11.5.2 Heart valve prostheses

The different types of prosthetic valves are divided into mechanical and biological valves. There are three main types of mechanical prostheses.

• Ball valve (Starr-Edwards), consisting of a Silastic™ ball oscillating in a cage made up of two arches fixed to a ring (steel) covered with Teflon™; when the valve is open, the flow passes around the ball; when the valve is closed, it is leakproof (Figure 11.34); the effective opening area is small and the size is considerable. This prosthesis is no longer implanted, but a few patients still have it.

Video: Starr valve in aortic position (long-axis view); the ball moves back and forth longitudinally during the cardiac cycle. 

 Figure 11.34: Starr-Edwards™ valve. Hemodynamically, it has 2 different openings: the annulus (large circular opening) and the annulus fibrosus (annular opening between the annulus fibrosus and the aortic or ventricular wall, with a smaller effective area) [From: Zipes DP et al, eds. Braunwald's Heart Disease, 7th ed. Philadelphia: Elsevier Saunders, 2005. Figure 57.47 ].

•  Oscillating disc valve (Medtronic-Hall™, Bjork-Shiley™): a pyrolocarbon disc oscillates about an eccentric pivot in a ring, allowing passage through two unequal-sized orifices; when closed, the valve has small leaks at the edges or at the central pivot (Figure 11.35). These prostheses are no longer used except in certain artificial hearts, but they are occasionally found.

 Video: Bjork-Shiley™ valve in mitral position; disc oscillates during cardiac cycle.

• Double-finned valve (St. Jude Medical™, Carbomedics™, ATS™, On-X™) in titanium and pyrolocarbon: When open, the fins form an angle of 85° with the flow, which passes through 2 crescent-shaped orifices on each side and a small central orifice with a rectangular cross-section where the gradient is greater. In the closed position, the fins form an angle of 25-30° with the plane of the ring and have small leaks at the edges (fin-ring junction) and at the pivots (Figure 11.36). The On-X™ unalloyed pyrolocarbon prosthesis is fixed in the supra-annular position, saving one size, i.e. 2 mm in diameter. Flow and gradient are measured in the two lateral orifices and not in the small central orifice where the gradient is up to 40-50% higher [43,47]. These stents have a lifetime of at least 25 years.

Figure 11.35: Oscillating disc valve (Medtronic-Hall™, Bjork-Shiley™). When opened, the valve has one large and one small opening. The angle of the disc is 70° to the flow axis, creating resistance in the large orifice and vortices in the small orifice. On closure, leaks occur at the edges and, in some models, at the pivot axis in the centre of the valve. disc [From: Zipes DP et al, eds. Braunwald's Heart Disease, 7th ed. Philadelphia: Elsevier Saunders, 2005. Figure 57.47] . 

 Figure 11.36: Double valve (St Jude Medical™, Carbomedics™, ATS™). A: mechanical double-finned valve (taken from: Zipes DP et al, eds. Braunwald's Heart Disease, 7th ed. Philadelphia: Elsevier Saunders, 2005. Figure 57.47). B: Diagram and TEE image of an open double-fin valve in the mitral position. When open, the wings form an angle of 85° with the flow, which passes through 2 crescent-shaped orifices on each side and a central slit of rectangular cross-section where the gradient is greater. C: Diagram and TEE image of a bi-fin valve closed in the mitral position. In the closed position, the leaflets are at an angle of 25° to the annulus plane and show autolavage leaks at the edges and at the fulcra. The prosthesis is placed with its fins in an anti-anatomical position in relation to the mitral leaflets, therefore the profile in which the two fins are visible on TEE is between 30° and 80°.

 The rings are covered with Dacron™ or braided Teflon™ to allow sutures to be anchored. Double-finned valves are the most commonly used as they offer the best haemodynamic profile. Mechanical prostheses typically have autolavage leaks, the purpose of which is to prevent the deposition of fibrin and platelets on the free edge of the ribs and on the inner part of the annulus, as this could block movement or prevent sealing. These leaks represent a fraction of regurgitation ≤ 5%.

On colour Doppler, they come from the inside of the annulus and the hinge; they are thin and extend over 1-2 cm. These characteristics distinguish them from paravalvular leaks originating outside the annulus, which are larger and originate from a visible solution of continuity between the annulus and the wall. Mechanical prostheses require lifelong anticoagulation, aiming for an INR of 3.0-3.5 for MVRs and 2.0-3.0 for AVRs (possibly 1.5-2.0 for On-X valves in the aortic position), with the addition of aspirin (75-250 mg/d) in the absence of contraindications (Table 11.6) [4,27,31,42]. Because they are less thrombogenic, the new prostheses do not require aspirin [36]. 

Biological valve prostheses

Biological valves are made from porcine valve tissue or bovine pericardium; this category also includes homografts (human tissue) and heterografts (animal tissue). The tissue is prepared in a glutaraldehyde bath, which reduces its antigenicity but alters its mechanical properties; the valves become less flexible, calcify and degenerate over time. They have a functional life of 12 to 20 years, which is inversely proportional to the patient's age, being longer in the elderly. In subjects aged 60 years, bioprostheses have an average life of 11.4 years in the mitral position and 17.6 years in the aortic position [1]. Bioprostheses may or may not be mounted on a frame, the diameter of which defines the size of the prosthesis, but the actual internal diameter is 2 mm smaller in porcine valves (Biocor™, Hancock™ II) and 1 mm smaller in pericardial valves (Perimount™, MagnaEase™); it is identical only in valves with leaflets implanted outside the frame (Mitroflow™, Trifecta™) [3]. Bioprostheses require anticoagulation for only 3 months (INR 2.5), after which endothelialisation is complete [4,27]. Omitting this period of anticoagulation triples the risk of thromboembolic events [40]. Long-term use of aspirin is controversial unless there are other reasons (coronary artery disease, stents, arterial disease, etc.) [4,42], but is most often recommended as routine [29] (Table 11.6) (Figure 11.37).

 Figure 11.37: Bioprosthetic heart valves. A: Valves assembled with bovine pericardial leaflets. B: Valves assembled with porcine pericardial leaflets. C: stentless valves [taken from: Arsalan M, Walther T. Durability of prostheses for transcatheter aortic valve implantation. Nat Rev Cardiol 2016; 13:360-7].

•  Conventionally mounted bioprostheses; porcine aortic valve (Biocor™, Hancock™ II) or bovine pericardial valves (Carpentier-Edwards Perimount™ or MagnaEase™, Trifecta™, Mitroflow™) mounted on a metal ring and suspended by their commissures on 3 vertical pins. The valves have a certain opening inertia due to their relative rigidity. For the same size, the effective orifices of these valves are smaller than those of mechanical valves and unmounted bioprostheses; their gradients are also higher. When closed, these valves are airtight or have small leaks, usually centrally but sometimes at the commissures. For the same size, bovine pericardial valves have a slightly better haemodynamic performance than porcine valves and a slightly lower attrition rate; their mean survival without revision is 19 years (Figure 11.38) [1,12].

 Video: Biological prosthesis in aortic position (short-axis view); the 3 cusps and 3 suspension posts are clearly visible.

Video: Biological prosthesis in mitral position; only 2 of the 3 struts and 3 leaflets are visible in an echocardiographic cross-section; the most anterior strut encroaches into the LV outflow tract.

 Figure 11.38: Biological valve prosthesis (from: Zipes DP et al, eds. Braunwald's Heart Disease, 7th ed. Philadelphia: Elsevier Saunders, 2005. Figure 57.49). A: Carpentier-Edwards bioprosthesis. B: Prosthesis closed in the aortic position; the 3 leaflets are in the same position as in the native valve. C: Open prosthesis in the aortic position; the opening is practically circular and easily measured by planimetry (TEE views short aortic axis 50°).

•  New generation of mounted bioprostheses; a thinner ring (Carpentier-Edwards Perimount Magna™, in bovine pericardium) increases the internal diameter by 2 mm for the same external diameter; its special configuration allows supra-annular fixation, saving one size compared to a standard prosthesis. Reduced spike size (St. Jude Biocor™, porcine valve) and special treatment of the porcine valve (Medtronic Mosaic™) provide a mechanical advantage, reduce the risk of calcification and increase longevity (70% at 20 years) [13]. Out-of-frame leaflets (Mitroflow™, Trifecta™) increase the effective opening area [3].
• Stentless bioprostheses (Freestyle™, Toronto SPV™, CryoLife™, Freedom Solo™, based on a porcine valve with an outer cover of polyester fabric): for the same diameter, they have a larger opening and a lower gradient than other valves. Their configuration and the absence of an annulus allow them to be used only in the aortic position; the diameter of the patient's aortic annulus and that of the sino-tubular junction must not differ by more than 10% for the prosthesis to be correctly suspended at the commissures (Figure 11.39). These valves are technically more difficult to implant because the fixation points on the prosthesis and on the patient's aortic annulus must be strictly equidistant, but their rate of endocarditis and thrombosis is very low. They are generally indicated for patients over 60 years of age with small aortic rings.

• Bioprostheses without fixation (sutureless) (Perceval S™, 3F Enable™, Intuity Elite™): Their implantation requires ECC, aortotomy and excision of the native valve, but is faster because the valve is placed by self-expansion or balloon inflation as with percutaneous prostheses (TAVI). The prosthesis is fixed under direct visual control, which reduces the risk of paravalvular leakage compared with TAVI (15% vs 45%) [11]. The results are equivalent to a conventional bioprosthesis, but the duration of ECC is halved. This technique is well suited for small aortic rings (< 2.2 cm), high-risk cases (age > 80 years) and fast-track patients [17,33].
• Homografts: mitral valve or aortic valve (with ascending aortic sleeve) harvested from human cadavers and preserved in liquid nitrogen (-196°C). Although they have an accelerated wear rate, they are resistant to infection and are particularly indicated for valve replacement in cases of endocarditis [23]. Their problem is their limited availability and the risk of malposition during implantation, which is more difficult than that of an attached valve.
• Heterografts: The bovine jugular vein is a valved conduit that can be prepared with glutaraldehyde and cryopreserved (Contegra™) to replace the right outflow tract (RVOT, pulmonary valve and PA trunk) [44].

 Video: Contegra™ heterograft in the pulmonary position (long-axis view of the valve and pulmonary artery in a short-axis image of the aortic arch).

•  Autograft: The Ross procedure consists of replacing the aortic valve with the patient's own pulmonary valve, which is replaced by a heterograft or homograft (see Aortic valve surgery and Figure 11.146). The main advantage of this technique is that it allows for growth, as the aortic neo-valve develops at the same pace as the child does.
• Valved LV conduit - descending aorta: In the case of multiple reoperations, it is possible to short-circuit the aortic root by means of a Dacron tube fitted with a prosthetic valve which is implanted in the apex of the LV and anastomosed termino-laterally to the descending aorta. The operation is performed via a left thoracotomy without ECC [9,28].

The major problem with bioprostheses is their long-term wear rate. With current prostheses, structural degeneration of the valve (tears, thickening, fibrosis, calcification) occurs in 4-8% of cases after 5 years, 10-15% after 10 years, 21% after 15 years and 50-60% after 20 years; reoperation for valve replacement is required in 2-4% of cases after 10 years and 15-20% after 20 years [1,8,12]. In the aortic position, deterioration is moderate when the mean gradient is 20-40 mmHg (V_LVOT /V_Ao ratio 0.25-0.35) and severe when it is > 40 mmHg (V_LVOT /V_Ao < 0.25); regurgitation is moderate or severe, respectively [8]. The durability of bioprostheses is poorer in young people whose cardiac erethism wears the valve prematurely and in patients with impaired calcium metabolism (renal failure, dialysis, parathyreosis) [1].

Endovascularly implanted bioprostheses (TAVI, transcatheter aortic valve implantation) (Figure 11.40) are indicated in patients with an intermediate or high surgical risk, preferably elderly patients, whose life expectancy does not exceed 5-10 years (see TAVI). Long-term outcomes are therefore not critical. The rate of deterioration with TAVI is 2-10% at 5 years; in the short term, the new prostheses give results that are almost comparable to those of surgically implanted valves [1]. However, laboratory fatigue tests show that the durability of TAVI valves is about half that of surgical valves (7.8 versus 16 years) [26]. In addition, the risk of valve thrombosis is around 7% with TAVI compared to less than 1% with surgical replacement [21]. If necessary, reoperation can be performed endovascularly (valve-in-valve technique) (see Chapter 10 Other valve procedures without ECC).

Figure 11.40: There are many types of valve available for transcatheter implantation (TAVI). A: Balloon-expandable prostheses. B: Self-expanding porcine pericardial prosthesis. C: Self-expanding bovine pericardial stent. D: Self-expanding prosthesis in native porcine valve. E: alternative bovine pericardial prosthesis [taken from: Arsalan M, Walther T. Durability of prostheses for transcatheter aortic valve implantation. Nat Rev Cardiol 2016; 13:360-7].

 Haemodynamics of prostheses

Each type of prosthesis has its own haemodynamic characteristics. However, the opening surface area of a prosthetic valve is smaller than the normal surface area of the native valve, varying from 1.1 cm² (aortic bioprosthesis 19) to 3.5 cm² (mechanical mitral valve 33). As a result, the pressure gradient is significant; the mean gradient varies from 3 mmHg (St Jude mitral valve) to 15 mmHg (aortic bioprosthesis) (Tables 11.7, 11.8 and 11.9) [6,29,31,42]. For the same size, the gradients are in ascending order: homografts < stentless bioprostheses < mechanical valves < stented bioprostheses. Stents fixed in the supra-annular position have a lower gradient than those fixed in the native annulus. The maximum gradient is considered abnormal if it is > 35 mmHg at the aortic level and > 10 mmHg at the mitral level. Most of the technical data for each type of prosthesis can be found in the reference Echo prothèses 2009.pdf (see under Guidelines) and on the open access website: www.valveguide.ch [15]. 

• The geometry of stents induces a pressure recovery phenomenon: the law of conservation of energy means that pressure decreases as velocity increases in the constricted zone, but the kinetic energy is converted back into pressure as soon as the flow slows beyond the constriction [38]. Doppler calculates ΔP from Vmax, which corresponds to the narrowest point of the flow (vena contracta); this zone is generally very short (1-3 mm) and pressure rises rapidly in the downstream chamber (see Figure 11.44). This phenomenon is more pronounced in prostheses than in native valves, and increases with valve size because the turbulence created by small prostheses causes a loss of kinetic energy. For these reasons, the mean gradient is a better criterion than the maximum gradient [47].
• The greater the mismatch between the prosthesis and the receiving chamber, the lower the pressure recovery because energy is wasted in turbulence; thus, a large aorta reduces the downstream pressure because the energy loss due to turbulence is greater. Therefore, the inflated gradient measured by Doppler tends to underestimate the true orifice area in small prostheses and large aortas [47].
• In the aortic position, the velocity in the LV outflow tract is often greater than 1.5 m/s after valve replacement in cases of stenosis and LVH. It is therefore essential to use the modified Bernoulli equation ΔP = 4 · (V2² - V1² ) and not its simplified version (ΔP = 4 · V² ). Failure to subtract V_max in the LVOT leads to an overestimation of the gradient by 9 to 30 mmHg.
• Surface calculations such as the half-pressure time or the equation of continuity (see Chapter 25, Half-pressure time, Equation of continuity) assume that the orifices are circular, which they are not in mechanical prostheses. Therefore, the half-pressure time is not applicable to assess the surface area of a mitral prosthesis. In the continuity equation used to measure aortic valve area, the reference value is generally the stroke volume in the LVOT (S_Ao = (S_LVOT · ITV_LVOT ) / ITV_VAo ). However, the cross-sectional area of the LVOT is oval rather than circular, and the long-axis slice used in echocardiography to assess its diameter actually measures the smallest diameter, which is 17% smaller, underestimating the area by an average of 24% [5,16]. In addition, measuring the diameter of the LVOT is difficult because of the brightness and shadows of the prosthesis.
• The gradient is increased by 40% in the central part of double-finned valves, so it must be measured with the Doppler axis in one of the two wide lateral orifices; in biological prostheses it is measured in the centre of the valve.
• On leaving bypass, the flow is accelerated due to catecholaminergic stimulation and the transient increase in systolic volume (transfusion, high flow, extrasystole); the artificially accelerated velocity through the prosthesis tends to underestimate the real surface area.
• Low pressure in the aortic root (vasoplegia, intra-aortic balloon pump, ascending aortic aneurysm) increases the gradient of an aortic prosthesis by reducing the downstream pressure. In case of IABP, this difference is of the order of 30-40 mmHg.