Prosthetic heart valves

Durability and long-term outcomes

Mechanical valves have excellent long-term durability, up to 50 years for the Starr-Edwards valve and more than 35 years for the SJM Regent valve. Structural valve deterioration (SVD), exemplified by some older-generation Björk-Shiley valves (i.e., strut fracture with disk embolization) and Starr-Edwards prostheses (i.e., ball variance), is now rare. The 10-year rate of freedom from valve-related death exceeds 90% for the SJM Regent and the Carbomedics bilealfet valves. The Medtronic-Hall prosthesis has achieved comparable longevity. Actuarial survival rates, which depend on several patient factors such as age, gender, ventricular function, coronary artery disease, functional status, and major comorbidities, range from 94% ± 2% at 10 years for SJM Regent valves to 85% ± 3% at 9 years for Omniscience valves and 60% to 70% at 10 years for Starr-Edwards valves ( Table 26.3 ).

All patients with mechanical valves require lifelong anticoagulation with a vitamin K antagonist (VKA). Direct-acting oral anticoagulants (DOACs) are not approved for use in this patient subset. In a small phase 2 study, dabigatran was associated with increased rates of thromboembolic and bleeding complications compared with warfarin. Higher-intensity anticoagulation is required for mechanical valves placed in the mitral versus the aortic position, for patients with multiple mechanical prostheses, and often for patients with additional risk factors for thromboembolism (e.g., atrial fibrillation). Even with appropriately targeted anticoagulation, reported rates of thromboembolism range from 0.6 to 3.3 per 100 patient-years for patients with bileaflet or tilting disk valves. ^{, , , ,}

Complications related to anticoagulation in this population occur at rates of 0.9 to 2.3 per 100 patient-years. Long-term issues associated with mechanical valves include infective endocarditis, paravalvular leak (PVL), hemolytic anemia, thromboembolism/valve thrombosis, pannus ingrowth, prosthesis–patient mismatch (PPM), and hemorrhagic complications related to anticoagulation ( Fig. 26.3 ).

Stented bioprostheses

The traditional design of a bioprosthetic valve consists of three biological leaflets made from a porcine aortic valve or bovine pericardium and treated with glutaraldehyde to reduce antigenicity. The leaflets are mounted on a metal or polymeric stented ring, and they open to a circular orifice in systole, resembling the anatomy of the native aortic valve (see Fig. 26.1 C, D). Most bioprosthetic valves are treated with anticalcifying agents or processes.

Newer-generation bovine pericardial valves (Carpentier-Edwards Magna, Edwards Lifesciences [see Fig. 26.1 D] or St. Jude Trifecta, LivaNova) offer improved hemodynamic performance compared with earlier-generation porcine bioprostheses (see Tables 26.1 and 26.2 ). In the aortic position, the antegrade velocity varies as a function of valve size but approximates 2.4 m/s, with a mean gradient of 14 mmHg and an indexed valve area of 1.04 cm ² /m ² . The pericardial aortic valve has a larger EOA at any given valve size between 19 and 29 mm. The average peak gradient in the mitral position is 9 ± 3 mmHg, with an EOA of 2.5 ± 0.6 cm ² . In a prospective randomized trial of patients with aortic valve disease, the Carpentier-Edwards Perimount Magna bovine pericardial valve (Edwards Lifesciences) demonstrated better hemodynamic performance and greater LV mass regression over 5 postoperative years than the newer-generation Medtronic Mosaic porcine valve (Medtronic, Inc.).

A small degree of regurgitation can be detected by color Doppler flow imaging in 10% of normally functioning bioprostheses. One limitation of earlier generations of bioprosthetic valves was their limited durability due to SVD, typically beginning within 5 to 7 years after implantation but varying by position and age at implantation, with tissue changes characterized by calcification, fibrosis, tears, and perforations (see Fig. 26.3 ).

SVD occurs earlier for mitral than for aortic bioprosthetic valves, perhaps because of exposure of the mitral prosthesis to relatively higher LV closing pressures. The process of SVD is accelerated in younger patients ( Fig. 26.4 ), in those with disordered calcium homeostasis (e.g., end-stage renal disease), and possibly in pregnant women independent of younger age. With most current-generation bioprosthetic pericardial valves, the durability is excellent, with SVD rates of 2% to 10% at 10 years, 10% to 20% at 15 years, and 40% at 20 years ^, ( Table 26.4 ; Fig. 26.5 ). Accelerated SVD and reduced survival have been reported with Mitroflow models 12A/LX.

Stentless bioprostheses

The rigid sewing ring and stent-based construction of certain bioprostheses allow easier implantation and maintenance of the three-dimensional relationships of the leaflets. However, these features also contribute to impaired hemodynamic performance and accelerated SVD. Stentless porcine valves (see Fig. 26.1 E) were developed in part to address these issues. Their use has been restricted to the aortic position. Implantation is technically more challenging, whether they are deployed in a subcoronary position or as part of a mini-root, and they are therefore preferred by only a minority of surgeons.

Early postoperative mean gradients can be less than 15 mmHg (with further improvement in valve performance over time due to aortic root remodeling), leading to lower peak exercise transvalvular gradients and more rapid reduction in LV mass. David et al. reported a rate of freedom from SVD with the stentless Toronto SPV (St. Jude Medical, St. Paul, MN) at 12 years of 69% ± 4%, 52% ± 8% for patients younger than 65 years, and 85% ± 4% for patients 65 years of age and older. The researchers limited the use of this stentless valve to older patients with small aortic annuli. They also emphasized the marked mortality hazard associated with reoperation for valve failure within 1 year of implantation. Sutureless bioprosthetic valves have been developed to decrease the complexity and duration of implantation of bioprosthetic valves (see Fig. 26.1 G).

Homografts

Aortic valve homografts are harvested from human cadavers within 24 hours of death and are treated with antibiotics and cryopreserved at −196°C. They have become most commonly implanted in the form of a total root replacement with reimplantation of the coronary arteries. Homograft valves appear to be resistant to infection and are preferred by many surgeons for management of aortic valve and root endocarditis in the active phase. Neither immune suppression nor routine anticoagulation is required.

Despite earlier expectations, long-term durability beyond 10 years is not superior to that for current-generation pericardial valves, and reoperation may be technically more challenging due to excessive root and leaflet calcification. In an echocardiographic follow-up study of 570 patients with aortic valve homografts, 72% had signs of valve dysfunction at 6.8 ± 4.1 years after implantation, with moderate to severe aortic regurgitation in 15.4%, moderate aortic stenosis in 10%, and severe aortic stenosis in 2.5%. Rates of homograft reoperation at 15 years for SVD, which do not account for all cases of SVD, approximate 20% for patients 41 to 60 years of age and 16% for those older than 60 years at the time of implantation.

Autografts

In the Ross procedure, the patient’s own pulmonic valve or autograft is harvested as a small tissue block containing the pulmonic valve, annulus, and proximal pulmonary artery; it is inserted in the aortic position, usually as a complete root replacement with reimplantation of the coronary arteries. The pulmonic valve and right ventricular outflow tract are then replaced with an aortic or a pulmonic homograft. The procedure therefore requires two separate valve operations, a longer time on cardiopulmonary bypass, and a steep learning curve. With appropriate selection of young patients by expert surgeons at experienced centers of excellence, operative mortality rates are less than 1%, and 20-year survival rates are as high as 95% and similar to those of the general population. Advantages of the autograft include its ability to increase in size during childhood growth, excellent hemodynamic performance characteristics, lack of thrombogenicity, and resistance to infection.

The hemodynamic performance characteristics of the pulmonary autograft are similar to those of a normal, native aortic valve. Early homograft stenosis occurs in 10% to 20% of patients and is caused by extrinsic compression from inflammation and adventitial fibrosis. ^, The procedure is usually reserved for children and young adults but should be avoided in patients with dilated aortic roots given the unacceptably high incidence of accelerated degeneration, pulmonary autograft dilatation, and significant regurgitation. Significant degrees of aortic regurgitation and the deposition of calcium are additional markers for suboptimal outcomes. In propensity-matched analyses, survival and functional outcomes with the Ross procedure have equaled or exceeded those observed with mechanical or bioprosthetic valve replacement in selected young patients.

Transcatheter bioprostheses

Transcatheter aortic valve replacement (TAVR) is a valuable alternative to surgical aortic valve replacement (SAVR) in patients with symptomatic severe aortic stenosis across the spectrum of operative risk. The TAVR case volume in the United States surpassed SAVR case volume for treatment of isolated aortic stenosis in 2016, and the gap has continued to widen. Two main types of transcatheter aortic valves are used: balloon-expandable valves and self-expanding valves (see Fig. 12.1 in Chapter 12 ).

The Edwards Sapien XT and Sapien 3 balloon-expandable valves (Edwards Lifesciences) consist of a three-leaflet pericardial bovine valve mounted in a cobalt-chromium frame. These valves are available in 20-, 23-, 26-, and 29-mm sizes. The most common access routes for TAVR are the transfemoral, transapical, and transaortic routes. Approximately 80% to 90% of TAVR procedures are performed using the transfemoral approach. As catheter sheath sizes decrease (i.e., 14 or 16 Fr for most valves), access is anticipated to shift even more toward the transfemoral approach. The transfemoral approach is associated with lower mortality rates and more rapid recovery compared with alternative access approaches.

The CoreValve Evolut R and Evolut Pro self-expanding valves (Medtronic, Inc.) consist of three leaflets of porcine pericardium seated relatively higher in a nitinol frame to provide true supra-annular positioning of valve leaflets. The valves are available in 23-, 26-, 29-, and 31-mm sizes. The CoreValve Evolut R and Evolut Pro are most frequently implanted using the transfemoral approach.

For a given aortic annulus size, transcatheter valves have larger EOAs and lower transvalvular gradients compared with surgical bioprosthetic valves. Rates of SVD have not been higher with TAVR valves than with SAVR valves. ^, Compared with SAVR, transcatheter valve implantation results in higher rates of PVL and heart block necessitating permanent pacemaker insertion. The risks of PVL and heart block have decreased over time with improved case selection and device performance. Moderate to severe PVL is associated with a 2.0- to 2.5-fold increase in mortality rates. Some studies suggest that even mild pulmonary valve regurgitation may have a deleterious impact on outcomes in vulnerable patient subsets (e.g., patients with severe LV concentric hypertrophy and severe diastolic dysfunction without preexisting AR).

Current-generation balloon-expandable valves (Sapien 3) are designed with a skirt at the inflow aspect of the valve stent to reduce the incidence of PVL. Self-expanding valves have slightly larger EOAs and lower gradients but somewhat higher rates of PVL than balloon-expandable valves. ^, Self-expanding valves are also associated with a higher incidence of postprocedural PPM. ^, Placement of a TAVR valve inside a failed surgically implanted bioprosthesis (valve-in-valve procedure) is approved for patients who are considered to be at high risk for reoperation.