Thoracic Aortic Aneurysm Treatment & Management

Indications and contraindications

Indications for surgical treatment of thoracic aortic aneurysms (TAAs) are based on size or growth rate and symptoms. Because the risk of rupture is proportional to the diameter of the aneurysm, aneurysmal size is the criterion for elective surgical repair.

Elefteriades published the natural history of TAAs and recommended elective repair of ascending aneurysms at 5.5 cm and descending aneurysms at 6.5 cm for patients without any familial disorders such as Marfan syndrome. ^{[41, 42]} These recommendations were based on the finding that the incidence of complications (rupture and dissection) exponentially increased when the size of the ascending aorta reached 6.0 cm (31% risk of complications) or when the size of the descending aorta reached 7.0 cm (43% risk). ^{[43, 42]} Patients with Marfan syndrome or familial aneurysms should undergo earlier repair, when the ascending aorta grows to 5.0 cm or the descending aorta grows to 6.0 cm.

In addition, relative aortic aneurysm size in relation to body surface area may be more important than absolute aortic size in predicting complications. ^[44] Using the aortic size index (ASI) of aortic diameter (cm) divided by body surface area (m²), one can stratify patients into the following three groups ^{[44, 45]} :

• ASI < 2.75 cm/m ² - Low risk for rupture (4%/year)
• ASI 2.75-4.25 cm/m ² - Moderate risk (8%/year)
• ASI >4.25 cm/m ² - High risk (20-25%/year)

Rapid expansion is also a surgical indication. Growth rates average 0.07 cm/year in the ascending aorta and 0.19 cm/year in the descending aorta. ^[42] A growth rate of 1 cm/year or faster is an indication for elective surgical repair.

Symptomatic patients should undergo aneurysm resection, regardless of aneurysm size. Acutely symptomatic patients require emergency operation. Emergency operation is indicated in the setting of acute rupture. Rupture of the ascending aorta may occur into the pericardium, resulting in acute tamponade. Rupture of the descending thoracic aorta may cause a left hemothorax.

Patients with acute aortic dissection of the ascending aorta require emergency operation. They may present with rupture, tamponade, acute aortic insufficiency, myocardial infarction, or end-organ ischemia. Acute dissection of the descending aorta does not necessitate surgical intervention, unless complicated by rupture, malperfusion (eg, visceral, renal, neurologic, leg ischemia), progressive dissection, persistent recurrent pain, or failure of medical management.

Patients who undergo surgery for symptomatic aortic insufficiency or stenosis with an associated enlarged aneurysmal aorta should have concomitant aortic replacement if the aorta reaches 5 cm in diameter. Concomitant aortic replacement should be consider for patients with bicuspid aortic valves with an aorta more than 4.5 cm in diameter.

As one may imagine, the quality of data and level of evidence supporting these recommendations widely vary. Given these variations and the importance of the subject matter, in 2010, a joint task force spearheaded by the American College of Cardiology (ACC) Foundation and the American Heart Association (AHA) produced an Executive Summary detailing guidelines for diagnosis and management of this disease. ^[46] These guidelines have been replaced by guidelines published in 2022 by the ACC and the AHA on the diagnosis and management of aortic disease. ^[2] (See Guidelines.)

Indications for surgical treatment may be summarized as follows:

• Increased aortic size - At centers with multidisciplinary aortic teams and experienced surgeons, the threshold for surgical intervention for sporadic aortic root and ascending aortic aneurysms is now 5.0 cm (from 5.5 cm) in select individuals, and it is even lower in specific settings among patients with heritable thoracic aortic aneurysms
• Rapid artic growth - Surgery is recommended for patients with aneurysms of aortic root and ascending thoracic aorta with a confirmed growth rate of ≥0.3 cm/y across 2 consecutive years or ≥0.5 cm in 1 year
• Symptomatic aneurysm
• Traumatic aortic rupture
• Acute type B aortic dissection with associated rupture, leak, distal ischemia
• Pseudoaneurysm
• Large saccular aneurysm
• Mycotic aneurysm
• Aortic coarctation
• Bronchial compression by aneurysm
• Aortobronchial or aortoesophageal fistula

Aneurysm surgery has no strict contraindications. The relative contraindications are individualized according to the patient's ability to undergo extensive surgery (ie, the risk-to-benefit ratio). Patients at higher risk for morbidity and mortality include elderly persons and individuals with end-stage renal disease, respiratory insufficiency, cirrhosis, or other comorbid conditions.

For descending thoracic aneurysms, endovascular stent grafting ^[1] is less invasive and is an ideal alternative (with appropriate anatomic considerations) to open repair for patients at high risk for complications of open repair. Stent grafts are also a reasonable alternative (with the appropriate anatomy) to open repair in patients who are not at high risk for complications. Patients must understand that life-long follow-up is required and that long-term durability is unknown.

Future and controversies

Early morbidity and mortality are related to bleeding, neurologic injury (eg, stroke), cardiac failure, and pulmonary failure (eg, acute respiratory distress syndrome [ARDS]). Risk factors include emergency operation, older age, dissection, congestive heart failure (CHF), prolonged cardiopulmonary bypass (CPB) time, arch replacement, previous cardiac surgery, need for concomitant coronary revascularization, and reoperation for bleeding. Late mortality is usually related to cardiac disease or distal aortic disease.

Bleeding is a potential complication for all aneurysm repairs. It is minimized by the use of antifibrinolytics, felt strips, and factors, including fresh frozen plasma and platelets. For patients who undergo hypothermic circulatory arrest, the use of aprotinin has been controversial, but most groups routinely use aminocaproic acid. Coagulopathy and bleeding in severe cases may warrant the use of recombinant factor VII.

Aprotinin, an antifibrinolytic agent used to reduce operative blood loss in patients undergoing open heart surgery, is now only available via a limited-access protocol. Fergusson et al reported an increased risk for death compared with tranexamic acid or aminocaproic acid in high-risk cardiac surgery. ^[47]

Stroke is a major cause of morbidity and mortality and typically results from embolization of atherosclerotic debris or clot. Transesophageal echocardiography (TEE) and epiaortic ultrasonography (US) may be beneficial in localizing appropriate areas to clamp. Patients undergoing arch repairs are at the highest risk of permanent and transient neurologic injury. Retrograde cerebral perfusion is helpful for flushing out embolic debris but may be detrimental, with increased intracranial pressure and cerebral edema. Antegrade cerebral perfusion is beneficial for reducing neurologic injury during hypothermic circulatory arrest. Stroke incidence for open surgical repair versus endovascular repair of descending TAAs is equivalent.

Myocardial infarction may occur with technical problems with coronary ostia implantation during root replacement for ascending aortic aneurysms and may require reoperation. Pulmonary dysfunction and renal dysfunction are other potentially morbid complications.

Paraparesis and paraplegia, either acute or delayed, are the most devastating complications of descending TAA and thoracoabdominal aortic aneurysm (TAAA) repairs. Despite cerebrospinal drainage, reimplantation of intercostal arteries, evoked potential monitoring, mild hypothermia, and atrial femoral bypass, spinal cord injury still occurs. Endovascular stent grafting has not eliminated spinal cord paraplegia; the incidence varies widely, with an overall incidence of 2.7%. ^{[31, 48, 49, 50]}

Complications specific to endovascular stenting include endoleaks, stent fractures, stent graft migration or thrombosis, iliac artery rupture, retrograde dissection, microembolization, aortoesophageal fistula, and complications at the site of delivery (eg, groin infection, lymphocele, seroma).

All aneurysms must be treated with risk-factor reduction. Systemic hypertension probably contributes to the formation of aneurysms and certainly contributes to expansion and rupture. This is especially true of TAAs. Strict control of hypertension is implemented in all patients, regardless of aortic aneurysm size.

Tobacco use contributes to aneurysm formation, though the exact pathophysiology is not well understood. Cessation of smoking is recommended. Control of other risk factors for peripheral arterial obstructive disease may be beneficial.

For acute aortic dissections, the first-line treatment of hypertension is a short-acting beta blocker (eg, esmolol). Beta blockade decreases the force of contraction, thus decreasing the dP/dt and shear force exerted on the dissection by minimizing the rate of rise of the aortic pressure. It also decreases the heart rate and the inotropic state of the myocardium, as well as reduces the likelihood of propagation of the dissection. A second agent added is a vasodilator (eg, nitroprusside), which reduces the systolic blood pressure so as, in turn, to decrease aortic-wall stress and the possibility of rupture.

Most aneurysm repairs involve aortic replacement with a Dacron tube graft. Dacron grafts allow ingrowth in the interstices to form a pseudoendothelial layer to minimize the risk of embolization. They may be knitted or woven. Knitted grafts are more porous and incorporate tissue well; however, they are prone to more bleeding. Woven grafts are more impervious and therefore are the most commonly used for aortic replacement. Grafts are typically impregnated with collagen to avoid preclotting the graft and to promote optimal healing.

Endovascular repair of a TAAA is shown in the video below.

Choice of procedure

Ascending aortic aneurysms

Surgical treatment of ascending aortic aneurysms depends on the extent of the aneurysm both proximally (eg, involvement of the aortic valve, annulus, sinuses of Valsalva, sinotubular junction, coronary orifices) and distally (eg, involvement to the level of the innominate artery). The choice of operation also depends on the underlying pathology of the disease, the patient's life expectancy, the desired anticoagulation status, and the surgeon's experience and preference. ^[51]

Ascending aortic aneurysms with normal aortic valve leaflets, annulus, and sinuses of Valsalva are typically replaced with a simple supracoronary Dacron tube graft from the sinotubular junction to the origin of the innominate artery, with the patient under CPB.

If the aortic valve is diseased but the aortic sinuses and annulus are normal, the aortic valve is replaced separately and the ascending aortic aneurysm is replaced with a supracoronary synthetic graft, with the coronary arteries left intact (ie, Wheat procedure).

Sinus of Valsalva aneurysms with normal aortic valve leaflets and aortic insufficiency due to dilated sinuses may be repaired with a valve-sparing aortic root replacement. Two valve-sparing procedures have been developed: the remodeling method and the reimplantation method. The remodeling method involves resecting the aneurysmal sinus tissue while maintaining the tissue along the valve leaflets and scalloping the Dacron graft to form new sinuses to remodel the root. The reimplantation method involves reimplanting the scalloped native valve into the Dacron graft. Both require reimplantation of the coronary ostia into the Dacron graft. ^[52]

Patients with an abnormal aortic valve and aortic root require aortic-root replacement (ARR). In nonelderly patients who can undergo anticoagulation with reasonable safety, the aortic root may be replaced with a composite valve-graft consisting of a mechanical valve inserted into a Dacron graft coronary artery reimplantation (eg, classic or modified Bentall procedure or Cabrol procedure). ^{[53, 54]}

For elderly patients, young active patients who do not desire anticoagulation, women of childbearing age, and patients with contraindications to warfarin, the options include stentless porcine roots, ^[55] aortic homografts, and pulmonary autografts (ie, Ross procedure). ^[56] For elderly patients who cannot undergo a complex operation, another option is reduction aortoplasty (ie, wrapping of the ascending aorta with a prosthetic graft).

Patients with Marfan syndrome have abnormal aortas and cannot undergo tube graft replacement alone. They must have either a valve-sparing ARR or a complete ARR.

ARR with a homograft is ideal in the setting of aortic-root abscess from endocarditis.

Aortic arch aneurysms

Arch aneurysms pose a formidable technical challenge. Deep hypothermic circulatory arrest (DHCA) with or without antegrade or retrograde cerebral perfusion is usually used to facilitate reanastomosis of the arch vessels. Aortic arch reconstruction techniques vary, depending on the arch pathology.

In patients with proximal arch involvement extending from the ascending aorta, a hemiarch replacement may be performed. The ascending aorta is replaced with a Dacron graft beveled as a tongue along the undersurface of the arch. In patients whose conditions mandate replacement of the entire arch, the distal anastomosis is the Dacron graft to the descending thoracic aorta. The head vessels are reimplanted individually or as an island. Grafts have been developed with a trifurcated head-vessel attachment and with a fourth attachment for the cannula. In this case, the head vessels are attached individually to the trifurcated branches.

For patients in whom the arch replacement is part of a staged procedure, preceding the delayed repair of a concomitant descending thoracic aneurysm, an "elephant trunk" is used. That is, the Dacron graft used to reconstruct the transverse arch ends distally in an extended sleeve that is telescoped into the descending thoracic aorta, facilitating later replacement of the descending thoracic/abdominal aneurysm (two-stage procedure).

The higher morbidity and mortality associated with ascending and arch aortic surgery combined with the increasing experience using thoracic endografts in the descending aorta has evolved into the use of endografts in the ascending and arch aortas.

The NEXUS Aortic Arch Stent Graft System is a single-branch two-stent graft system that is specifically intended for endovascular treatment of aortic arch pathologies. A study by Planer et al reported a high success rate in 17 patients with isolated aortic arch aneurysm, with excellent 1-year safety and performance. ^[57]

So-called "hybrid" procedures represent a combination of open and endovascular procedures. Although potentially still requiring a median sternotomy and, often, revascularization of some or all of the arch vessels, they offer the advantage of potentially less invasiveness through one-stage or single-incision procedures, avoidance of aortic cross-clamping or hypothermic circulatory arrest, or less total revascularization, especially if the ascending or descending aorta is also involved.

Further evolution of the hybrid procedure is demonstrated by the "frozen elephant trunk" technique, which involves standard ascending and arch repair as in the elephant trunk procedure, followed by antegrade reconstruction of the descending thoracic aorta, through the opened transverse arch. These potential advantages theoretically translate into decreased morbidity and mortality. Limited experience with these hybrid procedures has been reported; however, the long-term results and durability have yet to be defined.

Descending thoracic aortic aneurysms and thoracoabdominal aortic aneurysms

Descending TAAs may be repaired with open surgery or, if appropriate, with endovascular stent grafting techniques. ^{[31, 48, 30, 49, 45, 50]} Stent graft repair of descending TAAs should be performed if the predicted operative risk is lower than that of an open repair. Patient age, comorbidities, symptoms, life expectancy, aortic diameter, characteristics and extent of the aneurysm, and landing zones, should also be taken into consideration.

Surgically, descending TAAs may be repaired with or without the use of a bypass circuit from the left atrium to the femoral artery or femoral vein–femoral artery CPB, depending on the length of the anticipated ischemic cross-clamping and the experience of the surgeon. Discrete aneurysms with an anticipated clamp time of less than 30 minutes may be repaired without left-heart bypass or CPB (ie, "clamp and go" technique). More complex or larger aneurysms are probably safer to repair with the aid of either left-heart, partial, or full CPB with hypothermic circulatory arrest. The use of left-heart bypass or CPB is favored to reduce hemodynamic instability and the risk of spinal cord paraplegia.

Descending TAAs with the appropriate anatomy may be repaired by using endovascular stent grafts (eg, thoracic endovascular aortic repair [TEVAR]). ^[1] The GORE TAG nitinol-based stent graft, approved by the US Food and Drug Administration (FDA) in March 2005, was designed for descending TAA repair. Subsequently, the Zenith TX2 endovascular graft (Cook Medical Inc, Bloomington, IN) was approved in March 2008, followed by the Talent Thoracic Stent Graft (Medtronic Inc, Minneapolis, MN) in June 2008. The Valiant Thoracic Stent Graft (Medtronic Inc, Minneapolis, MN) is approved for use outside the United States.

With the GORE TAG, an appropriate proximal neck of 2 cm prior to the aneurysm is required. Ideally, the proximal landing zone is beyond the left subclavian artery, though in some circumstances the stent may be placed proximal to the left subclavian artery. Distally, a sufficient landing zone of 2 cm prior to the celiac artery is required. The aortic inner neck diameters in the proximal and distal landing zones must fall within 23-37 mm. In addition, appropriately sized femoral and iliac arteries (typically >8 mm in diameter) that lack tortuosity and calcium are required for implantation.

TAAAs, accounting for approximately 10% of thoracic aneurysms, may be repaired with the use of a partial bypass of the left atrium to the femoral artery.

Crawford type I TAAAs involve Dacron graft replacement of the aorta from the left subclavian artery to the visceral and renal arteries as a beveled distal anastomosis, using sequential cross-clamping of the aorta. Crawford type II TAAA repair requires a Dacron graft from the left subclavian to the aortic bifurcation with reattachment of the intercostal arteries, visceral arteries, and renal arteries. Crawford type III or IV TAAA repairs, which begin lower along the thoracic aorta or upper abdominal aorta, may use either the partial bypass of the left atrial artery to the femoral artery or a modified atriovisceral and/or renal bypass.

Prevention of paraplegia is one of the principal concerns in the repair of descending TAAs and TAAAs.

Previous investigational trials by Chuter at the University of California at San Francisco Medical Center and Greenberg at the Cleveland Clinic treated TAAAs with custom-built fenestrated and branched stent grafts. Such devices required precise anatomic tailoring of the grafts to the specific patient's anatomy for placement of the scallops (for visceral flow) or branches (for direct stenting into the visceral vessels) and resulted in prolonged operative delays.

Subsequent data and improvement in devices demonstrated that standardized multibranched endografts were applicable to approximately 90% of the patient population, thereby eliminating manufacturing delays and expanding the applicability of these devices in TAAAs. ^[58]

Preparation for surgery

Ascending aortic aneurysm

Preoperative assessment of coronary artery disease (CAD) is essential to determine the need for concomitant coronary artery bypass grafting (CABG). TEE is crucial preoperatively to examine the need for aortic valve replacement. Patients with aortic stenosis or aortic insufficiency in whom the valve leaflets are anatomically abnormal require replacement, whereas patients with aortic insufficiency and normal aortic valve leaflets may be candidates for valve-sparing procedures. TEE is valuable for accurate delineation of the aortic root at the sinuses of Valsalva and sinotubular junction.

Aortic arch aneurysm

The major morbidities from aortic arch aneurysm repair are neurologic, cardiac, and pulmonary in nature. All patients require preoperative assessment of cardiac function and evaluation for CAD. In the operating room, TEE is used to monitor ventricular function and to assess for atherosclerosis of the aorta.

A major concern in arch surgery is neurologic injury, both transient neurologic dysfunction and permanent neurologic injury. Patients with a higher risk of stroke undergo preoperative noninvasive carotid US, and those with a history of stroke undergo computed tomography (CT) of the brain. In the operating room, steroids are often given at the onset of the procedure if hypothermic circulatory arrest is anticipated. Evidence suggests that steroids given preoperatively several hours before the operation may have benefit. Some institutions monitor electroencephalographic (EEG) silence to assess for adequate duration and temperature of cerebral cooling for hypothermic circulatory arrest.

Descending thoracic aneurysms and thoracoabdominal aneurysms

A devastating complication of descending TAA and TAAA repair is spinal cord injury with paraparesis or paraplegia. Preoperatively, some groups perform spinal arteriograms to attempt to localize the artery of Adamkiewicz for reimplantation during surgery. Neurologic monitoring with somatosensory evoked potentials or motor evoked potentials is used by some to assess spinal cord ischemia and identify critical segmental arteries for spinal cord perfusion. Finally, preoperative placement of catheters for cerebrospinal fluid (CSF) drainage is performed to increase spinal cord perfusion pressure during aortic cross-clamping.

Spinal cord injury is less prevalent with endovascular stent grafting than with open repair but exists with both types of surgical treatment. ^{[31, 48, 49, 50]}  For endovascular stent grafting, CSF drainage and avoidance of hypotension are the primary mechanisms used to prevent paraplegia. The use of CSF drainage is selective among most centers. For some discrete aneurysms, stent graft coverage may allow preservation of spinal arteries. Others require coverage of the entire descending thoracic aorta. Indications for use of CSF drains include the following:

• Anticipated endograft coverage of T9-T12
• Coverage of the long segment of the thoracic aorta
• Compromised collateral pathways from prior infrarenal abdominal aortic aneurysm (AAA) repair
• Symptomatic spinal ischemia

Although not recommend as primary therapy, a report of an induced endoleak to allow spinal cord perfusion for persistent cord ischemia following endovascular repair, despite CSF drainage, proved successful and may represent a "bailout" technique to be considered in exceptional circumstances. ^[59]

Spinal cord ischemia is an uncommon complication following thoracic endovascular aortic repair, but its development can be identified by a preoperative renal insufficiency. Blood pressure augmentation alone, or in combination with CSF drainage, serves as an effective early detection process for most patients, the majority of whom enjoy a complete and long-term neurologic recovery. ^[60]

Brain protection

Methods used for brain protection during DHCA include intraoperative EEG monitoring, evoked somatosensory potential monitoring, hypothermia (to temperatures < 20^o C), packing the patient's head in ice, Trendelenburg positioning (ie, head down), mannitol, carbon dioxide flooding, thiopental, steroids, and antegrade and retrograde cerebral perfusion.

General monitoring and anesthesia

Venous access is obtained with two large-bore peripheral IVs and a central line. Filling pressures and cardiac output monitoring are performed with a pulmonary artery catheter. Continuous blood pressure monitoring is performed with a radial arterial line. Nasopharyngeal and bladder probes monitor systemic temperature. Intraoperative TEE is used to assess myocardial and valvular function.

Procedures

Ascending aortic replacement

CPB is established, and the aorta is cross-clamped just below the innominate artery. The heart is arrested with cardioplegia. The aorta is transected at the sinotubular junction and sized for the appropriate Dacron tube graft. The tube graft is sutured to the proximal aorta with running 4-0 polypropylene with a strip of felt. The tube graft is measured to length distally and sutured to the distal aorta with running 4-0 polypropylene and a strip of felt.

Valve-sparing aortic-root replacement

Once the aorta is transected at the sinotubular junction, the valve is inspected for normal anatomy. If sparing is feasible, the appropriate size tube graft is chosen to allow coaptation of the aortic valve leaflets without aortic insufficiency. In the remodeling technique, the tube graft is tailored to form aortic sinuses. The sinuses of Valsalva of the native aorta are removed, and the coronary ostia are mobilized. The neosinuses of the tube graft are sutured to the scalloped aortic valve with running 4-0 polypropylene and a strip of felt.

In the reimplantation technique, Tycron sutures are placed along the subannular horizontal plane and passed through the tube graft. The scalloped aortic valve is placed within the tube graft, and the proximal suture line is secured. The scalloped aortic valve is positioned in the graft to achieve valve competence, and the subcoronary suture line along the scalloped valve is performed with running 4-0 polypropylene. The valve is examined for competence within the graft. The coronary ostia are reimplanted in the graft. The graft is measured to length distally and sutured to the distal aorta.

In the reimplantation technique, Tycron sutures are placed along the subannular horizontal plane and passed through the tube graft. The scalloped aortic valve is placed within the tube graft, and the proximal suture line is secured. The scalloped aortic valve is positioned in the graft to achieve valve competence, and the subcoronary suture line along the scalloped valve is performed with running 4-0 polypropylene. The valve is examined for competence within the graft. The coronary ostia are reimplanted in the graft. The graft is measured to length distally and sutured to the distal aorta.

Aortic-root replacement

The aorta is transected, and the aortic valve is removed. The annulus is sized, and the appropriate valved conduit, stentless root, mechanical composite, or homograft is brought to the field. The coronary ostia are mobilized. Annular sutures are placed and are passed through the valve conduit. The proximal suture is thus secured. The coronary ostia are reimplanted. The distal suture line is performed for the mechanical valve composite, but an additional Dacron graft extension may be required for the stentless roots or homografts, depending on their length.

In the modified Bentall procedure ("buttons"), the right and left coronary arteries are dissected as a button, which is then reimplanted into the Dacron composite graft as an aortic button.

The Cabrol procedure, though rarely performed, may be used when the aortic tear or dissection extends into the coronary ostia. It may also be used when adequate mobilization of the coronary ostia is not possible (eg, from scarring in a reoperation), or when the coronary ostia are too low. The coronary buttons are dissected and anastomosed to a separate 6- or 8-mm Dacron interposition graft; this graft is then anastomosed into the Dacron composite graft.

This technique results in a tension-free anastomosis of the coronary buttons and also allows easier access for hemostasis. However, it is subject to twisting and kinking with resultant myocardial ischemia and thus is not as reproducible as the modified Bentall.

Open distal anastomosis

Deep hypothermic circulatory arrest with or without antegrade or retrograde cerebral perfusion is used. When cooled to 18°C (64.4°F), the pump is turned off and the arterial line is clamped. The patient is placed in the Trendelenburg position, and the aortic cross-clamp is removed. The distal anastomosis is performed open with running 4-0 polypropylene and a strip of felt. The distal anastomosis may be at the level of the innominate artery or, in the case of hemiarch replacement, along the undersurface of the arch to the level of the left subclavian artery.

Once the anastomosis has been completed, the pump is restarted with blood flow antegrade into the new graft and open proximal tube graft to flush out air and debris. The graft is then clamped; the proximal aortic reconstruction is performed during rewarming.

Hypothermia decreases oxygen consumption. For each drop in temperature by 1º C, the oxygen consumption by the tissues is reduced by 10%.

Air (ie, nitrogen) is poorly soluble in blood. The risk of air embolism is reduced by flooding the surgical field with carbon dioxide. Carbon dioxide is denser than air and displaces air. It is rapidly soluble in blood and causes less risk of embolization. Any carbon dioxide absorbed in the blood is removed by increasing the sweep speed of cardiopulmonary bypass.

Aortic arch aneurysm repairs

Cannulation for arch repairs varies among groups. They include the femoral artery, right axillary artery, and ascending aorta. Hypothermic circulatory arrest is required for arch repairs; the safe period of arrest to avoid neurologic injury is 30-45 minutes at 18°C (64.4°F), but some advocate a shorter period of 25 minutes. Antegrade cerebral perfusion to minimize neurologic injury is thus advocated. Others advocate cooling to 11-14°C (51.8-57.2°F).

Once the patient is cooled to the desired temperature, the circuit is turned off. For retrograde cerebral perfusion, flow is established through the superior vena cava as the arch reconstruction is performed. For antegrade cerebral perfusion, flow is continued through the axillary artery with the innominate artery clamped or individual perfusion catheters are placed into the innominate artery, left carotid artery, and left subclavian arteries.

The arch reconstructions are also varied. They basically involve performing the distal anastomosis to the aorta beyond the left subclavian artery as an open distal procedure with or without an elephant trunk. The three head vessels may be reanastomosed individually or as an island. They may be reimplanted directly to the graft or anastomosed to a separate graft, which is then attached to arch graft.

Descriptions of different hybrid procedures have been standardized according to the location of the most proximal placement of the endograft in relation to the arch vessels, under the Criado classification, as follows ^[61] :

• Zone 0 extends distally from the ascending aorta to the innominate artery
• Zone 1 extends from distal to the innominate artery origin to the left common carotid artery (CCA)
• Zone 2 extends from distal to the left CCA to the left subclavian artery (LSA)
• Zone 3 extends from distal to the LSA to the proximal descending thoracic aorta

Zone 0 pathology by definition involves all aortic arch vessels and requires revascularization of at least the innominate artery and left CCA and possibly revascularization of the LSA in the case of symptoms of left-arm ischemia, functional left internal mammary artery (LIMA) bypass graft, or dominant left vertebral artery circulation. Revascularization is usually accomplished via a median sternotomy and the use of a bifurcated or trifurcated graft from the ascending aorta to the arch vessels. Following revascularization and during the concomitant operation, a stent-graft is then implanted either in an antegrade or retrograde fashion.

Zone 1 placement, commonly avoids a median sternotomy, via revascularization of the left CCA by a right CCA–to–left CCA bypass, prior to endograft placement. Depending on the quality of angiographic resources in the operating room, this procedure may be performed in a single or staged procedure to allow use of a dedicated angiographic suite.

A zone 2 landing requires partial or complete coverage of the LSA. In general, this is well tolerated, however, several reports have detailed higher incidences of neurologic complications with LSA coverage and, therefore, a thorough assessment of the carotid, vertebral and circle of Willis circulations should be preoperatively performed. ^[62]

Descending thoracic aneurysm and thoracoabdominal aneurysm repairs

Measures to reduce spinal cord injury include CSF drainage, reimplantation of intercostal arteries, partial bypass, and mild hypothermia. A left thoracotomy or a thoracoabdominal incision is performed. The aorta is cross-clamped either just beyond the left subclavian or between the left carotid and left subclavian for Crawford types I and II. The cross clamp is placed more distally for Crawford types III and IV.

Atrial femoral bypass is established with a Bio-Medicus circuit, and the patient is cooled to 32-34°C (89.6-93.2°F). Distal cross-clamping is performed at T4-T7 to allow continued spinal cord, visceral, and renal perfusion. The proximal anastomosis is performed with running 4-0 polypropylene and a strip of felt. When complete, the proximal clamp is released and reapplied more distally on the tube graft. The distal cross-clamp is moved sequentially down, if feasible, to allow visceral and renal perfusion. The intercostal arteries may be reimplanted, if desired, or oversewn.

If sequential cross-clamping is not feasible, direct catheters may be placed in the visceral and renal vessels to allow continuous perfusion.

If the distal aneurysm extends to the renals, then the distal anastomosis may be beveled to incorporate the visceral and renal vessels and distal aorta. If the distal aneurysm extends to the bifurcation, the visceral and renal vessels are reattached to the tube graft. The left renal artery typically requires a separate anastomosis, but the celiac, superior mesenteric, and right renal arteries are often incorporated as a single island. The patient is rewarmed, and the partial bypass is discontinued as the tube graft perfuses the intercostals and abdominal vessels. The distal anastomosis at the bifurcation is performed as an open distal procedure.

For appropriate descending TAAs, endovascular stent grafting is a good alternative. Depending on the size of the patient's femoral or iliac arteries and size of the stent graft required, femoral or iliac artery exposure is performed under general or local anesthesia plus sedation. A sheath is placed and a wire guided under fluoroscopy into the arch. When in proper position, the floppy wire is exchanged with a soft catheter and rewired to a stiffer wire for device placement. The sheath is exchanged for the appropriate device sheath. The contralateral groin is used for angiocatheter placement.

After angiography and determination of stent placement, the device is loaded and, under fluoroscopic guidance, is positioned and deployed. More than one stent may be used, with as much overlap as is feasible, for stability. The proximal and distal landing zones are ballooned to seal the endograft to the aorta. The overlapping stent-graft segments are also ballooned. Angiography is performed to check for endoleaks. Endoleaks may require additional stents.

TAAAs may involve arteries supplying the abdominal viscera. In this case, for a completely endovascular repair, aortic stent grafts with fenestrations or branches oriented towards the intended covered arteries have been devised. These grafts have commonly been individualized to the specific anatomy of the patient, though recent data have demonstrated that noncustomized branch grafts may work for most patients. ^[58]

Initial placement of the aortic stent graft ensues, carefully aligning the fenestrations or branches to the abdominal viscera. The abdominal visceral arteries are then cannulated with separate guide wires in a retrograde fashion for cranially oriented arteries, or through the brachial artery for in an antegrade fashion for caudally oriented arteries. A bridging covered stent is then deployed to create a visceral seal zone.

Ross procedure (pulmonary autograft)

The aortic root and proximal ascending aorta are replaced with a pulmonary autograft. ^[56]  The pulmonary valve is then replaced with a pulmonary homograft. Most commonly performed in children with congenital disease, the Ross operation may be used for active young adults with aneurysmal disease (excluding those with connective-tissue disorders), women of childbearing age who desire pregnancy, or patients with contraindications to anticoagulation.

Patients who have undergone ascending aneurysm repairs are observed for signs of coronary ischemia, particularly if the coronary ostia were reimplanted, and for signs of aortic insufficiency when the aortic valve is repaired. After repair of arch aneurysms, particular attention must be given to neurologic status, and patients who have had the elephant trunk repair must be observed for signs of paraplegia because the telescoped sleeve in the descending aorta may obstruct critical spinal vessels.

Paraplegia is the main concern in patients who have had repair of the descending and thoracoabdominal aorta. CSF drainage may be continued for up to 72 hours postoperatively if necessary, along with motor evoked potential monitoring. Paraplegia and paraparesis may be acute or delayed postoperatively. If they are delayed, increased mean arterial pressure with pressors and reinstitution of CSF drainage may augment spinal cord perfusion to reverse this complication. Paraplegia due to occlusion of critical spinal arteries that were not reimplanted cannot be reversed by these maneuvers.

Acute postoperative renal dysfunction may be due to extended periods of ischemic cross-clamping or to hypothermic circulatory arrest.

Patients undergoing endovascular stenting are often extubated early postoperatively with a decreased length of stay in the intensive care unit (ICU).

Early morbidity and mortality are related to bleeding, neurologic injury (eg, stroke), cardiac failure, and pulmonary failure (eg, acute respiratory distress syndrome [ARDS]). Risk factors include emergency operation, older age, dissection, congestive heart failure (CHF), prolonged CPB time, arch replacement, previous cardiac surgery, need for concomitant coronary revascularization, and reoperation for bleeding. Late mortality is usually related to cardiac disease or distal aortic disease.

Bleeding is a potential complication for all aneurysm repairs. It is minimized by the use of antifibrinolytics, felt strips, and factors, including fresh frozen plasma (FFP) and platelets. For patients who undergo hypothermic circulatory arrest, the use of aprotinin is controversial, but most groups routinely use aminocaproic acid. Coagulopathy and bleeding in severe cases may warrant the use of recombinant factor VII.

Aprotinin, an antifibrinolytic agent used to reduce operative blood loss in patients undergoing open heart surgery, is now only available via a limited-access protocol. Fergusson et al reported an increased risk for death compared with tranexamic acid or aminocaproic acid in high-risk cardiac surgery. ^[47]

Stroke is a major cause of morbidity and mortality and typically results from embolization of atherosclerotic debris or clot. TEE and epiaortic US may be beneficial in localizing appropriate areas to clamp. Patients undergoing arch repairs are at the highest risk of permanent and transient neurologic injury. Retrograde cerebral perfusion is beneficial for flushing out embolic debris, but it may be detrimental, with increased intracranial pressure and cerebral edema. Antegrade cerebral perfusion is beneficial for reducing neurologic injury during hypothermic circulatory arrest. Stroke incidence for open surgical repair versus endovascular repair of descending TAAs is equivalent.

Myocardial infarction may occur with technical problems with coronary ostia implantation during root replacement for ascending aortic aneurysms and may require reoperation. Pulmonary dysfunction and renal dysfunction are other potentially morbid complications.

Paraparesis and paraplegia, either acute or delayed, are the most devastating complications of descending TAA and TAAA repairs. Despite CSF drainage, reimplantation of intercostal arteries, evoked potential monitoring, mild hypothermia, and atrial femoral bypass, spinal cord injury still occurs. Endovascular stent grafting has not eliminated spinal cord paraplegia; the incidence varies widely, with an overall incidence of 2.7%. ^{[31, 48, 49, 50]}

Complications specific to endovascular stenting include endoleaks, stent fractures, stent graft migration or thrombosis, iliac artery rupture, retrograde dissection, microembolization, aortoesophageal fistula, and complications at the site of delivery (eg, groin infection, lymphocele, seroma).

Development of another aneurysm postoperatively is not uncommon in these patients. For this reason, serial evaluations (ie, CT or magnetic resonance imaging [MRI] for ascending, arch, or descending aneurysms; echocardiography for ascending aneurysms) may be performed every 3-6 months during the first postoperative year and every 6 months thereafter.

In a study that evaluated the differences between male and female patients undergoing thoracic endovascular aneurysm repair in an FDA-approved trial, female patients had higher rates of periprocedural complications, required more blood transfusions, had longer hospital stays, and experienced more major adverse events after 30 days. ^[63] However, female patients also more often had successful aneurysm treatment at 1-year follow-up.