a distal thoracic aortic dissection.
Patients with a distal dissection are usually hypertensive, emphysematous, or older.
Long-term medical therapy involves a beta-adrenergic blocker combined with other antihypertensive medications. Avoid antihypertensives (eg, hydralazine, minoxidil) that produce a hyperdynamic response that would increase dP/dt (ie, alter the duration of P or T waves).
Survivors of surgical therapy also should receive beta-adrenergic blockers.
A series of patients with type B dissections demonstrated that aggressive use of distal perfusion, CSF drainage, and hypothermia with circulatory arrest improves early mortality and long-term survival rates.
Endovascular stenting remains an option for treatment of some type B dissections. Some studies recommend that patients with complicated acute type B dissections undergo endovascular stenting with the goal of covering the primary intimal tear.