Fig. 12.1
During different operations (top and bottom). The patented diagram for the heart lung machine (From United States Patent [1] 1 Weishaar APPARATUS FOR DRAINING BLOOD FROM A SURGICAL WOUND AND TRANSMISSION TO A HEART-LUNG MACHINE Inventor: Egon Georg Weishaar, 8000 Munich 2, Erzglessereistrasse 29, Munich, Germany Filed: Nov. 15, 1972 Appl. No.: 306.937 with permission)
Different aortic valve replacement prosthesis have developed over the years and include mechanical valves, bioprosthetic valves constructed from either bovine pericardial tissue or porcine or human aortic valve leaflets with or without stent struts, and pulmonary valve autograft. The currently available aortic prosthetic manufactures are ATS medical, Edwards life sciences, Medtronic, St. Jude Medical, Sorin group, and On-X life technologies. A list of manufacture specific valves is available on these company’s websites and surgeons have inherent preferences in their selection of valves.
Aortic Valve Surgery
The available methods for aortic valve surgery for aortic stenosis include:
Mechanical Aortic Valve Replacement
This was the first attempt to replace an aortic valve and in 1960 the first AVR was performed using a “ball and cage” mechanical valve. The most common of these mechanical valves is the Starr-Edwards valve, which was approved by the FDA in 1965. Mechanical valves have since evolved to a single leaflet by Beal, tilting disk valves by Bjork Shiley, Omnicarbon, monstrut, and the Hall Medronic approved by the FDA in 1977, bi–leaflet valves as Carbomedics, ATS open pivot, On-X, Coform-X, and St Jude that was also approved in 1977, and even tri–leaflets [2].
The Starr-Edwards valve required a higher anticoagulation threshold and greater concerns regarding hemolytic anemia. The Bjork Shiley suffered a design and construction flaw where the strut would fracture at the site it was welded onto the metal valve ring. The FDA withdrew its approval in 1986 in what was deemed as the “most infamous recall on record”. Currently, the most commonly implanted mechanical valve designs are the St Jude bi-leaflet valves and the longevity of these valves is an attractive feature. However, due to the need for anticoagulation, these valves pose a problem in women in the childbearing age and in the pediatric population. In addition, the inability to expand with somatic growth during child development guarantees the need for reoperation if implanted in children.
The prosthetic valves are constructed from material that has biocompatibility, strength, and hardness. The earlier mechanical valves were made of silicone rubber and other materials used for valve manufacturing include silicone carbide and pyrolitic carbon. The latter is a form of carbon developed for the nuclear fuel industry in the 1960s, which has great strength and has been used over the last 30 years in prosthetic valve construction.
Auto-graft Aortic Valve Replacement (Ross Procedure)
Commonly known as the Ross procedure, this involves replacing the aortic valve with the patient’s own pulmonary valve and then using a pulmonary allograft created from the patient’s tissues to replace the pulmonary valve. In a canine model in 1960, the concept was investigated using auto-transplantation of the pulmonic valve into the descending thoracic aorta of dogs [5]. Pillsbury and Shumway described auto transplantation into the aortic annulus in 1966 [6]. However, Donald Ross reported the first clinical application in 1967 and the surgical approach was named after him [7]. This method provides an excellent hemodynamic profile with no chance of prosthesis-patient mismatch, provides no increased risk of endocarditis, and can grow with child growth. As such, his procedure is highly attractive in the pediatric population obviating the need for anticoagulation or “resizing reoperation” due to child growth. A study comparing the Ross procedure in comparable patients, 18–60 years of age demonstrated no late survival benefit in the first post-operative decade between the Ross procedure and mechanical valve implantation with optimal anticoagulation self-management [8].
Bioprosthetic Aortic Valve Replacement
Bioprosthetic aortic valves can be either derived from animal or human tissue as noted below [9, 10].
A.
Animal tissue valves: These are either
(a)
Aortic valve tissue: typically stented porcine as the Hancock and Carpentier- Edwards. These valves are either obtained from single pig valve sewn onto a plastic stent the base of which is reinforced with metal as the standard Hancock valve. The system is then covered with Dacron and preserved with glutaraldehyde. On the other hand, the modified Hancock is obtained from two porcine valves where the septal porcine aortic valve leaflet in one of the valves, that is normally thicker and stiffer, is replaced by another leaflet from the other porcine valve. Sodium dodecyl sulfate, an anticalcification agent, is used to decrease leaflet mineralization. The Carpentier Edwards valve is sewn to a metal wire stent, which is bent to form three U-shaped prongs. A Dacron skirt covers the base of the wire and the stent
(b)
Bovine pericardium: Examples of these stented bioprosthetic valves are the Hancock and the Ionescu-Shiley valve that have been discontinued and the Carpentier- Edwards valve that has the pericardial tissue secured to the stent posts to avoid high stress regions and tears. Bovine pericardium valves have excellent hemodynamics and are preferred in patients with a small aortic root sizes (19–21 mm).
(c)
Stentless valves: These valves were suggested to have excellent hemodynamic profiles but are more difficult to implant and may require longer bypass times. They are preferred in relatively young active patients with impaired ventricular function and small aortic annulus. They are derived from either
(i)
The entire aortic root and adjacent aorta of a pig after trimming the coronary arteries and are implanted as a block as the Free Style and Prima Plus.
(ii)
Porcine valves as the Toronto, O’brien, and Biocor.
(iii)
Bovine pericardium as the Pericarbon
(iv)
Equine pericardial tubular valve as the 3F therapeutics valve.
B.
Human tissue valves: these can be either
(a)
Homografts: They are obtained from human tissue valves and are preserved in liquid nitrogen. They do not incite rejection in the host and need to be thawed overnight and as such require knowledge of the required valve size.
(b)
Autograft: The Ross described above.
A chronology of the development of bioprosthetic valves is highlighted below:
1961: Robert Frater starts free-hand utilization of autogenous pericardium to create valves or parts of valves. Denaturing the proteins with mercurial solutions, freeze-drying, or formalin treatment was preformed to remove the antigens and avoid tissue rejection.
1965: The first xenograft aortic valve was successfully implanted.
1965–1969: The labs of Robert Carpentier in Paris and Hancock and Nimni in Los Angeles utilize glutaraldehyde as a preservative for the xenografts.
1969: The first aortic porcine valve preserved with glutaraldehyde was implanted.
The Ionescu-Shiley pericardial bovine valve is subsequently widely used. However, a design error leads high stress forces to cause tears and premature failure and leads to the valve withdrawal.
1976: The Hancock Porcine Valve began being used but is no longer in use because the company went bankrupt.
1980: The Carpentier-Edwards stented pericardial and porcine valves were created.
Late 1980s–1990s: The Stentless valves are created.
1990s–early 2000s: Trans-aortic valve implantation of bioprosthetic valves in porcine and subsequently human models.
Current times: Sutureless aortic valve replacement via MIVS approaches in elderly patients.
Comparison Between Mechanical and Bioprosthetic Aortic Valve
As a general rule, there is no survival benefit of bioprosthetic versus mechanical valve implantation. The main differences is the higher need for repeat surgery combined with a lower risk of major bleeding in patients receiving a bioprosthetic valve compared to those receiving a mechanical valve [11]. In propensity-matched comparisons, actuarial 15-year mortality rates were 60.6 % with the bioprosthetic aortic valve and 62.1 % with the mechanical valve. Cumulative 15-year stroke rates were 7.7 % and 8.6 % in the two groups, respectively. The reoperation rate was 12.1 % in the bioprosthetic valve group at 15 years and 6.9 % in the mechanical valve group, while major bleeding occurred in 6.6 % of bioprosthesis patients and in 13.0 % of the mechanical-valve group [11].
Comparison Between Stented and Stentless Bioprosthetic Aortic Valves
The data is somewhat conflicting with some data suggesting improved mean gradients, effective orifice areas, faster and more significant ventricular mass regression with stentless valves in exchange for more difficult implantation techniques and longer bypass duration [12–18]. Improvement in the stented prosthetic valve technology somewhat dampened the initial enthusiasm regarding the stentless bioprosthesis share of the market. In 2008, only 12 % of the European market of AVR implantation was using stentless bioprosthesis [19].
Aortic Valve Decalcification
Surgical decalcification of the aortic valve in patients with aortic stenosis was one of the original cardiac surgical operations. The high incidence of restenosis and the emergence of prosthetic valves lead to the abandonment of the procedure. Ultrasound debridement was then introduced and was plagued by a high incidence of restenosis and early and severe aortic regurgitation. As such, surgical repair techniques of the aortic valve are now on limited in young patients with exclusive aortic regurgitation [20].
Transcatheter Aortic Valve Replacement
Finally, transcatheter aortic valve implantation developed from implantation in a porcine model in 1992 [21], to human experiments by Cribier and others in 2002 [22], initially via an antegrade trans venous-trans-septal- trans-mitral route was developed. With improved profile, trans arterial retrograde implantation via trans femoral, trans aortic, trans apical, trans subclavian, and trans caval-to aortic routes have been performed. Both balloon expandable (Carpentier Edwards) and self-expandable (Medtronic core valve) formats have been developed and have been discussed elsewhere in this book.
Approaches for Surgical Aortic Valve Replacement
Traditionally, conventional full sternotomy (FS) has been the mainstay for surgical aortic valve replacement (sAVR). At our institution, we are fortunate to have a group of progressive surgeons who believe in the value of minimally invasive techniques for aortic valve replacement surgery. Minimally invasive valve surgery (MIVS) for aortic valve replacement (MIAVR) is our mainstay for isolated aortic valve surgery as well as for other institutions with surgeons who are equipped and trained to perform. These techniques are backed by years of experience with conventional FS for AVR.
Right thoracotomy (RT) and upper hemisternotomy (HS) are minimally invasive approaches we are able to customize to each individual patient depending on certain variables outlined in Fig. 12.2. Patient anatomy, comorbidities, need for other cardiac surgeries, and patient preference are factors to consider when selecting the best approach. A brief comparison is noted in Table 12.1.
Fig. 12.2
Decision trees for surgical aortic valve replacement. (a) general overview, (b) with coronary artery disease, and © with other valve disease. MIVS/MIAVR minimally invasive valve surgery/aortic valve replacement, AVR aortic valve replacement, SVCAD and MVCAD single and multi vessel coronary artery disease, respectively, RAT right anterior thoracotomy, CAB coronary artery bypass, PCI percutaneous coronary intervention, LIMA left interior mammary artery, MR mitral regurgitation, MS mitral stenosis, TVR tricuspid valve regurgitation, RCA right coronary artery, Cx circumflex, LAD left anterior descending artery
Table 12.1
Comparison or right thoracotomy and hemisternotomy
Property | Hemi-sternotomy | Right anterior thoracotomy |
|---|---|---|
Learning curve | Short (<10 cases) | Long (>30 cases) |
Versatility | Ascending aortic cases | RCA bypass |
Vary incision length | Island maze lesion | |
Central cannulation | Coronary sinus catheterization | |
% MIAVR cases done internationally | 80 % | 20 % |
Cosmetic results | Better for men | Preferred by women |
In recent years our program has not only been able to survive, but thrive, due to the minimally invasive techniques. As patients have become more sophisticated and empowered in the decision making of their surgical care, the real advantages of MIAVR have patients and referring physicians seeking out programs for these procedures.
Studies that have compared conventional FS to MIAVR have revealed no difference in mortality. However, the major benefit is in the need for transfusion, time to extubation, cardiopulmonary bypass time, incidence of atrial fibrillation, and ICU and post-operative length of stay (Tables 12.2 and 12.3) [23, 24].
Table 12.2
AVR using ministernotomy versus full sternotomy
Event | Hemi-sternotomy | Full sternotomy | p value |
|---|---|---|---|
Mortality | 0.96 % | 0.96 % | NS |
Stroke | 1.3 % | 1.3 % | NS |
Renal failure | 0.72 % | 0.84 % | NS |
Sternal wound infection | 0.6 % | 0.8 % | NS |
RBC transfusion (any) | 24 % | 34 % | <0.0001 |
Respiratory insufficiency | 2.9 % | 5.4 % | <0.01 |
Median time to extubation | 5.2 h | 6.9 h | <0.0001 |
ICU length of stay | 2 days | 3 days | <0.0001 |
Post op length of stay | 9.2 days | 12 days | <0.0001 |
Table 12.3
AVR using right thoracotomy versus full sternotomy
Event | Right anterior thoracotomy | Full sternotomy | p value |
|---|---|---|---|
Mortality | 0.7 % | 0.7 % | NS |
Stroke | 0.7 % | 1.5 % | NS |
Wound infection | 0 | 0.7 % | NS |
Re-exploration bleeding | 6.5 % | 4.3 % | NS |
RBC transfusion (any) | 19 % | 34 % | <0.006 |
Median time to extubation | 6 h | 8 h | <0.02 |
Median post op LOS | 5 | 6 | <0.02
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