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Send to a FriendIntroduction and Cardiac Anatomy
The aortic valve has a three cusp architecture. The leaflet tissue ordinarily coapts to achieve a competent valve mechanism but there may be trivial incompetence at the central coaptation point. The leaflet tissue is hinged to the aortic wall by dense fibrous connective tissue. This is called the annulus, which is a misnomer because the hinge point is actually semilunar in shape. The aortic valve is inserted on the wall of the left ventricular outflow tract mostly above the anatomic ventriculoaortic junction, but the lowest point of the semilunar point of attachment is actually on the ventricular side of the junction. The highest point of attachment is called the commissure. The triangular space below the commissure is called interleaflet triangle. The tissues in this triangular space have much less dense connective tissue and are pliable and flexible.
This back lighted specimen highlights the thin connective tissues of the interleaflet triangle, in this case the posterior triangle, located between the left and the noncoronary sinuses of Valsalva. Notice the relationship of the triangle and the anterior leaflet of the mitral valve. The posterior commissure, that one between the left and the noncoronary sinus is directly above the mid-point of the anterior leaflet of the mitral valve. While it is commonly thought that the lowest point of the noncoronary sinus is centered over the anterior leaflet, the posterior commissure is actually at the center point.
There are no chordae tendineae at the midpoint of the anterior leaflet of the mitral valve making incision of the anterior leaflet safe. The relationships of the aortic root anteriorly are important whenever removal of the pulmonary trunk is considered. Incision of the septum to the right of the septal papillary muscle (papillary muscle of the conus) could involve the conduction system while incisions to the left of the papillary muscle could involve septal blood supply. The conduction system is very close to the aortic valve below the anterior commissure in the region of the membranous septum and to the mid point of the right coronary sinus. The conduction system heads down on the ventricular septum to the right of the septal papillary muscle.
The first septal branch of the left anterior descending coronary artery comes out below the pulmonary valve and crosses the infundibular septum heading directly toward the septal papillary muscle. Injury to the first septal artery could not only result is a significant septal myocardial infarct but also could cause heart block by interfering with the blood supply to the bundle of His. The first septal artery comes onto the ventricular septum below the posterior cusp of the pulmonary valve near the left posterior commissure. It takes a course through the septum toward the septal papillary muscle.
Kunselman and associates measured the dimensions of the normal aortic root and determined the ratio of diameters at various levels to the mid-point of the sinus of Valsalva. Unfortunately, that diameter is difficult, if not impossible to obtain in vivo. The data can be converted mathematically and related to the "aortic annulus" or that diameter measured at the ventriculoaortic junction. This diameter is the one measured by surgeons to determine size of prosthetic valves. The diameter sino-tubular junction is normally about 90% of the diameter of the annulus. For reconstructive operations on the aortic root, the surgeon should remember that 0.9 times the diameter of the annulus will give a good estimate of the diameter desired for the sinotubular junction.
Mechanical Prosthetic Valves
Replacement of the aortic valve with a mechanical prosthesis when the annular diameter is small, meaning 21 mm or less, can present significant hemodynamic and technical problems. There are three mechanical prosthetic devices available in the U.S.A. in 1997 for use in replacement of the aortic valve. The Medtronic-Hall valve has a single occluder disk which tilts on retaining strut. The St. Jude Medical valve is a bileaflet device. It is hinged by a fairly complex mechanism in pyrolyte carbon. The Carbomedics valve also has a bileaflet mechanism and the sewing ring is coated with pyrolyte carbon to resist thrombus formation. The Carbomedics Top Hat device has a modified sewing ring to allow implantation above the aortic annulus in the wider portion of the sinus of Valsalva.
These three mechanical prosthesis have equivalent performance as regards thromboembolism and thrombosis of the valve. All require anticoagulation with Coumadin at similar levels of prothrombin time and thus all have equivalent rates of anticoagulant related hemorrhage. While all of the modern mechanical valve have shown excellent wear performance related to the durability of pyrolyte ceramic carbon, there is some data that the carbon parts may wear somewhat over time. Mechanical valves all create turbulent blood flow across the prosthesis simply because the occluder is in the blood flow pathway. The flow velocity patterns create a fingerprint typical for each prosthesis with high flow and low flow areas as well as eddy currents. In addition there is the ridge created by the support ring of the prosthesis which protrudes varying distance into the blood flow pathway which is a fixed physical obstruction.
The standard of practice for choosing a prosthesis for replacement of the aortic valve has been stated by Kirklin and Barratt-Boyes in their text, Cardiac Surgery, Ed. 2. Prostheses less than 23 mm in diameter may create significant obstruction. A 21 mm prosthesis is acceptable only if the patient is small (<1.5 m2) and sedentary. a 19 mm prosthesis should not be used because of prohibitive pressure gradient. replacement device mismatch is a serious iatrogenic disease. surgeons should measure their own practice for choosing a prosthetic device against this standard.
Prosthetic valves are ordinarily sized in odd millimeter diameters. Medtronic introduced even numbered small size prostheses. These devices allow a larger mechanical device to be used by simply paring down the sewing ring. The company states the 20 mm device sizes like a 19 mm prosthesis but performs like a 21 mm device. That is simply because it is a 21 mm mechanical part in a shaved down sewing ring. Not to be outdone, St. Jude Medical gave us the Hemodynamic Plus valves which retain the same size labels but utilize a 2 mm larger mechanical device inside a pared down sewing ring.
The charts comparing the actual inside and outside dimensions tell the story: A 20 mm Medtronic is actually a 21 mm device which compares either to a 21 mm standard or 19 mm HP St. Jude prosthesis. Carbomedics haven't entered this game except to state that a 2 mm larger Top Hat device can be inserted supra-annular. It is the same situation for the Medtronic 22 mm device which is actually a 23 mm prosthesis and the St. Jude Medical 21 mm HP which contains the 23 mm mechanism.
Pressure gradient over the prosthesis related to increasing flow show that the 19 mm devices are not nearly as good as the 21 mm devices. The 23 mm devices and larger are superior. Gradient is not the only factor to consider. Gradient should be considered as it relates to the resolution of left ventricular hypertrophy. A recent study by Gonzalez-Juanatey and associates from Spain measured gradients in vivo at rest using echocardiogram. Prosthetic heart valves (Carbomedics and various stented bioprostheses) showed gradients which were too high on the 19 mm devices. Left ventricular mass index was reduced most significantly in 23 and 25 mm devices, less significantly in 21 mm prostheses, and not significantly after implanting a 19 mm device.
By way of review: 19 mm prosthetic heart valves have prohibitive pressure gradients, and 23 mm and above are satisfactory. The recommendation: Do not use a 19 mm prosthesis, enlarge the aortic root to accommodate a larger prosthesis. Only use a 21 mm prosthesis in small, sedentary patients. A 23 mm prosthesis will work for all patients.
Stented Bioprostheses
The problem with mechanical and stented bioprostheses is that there may be a 5-8 mm reduction of the left ventricular outflow tract to accommodate the support portions of the devices. Since the pulmonary annulus is naturally 2 mm larger than the normal aortic annulus the Ross procedure will actually be a small up-size and therefore the most ideal hemodynamic performance of all the available operations. Aortic allografts and nonstented porcine grafts used as freehand valve replacements and enclosed within the patient's aorta only reduce the size of the outflow tract about 2 mm, the thickness of the tissue being implanted.
Comparison of the inside diameter of exactly the same size external diameter biologic valve shows considerable advantage of nonstented porcine bioprostheses compared to stented porcine aortic valves or stented bovine pericardial valves. The bovine pericardial valve has been reported to be a good hemodynamic performer in the small aortic root. Aupert and associates reported ECHO gradients which were quite acceptable and a four center cooperative study showing 85% freedom from structural failure resulting in explantation at 14 years.
Aortic Root Enlargement Procedures
Aortic root enlargement procedures are important techniques to be applied in the small aortic root. The root can be enlarged anteriorly by Konno-Rastan aortoventriculoplasty. This was extensively used but there are some significant problems with the operation and it has largely been abandoned in favor of the Ross procedure with anterior enlargement (Ross-Konno procedure). Most root enlargement procedures are performed posteriorly in adult patients. Incision into the posterior commissure is the most anatomically correct of these procedures. Operating time is increased but not at the expense of increased morbidity or mortality.
The Konno procedure is historical, and the Nicks procedure with incision of the posterior commissure is the best anatomically of the posterior enlarging procedures. The Konno procedure involves a deep incision of the infundibular septum which can divide the first septal artery. The reconstruction is complex involving two patches to reconstruct the left ventricular outflow tract and the right ventricle while implanting a large prosthetic valve.
The Nicks operation involves extending the aortotomy into the posterior commissure. The incision enters the interleaflet triangle. The loose tissues of the triangle separate readily so that the annulus may enlarge sufficiently without incision of the anterior leaflet of the mitral valve or entering the left atrium. A prosthetic patch is placed into the posterior commissure and the noncoronary sinus to accommodate a prosthesis perhaps 2 or 3 mm larger than the original annulus.
When enlargement more than 2 or 3 mm is required, it is necessary to extend the incision across the mitral annulus into the anterior leaflet of the mitral valve to allow the tissues to separate further at the annulus. A prosthetic patch is used to close the defect and enlarge the left ventricular outflow tract. The patch is attached to the anterior leaflet of the mitral valve with interrupted stitches. While it is tempting to use a simple running stitch, the interrupted stitch method distributes suture tension more evenly in order to prevent dehiscence of the patch from leaflet tissuesA larger prosthetic valve may then be accommodated in the left ventricular outflow tract.
Cryopreserved Aortic Allograft
Tissue valves which are not mounted on stents provide excellent hemodynamic performance in the small aortic root, often without any enlargement. The tissue with which we have the most and longest experience is the cryopreserved aortic allograft. This tissue is taken from human donors and includes the entire aortic root with attached anterior leaflet of the mitral valve and usually the ascending aorta and some of the arch. It is versatile and flexible tissue which can be implanted in a number of ways.
The original method described by Barrett-Boyes and Kirklin for implantation of an aortic allograft was to remove the sinus aorta from all three sinuses and to implant the valve with the least possible amount of aorta. We used this technique for several years with good results. Later on, we used the method described by Ross in which the noncoronary sinus of the aorta remains intact. This effectively fixes the position of two or the three commissures and makes implantation of the graft more reliable. The valve is implanted below the coronary arteries, attaching the sinus aorta of the graft to the sinus aorta of the patient by continuous stitches below and around the coronary ostia. The intact noncoronary sinus makes positioning of the commissures of the graft in the aortic root easier to do.
Root enlarging procedures are also possible using aortic allografts. The posterior interleaflet triangle can be opened and the noncoronary sinus partially removed to accommodate a larger graft which is attached to the annulus of the mitral valve and roof of the left atrium. The anterior leaflet of the mitral valve may be incised in patients having a subvalvular component of left ventricular outflow tract obstruction. In these operations, the anterior leaflet of the mitral valve of the graft is used to widen the outflow tract and to fill the defect in the patient mitral valve caused by the separation of the tissues. The intact noncoronary sinus of the graft is used to close the aorta after attaching the valve below the coronary ostia.
Aortic allografts may also be implanted as a free-standing aortic root. This provides the best hemodynamic performance of this tissue because it is not enclosed within the natural aorta. Standard root replacement technique is employed with reimplantation of the coronary arteries to the graft. The graft is anastomosed in end-to-end fashion to the ascending aorta.
Recent study of over 100 patients having aortic valve replacement with aortic allograft followed for over ten years showed no patient suffered thromboembolism even thought no patient received anticoagulant medication. The graft also appears to resist endocarditis. This shows the Kaplan-Meier analysis for thromboembolism, with all patients free of this complication. The valve seems reasonably durable for the time period of the study with 92 percent of patients free of explantation for tissue failure.
Analysis of subgroups of patients having various methods of implantation of cryopreserved aortic allografts appears to indicate that subcoronary valve implant with the noncoronary sinus intact or its variant with root enlargement offers the best chance for freedom from explantation of the tissue. From these data we conclude that during the time period of the study of ten years, mortality from valve related causes is low, thrombolism is virtually nonexistent, endocarditis is rare, and reoperation for valve related causes is unusual.
Pulmonary Autograft
Aortic valve replacement with pulmonary autograft is another tissue valve procedure which appears to hold great promise, especially in patients with small aortic root. This operation is often called the Ross procedure in honor of the surgeon who described it 30 years ago. In this operation, the aortic valve and the aortic root are excised retaining only the fibrous tissues of the aortic annulus and the coronary ostia surrounded by some sinus aorta. The patient's own pulmonary trunk including the pulmonary valve is removed from the right ventricular outflow tract. Careful, shallow incision of the infundibular septum is required to protect the underlying first septal branch of the left anterior descending coronary artery is required during excision of the pulmonary trunk.
The pulmonary trunk is used to replace the aortic root. It is attached to the fibrous tissue of the aortic annulus with simple interrupted stitches of 3/0 polypropylene suture. The stitches are tied down around a Teflon felt or pericardial strip. This is called the supported root technique. The prosthetic or pericardium fixes the size of the left ventricular outflow tract to that of the pulmonary trunk at the valve and is thought to prevent dilation and to preserve competence of the valve. The coronary arteries are reimplanted to the pulmonary trunk and an end-to-end anastomosis of the pulmonary trunk to the ascending aorta is constructed. The right ventricular outflow tract is reconstructed using an allograft pulmonary trunk.
Aortic root enlarging procedures may be applied in the Ross procedure. Aortic valve stenosis is often accompanied by general hypoplasia of the left ventricular outflow tract. One simple method for gaining enlargement of the outflow tract at the level of the ventriculoaortic junction after excision of the aortic valve is to excise the commissures through the interleaflet triangles. The loose tissues of the triangle often expand enough to match the outflow tract to the size of the pulmonary trunk. This shows the left ventricular outflow tract after excision of the commissures and the relationship of the ventricular septum after removal of the pulmonary autograft from the right ventricular outflow tract.
The ventricular septum may be incised to widen the left ventricular outflow tract. This is the so-called Ross-Konno operation. The incision need not be as deep as in the Konno operation and will avoid injury to the first septal branch of the left anterior descending coronary artery. Myocardium on the left side of the septum may be shaved down to totally relieve left ventricular outflow tract obstruction. The pulmonary autograft is attached to the ventricular septum, seating it more deeply in the left ventricular outflow tract. Extra myocardium from the right ventricular outflow tract may be left on the pulmonary autograft to fill the defect in the ventricular septum. This approach has nearly replaced the Konno operation for relief of subaortic tunnel stenosis and is very useful in many patients with narrow left ventricular outflow tract to allow perfect matching of the diameter of the outflow tract to the pulmonary autograft.
Our series of patients having pulmonary autograft to replace the aortic valve is typical and features young patients mostly under age 55 with mean age 36. The etiology of the valvular heart disease is primarily congenital. We studied four athletic men after the Ross operation to determine cardiac and autograft performance at peak exercise. The patients were exercised to exhaustion on a bicycle ergometer. Hemodynamic performance was determined by echocardiography. High level of work was attained and oxygen consumption was very high. The gradient over the left ventricular outflow tract and the autograft valve was very low at peak exercise. From these data we conclude that after aortic valve replacement with pulmonary autograft, high levels of exercise performance can be obtained without any aortic pressure gradient. The Ross procedure is a good choice of athletic patients.
At the same time, we also conclude that the Ross procedure is more complex than simple aortic valve replacement but it can be performed safely. It may be a permanent cure for aortic valve disease but the pulmonary allograft used to reconstruct the right ventricular outflow tract may not last the lifetime of young patients. Techniques employed during operation will likely affect late results. There are multiple suture lines which are exposed to the pericardial sac which are subject to breakdown and may not be completely accessible even on cardiopulmonary bypass. Bleeding and false aneurysm are to be expected in this operation. Hemodynamic performance is clearly superior to any other form of aortic valve replacement. The Ross procedure may be the operation of choice for young patients requiring aortic valve replacement.
Since the results are comparable using aortic allografts and pulmonary autografts to replace the aortic valve, how does one choose the procedure to perform? We would recommend an aortic allograft for replacement of the aortic valve if the patient has life expectancy less than 20 years, or if there is associated medical problems such as Marfan syndrome, autoimmune disorder, coronary artery disease, morbid obesity, and so on. An aortic allograft would be used if the pulmonary valve was abnormal. While others have large experience with the Ross operation for bacterial endocarditis, we would not recommend it for patients having infection extending beyond the aortic valve cusps. We rely on the established practice of using aortic allograft for patients with aortic root abscess.
Stentless Heterografts
Stentless porcine aortic heterografts have recently become available for implantation for replacement of the aortic valve. Our experience is with the Medtronic Freestyle bioprosthesis. We employ the subcoronary valve replacement technique.
The Freestyle bioprosthesis shown here is the complete porcine aortic root with the coronary arteries ligated. There is a thin Dacron cloth covering of the inflow end of the root extending over the excised septal myocardium to strengthen this portion of the pig aorta and to prevent shrinkage which could accompany resorption of septal myocardium. Marking flags are placed below each commissure and circumferentially to guide suture placement.
The Freestyle bioprosthesis consists of the porcine aortic root fixed in glutaraldehyde. There is no supporting stent and minimal cloth covering. A variety of sizes are available. Unique features of the preparation of the device not shared by other similar devices include zero net pressure on the valve cusps and fixation of the aortic wall under 40 mm Hg pressure. Alpha amino oleic acid is added to reduce calcium deposition in the valve cusps after implantation.
The zero pressure fixation of the valve cusps results in preservation of the leaflet crimp and results in a more flexible cusp which folds back widely and smoothly during systole and is more resistant to pressure during diastolic closure of the valve. The preparation of the tissue in the Freestyle device is similar to that of the stent mounted porcine aortic valve in the Medtronic Intact bioprosthesis. The Intact bioprosthesis employs zero net pressure leaflet fixation and toluidine blue to retard calcium deposition. Fourteen years experience with this prosthesis at Green Lane Hospital in New Zealand has shown 95% freedom from structural failure. The Freestyle bioprosthesis should be at least as good as the Intact bioprosthesis, assuming that it is properly implanted.
This is the implant technique which we have used. The sinus aorta is trimmed from the right and the left sinuses of Valsalva. The noncoronary sinus remains intact. Continuous suture technique is employed for the inflow suture line. We use pulley stitches placed around every third stitch to aid in tightening the suture loops. The outflow suture line attaches the sinus aorta of the graft to the sinus aorta of the patient below the coronary ostia. The graft is trimmed just above the sinotubular junction in the noncoronary sinus and attached directly to the aorta.
We have had a large experience implanting this bioprosthesis in older patients, mostly over the age of 75 years. Echocardiogram at one year after implant shows low valve gradient and good effective orifice area even in small diameter implants. Implant techniques result in competent aortic valves. Ninety percent of patients have either no or trivial aortic valve incompetence.
These data from Walther compare left ventricular posterior wall dimension before and after aortic valve replacement. Stentless bioprostheses are superior to either conventional stented bioprostheses or bileaflet mechanical prostheses in terms of regression of left ventricular hypertrophy.
From these data we conclude that stentless porcine bioprosthesis for replacement of the aortic valve provide good hemodynamic performance even in small sizes. Subcoronary implant technique results in a competent aortic valve. This device is an excellent choice for aortic valve replacement in elderly patients.
Conclusion
Finally, these are our current practice recommendations for choice of prosthesis for replacement of the aortic valve in patients with small aortic root. For patients less than 55 years old, we use a pulmonary autograft -- the Ross procedure. The Ross-Konno modification is employed for small aortic root to allow perfect matching of the size of the autograft to the left ventricular outflow tract. Mechanical prostheses are used for patients 55 to 75 years of age, enlarging the aortic root to accommodate a 23 mm prosthesis. A 21 mm prosthesis is used only is the patient is small, less than 1.5 square meters and sedentary. For patients over the age of 75, we favor a stentless porcine bioprosthesis. These age limits are adjustable. We have found with more favorable experience with the Freestyle bioprosthesis, we are lowering the age limit to 70 or even lower is some circumstances, especially when the aortic root is small. Similarly, the age limit for the Ross procedure is occasionally raised above 55 for exceptionally active patients in order to avoid anticoagulants and to obtain the favorable hemodynamic performance associated with this operation.