The first thoracic aortic replacements were performed with homograft tissue primarily in the descending thoracic aorta. Continuing efforts at developing an artificial fabric graft for use for aortic replacement, culminated with Michael DeBakey’s discovery of Dacron.¹ During the late 1950s, the Houston group led by Michael DeBakey systematically developed operations for resection and graft replacement of the ascending aorta, followed by the descending and thoracoabdominal aorta. In 1968, Bentall and DeBono published a landmark article on replacement of the entire aortic root with anastomoses of the coronary ostia to the replacement graft.² The description of this technique included side-to-side anastomoses of the coronary arteries to the graft. The aneurysm sac then was closed completely around the graft. A common complication of this operation was pseudoaneurysmal development at the level of the coronary ostia, presumably caused by the tension placed on the coronary anastomoses. A subsequent modification of this technique by Kouchoukos and coworkers³ included complete excision of the aneurysm and aortic root, leaving both coronary arteries suspended by only a small “button-shaped” circular portion of aorta that then would be anastomosed directly into a hole created in the side of the Dacron graft, thus eliminating pseudoaneurysm formation. An extended ascending aortic replacement was described by Wheat and associates, effectively excluding most of the ascending aorta but leaving the coronary arteries attached to the remaining aortic root tissue in continuity.⁴

In 1979, acting on the observation that many patients presenting for aortic root replacement have normal aortic valve morphology, Sir Magdi Yacoub and colleagues at Harefield Hospital in the United Kingdom developed a strategy for aortic root replacement in which the aortic valve was preserved.⁵ In 1988, Tirone David introduced a different technique for valve-sparing aortic root replacement.⁶

Interest in homograft aortic root replacement began in the 1950s. At that time, Norman Shumway and colleagues at Stanford University were experimenting with excision of the right ventricular outflow tract, the pulmonary valve, and the proximal portions of the pulmonary artery in continuity in dogs and translocating this to the aortic root, with subsequent reconstruction of the pulmonary artery with homograft or tube graft material.⁷ The operation in this iteration ultimately would be doomed to failure because of right ventricular failure resulting from the lack of a pulmonary valve, but Donald Ross, building on that experience, later performed the first pulmonary autograft aortic root replacements in humans.⁸

The aortic root and the ascending aorta develop as part of a common truncus arteriosus that partitions itself into the ascending aorta and the aortic root and the pulmonary artery at approximately the fifth and sixth weeks of development. The aortic root is a tripartite structure owing to the presence of coronary sinuses. The aortic valve proper has no true fibrous annulus, although surgeons use the term aortic annulus to describe the junction of the aorta and the ventricle. The noncoronary sinus of the aortic valve tends to be the largest of the three sinuses, and therefore the size of the aortic valve leaflets reflects this, with the noncoronary leaflet generally being the largest. The length of the distance from the basal attachment of each aortic valve leaflet to the aorta is approximately 1.5 times the length of the free margin of the leaflet (Fig. 33-1). The commissures of each of the aortic valves extend right to or just below the sinotubular junction that marks the anatomic ridge between the end of the aortic root and the beginning of the ascending aorta. In general, the sinotubular ridge is approximately 10 to 15 percent smaller in diameter than the aortic annulus (Fig. 33-1). Anatomic disease states that cause dilatation of the sinotubular junction or dilatation of the aortic annulus will cause insufficiency of the aortic valve. There is an aortic–mitral continuity, which is a fibrous tissue attachment between the aorta and the mitral valves that constitutes approximately 55 percent of the circumference of the aortic root. The left side of the aortic root toward the pulmonary artery is attached to the ventricular muscle, corresponding to approximately 45 percent of the circumference.

The exact extent of occurrence of thoracic aortic aneurysms (TAAs) is unknown. Bickerstaff and associates⁹ found the prevalence of TAAs to be 5.9 per 100,000 per year in the Rochester, MN, area. Among a total of 72 patients with TAAs studied by Bickerstaff and colleagues, 51 percent, or 37 of the aneurysms in those patients, involved the ascending aorta, 8 (11 percent) the aortic arch, and 27 (38 percent) the descending thoracic aorta.

According to the National Center for Health Statistics, aortic aneurysms (AAs) were the 15th most common cause of death in the United States in the year 2000; approximately 0.6 percent of all females and 1.1 percent of all males die of aortic aneurysmal disease. TAA deaths occur in about 0.7 per 100,000 population per year.¹⁰ Among patients who die of TAAs, rupture is the cause of death in about 80 percent of cases.

The estimated growth rate for TAAs has been calculated to be 1.2 to 4.2 mm/year, and enlargement accelerates as an aneurysm gets larger.¹¹ Risk factors for accelerated growth include the presence of dissection in the enlarged segment, synchronous arch or abdominal AA, smoking, no β-blocker therapy, renal failure, and diastolic hypertension. Rupture is much more likely to occur when the aneurysm exceeds 5 cm in diameter, and the risk of rupture increases as the aneurysm increases in size. Aneurysms larger than 6 cm can be associated with a yearly rate of rupture or dissection of at least 6.9 percent and a death rate of 11.8 percent.¹²

Annuloaortic ectasia is a term used to describe an increase in the diameter of the aortic annulus coupled with an increase in the diameter of the aortic root. This ectasia tends to occur along the fibrous tissue of the ventricle, with the ventricular muscular portion of the aortic root generally preserved. This type of situation is seen in patients with Marfan syndrome (characterized by defect in the fibrilin gene), Ehlers–Danlos syndrome (characterized by a defect in type 3 collagen synthesis), Loeys–Dietz syndrome (characterized by a defect in Transforming Growth Factor beta gene), osteogenesis imperfecta, and pseudoxanthoma elasticum. It may have other familial origins or may be idiopathic. In these syndromes, the aortic sinuses become thinner and dilated and the sinotubular junction increases in diameter. When this happens, the aortic valve leaflets are not allowed to coapt properly because of separation of the commissures. These patients develop central jets of aortic insufficiency on echocardiographic assessment. Degenerative diseases of the aorta may cause dilatation of the ascending aorta and aortic sinuses with minimal or no dilatation of the aortic annulus. The aortic root also may be destroyed in the setting of acute or chronic aortic dissection and other congenital syndromes, such as those associated with bicuspid aortic valves. Recently, a significant body of research has focused on the involvement of endogenous extracellular matrix degrading enzymes in the involvement of aneurysms and aortic remodeling. Of greatest interest are the matrix metalloproteinases (MMPs), particularly those of the gelatinase class (MMP-2, MMP-9).^13,14 Another important cause of aortic root destruction is acute infective endocarditis with aggressive organisms such as Staphylococcus aureus.

A central common theme in the development of aortic root aneurysms is a condition referred to as cystic medial degeneration or simply medial degeneration. Histologically, the aorta is made up of three layers. The adventitia is composed primarily of a collagen-rich network. The tunica media, or media, is composed of alternating layers of vascular smooth muscle and elastin. Each successive pairing of smooth muscle cells and elastin is referred to as a lamellar unit. In humans, the media of the aortic root is composed of approximately 50 lamellar units. The third layer of the aorta is referred to as the intima and is composed of a single layer of epithelial cells. In many cases of AA, one sees gradual disruption of the media of the aorta, with the creation of small acellular spaces within it. This process of medial degeneration results ultimately in weakening of the aortic wall and inability to sustain the normal shear stresses associated with systole. As a result, a slow remodeling of the aortic root and ascending aorta occurs, leading to aneurysm formation.

Patients present for aortic root replacement in a number of circumstances (see the decision-making flowchart; Fig. 33-2). The most acute circumstance is a patient with a Stanford type A aortic dissection in whom the aortic root is virtually destroyed by the proximal extent of the dissection. The condition of these patients constitutes a surgical emergency, and as a result, aortic root replacement in these circumstances is associated with more morbidity and a higher mortality than is standard aortic root replacement. A second acute situation occurs with infective endocarditis. The most common situation in which patients present for aortic root replacement is an aortic root aneurysm. The age range of presentation is wide and starts in the 20s and 30s in patients with hereditary connective tissue disorders and extends into the 70s and even 80s in those with degenerative aneurysms. Other than a family history of AAs and connective tissue disorders, risk factors for AAs include hypertension and atherosclerosis. Patients with isolated aortic root aneurysms are usually asymptomatic. Those conditions are identified as a result of a workup for another disease process, such as a respiratory tract infection, that results in a chest x-ray. The chest x-ray may identify an abnormality in the mediastinum, resulting in a computed tomography (CT) scan or a magnetic resonance imaging (MRI) scan that illustrates the aortic root aneurysm. Alternatively, other patients with AAs have significant aortic valve pathology, such as stenosis or insufficiency. Symptoms from these conditions result in referral to a physician. An echocardiogram is performed and shows evidence of aortic stenosis or insufficiency, and often a dilated aortic root is identified. This often leads to a further imaging study, such as a CT scan or an MRI scan, which shows the aortic root aneurysm.

The workup in patients with an aortic root aneurysm involves a careful history and physical examination, assessing for a history of bleeding problems and a history of significant dental work. In addition, a family history of AA should be elicited. The physical examination is generally unremarkable apart from the features of aortic insufficiency and aortic stenosis. Patients with Marfan syndrome have a very characteristic appearance. They tend to be tall and thin, with joint laxity, pectus excavatum, and very characteristic facial features. After physical examination, patients should have standard blood work and be crossed and typed for 4 to 6 units of red blood cells. Clotting studies should be performed. The electrocardiogram shows no abnormalities specific to the aortic root aneurysm. Chest x-ray often reveals a prominent right mediastinal border, which represents an outpouching of the aortic root and the ascending aorta into the right chest. Transthoracic echocardiography is valuable in showing features of aortic stenosis and aortic insufficiency in addition to allowing assessment of the potential for valve preservation. The most useful test for the evaluation of aortic root aneurysms is a contrast CT scan or MRI, especially scans with three-dimensional reconstructions; this allows precise assessment of the aortic root and its dimensions (Fig. 33-3). Females older than 40 years and males older than 35 years with coronary risk factors should undergo coronary angiography to rule out the presence of coronary artery disease before surgery. In addition, patients with significant neck bruits should have carotid vascular studies and those with a substantial history of smoking should have pulmonary function tests. Finally, a dental examination is critical whenever a prosthetic valve replacement is considered in a patient since bacterial seeding from dental disease is a very important cause of prosthetic valve endocarditis (PVE) and graft infections.

At the present time, there is no medical therapy for aortic root dilatation that directly addresses the underlying etiology. Some treatment strategies have focused on the use of broad-spectrum MMP inhibitors to reduce or prevent aneurysm expansion. Thompson and Baxter gave doxycycline 100 mg twice a day for 7 days to five patients before abdominal AA repair, and aneurysm biopsy showed a three-fold reduction in MMP-2 and a four-fold reduction in MMP-9 expression compared with nontreated controls in whom those two enzymes were expressed abundantly.¹⁵ After this, a multi-institutional phase II prospective randomized trial was undertaken to assess the safety and potential efficacy of long-term doxycycline administration in 36 patients. Thirty-three patients completed the study, and significant treatment-related side effects occurred in five, or 13.9 percent, of those patients. There was no significant increase in aneurysm size at 6 months, and plasma MMP-9 levels dropped so that only 21 percent of the patients in the drug arm had MMP-9 that was considered to be elevated compared with 47 percent in the control arm.¹⁶ Recent work from our institution has similarly suggested a role for ARB2 blocker Losartan, in slowing the growth of thoracic aneurysms associated with Loeys–Dietz Syndrome.¹⁷ Hence, at present, the mainstays of management of aortic root aneurysms are judicious hypertension control using a combination of β-blockers and ARB2 receptor blockers in conjunction with regular follow-up with CT scans and transthoracic echocardiography to assess for interval changes in aortic diameter and follow the characteristics of the aortic valve.

Generally, resection of an aortic root aneurysm is indicated in patients in whom the aortic root diameter exceeds 5 to 5.5 or becomes twice the size of a comparable normal aortic segment.¹¹ In addition, a patient with an aortic root aneurysm that is seen to grow more than 0.5 cm in a 6-month period also should be considered for operation. Some latitude is given for patients with connective tissue disorders in whom it is known that the aorta is inherently weak and more susceptible to rapid dilatation, especially in patients with Loeys–Dietz syndrome (ascending aortic diameter cutoff 4 cm), those with bicuspid aortic valve (ascending aortic diameter cutoff 4.5 cm), and those with Marfan (ascending aortic diameter cutoff 5 cm). The clinical situation often also arises when patients have significant aortic valve pathology in association with an AA that does not quite meet the size criteria for resection. In these circumstances, aortic root replacement is indicated, since isolated replacement of the aortic valve in patients with a large aortic root and ascending aorta can pose technical difficulties in terms of closing the aorta after the aortic valve procedure and may predispose those patients to aortic dissections. In addition, a complicated redo operation for aortic root replacement may be necessary, since the ascending aorta continues to dilate. As was stated earlier, aortic root replacement occasionally is indicated in patients with type A aortic dissections with destruction of the aortic root and also in younger patients with significant aortic valve pathology who have a normal aortic root and ascending aorta and who are undergoing homograft root replacement or a Ross procedure to benefit from the increased durability of the aortic valve associated with this operation. Finally, severe aortic valve endocarditis with abscess or PVE may be best treated with homograft aortic root replacement.

Conduct of the Operation

The patient is placed in the supine position under general anesthesia. The chest, abdomen, perineum, and lower extremities are prepped and draped in sterile fashion. Standard median sternotomy and exposure of the heart are performed. The orientation of the aortic root is visualized more easily if only the right side of the pericardium is tacked upward. This rotates the heart counterclockwise and allows the apex of the heart to sink into the left chest, thus improving the exposure of the aortic root. Arterial and venous cannulation is undertaken, and appropriate connections are made to the pump oxygenator. If the patient has no significant aortic insufficiency, an antegrade cardioplegia tack is placed, and in all cases a retrograde cardioplegia cannula also is placed. When everything is in readiness for the initiation of cardiopulmonary bypass, careful dissection is undertaken to separate as much of the aortic root from the pulmonary artery and right ventricular outflow tract as possible. Care must be taken in dissecting anteriorly to avoid injury to the right coronary artery. A careful dissection at this point can result in significant exposure of the aortic root and therefore decrease the amount of time needed to perform the aortic root replacement when the aorta is cross-clamped. After confirmation of an activated clotting time (ACT) longer than 400 s, cardiopulmonary bypass is performed and the aorta is cross-clamped. Whenever possible, the heart should be arrested with antegrade cardioplegia to promote better and faster distribution of the cardioplegia. A switch to retrograde cold blood cardioplegia may be considered especially in the presence of significant aortic regurgitation. The authors give intermittent shots of 250 mL every 20 min during the cross-clamp period. This is supplemented with a cold saline-infused cooling jacket placed around the left ventricle. A right superior pulmonary vein vent is placed to keep the heart decompressed.