Acute Aortic Dissection

Morphologic Substrates

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In many patients in whom an aortic dissection develops, the aortic wall shows only changes commensurate with patient age. Thus, it appears that once blood enters the aortic media, cleavage of concentric elastic lamellar plates can occur in even an essentially normal aorta; this event permits rapid and extensive dissection. Dissection usually proceeds distally but may also extend proximally.

Medial degeneration (cystic medial necrosis) of the aorta of a greater degree than is normal for age is present in about 20% of patients with acute aortic dissection and may predispose to dissection. Rarely, aortitis is a predisposing factor.

Marfan syndrome is an important morphologic substrate for acute aortic dissection, and acute dissection develops in 20% to 40% of patients with this syndrome. In fact, aortic root dissection and rupture and chronic aortic regurgitation are the primary causes of death in these patients. Many patients with Marfan syndrome and aortic dissection exhibit no medial degeneration; therefore, this syndrome appears to be, per se, a risk factor for acute dissection.

Defective synthesis of fibrillin, a glycoprotein that is an important component of elastic tissue in the medial layer of the aorta, has been demonstrated in patients with Marfan syndrome. The genetic defect of Marfan syndrome and the fibrillin gene have been mapped to the same region in the long arm of chromosome 15. Turner, Noonan, vascular Ehlers-Danlos, and Loeys-Dietz syndromes are other genetic disorders associated with aortic dissection.

A genetic basis of nonsyndromic familial thoracic aortic aneurysm and dissection has recently been defined.

Anuloectasia without Marfan syndrome is present in some patients in whom acute dissection of the ascending aorta develops. It is important to recognize anuloectasia when it coexists, because aortic root replacement rather than cusp resuspension is indicated (see “Indications for Operation, Selection of Technique, and Choice of Device” in Chapter 12 ).

A bicuspid aortic valve is frequently associated with acute aortic dissection. In the study of Larson and Edwards, the process of acute dissection occurred nine times as frequently in patients with bicuspid as in those with tricuspid aortic valves. There may be a higher prevalence of congenital abnormalities of the aortic wall in patients with bicuspid than in those with tricuspid valves.

A dilated ascending aorta (>5.0-5.5 cm) occurring in combination with anuloectasia (with or without Marfan syndrome), with a bicuspid aortic valve, or with previous aortic valve replacement is associated with increased risk of aortic dissection.

The role of arteriosclerosis in development of acute aortic dissection has been debated. It is probably not a predisposing lesion, although occasionally an intimal tear may develop in a penetrating arteriosclerotic ulcer, resulting in dissection of the media.

Aortic coarctation is associated with acute dissection, but this is likely due to systemic arterial hypertension, an important risk factor for developing acute aortic dissection. Bicuspid aortic valve and an abnormal ascending aorta, frequently present in patients with coarctation, may also be contributing factors.

The role of pregnancy in genesis of acute aortic dissection is unresolved. Closed chest trauma may rarely result in true aortic dissection, as may aortic cannulation and aortic clamping during operations employing CPB.

Intramural hematoma may be a precursor of aortic dissection. It likely results from rupture of vasa vasorum and may, in its initial stages, exist in the absence of an intimal tear (see text that follows).

Intimal Tear

Controversy exists as to whether an intimal tear is consistently present in acute aortic dissection and, thus, whether an intimal tear is a requisite for dissection. One point of view is that rupture of aortic vasa vasorum is the inciting event and that it initiates an intramedial hemorrhage and subsequent dissection. This hypothesis is supported by studies of the role of intramural hematoma as a precursor of aortic dissection and by intraoperative and postmortem observations of aortic dissection without the presence of an intimal tear. However, Larson and Edwards found an intimal tear in each of 158 specimens personally examined, supporting the hypothesis of the primacy of an intimal tear, a view held by Murray and Edwards and by Roberts. The intimal tear develops commonly in the ascending aorta but also in the upper descending aorta just beyond the origin of the left subclavian artery. In the latter instance, dissection usually proceeds only distally (antegrade) but may extend proximally (retrograde) as far as the ascending aorta. Proximal dissection occurred in 38% (CL 29%-47%) of autopsied cases studied by Larson and Edwards. The intimal tear originates in the aortic arch in about 10% to 20% of patients and dissection is both retrograde into the ascending aorta and antegrade into the descending thoracic aorta. Rarely, the intimal tear is low in the descending thoracic or abdominal aorta.

Dissection

When medial dissection occurs, the walls of any of the branches of the aorta may be involved with the dissection, may be sheared off from the lumen and occluded by the dissecting media and intima, may stay in communication with the aorta but only by the false lumen, or may be uninvolved. Extension of dissection into the branch wall is more common in large arteries such as the brachiocephalic, carotid, subclavian, and renal than in smaller ones. Dissection more frequently involves the left rather than right iliac artery. Extent and nature of involvement of the branches, including coronary and iliac arteries, is an important determinant of the clinical syndrome with which the patient presents.

False Lumen

The false lumen develops in the outer half of the aortic media; as a consequence, its external wall is thinner than the internal wall (dissecting membrane). The false lumen usually involves half to two thirds of the circumference of the aorta and rarely the entire circumference. Although the false lumen may be contained initially by the thin outer layer of media and adventitia, it often ruptures into the pericardium, the pleural space (usually the left), or less commonly, the abdomen. Even when initial rupture does not occur, blood from the false lumen may extravasate through weak areas of media and adventitia to form a mediastinal or pericardial hematoma.

Usually, the false lumen gradually enlarges as time passes, producing a marked increase in wall thickness and size of involved portions of the aorta. In many instances, aortic enlargement in the acute stage is diffuse and does not reach aneurysmal proportions. In the ensuing years, the thin outer wall of the false aneurysm tends to weaken, the lumen tends to become aneurysmal, and eventually rupture may occur. The false lumen may become partially or totally thrombosed.

Types of Aortic Dissection

Two classifications of aortic dissection are widely used ( Fig. 25-1 ): DeBakey and Stanford ( Box 25-1 ). In DeBakey type I or Stanford type A dissection, the intimal tear is usually located in the anterior wall of the proximal portion of the ascending aorta. Occasionally it is in the aortic arch and less commonly in the descending aorta distal to the left subclavian artery. Aortic valve regurgitation and myocardial infarction (MI) may result from extension of the dissection into the most proximal portion of the ascending aorta and the aortic root. In DeBakey type II dissection, only the ascending aorta is involved, and dissection terminates proximal to the brachiocephalic artery. This type may be found incidentally during operations for ascending aortic aneurysms. In DeBakey type III or Stanford type B dissection, the dissecting hematoma may involve only the descending thoracic aorta (DeBakey type IIIa), but most commonly extends into the abdominal aorta and occasionally into the iliac arteries (DeBakey type IIIb). It may also extend proximally into the aortic arch and the ascending aorta. In these types, the intimal tear is usually located just distal to the left subclavian artery. The proportion of patients with the various types of dissection depends on the nature of the series reported. In the large surgical series of DeBakey and colleagues, which contained both acute and chronic dissections, type I and type II dissections comprised about 35% of cases. In other clinical and autopsy series, acute dissections involved the ascending aorta in 62% to 85% of cases.

Hemodynamic State

Sudden death may be the presenting feature, occurring shortly after onset of dissection, with free rupture of the false lumen through the thin outer wall into the pericardial, pleural, or peritoneal space. Sudden death can also follow shearing off of the coronary arteries from their aortic origins.

Alternatively, the presentation may be hypovolemic shock of varying degree. It may be the immediate result of acute dissection with loss of a considerable amount of blood from the false lumen into periaortic tissues and spaces. It may also result from development of acute aortic regurgitation when the dissection shears off the aortic attachment of valve commissures, which occurs in 35% to 60% of patients with acute dissection involving the ascending aorta. It may also be the result of acute cardiac tamponade after the site of rupture into the pericardium has temporarily sealed off. As time passes after the acute event, often with some improvement in arterial hypotension, further extravasation of blood can occur from the false lumen into periaortic tissues or spaces, leading to further hypovolemia and a worsening hemodynamic state.

In some patients, the hemodynamic state is good after the immediate event, and acute extravasation from the false lumen or rupture of the lumen may not occur for hours, days, or years, if at all. In a few patients, acute dissection results in no symptoms and passes unnoticed.

Demographics and Syndromes

Most patients (80%-90%) presenting with acute aortic dissection are age 60 or older and have a history of arterial hypertension. Severe acute arterial hypertension, such as occurs in weight lifters, appears to predispose to acute dissection, particularly of the ascending aorta. Use of cocaine, which induces hypertension and vasoconstriction, has also been associated with acute aortic dissection. Patients with acute dissection involving the ascending aorta tend to be younger than those with more distal dissections; thus, the mean age of patients with ascending aortic dissections in the experience of Fann and colleagues was 56 ± 14 years (range 15-85 years), whereas that of patients with involvement of only the descending aorta and beyond was 64 ± 13 years (range 32-86 years). Some patients have evidence of Marfan syndrome; in others, dissection develops during pregnancy. A few have a history of coarctation or its repair or previous aortic valve surgery (see Morphology earlier in this chapter). Occasionally, acute aortic dissection occurs in patients with Turner, Noonan, vascular Ehlers-Danlos, and Loeys-Dietz syndromes. In patients with these syndromes, dissection develops at a younger age, generally during the third or fourth decade of life. In them, and in younger persons in general, there is likely to be no history of hypertension, and dissection usually originates in the ascending aorta. Patients with nonsyndromic familial thoracic aortic aneurysm and dissection present at a younger age than patients with sporadic (non-genetically mediated) disease, but are older than patients with Marfan or Loeys-Dietz syndrome.

Symptoms and Signs

Although dissection may be painless and at times unknown to the patient, most patients experience sudden severe pain at the moment of dissection and a feeling of impending death. The pain is often interscapular, but it may be precordial and radiate into the neck or arms. It is at times difficult to distinguish from angina pectoris. Once acute dissection occurs, symptoms and signs can be produced by occlusion of a major vessel. Arch vessel occlusion causes stroke in 5% to 10% of patients with type I dissection. One leg (more commonly the left than right) may suddenly become numb, pale, and pulseless as dissection occludes the iliac artery or aortic bifurcation. Occasionally, the same process causes pulses to diminish or disappear in an upper extremity. Uncommonly (2%-5% of patients ), paraplegia suddenly develops as intercostal arteries are separated from the aortic lumen by dissection. Oliguria or anuria may appear with occlusion of the aortic origin of the renal arteries.

Chest Radiography

The chest radiograph frequently exhibits widening of the mediastinal shadow, particularly in its upper part and toward the left in DeBakey types I and III dissections. There may be cardiomegaly secondary to pericardial effusion or signs of pleural effusion, particularly in the left hemithorax. The aortic shadow is frequently prominent. However, as a diagnostic test, chest radiography is inadequately sensitive to definitively exclude the presence of aortic dissection in all but the lowest-risk patients. Sensitivity is lower for pathology confined to the ascending aorta than for disease involving distal aortic segments.

Imaging Studies

Echocardiography

Transesophageal echocardiography (TEE) with Doppler color flow imaging is emerging as the most useful and accurate diagnostic technique. It is distinctly superior to transthoracic echocardiography (TTE) and can be performed relatively rapidly with minimal morbidity. When TEE was compared with aortography or with findings at operation or autopsy, its sensitivity and specificity for type A dissection ranged from 88% to 100% and from 86% to 100%, respectively, and for type B dissection from 98% to 100% and from 96% to 100%, respectively. In addition to identifying the dissecting membrane ( Fig. 25-2, A ), TEE can identify pericardial fluid, evidence for pericardial tamponade, aortic regurgitation, involvement of proximal coronary arteries in the dissection process ( Fig. 25-2, B ), and wall motion abnormalities of right and left ventricles.

TTE may be useful in critically ill patients to establish a diagnosis rapidly. However, absence of positive findings does not preclude the presence of an acute dissection, and other diagnostic studies become necessary.

Computed Tomography

Computed tomography (CT) with use of contrast material is useful for diagnosing and delineating acute aortic dissection. It produces excellent images with relatively short scanning times ( Fig. 25-3 ). Because CT scanning equipment is available in emergency departments of most hospitals, CT is the most widely used diagnostic technique for aortic dissection. Advantages include ability to image the entire aorta including lumen, wall, and periaortic regions; identify anatomic variants and branch vessel involvement; and distinguish between the various acute aortic syndromes (intramural hematoma, penetrating arteriosclerotic ulcer, and acute aortic dissection). Electrocardiographic-gated techniques have made it possible to generate motion-free images of the aortic root and coronary arteries. Reports using newer-generation multidetector helical CT scanners have noted sensitivities of up to 100% and specificities of 98% to 99%. In comparative studies, however, CT has lower sensitivity and specificity than either TEE or magnetic resonance imaging (MRI) (see later). Dissection may be obscured by complete thrombosis of one lumen or similar opacification in both true and false lumens. Location of entry site and presence of aortic regurgitation cannot always be accurately determined. The technique requires use of contrast medium for accurate delineation of aortic pathology and may be contraindicated in patients with allergies to contrast agents or with renal insufficiency.

Magnetic Resonance Imaging

MRI is emerging as a premier imaging method for diagnosing diseases of the thoracic aorta, including acute aortic dissection. It does not require use of contrast medium. In some situations, a single study can provide information similar to that obtained from a combination of echocardiography, CT, and aortography, with high sensitivity and specificity. It provides superb imaging of both ascending and descending thoracic aortic dissections and can accurately identify sites of entry and thrombus formation ( Fig. 25-4 ). MR angiography (MRA) using gadolinium, a contrast agent, further enhances its utility. Disadvantages of MRI compared with CT and TEE include a longer time to complete the study, greater cost, inaccessibility to patients who are connected to ventilators and monitoring devices, and limited availability.

Aortography

Despite advances in noninvasive and minimally invasive techniques for diagnosing acute aortic dissection, aortography remains an important and highly accurate method for establishing the diagnosis. It is the benchmark against which other diagnostic studies are measured. The false lumen can be visualized, as can at times the intimal tear ( Fig. 25-5 ). It provides accurate information about branch artery involvement and presence of aortic valve regurgitation. It is an essential component of interventional procedures to treat acute aortic dissection, such as fenestration and endovascular stent-grafting (see “ Special Situations and Controversies ” later). Disadvantages of aortography compared with other diagnostic methods include need for arterial access and introduction of wires and catheters into the aorta, potential for false-negative results if the false lumen is thrombosed, risk of allergic reactions to contrast medium, and renal failure in patients with impaired renal function.

In the International Registry of Acute Aortic Dissection (IRAD), which included 618 patients with acute aortic dissection who had imaging studies between January 1996 and December 1999, the order for which the imaging studies were performed was known in 604 patients (98%). Among these, CT was performed first in 379 (63%), TEE in 192 (32%), aortography in 24 (4%), and MRI in 9 (1%). Among the 396 patients with a second study, TEE was performed in 229 (58%), CT in 68 (17%), aortography in 61 (15%), and MRI in 38 (10%). Data from the IRAD centers demonstrated a high diagnostic sensitivity for all four imaging modalities. However, false-negative CT, TEE, and aortography studies were frequent, so diagnosis of acute aortic dissection could not be confidently excluded on the basis of negative findings of a single test. The authors strongly recommended a second imaging study if the initial diagnostic test did not identify aortic dissection when the diagnosis was suspected clinically.

Differential Diagnosis

Symptoms associated with acute aortic dissection can mimic those of acute MI. The electrocardiogram (ECG) may demonstrate myocardial ischemia, and serum creatine kinase may be elevated. Because thrombolytic therapy is frequently administered to patients with acute MI and ST-segment abnormalities, thrombolytic agents might be administered to patients with acute aortic dissection with potentially disastrous results. ST-segment elevation occurs rarely in acute aortic dissection ; however, ST-segment depression occurs more commonly and was noted in 16 of 50 (32%; CL 25%-39%) patients analyzed by Weiss and colleagues. Thus, thrombolytic therapy can be safely administered to patients with ST-segment elevation and no physical signs of aortic dissection without need for further diagnostic studies. Additional studies may be indicated before thrombolytic agents are given to patients with ST-segment depression or other ECG evidence of myocardial ischemia.

Several serum markers have been investigated for their utility in diagnosing acute aortic dissection and differentiating it from other conditions associated with acute onset of chest pain, such as MI and pulmonary embolism. D-dimer, a degradation product of cross-linked fibrin in thrombus, is sensitive for ongoing intravascular thrombosis. It is highly elevated in patients with acute aortic dissection, with a sensitivity in pooled studies of 94% (95% CL, 91%-96%). It is also highly elevated in patients with acute pulmonary embolism. The lower specificity (40%-100%) in the pooled studies is not of sufficient magnitude to exclude the diagnosis, however, and in general, other diagnostic studies are required. Using 64-slice CT scanners, it is possible to establish or exclude a diagnosis of acute aortic dissection, acute pulmonary embolism, and obstructive coronary artery disease (the so-called triple rule-out CT), and this diagnostic study is being used with increasing frequency.

Survival

Information about survival of nonsurgically treated patients is sparse, but some general inferences may be drawn. Between 40% and 90% of patients survive 24 hours or more after the dissection, but this generalization ignores the well-established difference in prognosis between type A and type B dissections ( Fig. 25-6, A ). The hazard function for death also appears to be different in these two types of dissection ( Fig. 25-6, B ). Type B has a rapidly declining early hazard phase, a constant phase, and a rising late hazard phase beginning about 2 years after dissection; type A dissection has a rapidly declining early hazard phase and only a constant phase thereafter.

Involvement of the ascending aorta or aortic arch is, then, a risk factor for early death in patients with acute dissection. Hypertension is an important risk factor in untreated patients, both by inciting the intimal tear and unfavorably affecting survival once dissection occurs. Large size of the dissected aorta is also a risk factor. Complete or near-complete thrombosis of the false lumen appears to reduce risk of subsequent rupture and death.

Modes of Death

Most patients who die acutely succumb from false lumen rupture with hemopericardium, hemomediastinum, or hemothorax. Deaths later in the early period after dissection can result from delayed rupture or organ dysfunction secondary to arterial occlusions.

Course after Surviving Acute Aortic Dissection

Patients surviving acute dissection continue to be at greater risk of dying than the general population. This is because the false lumen generally persists (see Morphology [earlier] and Results [later]), usually and gradually becomes aneurysmal, and may rupture months or years after the acute episode. Also, a new dissection (redissection) may occur in a previously uninvolved portion of the wall of the aorta and present new risks.

Purpose of Surgical Treatment

Operation for acute aortic dissection is performed to prevent death from cardiac tamponade or exsanguination by excising and repairing or replacing areas of actual or impending rupture and, wherever possible, restoring blood flow to branches of the aorta that have been occluded by the dissection. Operation does not remove the entire false lumen in most patients. To this extent, operation is palliative rather than curative. Another purpose of operation when dissection involves the ascending aorta is to correct acutely developed or chronic coexisting aortic valve regurgitation. Resuspending detached commissures or replacing the aortic valve may be necessary. Replacing the aortic root may be required in some instances, and this is accomplished with a composite graft, an aortic allograft, or a stentless xenograft.

Repair of Acute DeBakey Type I or II (Type A) Dissection

General Considerations

Usual preparations are made for operations in which CPB is used (see Sections III and IV in Chapter 2 ). Pressures are monitored in the right radial artery and in the femoral artery opposite the cannulation site to promptly detect obstruction to retrograde flow within the aortic arch after CPB is established. Doppler sonography of the extracranial carotid arteries and monitoring of cerebral oxygen saturation are useful techniques for detecting compromised blood flow in brachiocephalic arteries.

The common femoral artery with the most normal pulse is exposed through a small vertical or oblique incision in the inguinal fold. If dissection is present upon opening it, the lumen in which blood is flowing should be cannulated. This may be either the false or the true lumen. In some cases, effective retrograde arterial perfusion may not be possible, because the aorta becomes obstructed (see “ Malperfusion Syndromes ” later under Special Situations and Controversies ). In this situation, provisions should be made for antegrade aortic perfusion through the ascending aorta, aortic arch, or apex of the left ventricle, or for perfusion of an axillary artery or the opposite femoral artery (see “Cardiopulmonary Bypass Established by Peripheral Cannulation” under Special Situations and Controversies in Section III of Chapter 2 ). If it is necessary to establish CPB urgently, the right common femoral vein should be used because the long venous cannula can be more easily positioned in the right atrium from the right side.

After median sternotomy, the pericardium is incised and stay sutures placed. The often hemorrhagic ascending aorta is not disturbed at this point ( Fig. 25-7, A ). If dissection is confined to the ascending aorta (DeBakey type II) and hypothermic circulatory arrest is not needed, a single two-stage cannula for venous drainage can be inserted through a purse-string suture into the right atrial appendage. TEE is useful in making this determination. In all other situations, purse-string sutures are placed for cannulation of both superior and inferior venae cavae.

After heparinization, the femoral artery and right atrium or the venae cavae are cannulated, CPB established, and the patient's body temperature reduced. If only the ascending aorta requires repair and circulatory arrest is not required, perfusate temperature is taken to 28°C to 32°C and flow rate appropriately reduced. If circulatory arrest is necessary, the patient's body temperature is reduced to less than 20°C. A venting catheter is inserted through the right superior pulmonary vein and advanced into the left ventricle (see “Commencing Cardiopulmonary Bypass and Left Heart Venting” in Section III of Chapter 2 ). A balloon-tipped catheter is placed into the coronary sinus through a purse-string suture in the right atrium for delivery of retrograde cardioplegia (see “Technique of Retrograde Infusion” in Chapter 3 ).

Limited dissection is carried out, carefully separating the ascending aorta from the pulmonary trunk proximal to the origin of the brachiocephalic artery, and the aorta is clamped at this point after transiently reducing CPB flow to a low level (0.5 L · min ⁻¹ · m ⁻² ). Whenever possible, the clamp should be placed several centimeters proximal to the brachiocephalic artery to avoid further injury and fragmentation of the aorta at a site where an anastomosis to a graft may be performed. A longitudinal incision is made in the ascending aorta extending to, but not into, the noncoronary sinus ( Fig. 25-7, B ). This incision often enters into the false lumen, which is usually anterior and to the right of the true lumen. If clot is present in the false lumen, it is carefully removed. The dissecting membrane is incised, and retraction sutures are placed in the aortic wall. Cardioplegic solution is promptly infused into the coronary ostia. Retrograde cardioplegia should be administered if the coronary ostia are compromised by the dissection (see “Methods of Myocardial Management during Cardiac Surgery” in Chapter 3 ). Cardioplegic solution is infused every 12 to 15 minutes. External cooling of the ventricles with topical slush or a cooling jacket may also be used. The interior of the ascending aorta is examined and the aortic wall transected circumferentially 4 to 5 mm above the level of the aortic commissures ( Fig. 25-7, C ).