By 1961, Dr. Albert Starr and Lowell Edwards had successfully implanted the world’s first mechanical valve into a human to replace a mitral valve that had been deformed by rheumatic fever [3]. Initially, this steel ball and cage design was successful in approximately 50% of implantations. Major complications were soon recognized including (1) clot formation resulting in embolic strokes, (2) significant noise, (3) red blood cell destruction, and/or (4) tissue in-growth causing subsequent valve obstruction. A complete history of the development of currently used mechanical prostheses is beyond the scope of this text. However, it is important to mention two key aspects of any successful new valve design—improved valve hemodynamics and reduced thrombogenic (or clot forming) potential. Efforts to optimize valve hemodynamic function date back to the development of the Lillehei/Kaster tilting disk valve which allowed blood to flow centrally through the valve. At that time, this new type of device emphasized the requirement to design a valve that would reduce turbulent blood flow, reduce cell destruction, and minimize the transvalvular gradients [4]. A transvalvular gradient is defined as the pressure difference across the valve. Despite the advantages of a new steel tilting disk design, careful strict anticoagulation therapy was still required to reduce the risk of clot formation [5]. The next improvement of valves came with the development of the pyrolytic carbon valve leaflets. The nonthrombogenic weight and strength properties were determined by Drs. Jack Bokros and Vincent Gott. Subsequently, pyrolytic carbon was used in the creation of a bileaflet valve, inspired by Dr. Kalke. This valve, originally manufactured by St. Jude Medical (St. Paul, MN), provided exceptional performance, and today this design remains the gold standard of mechanical valve designs [6]. To date, all patients with mechanical valves require anticoagulation, e.g., with oral warfarin therapy, which reduces the risk of thromboembolism to 1–2% per year (Table 6.1) [7]. It should be noted that numerous studies have demonstrated that the risk of thromboembolism is directly related to the valve implant position, i.e., in the descending order of tricuspid, mitral, and aortic. In addition, this risk of emboli appears to be greatest in the early post-implant period and then becomes reduced as the valve sewing cuff becomes fully endothelialized.

In general, management of anticoagulation must be individualized to the patient to minimize risk of thromboembolism and, at the same time, prevent bleeding complications. In situations where a patient with valve prosthesis requires noncardiac surgery, warfarin therapy should be stopped only for procedures where risk of bleeding is substantial. A complete discussion of anticoagulation therapy is beyond the scope of this chapter; however, it is noted that several excellent reviews are available on this subject [7].

Due to problems related to anticoagulation, the majority of subsequent valve research has focused on developing a tissue alternative that avoids the need for anticoagulation. From a historical perspective, Drs. Lower and Shumway performed the first pulmonary valve autotransplant in an animal model [8]. Later in 1967, Dr. Donald Ross completed the first successful replacement in a human. The Ross Procedure is a well-established method still used today to replace a diseased aortic valve with the patient’s own pulmonary valve (Fig. 6.2). A donor tissue valve or homograft (Table 6.2) is then used as a prosthetic pulmonary valve. In general, tissue valves are significantly more biocompatible than their mechanical counterparts. These valves are naturally less thrombogenic, and thus the patient does not require aggressive anticoagulation. Specifically, a risk of <0.7% per year of clinical thromboembolism has been reported in valve replacement patients eliciting sinus rhythm without warfarin therapy [7]. Therefore, this treatment option is advantageous in clinical situations where the use of anticoagulation would significantly increase morbidity and mortality. Yet, to date, a potential major disadvantage of tissue valve implantation is early valvular degeneration as a result of leaflet calcification. Thus, methods for tissue preservation to prevent such calcifications are currently a major focus of research in this field.

The choice of a mechanical or biologic valve for implant will typically depend on various factors: (1) the patient’s current disease status, (2) the specific native valve involved, and/or (3) the surgeon’s preference and experience. If these factors are not limiting, the choice of valve type should be based on the maximization of benefits over risks for the individual patient. Unfortunately, the ideal prosthetic valve that combines excellent hemodynamic performance and long-term durability without increased thromboembolic risk or the need for lifelong anticoagulation remains an illusion. In general, mechanical valves offer greater durability at the cost of requiring lifelong anticoagulation as well as the risk of thromboembolism. In contrast, bioprosthetic valves have a much lower thromboembolic risk without the need for anticoagulation but elicit a higher risk for structural degeneration and thus potential need for reoperation. As such, mechanical valves are perhaps most well suited for the younger patient who does not desire future reoperations. Currently, mechanical valve replacement in the USA has been standardized and is commonplace, yielding satisfactory valve function that is quite reproducible from patient to patient. Furthermore, the flow gradients with newer bileaflet mechanical valves have dramatically improved from the early ball valve type; currently, a trileaflet valve is in the preclinical stages of development and may eventually not require anticoagulation therapy. In the interim, bioprosthetic or tissue valves offer a safe alternative for patients in whom the risk of anticoagulation is prohibitively high (e.g., elderly patients >70 years of age or women of childbearing years desiring pregnancy). Yet, the length of durability remains a serious concern for tissue valves, and thus a patient whose life expectancy is greater than that of the prosthesis will encounter the risk of another surgery for a second valve replacement. The current concept of a tissue replacement valve via a transcatheter delivered procedure may dramatically change this clinical limitation.

It is important to note that two historic randomized clinical trials have compared outcomes between early generation tissue and mechanical valves—the Edinburgh Heart Valve trial and the Veteran Affairs Cooperative Study on Valvular Heart Disease [8–10]. Both trials showed increased bleeding associated with mechanical valves and increased reoperation with tissue valves. While the strength of these trials is a prospective randomized design, the disadvantages are that the valves used in these trials are currently obsolete. More recently, a large meta-analysis comparing mechanical vs. bioprosthetic aortic valves found no difference in risk-corrected mortality regardless of patient age [11]. Based on this and other studies, the choice of valve should not be based on age alone. Clearly, there is a trend towards increasing use of bioprosthetic valves in younger patients; this is based on the fact that advances in tissue fixation and improved anti-calcification treatments have resulted in superior durability of the newer generation bioprosthetic valves. Specifically, third-generation bioprosthetic valves have been shown to have a greater than 90% freedom from structural deterioration at 12-year follow-up [12]. Furthermore, improvements in cardiac surgery including better techniques for myocardial preservation, less invasive procedures (i.e., robotic surgery), and strategies for cardiac reoperation have significantly reduced the risks for cardiac reoperation. This has further allowed an increasing application of bioprosthetic valves in patients younger than 55–60 years old. In conclusion, in the absence of current randomized trials, physicians must make a choice based on existing data and individualize that choice based on patient-related factors such as age, lifestyle, tolerance for anticoagulation, and/or position of the replacement valve.

All patients with prosthetic valves also need appropriate antibiotics for prophylaxis against infective endocarditis. Details of these therapies are beyond the scope of this chapter, but the reader is referred to guidelines published by a joint committee from the American Heart Association (AHA) and American College of Cardiology (ACC) for the applicable protocols. In addition, a registry has been established to track the long-term performance of all clinically approved implanted valve prostheses. Established standards were revised in 1996 and are briefly summarized in Table 6.3. As alluded to in Chap. 14 on animal models for cardiac research, investigators seeking approval for all new valves must also report any complications that occur in the preclinical animal testing phase to the appropriate regulatory authority.

Anatomically, the normal aortic valve is composed of the annulus and the left, right, and noncoronary leaflets (sometimes referred to as cusps) (Fig. 6.3). Diseases affecting these structures can be subdivided into aortic stenosis or regurgitation or some combination thereof. Overall, aortic stenosis is considered a surgical disease with aortic valve replacement considered to be the standard of care. Treatment of aortic regurgitation is also typically surgical, though the exact method chosen will vary widely based on the etiology of the disease.