Surgery For Valvular Heart Disease

Harry J. D'Agostino, Jr., M.D., Assistant Professor of Surgery, Division of Cardiothoracic Surgery, University of Florida / Jacksonville

 

This past year marked the golden anniversary of cardiac valve replacement surgery. In 1960, Harken1 and Starr2 implanted the first man-made valve prostheses in the aortic and mitral positions, respectively, ushering in a new age of treatment for a difficult medical problem. The last forty years have seen major advances in the devices, the procedures and the results. Heart valve surgery is now routine with low perioperative morbidity and mortality, as well as long-term improvement in survival and quality of life. The goal of this article is to review some the issues of heart valve surgery, as well as the numerous valves and procedures that are available to patients with valvular heart disease. The discussion will be limited to aortic and mitral valves, as these constitute the bulk of valvular heart disease.

Prosthetic Valve Disease

Even as physicians welcomed valve surgery for their patients, they quickly realized that the morbidity and mortality of valvular heart disease did not vanish overnight. The presence of a non-native valve in the circulation created difficulties of its own. In effect, the patients had traded their disease for an entirely new set of problems, collectively termed prosthetic valve disease. These problems included thromboembolism, anticoagulant-related hemorrhage, endocarditis, structural valve deterioration, and patient-prosthesis mismatch.

The prosthetic valve can be a potential nidus for thrombus formation, creating the problem of thromboembolism (TE). Anticoagulating patients for this condition can result in the opposite problem, anticoagulant-related hemorrhage (ACH). These complications can be mild or life threatening. Coumadin and aspirin are the most common agents used. With both agents, there is no toxic - therapeutic window, i.e. there is no dose at which TE is completely prevented and ACH is absent. Therefore dosages are adjusted to provide the lowest risk of each event. Coumadin dosage is currently adjusted according to the International Normalized Ratio (INR). Aspirin is titrated empirically. The risks of TE and ACH vary significantly among different studies because of the patient populations, the prostheses chosen, the valve position, the intensity of anticoagulation, etc. In recent studies, the event rates of TE and ACH are each approximately 1-2 % per patient-year.3

The placement of any foreign body into the circulation poses a potential risk of early or late infection. Prosthetic valve endocarditis (PVE) is especially worrisome because it is often difficult to treat, and a reoperation carries a very high risk. Patients must strictly observe antibiotic prophylactic measures, such as those for dental procedures. Fortunately, the risk of PVE is low (0.0 - 0.5 % per year)3 and is greatest in the first three months following valve implantation.

It is humbling to note that modern technology is still unable to create a prosthesis with the same durability as the human heart valve. Thus, structural valve deterioration is a great concern for any patient with a prosthesis. Improvements in materials engineering have essentially eliminated this problem in valves composed of man-made materials. However, those valves made from biologic tissues are more prone to wear out over time (usually beginning at 10 years).

Determining the durability (i.e. longevity) of prosthetic valves is very complex. It is usually expressed as freedom from structural deterioration or reoperation. This gives patients and physicians some idea of the long-term performance for a given valve. However, comparing studies and valves is difficult. Factors that affect valve (and patient) life span include the type of valve, the age of the patient at implantation, and the valve position (aortic or mitral). For example, tissue valves in the aortic position perform very well in patients over the age of 70. In a recent study of one particular biologic prosthesis, the freedom from structural deterioration was 93% at 15 years4. This is an impressive performance. It should be noted that the overall 15-year survival for this group of patients was 17%.4 Most patients succumbed to non-valvular causes and never required reoperation. This indicates that the valve served the patient until the end of life. However, if these data are used to compare performance of this valve with other valves in other clinical settings, erroneous conclusions can result.

Manufactured valves do not have perfect hemodynamic performance. The valve leaflets, orifice, and housing offer a certain amount of resistance to blood flow, thus creating a transvalvular pressure gradient. In larger valve sizes, this gradient is small. However in smaller sizes, the gradient can be significant. Patient - prosthesis mismatch occurs when the valve orifice is too small relative to the patient's body size, causing a high transvalvular pressure gradient. This usually occurs in the aortic position in patients with small aortic roots. The consequences of this mismatch may be significant. At exercise the patient may not be able to increase cardiac output because of increasing transvalvular pressure gradients. Furthermore, left ventricular hypertrophy may not regress with implantation of the new valve, symptoms may not improve, and survival may be decreased.

This problem can be avoided by choosing a more hemodynamically efficient valve, or enlarging the valve annulus to accommodate a larger valve.

Indications for Heart Valve Surgery

The presence of valve pathology exacts a progressive toll on cardiac anatomy and physiology. Generally, in stenotic lesions, the heart responds by becoming hypertrophic. In regurgitant lesions, the heart progressively dilates. Both pathophysiologic processes result in altered cardiac function, decreased efficiency, arrhythmias, and ultimately decreased survival. A valve procedure can reverse these processes if the intervention occurs before the damage becomes permanent.

Natural history studies of aortic stenosis have shown that patients who develop significant symptoms have significantly decreased survival, only 2 to 5 years. Decreased survival has also been observed in patients with mitral stenosis who exhibit significant symptoms. Therefore, patients with valvular stenosis are usually considered for surgery when they become symptomatic.
 
Evaluation of asymptomatic patients with valvular heart disease is more difficult. As the heart compensates for the diseased valve, the patient may be unaware of any change. Only a careful history will reveal a gradual decrease in exercise capacity. Echocardiography has become an indispensable tool in evaluating these patients. Not only can cardiac function, hypertrophy, and dilatation be measured quickly and non-invasively, but the patient can be continually assessed over time and any deterioration discovered.

Patients often develop arrhythmias, especially atrial fibrillation, as a consequence of their valvular heart disease. The discovery of atrial fibrillation in this setting should prompt consideration of surgery for a number of reasons. First, the presence of the arrhythmia signals that the diseased valve has already had a negative impact on cardiac anatomy and physiology. Second, atrial fibrillation carries its own morbidity and mortality, especially from thromboembolism. Finally, it has been noted that the less time that the patient has been experiencing atrial fibrillation (e.g. 6 months), the greater the likelihood that normal sinus rhythm can be restored following valve surgery.

The American College Cardiology and American Heart Association have jointly published Practice Guidelines which offer insight into the indications and timing of valvular heart surgery5. The reader is referred to this source for an excellent in-depth discussion of this complex topic. A brief summary of these guidelines appears in Table 1.

Types of Valves

The perfect valve, which would be devoid of the problems associated with prosthetic valve disease, has yet to be developed. However, a wide variety of valves is available, each of which attempts to deal with each of these problems. Therefore, the particular choice of valve depends heavily on the assessment of the individual patient's risk for each of the components of prosthetic valve disease.

Mechanical Valves

The first widely available valve prostheses were manufactured from plastics and metals, hence the name "mechanical" or "metal". The prototype was the Starr-Edwards® valve, a "caged ball" prosthesis that functioned surprisingly well. Concerns about hemodynamics, hemolysis, and thrombosis led to the development of the present day valve. The ball has been replaced with a disk, either single or divided in half and mounted on hinges (bileaflet valve). Most prostheses are now manufactured from a metal-like material called pyrolitic carbon. The most popular models are the St. Jude®, Medtronic Hall®, and Carbomedics® valves. These are non-magnetic and pose no problem for patients who require MRI scanning. The valves have very low rates of structural valve deterioration and, in most cases, last for the lifetime of the patient. The hemodynamics are fairly efficient, so that patient-prosthesis mismatch is less likely. The drawback is that the valves are thrombogenic. Coumadin is required for all patients with mechanical valves. The American College of Chest physicians has established recommendations for anticoagulation of patients with prosthetic heart valves. These are summarized in Table 2.
 
Bioprosthetic Valves

The bioprosthetic valves are made from animal tissues. The prototype is the porcine valve in which swine aortic leaflets are treated with various chemical solutions to enhance durability and then mounted in a plastic frame. The treatment process also renders the valves non-immunogenic so that rejection is not a problem. (A surprising number of patients will ask about this!) These valves have very low thrombogenicity. Coumadin is recommended only for the first three months following valve, after which the patients can be maintained on aspirin alone6. The most popular models are the Carpentier-Edwards® and Hancock® porcine valves.

The major issue with all biologic valves is durability. The continual motion of the tissue with each heartbeat tends to weaken the valve over time.

Tears and calcification of the leaflets can occur. In general the valves begin showing evidence of structural deterioration at about 10 years and require replacement when they fail. Thus, they are usually recommended for patients over the age of 70.

Major research effort has focused on improving the longevity of bioprosthetic valves. Newer techniques of preserving the valves such as low-pressure fixation and anti-calcification treatments may enhance valve durability. Evaluation of other biologic tissues such as bovine pericardium has also produced results. Experience with the bovine pericardial valve (e.g. Carpentier-Edwards Perimount® valve) has shown that the valve longevity may have been increased by up to five years, making these valves suitable for patients over 65 years of age.7

Bioprosthetic valves are less hemodynamically efficient than mechanical valves and patient _ prosthesis mismatch can be a problem. Recently a new generation of bioprosthetic valves has been developed with improved hemodynamic performance. Known as "stentless" valves, these are made by removing the porcine aortic valve along with its supporting aortic wall. The valve is then treated chemically to enhance durability and resist calcification. In certain models, the outside of the aortic wall is covered with cloth to increase the structural support. Short-term studies suggest that the transvalvular pressure gradients are very low and the durability is similar to that of other bioprosthetic valves. Long-term data are not available. The Toronto Stentless Porcine Valve® and the Medtronic Freestyle® valve are examples of stentless valves.

Homografts and Autografts

A homograft is a transplant of tissue between members of the same species. An autograft involves transplant of tissue from the same individual. Both have found roles in the surgery for heart valve disease, in particular, as replacements for the aortic valve.

Homograft aortic valves are procured from cadavers immediately after death. In order to preserve anatomy and function, the valve is removed as a cylinder, containing the valve leaflets, aortic wall, adjacent cardiac muscle and ascending aorta. It is sterilized with antibiotics, and cryopreserved. The process renders the valve non-immunogenic. Furthermore the rate of thromboembolism is very low and the durability appears to be somewhat better than that of bioprosthetic valves. A recent study of homograft valves in adult patients noted a freedom from reoperation of 85% at 15 years.8 Another advantage is that the valves are more hemodynamically efficient in the small sizes and less likely to cause patient - prosthesis mismatch.

The implantation of a homograft aortic valve requires more surgical expertise than for a bioprosthetic valve. The base of the valve is sutured to the aortic annulus, and the aortic portion to the native aorta. The valve must be implanted without any distortion of its structure, or aortic insufficiency may result. Furthermore, the origins of the coronary artery must be implanted into the cylinder, which can be a major problem if they are kinked or oriented incorrectly.

The autograft procedure involves the implantation of the patient's own pulmonary valve into the aortic position. The procedure is identical to that of a homograft valve, and includes the concern about distortion of the coronary arteries. The right ventricular outflow tract is then reconstructed with a homograft valve, usually a cryopreserved pulmonary valve. Thus, the procedure actually involves two valve replacements. The operation is extensive, technically challenging, and should only be performed by an experienced surgeon. Mr. Donald Ross of the United Kingdom first popularized the procedure, which now bears his name.

The pulmonic valve is composed of living tissue and is capable of growth, which makes it the valve of choice in pediatric patients. Thromboembolism is rare and anticoagulation is not needed. However, over the long term, the pulmonic valve in the aortic position can develop regurgitation. Furthermore, the homograft in the pulmonic position can also deteriorate. According to recent data, approximately 7% of patients required a reoperation for one of these problems over a 13-year period.9 For more information on this procedure, the reader is referred to the article by Ceithaml in this issue.

Mitral Valve Repair

In 1925, Souttar performed a repair of a stenotic mitral valve by inserting a finger across the valve in a beating heart, splitting the valve along its commissures and relieving the stenosis.10 Later a mechanical dilator was introduced which opened the valve more completely and numerous "closed" commissurotomies were performed. Cardiopulmonary bypass has allowed the procedure to be performed under direct vision and "open" commissurotomy remains a viable option for mitral stenosis. However, the recurrence rate is high. One study found that the freedom from valve reoperation was 58% at 10 years for open commissurotomy compared to 98 % for replacement with a mechanical valve.11 For replacement of a stenotic mitral valve, the current mechanical and bioprosthetic devices perform well. Research on mitral valve homografts is currently underway.

In the 1970's, Carpentier studied mitral valve anatomy exhaustively and developed a series of techniques for repair of mitral regurgitation. Over the past 30 years, the Carpentier techniques have become widely accepted and applied. Using these techniques, approximately 70% of regurgitant mitral valves can be repaired.

The mitral valve complex bears a remarkable resemblance to a parachute, with a billowing canopy (leaflets), supporting lines (chordae tendinae), and harness attachments (papillary muscles). The competence of the mitral valve depends on the ability of the two leaflets to press against one another during systole (coaptation). Any process that disturbs this coaptation can cause regurgitation.

A common problem is tearing or stretching of chordae tendinae, which support the leaflets. This allows a portion of leaflet to rise above its counterparts, disrupting coaptation. One common corrective procedure is to remove that portion of the leaflet and suture the remaining leaflet back together, thus restoring competence. Another solution is to shorten a stretched chord by suturing the redundant portion to its papillary muscle. A third option is to replace the chord with Gore-Tex® suture.

Chronic mitral regurgitation is usually associated with a progressive dilatation of the mitral annulus along the posterior leaflet. This widens the orifice and prevents effective coaptation of the leaflets. Therefore, a fundamental part of mitral valve repair is the restoration of annular size. This is done by a measured reduction (annuloplasty) of the posterior annulus, usually by suturing to it a cloth-covered ring or band of the correct size. Annuloplasty can be performed as an isolated procedure or in conjunction with the repair techniques described above. There is general agreement that an annuloplasty should be part of every mitral valve repair.

Mitral valve repair offers a number of advantages over replacement. Because the repair is done with native tissue, most of the problems associated with prosthetic valve disease are avoided. Thromboembolism is unusual, anticoagulation is not necessary, and endocarditis is rare.

The quality of a mitral valve repair is highly dependent on the skill and experience of the surgeon. He or she must be able to evaluate the entire valve anatomy, decide whether or not the valve is repairable, determine the cause(s) of the regurgitation, select the proper repair techniques, and perform them correctly. Any breakdown in this process can result in a poor repair. The quality of the repair is assessed intraoperatively by transesophageal echocardiography after the heart is closed and beating. Any deficits in the repair can be corrected at this time.

A major concern of valve repair is that it will fail over time, i.e. experience structural deterioration. Fortunately, with an experienced surgeon, the repairs are remarkably durable. A recent study of valve repair for mitral regurgitation secondary to degenerative diseases found that the freedom from reoperation was 85% at 15 years.12 Repairs of mitral regurgitation due to rheumatic disease do not appear to be as durable, and valve replacement is more common. The quality and durability of mitral valve repairs are significantly related to the experience and skill of the surgeon.

Choosing a Valve

The patient with valvular heart disease facing a surgical procedure is confronted with a dizzying array of choices for valve replacement or repair, each with its own advantages and disadvantages. The patient should make a selection after careful consultation with the surgeon and the referring physician, because that choice affects all three participants. The patient will be subject to the various risks of prosthetic valve disease inherent in the particular valve or procedure chosen. The surgeon must be able to provide the particular operation with appropriate morbidity and mortality. The referring physician must be comfortable with the choice, as he or she will be involved in the long-term management of the patient.

A mechanical valve is suitable for most patients. The expected durability is lifelong and the risks of coumadin are well understood. For those patients who require anticoagulation for other reasons, such as atrial fibrillation or stroke, a mechanical valve is a natural choice. In addition, those patients with hypercalcemia (e.g. renal failure) should receive a mechanical valve because bioprostheses tend to calcify and fail at an accelerated rate in these patients.

Certain groups of patients may experience difficulty with coumadin. Such patients include those with prior bleeding episodes, risk for future bleeding episodes (e.g. the elderly), demonstrated non-compliance, high-risk activities such as contact sports, females of childbearing age, infants and children, difficulty with follow-up, and personal preference.

For patients in whom coumadin is not appropriate, there is a wide range of valve choices. For patients over age 65, a standard bioprosthesis (porcine or bovine) is acceptable, and will likely last for the lifetime of the patient. In patients at risk for patient-prosthesis mismatch, a stentless valve or homograft provides a better hemodynamic profile. The pulmonary autograft (Ross procedure) is ideal for pediatric patients because of its growth potential. The Ross procedure may also be useful in good risk adult patients, especially females of childbearing age, who wish to take advantage of a more favorable rate of freedom from reoperation, and for whom an experienced surgeon is available.

In patients with mitral stenosis, repair (i.e. open commissurotomy) is an option in selected cases, but most patients will undergo replacement. Patients with mitral regurgitation will usually be able to have a repair. For those valves that cannot be repaired, replacement is necessary. The choice of mechanical or bioprosthetic valve can be made using the above reasoning.

Morbidity and Mortality

Complications of valve surgery have progressively decreased over the last three decades, thanks to improvements in devices, anesthesia, cardiopulmonary bypass, surgical skill, and intensive care management. The most common complications are hemorrhage, infection, stroke, heart block, and death, and can occur with any open-heart procedure.
Heart block is more common in valve surgery than in other cardiac procedures. Portions of the aortic, mitral, and tricuspid valves share fibrous continuity. The conduction system (i.e. atrioventricular node _ His bundle complex) courses through this area. Therefore, surgery for any of these valves carries a risk of injury to the conduction system. Fortunately, the risk of complete heart block and pacemaker implantation is very low (< 2 %).

Mortality for valve surgery has decreased significantly over the last several decades. The National Database of the Society of Thoracic Surgeons, which includes the bulk of cardiac surgery performed in the United States, tracks the mortality rates for various procedures.13 Risk-adjusted operative mortality for valve procedures is listed in Table 3.

The addition of coronary artery bypass grafting (CABG) to a valve procedure increases the mortality rate, but this remains within acceptable limits. The mortality for combined mitral valve replacement and CABG is surprisingly high (11 %).

Minimally Invasive (Minimal Access) Valve Surgery

Surgical practice has trended toward procedures performed through smaller incisions, often with video camera assistance. Since all of the cardiac valves are close to one another, and only the valve need be exposed to operate upon it, valve surgery lends itself very well to minimal access techniques. A number of minimally invasive procedures are currently performed. All result in smaller incisions and diminution or avoidance of a sternotomy. All claim to reduce morbidity, patient discomfort, bleeding, hospital length of stay, cost, and recovery time. However, all require cardiopulmonary bypass. Connections between the patient and the heart-lung machine can be established either through the primary incision, or incisions in other areas of the body such as the groin.

The two most common minimal access procedures for valve surgery are the parasternal approach and the mini-sternotomy. The former involves a 6 to 8 cm incision over the 2nd and 3rd costal cartilages on the right. These are then removed to expose the aorta and right atrium. Access to aortic, mitral, and tricuspid valves is possible. One disadvantage of this approach is the possibility of chest wall instability and lung herniation postoperatively. Some patients experience increased, rather than decreased pain. The mini-sternotomy involves a 6 to 8 cm incision over the upper sternum, and a partial sternotomy. Exposure is excellent and the patients do seem to have decreased pain. This is the author's preferred approach.

Most studies of minimally invasive valve surgery have shown a decrease in postoperative pain and bleeding14. Some studies have also shown decreases in hospital stay and cost, as well as more rapid recovery14. More research is needed, but the known advantages make this approach worthwhile in appropriate patients.

The Future

Progress in multiple areas has contributed to the continued success of valve procedures. The near future will predictably focus on new ways to perform the procedure and improved biocompatability of the prostheses.

The first steps toward futurization of the surgical valve procedure have already been made. The minimally invasive (minimal access) techniques for valve repair and replacement have been used successfully. The next step will involve robotics, i.e. operative maneuvers performed with mechanical devices inserted through tiny incisions with vision augmented by video camera, controlled by a surgeon seated at an operative console. Indeed, Chitwood has reported significant progress with the so-called "micro-mitral" and robotic mitral valve repairs.15

Research in tissue engineering has resulted in a potential valve prosthesis. In this process, known as SynerGraft®, the valve is treated so that the original cells are removed, leaving a connective tissue framework.16 Following implantation, the valve matrix is repopulated by cells from the patient, resulting in a new valve of living tissue. The SynerGraft® valve may have the same durability and biocompatability as the original and may represent the closest approximation yet to the perfect valve.

Summary

  1. Indications for valve surgery have become refined.
  2. There is no perfect valve prosthesis or procedure. However, there are many choices.
  3. Aortic valve replacement has a wide range of choices, each with advantages and disadvantages.
  4. Approximately 70 % of regurgitant mitral valves can be repaired.
  5. Long-term complications, such as thromboembolism, anticoagulant-related hemorrhage, and structural valve deterioration vary with the choice of prosthesis and procedure.
  6. Minimal access procedures can reduce postoperative pain, bleeding, hospital stay, and cost.
  7. Improved prostheses and procedures will be available in the near future.

References

  1. Harken DE, Soroff HS, Taylor WJ, et al. "Partial and complete prostheses in aortic insufficiency." J Thorac and Cardiovasc Surg 1960;40:744-62.
  2. Starr A, Edwards ML. "Mitral replacement: Clinical experience with a ball-valve prosthesis." Ann Surg 1961;154:726-40.
  3. Milano A, Guglielmi C, De Carlo M, et al. Valve-related complications in elderly patients with biological and mechanical aortic valves. Ann Thorac Surg 1998;66:S82-7.
  4. Corbineau H, De La Tour B, Verhoye J-P, et al. Carpentier-Edwards supraannular porcine bioprosthesis in aortic position: 16-year experience. Ann Thorac Surg 2001;71:S228-31.
  5. Bonow RO, Carabello B, de Leon AC Jr, et al. ACC/AHA guidelines for the management of patient with valvular heart disease: executive summary. A report of the American College of Cardiology / American Heart Association Task Force on Practice Guidelines (Committee on Management of Patients With Valvular Heart Disease). Circulation 1998;98:1949-84.
  6. Sixth ACCP Consensus Conference on Antithrombotic Therapy. Chest 2001;119(Suppl 1).
  7. Banbury MK, Cosgrove III DM, Lytle BW, et al. Long-term results of the Carpentier-Edwards pericardial aortic valve: A 12-year follow-up. Ann Thorac Surg 1998;66:S73-6.
  8. O'Brien MF, Harrocks S, Stafford EG, et al. The homograft aortic valve: a 29-year follow up of 1,022 valve replacements. J Heart Valve Dis 2001;10(3):334-44.
  9. Oury JH, Hiro SP, Maxwell JM, et al. The Ross procedure: Current registry results. Ann Thorac Surg 1998;66:S162-5.
  10. Souttar HS. The surgical treatment of mitral stenosis. Br Med J 1925;Oct 3:603-6.
  11. Glower DD, Landolfo KP, Davis RD, et al. Comparison of open mitral commissurotomy with mitral valve replacement with or without chordal preservation in patients with mitral stenosis. Circulation 1998;98:II120-3.
  12. Alvarez JM, Deal CW, Loveridge K, et al. Repairing the degenerative mitral valve: ten- to fifteen-year follow-up. J Thorac Cardiovasc Surg 1997;113:957-8.
  13. STS National Database 2000 Executive Summary. http://www.sts.org. Accessed June 29, 2001.
  14. Grossi EA, Galloway AC, Ribakove Gh, et al. Impact of minimally invasive valvular heart surgery: a case-control study. Ann Thorac Surg 2001;71(3):807-10.
  15. Chitwood WR, Nifong LW, Elbeery J, et al. Robotic mitral valve repair: Trapezoidal resection and prosthetic annuloplasty with the Da Vinci Surgical System. J Thorac Cardiovasc Surg 2000;120:1171-2.
  16. Elkins RC, Dawson PE, Goldstein S, et al. Decellularized human valve allografts. Ann Thorac Surg 2001;71(5):S428-32.

October, 2001/ Jacksonville Medicine

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