Surgical prosthetic valves

Comments

The two major categories of prosthetic valves are tissue valves and mechanical valves. Bioprosthetic (or tissue) valve leaflets are fashioned from bovine pericardium or a porcine aortic valve. The leaflets are supported by a rigid ring around the annulus with metal or polymer stents that support the commissures of the valve leaflets or by the cylindrical stent of the self-expandable valve. Implantation of these “stented” bioprostheses involves sewing an appropriately sized valve into the annulus, with the valve height and symmetry ensured by the annular ring and struts. In contrast, stentless tissue valves are supported only by a cylinder of flexible fabric or tissue. Implantation of stentless valves involves placement and suturing both at the annulus, and at the top of the commissures at the appropriate height.

Tissue valves open with a central circular orifice with a valve opening and closing motion similar to a native trileaflet aortic valve. However, the antegrade velocities (and pressure gradient) are higher than expected for a native valve because the sewing ring reduces the effective orifice area. At smaller valve sizes, the degree of functional stenosis can be significant, with a smaller effective orifice area than a similar-size mechanical valve. Thus the optimal valve choice in each patient depends on the size of valve that can be implanted, in addition to considerations of valve durability and long-term anticoagulation. If the implanted valve is too small for the patient size, patient-prosthesis mismatch (defined as an indexed effective valve area <0.85 cm ² /m ² ) is associated with increased short-term mortality and suboptimal long-term outcomes.

Echocardiographic evaluation of the prosthetic valve after implantation should follow a standard format as shown in Fig 5.1

Suggested reading

• 1.

  Yoganathan AP, Raghav V. Fluid dynamics of prosthetic valves. In Otto CM, editor: The practice of clinical echocardiography, ed 5, Philadelphia, 2016, Elsevier.

• 2.

  Maslow AD, Bert AA: Echocardiographic evaluation of prosthetic valves. In Oxorn D, editor: Intraoperative echocardiography. Practical echocardiography series, Philadelphia, 2012, Elsevier, pp 95–130.

Comments

In the mitral position, mechanical valves are often used, as many of these patients are on chronic anticoagulation for atrial fibrillation. When bioprosthetic valves are used, stented valves are needed, rather than stentless valves, given the anatomy of the mitral annulus and left ventricle. The appearance of the valve in the mitral position is similar to a native aortic valve, with the stents protruding into the LV outflow tract. In the absence of a small and hypertrophied left ventricle, the stents rarely produce outflow tract obstruction ( Fig 5.16 ). The flow through the mitral prosthesis is similar to a normal mitral valve, with an early diastolic peak (E-velocity), normal deceleration time and an atrial velocity peak (A-velocity) if the patient is in sinus rhythm. Velocities are only slightly higher than for a native valve, due to the large effective valve area and low left atrial to left ventricular pressure gradient in diastole.

Suggested reading

• 1.

  O’Gara PT: Prosthetic heart valves. In Otto CM, Bonow RO, editors: Valvular heart disease, ed 4, Philadelphia, 2014, Elsevier, pp 420–438.

Comments

Replacement of right-sided valves is less common than left-sided valves in adults. Mechanical valves in the tricuspid position have a high rate of valve thrombosis, whereas tissue valves have rapid valve deterioration. Thus tricuspid valve repair is preferred whenever possible. Bioprosthetic tricuspid valves have an appearance and flow dynamics similar to a mitral bioprosthesis. A small degree of central regurgitation is normal with bioprosthetic valves in any position, although not seen in this case.

Suggested reading

• 1.

  Lin G, Bruce CJ, Connolly HM: Diseases of the tricuspid and pulmonic valves. In Otto CM, Bonow RO, editors: Valvular heart disease, ed 4, Philadelphia, 2014, Elsevier, pp 375–395.

Comment

The most common type of mechanical valve is a bileaflet valve with two semicircular leaflet occluders or a tilting disk valve with a single circular occluder that pivots on hinges or a central strut. Older mechanical valve types, such as ball-in-cage valves now are rarely seen. The normal fluid dynamics of bileaflet mechanical valve prosthesis are characterized by a small amount of regurgitation due to the closure of the valve occluders. With a bileaflet valve, there are typically two converging jets from the pivot points, a small central jet, and a variable number of peripheral jets, with little signal aliasing. In addition, these normal regurgitant jets are small in size, originating from within the sewing ring.

In contrast, pathologic regurgitation occurs in unexpected locations, often paravalvular, and is usually associated with larger, more eccentric jets. Prosthetic valve regurgitation is evaluated using the same approaches as for native valves, including measurement of the vena contracta width, evaluation of the intensity and shape of the continuous wave Doppler curve, and calculation of regurgitant volume and orifice area. However, evaluation of prosthetic valves, particularly in the mitral position, requires transesophageal imaging because shadowing and reverberations from the prosthesis preclude evaluation from the transthoracic approach. Detection of the proximal isovelocity surface area (PISA) on the ventricular side of the valve is helpful for identification of the origin of the regurgitant jet, but measurements are often difficult due to PISA asymmetry and poor image quality.

Suggested reading

• 1.

  Pibarot P: Prosthetic valve dysfunction: echocardiographic recognition and quantitation (including paravalvular regurgitation and closure) In Otto CM, editor: The practice of clinical echocardiography, ed 5, Philadelphia, 2016, Elsevier.

• 2.

  Beigel R, Siegel RJ: Evaluation of prosthetic valve dysfunction with the use of echocardiography, Rev Cardiovasc Med 15(4): 332–350, 2014.

• 3.

  Hahn RT: Mitral prosthetic valve assessment by echocardiographic guidelines, Cardiol Clin 31(2):287–309, 2013.

Comments

The bileaflet valve consists of two pyrolytic carbon disks attached to a rigid ring by two small hinges. This design results in a small central slit-like orifice and two larger lateral semicircular orifices when the valve is open. As for tissue valves, the hemodynamics of a normally functioning mechanical valve are inherently stenotic, compared with a normal native valve, with tables available listing the expected transvalvular velocities, pressure gradients, and expected orifice areas for each valve type and size. Even higher velocities may be recorded with normally functioning valves due to the fluid dynamics of the central slit-like orifice. Effective orifice areas can be calculated using the continuity equation, as for native valves. Because the velocity and pressure gradient across a prosthesis valve depend on transvalvular flow rate, as well as valve type and size, a baseline examination when valve function is clinically normal is useful for distinguishing valve stenosis from normal hemodynamics on serial examinations.

Suggested reading

• 1.

  Blauwet LA, Miller FA Jr: Echocardiographic assessment of prosthetic heart valves, Prog Cardiovasc Dis 57(1):100–110, 2014.

• 2.

  Eleid MF, Thomas JD, Nishimura RA: Increased prosthetic valve gradients: abnormal prosthetic function or pressure recovery? Catheter Cardiovasc Interv 84(6):908–911, 2014.

Comments

Although the most straightforward way to determine the type and location of valve prostheses in a patient is to review the medical record or the valve card patients are given to carry with them. In some cases valve position and type must be inferred from physical examination or chest radiography. As shown in this example, a standard postero-anterior and lateral chest radiograph allows identification of valve position, based on the normal position of the valves within the cardiac silhouette ( Figs 5.37 , 5.42 ).

A valve prosthesis in the tricuspid position has an appearance and flow dynamics similar to the prosthetic mitral valve, although velocities and pressure gradients may be even lower because a larger prosthesis can be placed in the tricuspid annulus. In addition to the mean pressure gradient, evaluation of mechanical atrioventricular valves includes measurement of the pressure half-time. The empiric constant of 220, derived from studies of native mitral stenosis, can also be used for central orifice bioprosthetic valves and for mechanical valves to estimate valve area.

Suggested reading

• 1.

  Otto CM: Prosthetic valves. In Otto CM, editor: Textbook of clinical echocardiography, ed 5, Philadelphia, 2013, Elsevier, pp 342–371.