Fig. 10.1
Different types of prosthetic valves. (a) Bileaflet mechanical valve (St Jude); (b) monoleaflet mechanical valve (Medtronic Hall); (c) caged ball valve (Starr-Edwards); (d) stented porcine bioprosthesis (Medtronic Mosaic); (e) stented pericardial bioprosthesis (Carpentier-Edwards Magna); (f) stentless porcine bioprosthesis (Medtronic Freestyle); (g) percutaneous bioprosthesis expanded over a balloon (Edwards Sapien); (h) self-expandable percutaneous bioprosthesis (CoreValve) (From Pibarot and Demesnil [3] with permission)
Bioprosthetic Valves
Bioprosthetic valves can be classified according to their species origin, nature of tissue used, whether or not they are mounted on a stent, whether they are placed surgically or percutaneously, and the site of placement.
A.
A tissue valve is an actual valve or biological tissue derived from:
(a)
Animals (heterograft or xenograft): Bovine or porcine
(b)
Humans (homograft or allograft): the Ross procedure describes transposing of the patient’s pulmonic valve in the aortic position and placing a bioprosthetic valve in the pulmonic position.
B.
A tissue valve is made of biologic tissue derived from:
(a)
Aortic valve tissue: usually porcine in origin and consists of three porcine aortic valve leaflets cross-linked with glutaraldehyde.
(b)
Pericardial tissue: pericardial valves are usually derived from sheets of bovine pericardium.
C.
A tissue valve may either be a:
(a)
Stented bioprosthesis: This may consistent of sheets of bovine pericardium mounted inside or outside a supporting stent or a porcine aortic valve mounted on a metallic or polymer supported stent.
(b)
Stentless bioprosthesis: may also be either porcine aortic valves or derived from bovine pericardium and are devoid of stent in an attempt to improve valve hemodynamics and durability.
D.
A tissue valve may either be placed:
(a)
Surgically: surgical aortic valve replacement (SAVR) through either a full sternotomy or minimally invasive surgery through a hemisternotomy or a right thoracotomy.
(b)
Percutaneously: Transaortic valve replacement (TAVR) that may be implanted through a transfemoral, transaortic, and transapical approaches. Other alternatives may include a transcaval approach where the valve is placed through the femoral vein and then through an IVC-aortic created connection as well as a transsubclavian approach. TAVR valves may either be placed as balloon expandable or self expandable techniques.
E.
A tissue valve may either be placed:
(a)
Annular: at the level of the aortic annulus
(b)
Supra-annular: This is designed to lift the valve out of the annulus in order to minimize the resistance contained in the annulus.
The most common aortic position tissue valve is a stented aortic valve xenograft. These aortic valves are extracted, preserved, and fixed within a mount attached to a Dacron sewing ring. Pericardial prosthetic valve leaflets are typically comprised of pericardial tissue sewn on to stent posts.
Stentless bioprostheses are also commonly used in the aortic position. Stentless xenograft valves are usually made from a preparation of a porcine aorta. This type of valve is supported by a “cuff” and does not require rigid stents. These valves (e.g. Medtronic Freestyle) are porcine aortic valves that include the annulus, valve and aortic root. Tissue valves have the advantage of non-thrombogenicity such that long term anticoagulation is not necessary, however the durability of a tissue valve is limited. Stentless bioprosthesis and pulmonary autografts may have a temporary increase in the gradients for 3 months after surgery due to edema between the prosthesis and the aortic wall or because of outflow tract remodelling that usually regress.
Mechanical Valves
Mechanical valves are classified as caged ball and tilting disk designs.
A.
The caged ball (e.g. Starr-Edwards) valves: they are no longer implanted. They consisted of a silastic ball with a circular sewing ring and a cage formed by three metal arches. Patients with these types of valves implanted require physicians to be versed in their special characteristics and echo image.
B.
Tilting Discs: Several tilting disk valves are in use, including:
(a)
Single tilting disk valves or monoleaflet valves (e.g. Bjork Shiley, Medtronic Hall, Omniscience): these are secured by lateral or central struts and the resultant two valve orifices are of different sizes
(b)
Bileaflet tilting disk valves (e.g. St Jude Medical and Carbomedics): these are made of two semilunar disks attached to a rigid valve ring by small hinges and there are three resultant valve orifices; one smaller central and two larger peripheral ones.
The most frequent mechanical aortic valve implanted is the bileaflet tilting disk valve. These valves differ among manufacturers based upon the design, shape and angle of opening of the leaflets, and design and shape of the housing and sewing ring. The major disadvantages of mechanical valves are related to the necessity for life long anticoagulation with warfarin, whereas durability is the major advantage.
Causes of Prosthetic Valve Aortic Stenosis
The most common causes of prosthetic aortic valve stenosis are valve degeneration (causing bioprosthetic valve stenosis or regurgitation), pannus formation, valve thrombosis, and rarely endocarditis (Fig. 10.2). Both bioprosthetic and mechanical vales are at risk of fibrous tissue or pannus overgrowth causing prosthetic stenosis. This is more common than valve thrombosis, which occurs more commonly in mechanical valves and presents with thrombo-embolic complications or an incidental finding on echocardiography, although critical valve thrombosis is uncommon. In addition to type of valve, the risk of thrombosis is also related to patient factors as left ventricular function, left atrial size, atrial fibrillation and most commonly is related to coagulopathy or lack of adherence to anticoagulation. The role of the novel anticoagulants in thromboembolic prevention in patients with mechanical valves is unclear. The CATHAR trial (Comparison of Antithrombotic Treatments After Aortic Valve Replacement, clinical trial number NCT02128841) will compare the role of Rivaroxaban (Xarelto) to warfarin in patients with mechanical valves. However, RE-ALIGN study found an increased risk of stroke and higher bleeding in patients who received a mechanical heart valve and were treated with Dabigatran (Pradaxa) as compared to warfarin [4].
Fig. 10.2
Causes of prosthetic aortic valve stenosis: pannus formation (1–2), thrombus formation (3–6), and vegetation (7–8): (From Zoghbi et al. [2]; Extract from the Educational Case on “Mechanical Valve Thrombosis” by Prof. D. Messika-Zeitoun and Dr C. Cimadevilla, members of the ESC Working Group on valvular heart diseases. Retrieve the full case on http://www.escardio.org/communities/Working-Groups/valvular/education/featured-cases/Pages/mechanical-valve-thrombosis.aspx; and Orban et al. [25] with permission)
Obstruction of homografts or Stentless bioprosthesis with thrombus or pannus is less frequent than mechanical or stented bioprosthesis. In a study of 251 patient with prosthetic valve malfunction requiring reoperation, the linearinzed rate of pannus formation was 0.24 %/patient-year (48/251) and that of thrombosis was 0.15 % (29/251) [2].
The distinction between thrombus, pannus, and endocarditis as the underlying etiology of obstruction is essential if thrombolytic therapy or surgery is contemplated. Using TEE along with clinical parameters, the following features may help differentiate the different etiologies of prosthetic valve stenosis (Fig. 10.2):
1.
Pannus Formation: this includes a small echodense echocardiographic mass on the valve that may not be visible in 30 % of the time and is more common in the aortic position. It is usually associated with a longer and gradual duration and onset of symptoms.
2.
Valve Thrombosis: thrombi present as larger masses with more echolucency compared to pannus. They more commonly present with shorter duration of symptoms and more abrupt onset, with a history of inadequate anticoagulation. Moreover, TEE is more likely to detect abnormal prosthetic valve motion with an occasional catastrophic presentation.
Patients with NYHA class I and II and who have recent symptom onset and thrombi with a resultant aortic valve area <0.8 cm2 are candidates for lytic therapy if IV heparin fails. Conversely, patients with mobile or larger thrombi or class III and IV symptoms should undergo emergency surgery. Baseline TEE and follow up serial Doppler of the prosthetic valves is essential with thrombolytic therapy. It is to be noted that both pannus and thrombus can occur together and after successful lytic therapy. Follow up Doppler to diagnose residual pannus is essential to avoid valve re-thrombosis.
3.
Vegetations from endocarditis: These tend to occur in the valve ring area and can affect valve function by interrupting the valve leaflet, stent, or occluder and cause prosthetic stenosis (or regurgitation). Vegetations are usually irregularly shaped and mobile with independent movement. Differentiating vegetations from thrombus, sutures, and pledgets, can be difficult, and comparison to previous or post-operative baseline studies may be essential. Moreover, interpreting the echocardiographic image in the context of the clinical picture (fever, positive blood cultures, etc.) will also aid with the diagnosis of prosthetic valve endocarditis [2].
Hemodynamics and Anticipated Gradients of Prosthetic Valves
The initial suspicion of abnormal prosthetic valve function is often the discovery of a new murmur or the incidental finding of a high trans-prosthetic gradient. Most of the relevant information regarding the function of a prosthetic valve is obtained from a thorough and quantitative Doppler examination. The range of normal, or anticipated gradients across a prosthetic aortic valve vary depends on the size of the prosthesis as well as the type of valve (i.e. mechanical valve, stented and stent-less bioprostheses, trans-catheter valves, have a decreasing expected trans valve gradient in that order) [2].
In general, Doppler gradients across normally functioning prosthetic valves resemble those obtained across a native valve with mild aortic stenosis. The hemodynamics and blood flow characteristics can differ substantially between the various prosthetic aortic valve types and according to patient characteristics.
The normal pattern of flow through stented aortic bioprostheses is typically a circular central flow field with a peak velocity between 2 and 3 m/s, corresponding to a mean pressure gradient of 10–15 mmHg, along with a triangular shape of the velocity contour with an early systolic maximal velocity. Importantly, high gradients may be seen across normal valves with a small size: for example a 29 mm St. Jude bi-leaflet valve may have a maximum gradient of 18 mmHg, while a 19 mm Carpentier-Edwards bio-prosthetic stented valve may have a normal maximum gradient of 43 mmHg. The hemodynamics of tissue valves vary significantly based upon the valve type, structure and size [5].
In general, lower profile valves such as stent-less substitutes (stentless bioprostheses, aortic homografts) tend to have lower trans-prosthetic gradients than stented valves. In addition, trans-catheter aortic valve replacement (TAVR) is associated with minimal gradients owing to the low profile of the valve.
See Table 10.1 for the different types of prosthetic valves and range of “normal” (anticipated) gradients depending on the type and size of valve prosthesis.
Table 10.1
Normal Doppler echocardiography values for selected prosthetic aortic valves
Cryolilfe Stentless | 19 21 23 25 27 | 9.0 ± 2.0 6.6 ± 2.9 6.0 ± 2.3 6.1 ± 2.6 4.0 ± 2.4 | 1.5 ± 0.3 1.7 ± 0.4 2.3 ± 0.2 2.6 ± 0.2 2.8 ± 0.3 | |
Edwards Duromedics Bileaflet | 21 23 25 27 | 39.0 ± 13 32.0 ± 8.0 26.0 ± 10.0 24.0 ± 10.0 | ||
Edwards Mira Bileaflet | 19 21 23 25 | 18.2 ± 5.3 13.3 ± 4.3 14.7 ± 2.8 13.1 ± 3.8 | 1.2 ± 0.4 1.6 ± 0.4 1.6 ± 0.6 1.9 | |
Hancock Stented porcine | 21 23 25 | 18.0 ± 6.0 16.0 ± 2.0 15.0 ± 3.0 | 12.0 ± 2.0 11.0 ± 2.0 10.0 ± 3.0 | |
Hancock II Stented porcine | 21 23 25 29 | 34.0 ± 13.0 22.0 ± 5.3 16.2 ± 1.5 | 14.8 ± 4.1 16.6 ± 8.5 10.8 ± 2.8 8.2 ± 1.7 | 1.3 ± 0.4 1.3 ± 0.4 1.6 ± 0.4 1.6 ± 0.2 |
Homograft Homograft valves | 17–19 19–21 20–21 20–22 22 22–23 22–24 24–27 26 25–28 | 1.7 ± 0.3 0.4 ± 0.6 | 9.7 ± 4.2 7.9 ± 4.0 7.2 ± 3.0 5.6 ± 3.1 6.2 ± 2.6 | 4.2 ± 1.8 5.4 ± 0.9 3.6 ± 2.0 3.5 ± 1.5 5.8 ± 3.2 2.6 ± 1.4 5.6 ± 1.7 2.8 ± 1.1 6.8 ± 2.9 6.2 ± 2.5 |
Intact Stented porcine | 19 21 23 25 27 | 40.4 ± 15.4 40.9 ± 15.6 32.7 ± 9.6 29.7 ± 15.0 25.0 ± 7.6 | 24.5 ± 9.3 19.6 ± 8.1 19.0 ± 6.1 17.7 ± 7.9 15.0 ± 4.5 | 1.6 ± 0.4 1.6 ± 0.4 1.7 ± 0.3 |
Ionescu-Shiley Stented bovine pericardial | 17 19 21 23 | 23.8 ± 3.4 19.7 ± 5.9 26.6 ± 9.0 | 13.3 ± 3.9 15.6 ± 4.4 | 0.9 ± 0.1 1.1 ± 0.1 |
Labcor Santiago Stented bovine pericardial | 19 21 23 25 | 18.6 ± 5.0 17.5 ± 6.6 14.8 ± 5.2 12.3 ± 3.4 | 11.8 ± 3.3 8.2 ± 4.5 7.8 ± 2.9 6.8 ± 2.0 | 1.2 ± 0.1 1.3 ± 0.1 1.8 ± 0.2 2.1 ± 0.3 |
Labcor Synergy Stented porcine | 21 23 25 27 | 24.3 ± 8.1 27.3 ± 13.7 22.5 ± 11.9 17.8 ± 7.0 | 13.3 ± 4.2 15.3 ± 6.9 13.2 ± 6.4 10.6 ± 4.6 | 1.1 ± 0.3 1.4 ± 0.4 1.5 ± 0.4 1.8 ± 0.5 |
MCRI On-X Bileaflet | 19 21 23 25 | 21.3 ± 10.8 16.4 ± 5.9 15.9 ± 6.4 16.5 ± 10.2 | 11.8 ± 3.4 9.9 ± 3.6 8.6 ± 3.4 6.9 ± 4.3 | 1.5 ± 0.2 1.7 ± 0.4 1.9 ± 0.6 2.4 ± 0.6 |
Medtronic Advantage Bileaflet | 23 25 27 29 | 10.4 ± 3.1 9.0 ± 3.7 7.6 ± 3.6 6.1 ± 3.8 | 2.2 ± 0.3 2.8 ± 0.6 3.3 ± 0.7 3.9 ± 0.7 | |
Medtronic Freestyle Stentless | 19 21 23 25 27 | 11.0 ± 4.0 | 13.0 ± 3.9 9.1 ± 5.1 8.1 ± 4.6 5.3 ± 3.1 4.6 ± 3.1 | 1.4 ± 0.3 1.7 ± 0.5 2.1 ± 0.5 2.5 ± 0.1 |
Medtronic Hall Single tilting disc | 20 21 23 25 27 | 34.4 ± 13.1 26.9 ± 10.5 26.9 ± 8.9 17.1 ± 7.0 18.9 ± 9.7 | 17.1 ± 5.3 14.1 ± 5.9 13.5 ± 4.8 9.5 ± 4.3 8.7 ± 5.6 | 1.2 ± 0.5 1.1 ± 0.2 1.4 ± 0.4 1.5 ± 0.5 1.9 ± 0.2 |
Medtronic Mosaic Stented porcine | 21 23 25 27 29 | 23.8 ± 11.0 22.5 ± 10.0 | 14.2 ± 5.0 13.7 ± 4.8 11.7 ± 5.1 10.4 ± 4.3 11.1 ± 4.3 | 1.4 ± 0.4 1.5 ± 0.4 1.8 ± 0.5 1.9 ± 0.1 2.1 ± 0.2 |
Mitroflow Stented bovine pericardial | 19 | 18.6 ± 5.3 | 13.1 ± 3.3 | 1.1 ± 0.2 |
Monostrut Bjork-Shiley Single tilting disc | 19 21 23 25 27 | 27.5 ± 3.1 20.3 ± 0.7 | 27.4 ± 8.8 20.5 ± 6.2 17.4 ± 6.4 16.1 ± 4.9 11.4 ± 3.8 | |
Prima Stentless | 21 23 25 | 28.8 ± 6.0 21.5 ± 7.5 22.1 ± 12.5 | 13.7 ± 1.9 11.5 ± 4.9 11.6 ± 7.2 | 1.4 ± 0.7 1.5 ± 0.3 1.8 ± 0.5 |
Omnicarbon Single tilting disc | 21 23 25 27 | 37.4 ± 12.8 28.8 ± 9.1 23.7 ± 8.1 20.1 ± 4.2 | 20.4 ± 5.4 17.4 ± 4.9 13.2 ± 4.6 12.4 ± 2.9 | 1.3 ± 0.5 1.5 ± 0.3 1.9 ± 0.5 2.1 ± 0.4 |
Omniscience Single tilting disc | 21 23 | 50.8 ± 2.8 39.8 ± 8.7 | 28.2 ± 2.2 20.1 ± 5.1 | 0.9 ± 0.1 1.0 ± 0.1 |
Starr Edwards Caged ball | 23 24 26 27 29 | 32.6 ± 12.8 34.1 ± 10.3 31.8 ± 9.0 30.8 ± 6.3 29.0 ± 9.3 | 22.0 ± 9.0 22.1 ± 7.5 19.7 ± 6.1 18.5 ± 3.7
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