Prosthetic Heart Valves

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Chapter 32

Prosthetic Heart Valves

Heart valve diseases are common and may result from congenital, inflammatory, infectious, or degenerative causes. The decisions regarding how to surgically correct these valvular problems, how to follow the patients after surgery, and what problems to anticipate are addressed in this chapter.

1. What are important considerations in the preoperative evaluation and planning of patients who are to undergo valve repair or replacement?

    Patients who are advised to have valve surgery require careful preoperative planning and assessment. Cardiac catheterization to rule out coronary artery obstructions that might require bypass is advised in adults more than 40 years old. Dental evaluation to identify abscesses or other potential sources of postoperative valvular infection is advisable. Carotid ultrasound to exclude significant stenosis in patients with bruits or neurologic symptoms is often requested. Visualization of the aorta, often with computed tomography (CT) scan, to assess atherosclerosis at potential cannulation sites and root dimensions, may occasionally be required before valve replacement.

2. What types of prosthetic valves are used for valve replacement, and which ones are most commonly used in current practice?

    Representative bioprosthetic and mechanical heart valve types are shown in Figure 32-1 and discussed here.

    A stentless bioprosthesis is a bovine or porcine heart valve implanted without a frame. In the aortic position, the aortic root is used to attach the valve. The major advantage is that the size of the implanted valve can be larger, because no space is required for a supporting frame. Data suggest that left ventricular function is better protected by nonstented compared with stented valves. Anticoagulation is recommended for the first 3 months after implantation.

    A stented bioprosthesis has a wire frame that provides the structure for the biologic material. The biologic material is usually bovine or porcine pericardium, which is specially treated to reduce antigenicity. Anticoagulation is recommended for the first 3 months after implantation.

    A single tilting disc mechanical valve opens with a minor and major orifice. The disc material is polycarbonate. The first successful valve was the Bjork-Shiley valve, introduced in 1969. These valves are rarely used in current practice.

    A bileaflet mechanical valve has two semicircular discs. The disc material is polycarbonate. The first bileaflet valve was introduced in 1977 by St. Jude Medical. Bileaflet valves are the most common mechanical heart valves used in current practice.

    Transcatheter aortic valve implantation (TAVI) or transcatheter aortic valve replacement (TAVR) has been performed at investigational sites for nearly 10 years, mostly commonly in Europe. In the United States, it has recently been approved, though programs that can implant such valves must meet strict standards. It is intended for patients with severe aortic stenosis who are not good surgical candidates for conventional valve replacement. The valve is implanted via a transarterial (femoral or axillary) or transapical approach. In Europe, the valve may also be implanted by a direct aortic approach. Significant risks of serious adverse events, such as valve displacement, cardiac tamponade, myocardial infarction, stroke, or injury of the aorta or femoral or axillary artery, are associated with this procedure. The procedure is illustrated in Figure 32-2.

    The ball-in-cage model is an older type of valve that is still encountered in some patients today. The Starr-Edwards mechanical valve prosthesis was introduced by Professor Albert Starr in 1961. It was the first mechanical heart valve. Some patients still have functioning Starr-Edwards prostheses after more than 30 years. The main drawback of this valve is its high-pressure gradient and the nonphysiologic streaming of flow through the valve. For these reasons, as well as the need for higher degrees of anticoagulation, the valve is no longer used in clinical practice.

3. What is a heterograft, and what is an allograft (homograft), and what is a Ross procedure?

    A heterograft is a biologic valve derived from an animal. An allograft (also called a homograft) is a heart valve taken from a human cadaver. Allografts have the advantage of natural configuration and the fact that they include a portion of the aortic root that can be incorporated in the valve replacement procedure if necessary (e.g., aortic root abscess). Careful matching of allograft size to the patient is critical to the performance and durability of the valve. The Ross procedure uses the patient’s own pulmonic valve to replace their aortic valve. The pulmonic valve is then replaced with a bioprosthesis. Advantages are a lack of antigenicity and the potential for growth of the valve over time, as required in children.

4. How does one choose between bioprosthetic and mechanical valves?

    A bioprosthesis is preferred in older patients and in patients in whom lifetime anticoagulation poses important risks. This includes persons with high trauma risk, clotting disorders, gastrointestinal problems with the potential for bleeding, and persons who may not be able to comply with required anticoagulant medication and follow-up testing. The major disadvantage of biologic prostheses is primary valve failure as a result of leaflet degeneration, which limits their functional life span. Mechanical heart valves, which have greater durability than bioprosthetic valves, are usually preferred in patients younger than 65 years and without contraindications to long-term anticoagulation.

5. What are the anticoagulation strategies for patients with prosthetic valves, and what are the risks for thromboembolic or hemorrhagic events?

    The degree and type of anticoagulation depends on the valve type and location, and the overall risk of thrombosis. Guidelines on anticoagulation of prosthetic heart valves have been published by both the American College of Cardiology/American Heart Association (ACC/AHA) and the American College of Chest Physicians (ACCP). Suggested anticoagulation regimens are given in Table 32-1.

6. How does one handle anticoagulation issues in patients with mechanical valve prostheses before elective surgery or invasive procedures?

    When warfarin must be discontinued for elective procedures, the patient can be treated with unfractionated heparin (UFH) (target activated partial thromboplastin time [aPTT] 55-70 seconds) while the international normalized ratio (INR) decreases. Low-molecular-weight heparin (LMWH) is not approved for protection against thromboembolism from mechanical valve prostheses; a recent ACCP practice guidelines does discuss the use of therapeutic-dose LMWH for bridging patients off and back on warfarin. Vitamin K administration can create a hypercoagulable state, which increases the risk of thromboembolism, and should not be used to accelerate the normalization of the INR.

7. How are anticoagulation issues managed when patients with mechanical heart valves become pregnant?

    Female patients who anticipate pregnancy may initially opt for valve prostheses that avoid the need for anticoagulation. For patients with mechanical prostheses, anticoagulation must be continued during pregnancy, but the risk of adverse fetal effects from warfarin mandates a different anticoagulation strategy. The ACCP’s Conference on Antithrombotic and Thrombolytic Therapy recommends one of the following three regimens:

8. How should prosthetic valve thrombosis be treated?

    Patients with thrombosis of a right-sided prosthetic valve should be considered for fibrinolytic therapy if there are no contraindications. The treatment for patients with left-sided valve thrombosis depends on the size of the thrombus. If the cross-sectional area is smaller than 0.8 cm2, fibrinolytic therapy is favored. For larger valve thrombosis, a surgical approach should be considered.

9. How does one prevent, diagnose, or treat prosthetic valve endocarditis?

    Endocarditis on a prosthetic valve is a potentially catastrophic event that may require repeat surgery to cure the infection. Such infections usually originate on the prosthetic sewing ring and rapidly extend into perivalvular tissues. The risk of endocarditis is approximately the same for mechanical and biologic valve prostheses, although some studies have suggested slightly higher risks for biologic valves. The AHA published new prophylaxis guidelines in 2007 suggesting that patients with prosthetic heart valves should be considered for endocarditis prophylaxis for procedures likely to create bacteremia with an endocarditis-causing organisms. In the UK, the National Institute of Clinical Excellence Guidelines now do not recommend antibiotic prophylaxis before dental procedures for any patient.

    Because artificial valves often create artifacts that complicate valve imaging, transesophageal echocardiography (TEE) is almost always required to adequately assess prosthetic heart valves for endocarditis, and to rule out perivalvular extension of infection. In cases of mechanical heart valve endocarditis, early surgery and re-replacement is often required, especially in the presence of perivalvular extension of infection, and may improve patient survival. Attempted medical treatment of prosthetic valve endocarditis requires at least 4 to 6 weeks of antibiotic therapy.

10. How does one use echocardiography to follow patients after heart valve surgery?

    All prosthetic valves are stenotic compared with natural valves, because of the extra space required for their sewing ring. As a result, the transvalvular flow velocity is expected to be higher than normal across prostheses; each valve type and size has an expected range of velocities and pressure gradients defined by the manufacturer. Flow patterns at closure are also valve specific; mechanical prostheses normally have multiple regurgitation jets (an exception to this being the ball-in-cage prosthesis).

    In evaluating prosthetic valves, it is important to know the valve type and size. The forward flow across the valve is measured with Doppler imaging, and used to calculate the peak and mean gradients for comparison to normal values. The gradients will increase with higher heart rate and with increased flow volume. Unexplained increases in valve gradients may indicate obstruction from endocarditis, thrombosis, leaflet calcification, or tissue overgrowth.

    During phases of the cardiac cycle when the valve is closed, spectral and color Doppler are used to identify valvular insufficiency. Some mild valvular insufficiency may be normally seen with mechanical valves due to leaflet closure and hinge points. With biologic prostheses, transvalvular insufficiency jets indicate valve dysfunction. Eccentric regurgitation in any type of prosthesis may result from paravalvular leaks caused by suture dehiscence, with or without endocarditis.

    Other echocardiographic clues to prosthetic valve dysfunction are progressive chamber enlargement, abnormal pulmonary vein flow patterns (for mitral regurgitation), diastolic reversal of aortic flow (aortic prosthetic regurgitation), and pulmonary hypertension (as estimated from the tricuspid insufficiency jet).

11. What can cause recurrent symptoms when the prosthetic valve appears normal?

    A mitral prosthesis that extends into the left ventricle (high profile) may crowd the outflow tract and cause signs and symptoms of outflow obstruction. This can also be seen after mitral valve repair, where redundant components of the native valve oppose the septum in systole. Intraventricular pressure gradients in the outflow tract can be identified with Doppler echocardiography. Low-profile valves have largely addressed this problem. All prostheses are smaller than the natural valves that they replace because of the space required for their sewing ring. Undersized valves relative to patient requirements can result in relative valvular stenosis. This can be investigated with Doppler studies.

12. Should magnetic resonance imaging (MRI) be used after valve replacement?

    According to a recent AHA scientific statement, the majority of prosthetic heart valves that have been tested have been labeled as MR safe; the remainder of heart valves and rings that have been tested have been labeled as MR conditional. On the basis of various studies and findings, the presence of a prosthetic heart valve that has been formally evaluated for MR safety should not be considered a contraindication to an MR examination at 3 Tesla or less (and possibly even 4.7 Tesla in some cases) any time after implantation. In cases where there is any doubt as to the safety of MRI scanning of a specific valve, the safety of the study should be discussed with an MRI specialist.

Bibliography, Suggested Readings, and Websites

1. Ali, A., Halstead, J.C., Cafferty, F., et al. Are stentless valves superior to modern stented valves? A prospective randomized trial. Circulation. 2006;114:1535–1540.

2. Aurigemma G.P., Gaasch W.H.: Routine Management of Patients with Prosthetic Heart Valves. In Basow, DS, editor: UpToDate, Waltham, MA, 2013, UpToDate. Available at: http://www.uptodate.com/contents/management-of-patients-with-prosthetic-heart-valves. Accessed March 26, 2013.

3. Bates, S.M., Greer, I.A., Hirsh, J., et al. Use of antithrombotic agents during pregnancy: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. Chest. 2004;126:627S–644S.

4. Bonow, R.O., Carabello, B., Chatterjee, K., et al. ACC/AHA 2006 guidelines for the management of patients with valvular heart disease: executive summary. Circulation. 2006;114:e84–e231.

5. Douketis, J.D., et al. Perioperative management of antithrombotic therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012;141:e326S–e350S.

6. Edwards, M.B., Taylor, K.M., Shellock, F.G., et al. Prosthetic heart valves: evaluation of magnetic field interactions, heating, and artifacts at 1.5 T. J Magn Reson Imaging. 2000;12:363–369.

7. Elkayam, U., Bitar, F. Valvular heart disease and pregnancy: part II: prosthetic valves. J Am Coll Cardiol. 2005;46:403–410.

8. Gott, V.L., Alejo, D.E., Cameron, D.E. Mechanical heart valves: 50 years of evolution. Ann Thorac Surg. 2003;76:S2230–S2239.

9. Hammermeister, K., Sethi, G.K., Henderson, W.G., et al. Outcomes 15 years after valve replacement with a mechanical versus a bioprosthetic valve: final report of the Veterans Affairs randomized trial. J Am Coll Cardiol. 2000;36:1152–1158.

10. Kovacs, M.J., Kearon, C., Rodger, M., et al. Single-arm study of bridging therapy with low-molecular-weight heparin for patients at risk of arterial embolism who require temporary interruption of warfarin. Circulation. 2004;110:1658–1663.

11. Kulik, A., Be´dard, P., Lam, B.-K., et al. Mechanical versus bioprosthetic valve replacement in middle-aged patients. Eur J Cardiothorac Surg. 2006;30:485–491.

12. Levine, G.N., Arai, A., Bleumke, D., et al. Safety of MRI/MRA in patients with cardiovascular devices; a scientific statement: statement by the AHA Council on Clinical Cardiology. Circulation. 2007;116:2878–2921.

13. Mohty, D., Orszulak, T.A., Schaff, H.V., et al. Very long-term survival and durability of mitral valve repair for mitral valve prolapse. Circulation. 2001;104:I1–I7.

14. Stein, P.D., Alpert, J.S., Bussey, H.I., et al. Antithrombotic therapy in patients with mechanical and biological prosthetic heart valves. Chest. 2001;119:220S–227S.

15. Whitlock, R.P., et al. Antithrombotic and thrombolytic therapy for valvular disease: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines. Chest. 2012;141:e576S–e600S.

16. Wilson, W., Taubert, K.A., Gewitz, M., et al. Prevention of infective endocarditis: guidelines from the American Heart Association. J Am Dent Assoc. 2007;138:739–745. 747–60

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