Aortic valve apparatus.
The aortic root has a complex anatomy due to a combination of several anatomical structures based on simple and consistent work in it. It is a hollow cylinder with three bulges, which have the main functional effect on the aortic valve opening-closing cycle and coronary circulation. Aneurysm is defined as a dilation of a blood vessel segment having ≥50% increase in diameter, whereas annuloaortic ectasia represents a diffuse dilation <50% of the normal diameter of the related vessel segment. Aortic root aneurysms mostly occur by degenerative processes as compared with primarily atherosclerotic changes in the descending and abdominal aortas: medial fragmentation, smooth muscle cells necrosis, and elastic fiber fragmentations with cystic spaces in the media filled with mucoid material. Because of the elevated mortality risk associated with complications, an effective aortic root aneurysm management depends on reduction the risk of death, rupture, and dissection. Conventional open heart surgery is the essential procedure for isolated aortic root replacement and a type of procedure (valve replacement or sparing) could be selected due to the pathology. An extensive aortic root replacement technique is the only option to rebuild the left ventricular outflow tract due to the reconstruction of the neo aortoventricular continuity in the aortic root abscess.
- aortic root
- root aneurysm
- annuloaortic ectasia
- aortic root replacement
- aortic valve sparing
- extensive root replacement
The aortic root is the first anatomical part of the aorta and also a functional bridge between the left ventricular outflow tract and ascending aorta, which lies between the ventriculoaortic junction (VAJ) and sinotubular junction (STJ). The aortic root has a complex anatomy due to a combination of several anatomical structures based on simple and consistent work in it. This complex combination provides several unique functional futures. On the other hand, several pathological processes disrupting this anatomo-physiological structure lead to functional, morphological, and hemodynamic disturbances. Detailed understanding of the complex anatomy of the aortic root leads to the development of many sophisticated, but functional and artistic surgical techniques. This chapter focuses only on the abnormal enlargement of the aortic root caused by different pathologies, complicated or not, and surgical treatment options.
Generally, two terms are used to describe the aortic root enlargement: aneurysm and annuloaortic ectasia. Aneurysm is defined as a dilation of a blood vessel segment having ≥50% increase in diameter compared with the expected normal diameter. True aneurysm involves all three layers of the arterial wall, but a pseudoaneurysm does not involve any anatomical layer of the aortic wall and is surrounded by thrombosis and/or surrounding tissues. Annuloaortic ectasia represents a diffuse dilation <50% of the normal diameter of the related vessel segment. The aortic root does not have a tubular or cylindrical shape with the same diameter at all levels; on the contrary, it has variable diameters at different levels. Normal dimensions of the aortic root are different between genders and they also varies depending on age and body surface area (BSA): annulus diameter is approximately 26 ± 3 and 23 ± 2 mm, sinus Valsalva diameter is 34 ± 3 and 30 ± 3 mm, and STJ diameter 29 ± 3 and 27 ± 4 mm in male and female population, respectively . Calculated normal aortic diameter indexes (diameter/BSA) can be useful during surgical treatment to choice a patient-appropriate tubular graft, especially to prevent any patient-prosthesis mismatch: annular diameter index is approximately 13 ± 1 mm/m2, sinus Valsalva diameter index 19 ± 1 mm/m2, and STJ diameter index 15 ± 1 mm/m2 .
3. Functional anatomy
The aortic root has a truncated corn shape with the semilunar attachments of the leaflets, sinuses, interleaflet triangles, and commissures (Figure 1). The top of the aortic root is created by the distal circumferential plane joining the crests of three commissures and named as the STJ. The bulged mid-portion of the aortic root has three sinuses of Valsalva and it is like a three-leaflet clover in two-dimensional view (Figure 2): left coronary sinus (LCS), right coronary sinus (RCS), and noncoronary sinus (NCS). The base of the aortic root is shaped as a zone between the left ventricle and aorta because there is not a true single-circular annular attachment of three aortic leaflets. This anatomic VAJ is a circular zone between lower and upper rings:
The aortic root is a hollow cylinder with three bulges, which have the main functional effect on the aortic valve opening-closing cycle and coronary circulation. In fact, the aortic root has two-sided asymmetrical structure .
The aortic root changes its overall configuration from a cone to a cylinder and from a cylinder to an inverted cone according to left ventricular filling and contraction . Because the aortic leaflets attach in the shape of a three-pointed crown spanning the entire vertical extent of the aortic root from the VAJ to the STJ, it is more meaningful to discuss “the aortic valve apparatus” (Table 1). This functional apparatus organizes ventricular and arterial hemodynamics, and it works with an elegant mechanism. Every part of this apparatus has own function during the cardiac cycle. The aortic valve passively opens and closes in response to pressure differences between the left ventricle and aorta. The expansion of the VAJ at the preejectional phase before aortic valve opening helps to decrease the coaptation area among the leaflets and to separate the leaflets under minimal stresses (20% of total opening), and the aortic valve opening is completed rapidly by the ejection phase. The STJ expands approximately 12% during systole and the aortic root gets more cylindrical shape in the longitudinal axis, where inflow and outflow effective orifice areas of the aortic root become similar, and this configuration allows a zero resistance to flow throughout the aortic root during every ejection. Sinuses of Valsalva have the main effect on the coronary blood circulation, whereas they permit aortic valve opening during systole without blocking of the coronary artery ostia and simplify aortic valve closing at the end of systole without any tension.
|2.||Sinotubular ring (junction)|
|3.||Sinuses of Valsalva|
|7.||Ventriculoaortic ring (junction)|
The aortic root with this complex structure maintains the functional capacity of the aortic valve and the conversion of intermittent and irregular high blood volume from the left ventricle to regular and laminar blood flow even in variable hemodynamic conditions. Both the STJ and VAJ provide aortic valve coaptation, but the outflow part of the aortic root is the main structure that promotes aortic valve coaptation, whereas it hangs all three commissures during the diastole. Any significant dilatation of these rings can cause aortic regurgitation, whereas enlargement of the STJ more than 110% of the VAJ diameter results severe aortic insufficiency.
4. Histology and pathohistology
The aortic leaflets are covered by a continuous layer of endothelial cells with a smooth surface on the ventricular side and numerous ridges on the arterial side. The aortic leaflets are attached to the sinus wall via a very dense collagenous meshwork hinged to the annulus, so they transmit the stress on the leaflets to the aortic wall during diastole. The right and posterior interleaflet triangles consist of a thinned fibrous wall of the aorta, whereas the left interleaflet triangle is supported by muscular tissue and only fibrous at its apex. Interleaflet triangles contain primary collagen fibers; in contrast, sinuses of Valsalva contain concentric elastic lamella.
The aortic wall has three layers: tunica intima, tunica media, and tunica externa (adventitia), which are separated from each other with a thin membrane: membrana elastica interna (between intima and media) and membrana elastica externa (between media and adventitia). The intima is composed of single layers of endothelial cells arranged in the direction of the vessel. The subendothelial connective tissue is arranged in the same manner as the endothelial cells. The media is composed of circular arranged structures: smooth muscle cells, elastic fibers, collagen, and proteoglycans. The adventitia is arranged in a longitudinal fashion and composed of collagen fibers of type I. Although the wall of the sinuses is principally arranged in this manner, the thickness of its wall is significantly thinner compared with the ascending aorta and STJ .
The tunica media of the aortic root comprises concentric layers around the lumen, which consist two elastic lamellae and intervening tissue between them. The elastic lamellae include collagen fibers (type I-III-V) and fibronectin, and they are interconnected by a network of elastic and collagen fibers, and proteoglycans. Smooth muscle cells are also in contact with fibrillin-1 and type VI collagen containing bundles of microfibrils (oxytalan fibers), and they have a basal lamina-like layer connecting them to each other and oxytalan fibers . There are many types of proteoglycans in vascular walls such as collagen-associated dermatan sulfate proteoglycan, cell-associated heparin sulfate proteoglycan, and interstitial chondroitin sulfate proteoglycan. The constituents of the arterial wall are responsible for mechanical properties, and the ability of the arterial vessel is essential to prevent or limit any permanent deformation. Elastic fibers providing the elasticity and compliance of the arterial wall comprise elastin and microfibrils such as fibulins, fibrillins, and microfibril-associated glycoproteins.
Aortic root aneurysms mostly occur by degenerative processes as compared with primarily atherosclerotic changes in the descending and abdominal aortas. The elastin content of the aorta decreases distally and in the abdominal aorta it is less than half of that in the ascending aorta. Degenerative processes mean medial fragmentation, smooth muscle cells necrosis, and elastic fiber fragmentations with cystic spaces in the media filled with mucoid material. There is a disorganization and breakdown of the elastin network and its interconnections with the collagen network and other components of the aortic wall. The collagen structure also alters specifically and significantly: collagens type I and III decrease, while collagens alpha-1 (XI) and V increase . Smooth muscle cell impairment and increased amounts of the vacuolated basophilic material are prominent.
Matrix metalloproteinases play an important role in connective tissue homeostasis, and in fragmentation of extracellular matrix elements via digestion of elastin and collagen fibers [10, 11]. Disharmony of matrix metalloproteinases activity causes connective tissue impairment. Blockade of plasmin formation by overexpression of plasminogen activator inhibitor-1 prevents the formation of aneurysms by inhibiting metalloproteinase activation. And also, local overexpression of the tissue inhibitor of matrix metalloproteinases can prevent aneurysmal degeneration and rupture. Inflammation characterized with leukocyte and lymphocyte infiltration is greater in the aneurysmal aortic wall compared to nonaneurysmal aortas, especially in some inflammatory disorders. Many inflammatory mediators (interferon-γ, interleukin-1 β, TNF-α, IL-6, TGF-β) increase in degenerative aortic aneurysms and their disharmony stimulates aneurysm formation [12, 13]. Genetic abnormalities can also cause this disharmony that results abnormal functions and structures in aortic wall. These disorganizations can cause early degeneration of the vascular wall and aneurysmal formation. Some genetic defects or mutations causing specific diseases are well described:
Aortic root aneurysm occurs due to decreased connective tissue strength or elevated pressure in the aorta. High content of an elastic fiber in the medial layer of the aortic root provides expansion during systole acting as a reservoir and stores kinetic energy resulting by left ventricular contraction as potential energy in the aortic wall. In diastole, elastic recoil returns the aortic root to its original form back and transforms the stored potential energy back to kinetic energy to start aortic wave motion for forward blood flow. Several pathological changes lead to degenerative processes in the elastic media and decreasing the strength of the aortic wall connective tissue.
The coupling between mechanical stress and the biochemical changes leading an aneurysmal formation is not well elucidated. Several adversely changed mechanisms such as decreased aortic wall compliance, broken cross-sectional symmetry, and disrupted stress-strain relationships increase intraluminal systolic impulse stress and aortic wall tension, which active predate dilatation formation (tension = pressure × radius). Because the inner curvature of the aortic root is adherent to the pulmonary artery, the NCS is the most affected part of the root and the typical enlargement occurs at the STJ level, especially in aortic root dilatation associated with ascending aortic aneurysm. Synchronously involvement of three sinuses causes a symmetrical aneurysmal dilatation (resembling a pear).
There are different specific etiologies causing aortic root aneurysm (Table 2). Most of them have genetic origin, but the most common etiology is
Although clinical symptoms (retrosternal pain, hoarseness, breathless, etc.), examination (aortic valve murmur), and several laboratory testing play a minor role in the diagnosis of the aortic root aneurysms, noninvasive imaging techniques have a major role yielding a view of the total intrathoracic aorta . In the daily practice, transthoracic echocardiography associated with transoesophageal echocardiography is the most frequently used technique for the diagnosis, which should be completed with thoracic computed tomography and/or magnetic resonance imaging, as well as ultrasonography and/or computed tomography for the abdominal aorta. It is recommended to measure diameters at anatomical landmarks perpendicular to the longitudinal axis (antegrade flow in the aortic root and ascending aorta) and to use the same imaging modality with a similar method of measurement in the case of repetitive imaging of the aorta. The aortic annulus should be measured at midsystole from the inner-edge-to-inner edge and all other aortic root measurements (i.e., maximal diameter of the sinuses of Valsalva, the sinotubular junction, and the proximal ascending aorta) should be made at end-diastole. Measurements of maximal diameters of the aortic root should be compared with age- and BSA-related nomograms or to values calculated from specific allometric equations (Table 3).
|Aortic root||Absolute values (cm)||Indexed values (cm/m2)|
|Annulus||2.6 ± 0.3||2.3 ± 0.2||1.3 ± 0.1||1.3 ± 0.1|
|Sinuses of Valsalva||3.4 ± 0.3||3.0 ± 0.3||1.7 ± 0.2||1.8 ± 0.2|
|Sinotubular junction||2.9 ± 0.3||2.6 ± 0.3||1.5 ± 0.2||1.5 ± 0.2|
|Proximal ascending aorta||3.0 ± 0.4||2.7 ± 0.4||1.5 ± 0.2||1.6 ± 0.3|
Aneurysm surveillance in asymptomatic patients includes ongoing clinical evaluation, the development of symptoms, signs of aneurysm complications, and serial imaging to evaluate the diameter and structure of the aneurysm. Ideally, the serial studies should be performed using the same technique (echocardiography, computed tomographic angiography or magnetic resonance angiography) in the same center for consistency with future comparisons. Aortic valve function and morphology should be evaluated during follow-up; therefore, echocardiographic assessment should be considered in aortic root aneurysm. The surveillance program may be modified based upon the etiology, site, and diameter of the aneurysm at presentation. In general, imaging at 6 months after the initial diagnosis could ensure the stability of the aneurysm and expansion rate. The thoracic aorta expands slowly with the age at a rate of 0.7 mm in women and 0.9 mm in men per decade of life. The expansion rate of the aortic aneurysms is much higher than these values, and a larger diameter of aortic root aneurysms could expands more rapidly than smaller ones like other segments of the aorta: the rate of expansion for large aneurysms (>5 cm) was about 8 mm per year while 2 mm per year for smaller aneurysms (<5 cm).
Once the aortic root aneurysm detected patient should be examined for concomitant diseases, genetically mediated disorders and risk factors. Major life threatening complications are dissection, rupture, and aortic valve regurgitation. Because of the elevated mortality risk associated with complications, an effective aortic root aneurysm management depends on reduction the risk of death, rupture, and dissection. The most important determinant is the diameter of the aneurysm. Other factors, such as a rapidly expanding aortic diameter, concomitant bicuspid aortic valve, or connective tissue disease also increase the risk of rupture.
Medical treatment depends on reducing aortic wall stress and slow down medial degeneration. Lifestyle modifications contain smoking cessation, avoiding intensive exercise, and patient education. Aggressive antihypertensive therapy is the mainstay of pharmacologic management to prevent increased wall stress. If tolerated, a goal of the therapy is maintaining a systolic pressure below 120 mmHg. Beta-blockers reduce the contractility of the heart, decrease shear stress, and the impact force of ejected blood on the aorta. Although these beneficial effects, it should be noted that the β-blocker therapy will block the compensatory tachycardia and could precipitate clinical deterioration in aortic regurgitation. Reduction in heart rate is also associated with an even higher stroke volume, which contributes to the elevated systolic pressure in patients with chronic severe aortic regurgitation. On the other hand, patients treated with β-blockers have a significantly lower growth rate (1.2 mm/year) than those without β-blocker treatment (4.2 mm/year), which shows the beneficial effect and the importance of β-blocker medical therapy on aneurysm stabilization .
Surgical indication is dependent on the presence of symptoms and diameter measurements of several parts of the ascending aorta. In general, surgical repair of asymptomatic aortic root aneurysm is not recommended until the risk of dissection, rupture or other complications exceeds the risks associated with surgery (Table 4). In general, asymptomatic aortic root aneurysms without dissection must be directed to surgical treatment depending upon an aortic diameter (≥5 cm) and speedy expansion rate (≥5 mm/year); however, in the presence of underlying special contributing etiologies these limits should be decreased (≥4.5 cm or ≥3 mm/year or the presence of acute or chronic aortic dissection). Aneurysms can cause symptoms especially in larger diameters. Symptoms could be pioneers of fatal complications, therefore surgery should be considered for symptomatic patients either at smaller diameters of aneurysm.
Aortic valve replacement (AVR) with supracoronary ascending aorta replacement is the first aortic root surgery operation. The Bentall technique is the first true total aortic root replacement (ARR) procedure, which contains en bloc replacement of the ascending aorta. The modified Bentall technique eliminates wrapping of the native aortic wall over the tubular graft. The Button technique eliminates most of problems regarding to coronary ostial anastomoses via end-to-side reimplantation of the coronary ostia. A interposition graft between the coronary ostia and the composite graft to prevent coronary malposition is especially useful when mobilization of the coronary buttons is often difficult or impossible. Other modifications include leaving a small part of the tubular graft below the prosthetic valve to simplify and secure aortic annular anastomosis [20–22], To simplify proximal annular anastomosis, a prefabricated composite graft with sinus of Valsalva can be chosen .
Conventional open heart surgery is the essential procedure for isolated ARR. Urgent surgery is usually preferred for life-threating pathologies of the aortic root, and standard approaches should initially be used for emergent situations or with concomitant cardiac procedures. Some more recent approaches toward more noninvasive surgery can be considered in elective, noncomplicated-isolated aortic root surgery. General anesthesia with full median sternotomy is the standard approach for aortic root surgery, and transesophageal echocardiography is mandatory in cases of aortic valve-sparing procedures. Awake cardiac surgery offers several advantages over general anesthesia, including the absence of tracheal intubation, reduced stress response, lower postoperative arrhythmias, and improved pulmonary outcome [24–26]. This approach may be more beneficial and safer in patients with chronic obstructive pulmonary disease who are frequently rejected for cardiac surgery [27, 28]. This approach should be only used in elective cases with noncomplicated aortic root surgery, whereas general anesthesia remains the preferred option in aortic valve-sparing procedures or in emergency situations. Full median sternotomy is the versatile and most reliable option to reach all sites of the heart, but minimal invasive incisions may be preferred in order to reduce invasivity and adverse effects of full median sternotomy. The full median sternotomy can be restricted only to perform aortic root surgery in the presence of other concomitant cardiac pathologies, otherwise limited median sternotomy techniques are the best approach for isolated aortic root surgery due to shorter hospitalization periods, improved lung functions, reduced trauma effects, and early mobilization. There are two most preferred minimal invasive incision techniques for access into the mediastinum:
Conventional open heart surgery is the essential procedure for isolated ARR and a type of procedure could be selected due to the pathology (Table 5). It is always a good idea to keep in mind that the ARR surgery is a life-saving procedure and there are not many drawbacks to the conventional procedures. On the other hand, less invasive or aggressive interventions may be chosen in elective and selective cases . The first goal during ARR is to spare the competent aortic valve if possible. If not possible, the use of a composite graft to replace whole ascending aorta seems the most preferred option during ARR. The first choice of a prosthetic valve for ARR is a mechanical valve due to its simple handling, easy sizing, lower profile, long-term durability, and resistance to mechanical stress. Composite grafts with stentless bioprosthetic valves can be chosen in older patients to avoid valve-related late reoperations. But, using mechanical valve conduits could be changed to the use of bioprosthetic composite grafts in all age groups if the transcatheter methods offer similar or better results when compared to reoperative ARR procedures. Although allografts can be the best option for the total ARR, their availability is very limited in most countries.
9.1. Total aortic root replacement procedures with aortic valve replacement
9.2. Total aortic root replacement with aortic valve sparing procedures
Aortic root pathologies with normal anatomic structure of the aortic leaflets causing significant aortic regurgitation are the primary indication for sparing aortic valve with/without aortic valve repair during the total ARR. Aortic valve sparing procedures allow avoidance of anticoagulation, prosthetic material, and postoperative transvalvular gradient. There are two major techniques with own advantages and disadvantages. The remodeling technique is preferred if any annular stabilization is not necessary, while the reimplantation technique is essential if annular stabilization is inevitable. The proximal anastomosis is completely different from valvular composite graft procedures, but coronary and distal anastomoses have similar surgical technical details to the modified Bentall procedure.
9.3. Subtotal aortic root replacement procedures
These techniques are subtypes of the standard remodeling technique and popularized to avoid either a total composite or aortic valve sparing root replacement procedure. The aortic root seems more or less intact, and may distort aortic valve functions. Subtotal aortic root remodeling techniques can be preferred if all three sinuses are not involved without an aortic annular dilatation. Significant aortic annular dilatation requiring annular fixation is a contraindication for these approaches. Another indication is supravalvular stenosis, which can be congenital or acquired, and the repair option of this pathology depends on its involvement: single , double , or triple  sinus replacement. Isolated sinusal involvement usually affects NCS, especially in a bicuspid aortic valve. Aortic root enlargement is not diffuse and both coronary sinuses seem normal. The other rare pathology is restricted aortic dissection with/without involving the ascending aorta and chronic healing surrounds the dissection tear. The affected sinus can be replaced with a patch or with prosthetic tubular graft having a tongue-shape extension. If ascending aorta replacement is not necessary, a Dacron patch is tailored as a new sinus and sutured to the annulus of the resected sinus of Valsalva. The shape of the patch should be appropriate to the sinus, but the width should be <10 mm more than the diameter between two commissures and the height <10 mm more than the diameter from the annulus to the sinotubular junction. Subtotal root remodeling on the two sinuses can be chosen for acute aortic dissection without left coronary ostial involvement if aortic root dilatation is not greater than 35 mm, which can prevent the reimplantation of the left coronary ostium. On the contrary, if both coronary sinuses are involved or the presence of aortic annular dilatation requires aortic annular stabilization, any total ARR is more meaningful.
9.4. Sinus of Valsalva aneurysm repair techniques
Repair is recommended for ruptured aneurysms, significant aortic valve regurgitation, associated intracardiac abnormalities, or symptomatic unruptured or enlarging aneurysm. Rupture of sinus of Valsalva aneurysm is a life-threatening complication and requires immediate surgical or interventional closure. Surgical closure is the gold standard treatment, but percutaneous closure can be chosen in very sick patients . Different surgical approaches can be used: transaortic, double-chamber, or involved chamber. The transaortic approach is used for isolated sinus of Valsalva aneurysm with/without aortic regurgitation, especially in unruptured cases. The double-chamber approach is chosen mostly in ruptured aneurysms because of closing defect from both sides or the presence of any intracardiac pathology . The last approach is used very seldom because of possible bacterial colonization or thrombus formation inside the aneurysm, or recurrent fistula formation or rupture of aneurysmal sac. The goals of repair are removing the aneurysmal sac, closing the defect primary or with a patch or with valve replacement, and repairing any associated defects. Patch closure at the aortic end is the most preferred technique, which minimizes aortic leaflet distortion, with/without a concomitant surgery for aortic valve repair.
9.5. The extensive aortic root replacement technique
Aortic root abscess continues to challenge cardiovascular surgeons, because uncontrolled aortic root abscess can manifest itself as a burrowing pathology destroying the whole aortic annulus and extending proximally into the left ventricular outflow tract, a cardiac fistula or a rupture into a cardiac chamber, a pseudoaneurysm, or an arrhythmia leading to hemodynamic instability. Early and extensive surgical intervention of aortic root abscesses is essential, and the complexity of the surgical treatment ranges from partial resection of the aortic annulus and surrounding tissues to radical removal of the base of the heart—including the entire aortic root, the intervalvular fibrous body, and part of the interventricular septum.
The extensive ARR technique is the only option to rebuild the left ventricular outflow tract due to the reconstruction of the neo aortoventricular continuity in the isolated aortic root abscess. The flanged technique with the elongated tubular graft below the prosthetic valve is the best option for solving this life-threatening sequel and reconstructing the aortic root . A larger (3 cm) segment of the proximal end of the tubular graft is implanted in a circular manner with 2–0 interrupted sutures supported by large Teflon pledgets placed subannularly on healthy tissue at the native left ventricular outflow tract. Both ends of the sutures are passed through the proximal free end of the flanged portion of the conduit in order to use that part as a strip between knots and the myocardial aortic wall.