Aortic Diseases



Aortic Diseases


Dawn S. Hui

Lalithapriya Jayakumar

Andrea J. Carpenter



GENERAL INTRODUCTION AORTIC DISSECTION

An aortic dissection is a tear in the aortic intima, with the separation of the intima and media and the creation of a false lumen. Clinical manifestations are related to structural changes in the aortic wall and the subsequent rheologic alterations, both in the aorta itself and in the branch vessels. Because all end-organ perfusion originates from the aorta, aortic dissection is one of the most fatal vascular diseases.


Epidemiology

In the United States, the estimated incidence of thoracic aortic dissection is 2.9 to 4.3 cases per 100,000 persons annually.1 The increasing incidence over time is likely due to the growing use and improved quality of diagnostic imaging; however, the true incidence may remain underestimated due to the exclusion of prehospital deaths from studies. There is a gender and age differential, with the age-adjusted incidence of men being 5.2 compared with 2.2 per 100,000 in women.2 The average age of onset is 65 years, although specific risk factors are associated with aortic dissection at a younger age, including connective tissue disorders, bicuspid aortic valve, inflammatory or infectious conditions leading to arteritis, a family history of aortic dissection, cocaine use, and pregnancy. Other risk factors associated with aortic dissection include male gender, hypertension, smoking, and atherosclerosis. A well-known risk factor is an aortic aneurysm, with dissection risk related to aneurysm diameter.


Pathogenesis

The sine qua non of an aortic dissection is an intimal tear. However, the inciting event leading to intimal disruption remains under investigation, with classical theory holding that the intimal tear is the inciting event. Newer evidence suggests an alternate theory highlighting the role of the tunica media.3,4,5 This layer, rich in lamellar units with elastic laminae, smooth muscle cells, collagen fibers, and elastic fibers, provides the strength of the aortic wall, tolerating pressures up to 600 mm Hg.3 Changes in medial architecture occur with processes such as cystic medial necrosis or ischemia from longstanding hypertension causing rupture of the vasa vasorum.4 Medial weakness then leads to the intimal tear.

Regardless of the preceding events, the intimal tear occurs most commonly as a spontaneous event, while trauma and iatrogenic etiologies (such as coronary angiography) account for a minority of events. Pressurized blood flows through this tear, leading to progressive separation of the intima from the media and adventitia. This channel is known as the false lumen, with the intimo-medial septum (“intimal flap”) separating it from the true lumen. As flowing blood propagates longitudinally, either antegrade or retrograde, more and more aortic segments become involved. Shear forces may produce additional tears along the aorta, leading to additional communication sites between the two lumens. Thrombus may develop, limiting propagation of the dissection.

Complications of dissection include malperfusion, aneurysm, and rupture. Malperfusion occurs when localized thrombus, or the intimal flap, obstructs the ostium of a branch vessel. The intimal flap location is dynamic and sensitive to relative true and false lumen pressurization. The compromised integrity of the media and adventitia can lead to aneurysmal dilation. If an aortic rupture occurs, the cardiac output is displaced into the extravascular space. Localized thrombi may limit egress of blood into the extravascular space (ie, “contained rupture”), but aortic rupture is a near-universally fatal event. Immediate treatment priorities in prehospital and emergency room settings are aimed at preventing or mitigating these complications until definitive treatment can be rendered.


CLINICAL PRESENTATION

The most common presenting symptom of an acute aortic dissection is pain in the chest (79%), back (47%), or abdomen (22%). Common descriptors of the pain include abrupt (85%), severe or “the worst ever” (90%), sharp (62%), and a tearing or ripping sensation (49%).6 When malperfusion exists, symptoms related to the end-organ ischemia predominate, such as syncope or focal weakness with cerebrovascular malperfusion and extremity pain or numbness with extremity malperfusion. Visceral malperfusion may present as abdominal pain, nausea, vomiting, or renal failure.

The differential diagnosis is variable, depending on the presenting symptoms. Since many thoracic aortic dissection patients have risk factors for coronary artery atherosclerosis, the differential diagnosis includes myocardial ischemia or infarction (MI). Other chest pain- related differential diagnoses
include pulmonary embolism or pneumonia. Because these diagnoses are generally more common or more often suspected than thoracic aortic dissection, workup including electrocardiogram (ECG), cardiac enzymes, or pulmonary embolism-protocol computed tomography (CT) can lead to a delay in the true diagnosis. Malperfusion syndromes may direct suspicion toward more localized processes, such as intra-abdominal processes in the case of visceral malperfusion, classical ischemic or hemorrhagic stroke in the case of cerebral malperfusion, and peripheral vascular disease in the case of extremity malperfusion. When malperfusion is advanced, particularly with visceral ischemia, mentation may be altered, limiting the reliability of the history or physical examination.

Propagation of the dissection into the aortic root can lead to a special subset of clinical sequelae. If the flap involves the coronary ostia leading to coronary malperfusion, the patient may present with a concomitant acute MI, clouding the picture or masking the true diagnosis. Aortic valve commissures may shear from their aortic wall attachments, leading to acute aortic insufficiency and acute heart failure.

Patients may not present for evaluation at the time of the initial dissection event. In etiologies such as catheter-induced dissection or trauma, the patient may not recall a history of symptoms. Rarely, the dissection may be undetected for years and discovered on chest imaging incidentally or due to symptoms from aneurysm degeneration of a longstanding dissection. The classification by time course is acute (≤2 weeks since the onset of symptoms), subacute (15-90 days from symptoms), and chronic (91 days or more).




DIAGNOSIS

History should focus on the characteristics and location of pain in addition to risk factors, particularly hypertension, known connective tissue disorders, bicuspid aortic valve, or a family history of dissection. Physical examination should include a focused heart and lung exam, a gross neurologic assessment, vascular exam, abdominal exam, and bilateral arm blood pressure measurement. ECG and chest radiography are routinely obtained in the acute evaluation of chest pain; chest radiograph may show a widened mediastinum or otherwise unexplained pleural effusion, raising the suspicion for thoracic aortic dissection.

The diagnostic test of choice is CT angiography (CTA). While magnetic resonance angiography has a sensitivity and specificity of ˜100%, the long acquisition time and lack of emergent availability make it less desirable. In the contemporary era, multidetector helical CT (MDCT) is an excellent rapid, minimally invasive, and safe imaging modality. It allows assessment of both the thoracic and abdominal aorta. False-positive MDCT examinations can be minimized with attention to optimal contrast bolus timing, ECG gating to reduce motion artifact, and narrow collimation to improve resolution. False negatives can be minimized with “thin cuts” of images (ie, ≤3 mm). Recent studies have shown MDCT to have 100% sensitivity and specificity. Other modalities include angiography and transesophageal echocardiography which are invasive and require sedation. Transthoracic echocardiography, while noninvasive and not requiring sedation, has a sensitivity of only 59% to 85% and specificity of 93% to 96%, but it may be the initial test of choice for hemodynamically unstable patients.7

Because the differential for chest pain includes MI, pulmonary embolism, and thoracic aortic dissection, CT protocols to evaluate all three diagnoses in a single examination have been investigated. However, these protocols use higher contrast and radiation doses, often exclude the abdominal aorta, and lack data on sensitivity and specificity. If the clinical judgment can differentiate among the likelihood of these three diagnoses, the most appropriate test for that diagnosis should be chosen. Regardless, ECG and cardiac enzymes are recommended as part of the initial workup. D-dimer testing may be useful in differentiating pulmonary embolism and thoracic aortic dissection, with a serum D-dimer level less than 500 ng/dL less likely to be associated with aortic dissection.8

An important part of the evaluation includes the identification of the dissection location. The Stanford classification system is practical and widely used due to its simplicity in stratifying patients for treatment (Figure 79.1). Stanford type A thoracic aortic dissection is one involving the ascending aorta, regardless of whether the descending aorta is involved. Stanford type B thoracic aortic dissection involves only the descending aorta. This classification system becomes unclear when the dissection originates or propagates retrograde to the aortic arch, leading to the contemporary term “acute non-A non-B aortic dissection.”9 Which aortic segments need to be treated varies according to the extent of dissection, long-term rupture risk, and the surgeon’s clinical judgment balancing operative complexity and benefit. The details of surgical repair exceed the scope of this chapter, but some references may be of interest to the reader.9,10


MANAGEMENT OF PATIENT

When a thoracic aortic dissection is suspected, immediate management priorities include (1) performing a prompt history and physical examination; (2) confirmation of diagnosis with testing; (3) minimizing the rupture risk and optimizing perfusion of the true lumen; and (4) excluding other cardiovascular diagnoses. The differential of MI, pulmonary embolism, and thoracic aortic dissection have important implications for treatment, due to the impact of immediate management decisions. Antiplatelet and anticoagulation therapy are indicated for acute coronary syndrome and pulmonary embolism respectively, but in the 39% of thoracic aortic dissections initially misdiagnosed, these therapies lead to worse outcomes.11

Immediate medical management of thoracic aortic dissection is blood pressure and heart rate control using medications that decrease pulsatile pressure in the aorta, including treating pain to mitigate the catecholamine surge. Goal systolic pressure is 100 to 120 mm Hg and heart rate is 60 to

80 beats per minute. A large-bore intravenous catheter should be placed for rapid resuscitation, and hemodynamic monitoring should be established. Short-acting intravenous agents are used due to the potential for rapid hemodynamic changes and instability. Beta-blockers are first-line agents given their effect on both blood pressure and heart rate (Class I, LOE C).12 If vasodilators are initiated before heart rate control (Class III), reflex tachycardia may ensue, thereby increasing aortic shear stress. For patients with suspected or confirmed cocaine use, selection of a β-blocker with α and β dual receptor blockade, such as carvedilol or labetalol, is preferred to avoid unopposed α-stimulation. For patients who cannot tolerate β-blockade due to bradycardia, calcium channel blockers may be administered. Vasodilators are considered once heart rate is controlled (Table 79.1). Infusion concentrations should be maximized to avoid administering large volumes of crystalloid and subsequent volume overload. Intravenous opioids for pain may aid in managing hypertension. Finally, a blood sample for type and crossmatch should be submitted in anticipation of emergent intervention, along with electrolytes, complete blood count, and coagulation parameters. Metabolic derangements, as a sequela of malperfusion, should be promptly treated.






If patients present with or develop sudden hypotension, concern for acute rupture or hemopericardium with tamponade must be considered immediately. Permissive relative hypotension to a systolic blood pressure of 80 mm Hg and judicious administration of fluids may mitigate sequela of rupture, but emergent intervention is critical for survival.







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May 8, 2022 | Posted by in CARDIOLOGY | Comments Off on Aortic Diseases

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