Principles of Surgery, Companion Handbook - page 21

Chapter 19 Thoracic Aneurysms and Aortic Dissection

Principles of Surgery Companion Handbook

CHAPTER
19
THORACIC ANEURYSMS AND AORTIC DISSECTION

Thoracic Aneurysms
Aortic Root and Ascending Aortic Aneurysms
Aortic Root and Aortic Arch Aneurysms
 Descending Thoracic Aneurysms
 Thoracoabdominal Aneurysms
Aortic Dissection

THORACIC ANEURYSMS

General Considerations Excisional therapy and endoaneurysmorrhaphy with internal graft replacement have been used for repair of thoracic aortic aneurysms. If the aneurysm is well localized, total excision is feasible, and the involved area is replaced with an end-to-end Dacron graft (graft interposition method). This method is associated with a lower risk of reoperation for pseudoaneurysm. Patients with more extensive aneurysmal disease may require replacement of the diseased segment from within by placing a Dacron graft without excising the aorta itself. An end-to-end anastomosis is performed within the open aneurysm; the wall of the aneurysm is wrapped around the graft for tissue coverage (graft inclusion method).

Etiology An aortic aneurysm is a localized or diffuse aortic dilatation greater than 5–6 cm in diameter that develops from a weakness or defect in the aortic wall. Atherosclerosis associated with increased proteolytic enzyme activity has been implicated. There is a degeneration of elastin and collagen within the vessel wall, as well as decreased protease inhibitor activity, resulting in progressive dilation of the vessel. Late in the process, there is degradation of the adventitial layer with additional dilation or rupture of the aneurysm.

Causative factors are age, hypertension, smoking, atherosclerosis, aortic dissection, and connective tissue disorders. Chronic atherosclerosis is responsible in a minority of patients usually in conjunction with other factors such as inflammation or a genetic tendency for increased proteolytic activity. The most common connective tissue disorders associated with aneurysm formation are Erdheim's cystic medial necrosis, Marfan syndrome, and Ehlers-Danlos syndrome. Familiarity with genetic defects associated with aneurysmal disease can lead to earlier, more effective operative treatment. Less common causes are trauma, infection (mycotic), inflammatory disease, and autoimmune diseases.

Classification Aneurysms are classified by anatomic location: aortic root and ascending aorta, transverse arch, descending thoracic, and thoracoabdominal aneurysms. The most common are aneurysms of the aortic root and ascending aorta. Other classifications are as follows: Aortoannular ectasia is degenerative dilatation of the aortic annulus and the sinuses of Valsalva; traumatic aneurysms may occur after blunt trauma; and aortic dissection may occur as a result of an intimal tear in the aortic wall.

Clinical Manifestations Sudden and severe pain is the most common symptom associated with a large or expanding aneurysm. Chronic pressure or low-grade, aching pain may suggest chronic aneurysm. In a patient with known aneurysms, new onset of pain may indicate rapid expansion, leakage, or impending rupture. Large aneurysms may produce symptoms from compression of adjacent structures, which can result in pain, pulmonary symptoms, and hoarseness. Most patients with moderate-sized aneurysms are asymptomatic and are discovered incidentally.

Diagnosis Diagnosis is established by echocardiography, computed tomographic (CT) scan, magnetic resonance imaging (MRI), and rarely, aortography. Patients often have associated coronary artery disease, and screening studies or coronary angiography should be performed before repair.

Natural History and Operative Indications The history is one of progressive enlargement with rupture. Risk factors for rupture include size of the aneurysm, change in size, smoking, and chronic obstructive pulmonary disease (COPD). Risk of rupture by size is as follows: <3 cm, 0 percent; 3–4 cm, 6 percent; 4–5 cm, 12 percent; 5–6 cm, 36 percent; 6–7 cm, 50 percent; and 7–8 cm, 100 percent. A few patients with smaller aneurysms that were not enlarging have had sudden ruptures; if treated nonoperatively, patients should be aware of this fact. Overall survival if untreated is 60 and 13 percent at 1 and 5 years, respectively. Operation should be strongly considered in patients with saccular aneurysms or connective tissue diseases when the aneurysm is greater than 4.5–5 cm in size.

AORTIC ROOT AND ASCENDING AORTIC ANEURYSMS

Operative Treatment Standard operative repair is through a median sternotomy incision using cardiopulmonary bypass and cardioplegia. When the aneurysm is confined to the ascending aorta without involvement of the aortic root, the ascending aorta is replaced with a woven Dacron graft, beginning just distal to the sinotubular ridge and ending proximal to the innominate artery. Concomitant valve replacement is performed if aortic valve disease is present.

The operation most commonly used for root replacement is a composite valve graft, which involves replacement of the aortic valve and aortic root with aortic valve–Dacron graft conduit placed from the aortic annulus to the distal aorta beyond the aneurysm; this requires reimplantation of the coronary arteries into the composite graft. The need for root replacement can be determined preoperatively by MRI, CT, echocardiogram, or angiogram during cardiac catheterization. Long-term operative results are excellent with low mortality (2–5 percent) depending on the number of associated risk factors.

AORTIC ROOT AND AORTIC ARCH ANEURYSMS

These may be isolated or part of a continuous aneurysmal process involving the ascending and descending aorta. Most common causes are atherosclerosis, aortic dissection, and connective tissue disorders. Diagnosis is made after chest x-ray by MRI, CT, or aortography. The innominate, carotid, or subclavian artery also may be aneurysmal. The extent of disease and great vessel involved should be defined preoperatively in order to determine the operative approach.

Treatment The extent of arch involvement may make it necessary to perform total arch replacement with reimplantation of the arch vessels into the graft or partial hemiarch replacement with placement of a beveled graft leaving the arch vessels to arise from the native aortic arch superiorly. Hemiarch repair allows replacement of 25–85 percent of the aortic arch as necessary. The elephant trunk technique is used for patients who require a subsequent operation for a descending aneurysm. In this technique, the distal graft is invaginated into itself while the anastomosis is constructed, and the invaginated portion is subsequently unfolded so that it lies free in the descending aorta, allowing easier access to the distal graft during the subsequent descending aneurysm repair.

Use of hypothermic circulatory arrest for cerebral protection allows blood flow to be stopped so that a precise, sound technical arch anastomosis can be performed and avoids clamping and manipulation of the disease aorta, lessening the risk of aortic injury or embolization. Risk of permanent neurologic injury (stroke), caused by embolic events, after circulatory arrest is low if the cerebral ischemic time does not exceed 45 min. Temporary neurologic dysfunction such as transient confusion, agitation, or obtundation occurs in 20–30 percent of patients in whom circulatory arrest exceeds 50 min.

A new technique for cerebral protection involving retrograde cerebral perfusion through the superior vena cava has minimized the risk of cerebral ischemia when the circulatory arrest time is greater than 45 min.

Descending Thoracic Aneurysms

Etiology Descending thoracic aneurysms may result from atherosclerosis, cystic medial necrosis, connective tissue diseases, dissection, infection, inflammation, or prior trauma. Most begin in the proximal descending thoracic aorta, just distal to the left subclavian artery and can extend to involve the entire descending thoracic aorta. Atherosclerotic aneurysms usually are fusiform; some are localized and saccular. Concomitant abdominal aneurysms occur in 25–30 percent of patients; the entire thoracoabdominal aorta may be involved in 10 percent of patients.

Clinical Manifestations Most are asymptomatic and diagnosed on routine chest x-ray. Aneurysmal enlargement or compression of adjacent structures may produce symptoms that result in pain, cough, dyspnea, vocal cord paralysis, or hemoptysis. Physical examination usually is normal.

Diagnosis Thoracic aneurysm is usually diagnosed after chest x-ray and is confirmed by CT or MRI. Concomitant atherosclerosis often is present in the coronary, renal, or carotid arteries. Stress testing, coronary arteriography, and carotid studies should be performed to determine whether concomitant cardiovascular disease requires treatment. Patients with symptomatic coronary artery disease or a positive stress test should undergo cardiac catheterization and revascularization before aneurysm repair.

Operative Indications In patients with descending aneurysms larger than 5–6 cm in diameter, elective repair is suggested. Patients with aneurysms less than 5 cm in diameter require frequent follow-up and imaging studies; operation is indicated if the aneurysm expands or the patient becomes symptomatic.

Treatment Operative techniques include unprotected cross-clamping, in which cross-clamps are placed proximally and distally without distal perfusion or with a single proximal cross-clamp with controlled distal exsanguination. In perfusion or shunting, bypass or passive shunts are used to maintain distal aortic perfusion during the cross-clamp time. Perfusion may be done with left atriofemoral bypass, with femoral-femoral bypass with an oxygenator, or with a Gott shunt from the proximal to the distal aorta.

Operative exposure is achieved with a left posterolateral thoracotomy through the fourth, fifth, or sixth interspace; the fifth is used for most middle descending aneurysms. The aorta initially is mobilized and encircled proximal and distal to the aneurysm; occasionally, the aorta is controlled proximally between the left carotid and left subclavian arteries. Almost 25 percent of descending aneurysms require placement of the cross-clamp proximal to the left subclavian artery. Operative dissection is facilitated by opening the pericardium to expose the intrapericardial portion of the transverse aortic arch. The vagus nerve and recurrent laryngeal nerve should be mobilized and protected.

To place the graph, an initial dissection is performed to isolate the aorta proximally and distally. The aorta must be sufficiently mobilized to allow precise placement of the cross-clamps, with proximal control usually obtained first. Decision is then made as to whether a simple cross-clamp or perfusion technique is to be used.

When the aorta is clamped and opened widely, thrombus is removed from the lumen. Ostia of intercostal vessels are oversewn from within the aneurysm unless they are to be reimplanted into the side of the graft. In most patients, the aneurysm is not resected; the graft is placed internally after the aneurysmal contents are evacuated. A woven Dacron graft is inserted; end-to-end anastomoses are performed proximally and distally with continuous vascular suture. After graft placement, the clamps are temporarily opened to remove air or thrombus; the suture lines are then tied. The cross-clamps are removed slowly; the distal body is perfused through the graft. Reperfusion can result in transient hypotension, which is corrected by volume infusion and sodium bicarbonate. When hemostasis is obtained, the aneurysmal sac is wrapped around the graft.

Complications after repair can include paraplegia. Unprotected cross-clamping for more than 20 min is associated with an increased risk of paraplegia. Data suggest that when cross-clamp time is greater than 30–40 min, maintaining perfusion of the distal aorta at more than 60 mmHg reduces the risk of paraplegia, renal insufficiency, intestinal ischemia, and reperfusion-related white blood cell activation with multiorgan dysfunction.

Perfusion Technique Perfusion options include using femoral vein–femoral artery perfusion with an oxygenator or left atriofemoral artery bypass using heparin-bonded circuits, no oxygenator, minimal heparin, and a cell-saver–rapid infuser. Left atriofemoral bypass is preferred because it allows better control of the proximal blood pressure and minimizes the need for nitroprusside.

In addition, somatosensory evoked potential monitoring (SEP) can evaluate spinal cord ischemia or the adequacy of perfusion while the aorta is occluded, although some have reported no decreased incidence of paraplegia when using distal perfusion and SEP monitoring versus the simple cross-clamp method.

Single Cross-Clamping with Controlled Distal Exsanguination Controlled distal exsanguination reduces venous distention and lowers cerebrospinal fluid pressure, improving spinal cord perfusion during the cross-clamp period (as long as the cross-clamp period is not more than 30–40 min). This method limits spinal cord ischemia and edema. Because no bypass circuit for distal perfusion is required and no heparin is used, the risk of bleeding and coagulopathy may be lower.

Results Long-term prognosis after repair depends on concomitant coronary artery disease, cerebrovascular disease, or aneurysmal disease in other parts of the aorta. Approximately 30 percent of patients with descending thoracic aneurysms develop aneurysmal disease elsewhere; follow-up imaging studies on a yearly basis are recommended.

Thoracoabdominal Aneurysms

Etiology and Classification These usually are a result of atherosclerosis, connective tissue disease, or aortic dissection and are classified as follows:

Type I—proximal descending to upper abdominal
Type II—proximal descending to below renal arteries
Type III—distal descending and abdominal
Type IV—suprarenal and infrarenal abdominal

Operative Treatment Because thoracoabdominal aneurysms usually involve the segment of aorta where the celiac axis, superior mesenteric artery, and renal arteries arise, surgical repair may result in transient spinal cord, renal, and visceral organ ischemia, with subsequent ischemia-reperfusion-related white blood cell activation, which can lead to multiorgan injury. Coagulopathy and bleeding may be significant; if massive transfusion is needed, the risk of multiple organ failure is increased.

Operative exposure requires a large thoracoabdominal incision in the sixth or seventh intercostal space of the left chest; the costal cartilage is divided; the incision is extended below the umbilicus. The diaphragm is divided circumferentially along the radius, preserving the central innervation. The descending thoracic aorta is isolated proximal to the aneurysm; the spleen, left colon, and kidney are reflected medially; the retroperitoneum is entered to expose the aorta distally to the bifurcation; the retroperitoneal lymphatics are tied in continuity.

When the aorta is exposed, the aorta is clamped proximally. Distally, the aorta is left open or clamped and perfused. The aneurysm is incised, and an anastomosis is performed between the graft and the proximal aorta. Segmental intercostal arteries or the cluster of arteries above and below the diaphragm are reimplanted into the graft. The cross-clamp is sequentially moved distally beyond the reimplanted intercostal arteries. The visceral vessels are reimplanted and reperfused, and an anastomosis between the graft and the distal aorta is performed.

Reimplanting large intercostal vessels or clusters of vessels in the aortic graft can reduce the risk of paraplegia. Other methods of spinal cord protection include bypass methods to increase spinal cord blood flow; the use of intrathecal vasodilators such as papaverine, systemic steroids, intravenous lidocaine, systemic hypothermia, barbiturates, calcium channel blockers, and oxygen radical scavengers; and cerebrospinal fluid drainage to decrease spinal cord pressure.

AORTIC DISSECTION

Etiology This begins as a tear in the intima with entry of blood and separation of the media, which results in blood flow down a “false lumen.” A localized aneurysm may develop immediately or months or years later. The disease is three to four times more common in males, occurring mostly in older patients (after the fifth decade). Dissection may occur in any age group; certain cases occur in childhood secondary to coarctation of the aorta.

Aortic dissection usually results from a combination of hypertension and degenerative connective tissue disease. The strongest predisposing factor is cystic medial necrosis, but it also may be associated with congential bicuspid aortic valve. Aortic dissection is a disease of the media, usually originating in the thoracic aorta, and it may continue distally to the aortic bifurcation.

Pathology A transverse tear of intima and media is the initial event, which permits blood to enter into the media and dissect distally. Most prominent sites are 70 percent ascending, 10 percent arch, 20 percent upper thoracic, and 2 percent abdominal aorta. The aortic wall progressively separates (“dissects”) with an inner (true) lumen composed of intima and an outer (false) lumen composed of media and adventitia. It usually extends rapidly through the thoracic and abdominal aorta in the peripheral arteries.

As dissection proceeds, branches are obliterated unless communication with the false lumen is established. Proximally, the coronary arteries may be involved, and often one or more aortic valve commissures are detached, creating aortic insufficiency. The commissure between the right sinus and the noncoronary sinus is most commonly involved. Distally, any vessel may be involved. Innominate or carotid artery involvement may produce neurologic injury. Obstruction of a subclavian artery may produce arm ischemia, and occlusion of intercostal arteries may cause spinal cord injury with paraparesis or paraplegia. Dissection of renal arteries may produce renal insufficiency, hematuria, oliguria, or anuria. Acute obstruction of the iliac or femoral arteries may cause leg ischemia.

Rupture into the pericardial cavity with cardiac tamponade is the most common fatal complication.

Clinical Manifestations Abrupt onset of excruciating pain is characteristic. Pain may radiate from the anterior chest to the neck, arms, epigastrium, or legs. Other presenting symptoms include congestive heart failure, tamponade, syncope, stroke, peripheral neurologic injury, leg or arm ischemia, paraplegia, gastrointestinal hemorrhage, hematuria or anuria, hoarseness, dysphagia, superior vena cava syndrome, and aortic insufficiency.

Classification The Stanford classification based on clinical course and surgical significance is as follows: Stanford type A dissection includes any dissection involving the ascending aorta, and Stanford type B dissection involves only the descending aorta.

Diagnostic Studies A chest x-ray will show a widened mediastinum or left pleural effusion. Transesophageal echocardiography (TEE) is the procedure of choice. CT and MRI can be useful; aortography is seldom necessary.

Medical Treatment Immediate drug therapy is initiated to control blood pressure and decrease the contractility of the left ventricle (dp/dt). Combination drug therapy is used to achieve beta-adrenergic blockade and afterload reduction. The systolic blood pressure should be kept below 110–120 mmHg.

Natural History and Operative Indications Patients with Stanford type A dissections are at risk for early death and need an emergent operation. Patients with Stanford type B dissections are treated medically initially, and early operation is recommended only for those with complications such as rupture, hemodynamic compromise, prolonged pain, aneurysm expansion, visceral or limb ischemia, and neurologic signs. Close observation is essential, including serial hematocrits, chest x-rays, and imaging studies.

Operative Treatment Stanford Type A Dissection The objective is to remove the intimal tear, replace the diseased aorta, obliterate the false lumen and redirect flow into the true lumen, and correct associated valvular insufficiency or coronary ischemia. This is accomplished usually with deep hypothermia and circulatory arrest with performance of an “open” distal anastomosis. Alternatively, the aorta may be excised totally with removal of the tear site and aneurysmal segments; the risk of early bleeding may be increased with this method.

When the aortic valve is involved, resuspension of the valve is highly effective. If a competent valve cannot be ensured, aortic valve replacement should be performed.

Stanford Type B Dissection Most advocate initial medical therapy. Urgent operation is indicated for complications such as recurrent pain, progressive mediastinal hematoma, leakage, acute expansion, rupture, visceral organ ischemia, extremity ischemia, progressive neurologic dysfunction, and retrograde dissection with aortic valve involvement. Operation is performed through a left thoracotomy. The goal is to exclude the tear site from the circulation, to obliterate the false lumen, and to redirect blood flow into true lumen distally. The technique is that previously described for aneurysms of the descending thoracic aorta.

For a more detailed discussion, see Galloway AC, Miller JS, Spencer FC, and Colvin SB: Thoracic Aneurysms and Aortic Dissection, chap. 19 in Principles of Surgery, 7th ed.

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Seymour I. Schwartz
Principles of Surgery Companion Handbook