Prosthetic Valves

Published on 21/06/2015 by admin

Last modified 21/06/2015

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8 Prosthetic Valves

Scanning Issues

The expected appearance and hemodynamics of prostheses differ widely for different sizes and models of valve prostheses. Interpretation of appearance and hemodynamics requires knowing the specific size and design of the prosthesis. Therefore, know (and record) the following parameters of each prosthetic valve you scan:

Type, including model and year

Size

Notes

Use zoom views liberally.

Recall that the correlation of pressure half time (PHT) to mitral valve effective oriface area is poor for all forms of mechanical mitral valve replacement (MVR), and that the PHT method should not be emphasized for the assessment of mechanical MVRs.

Diastolic turbulent flow into the left ventricular outflow tract in the presence of a mechanical MVR suggests mechanical MVR obstruction.

The presence of a bileaflet occluder mechanical prosthesis requires the attempt to visualize the motion of both occluders. Prosthesis thrombosis usually occurs with one leaflet more frozen and one leaflet less so—so do not extrapolate the impression of one occluder to the other. Visualize both.

The range of design of valve prostheses is extensive, and familiarity with their design and components is essential to analysis of the two-dimensional and Doppler findings of prostheses, and also to the fluoroscopic findings. The range of mechanical prosthesis design (e.g., ball-in-cage, single tilting disk occluder, and bileaflet occluder designs) is generally understood in simplified terms. However, important design details that influence normal findings—such as the strut penetration into the single disk of the Medtronic Hall prosthesis (which normally emanates a central jet of insufficiency), and the variable profile of mechanical prosthesis sewing rings and of their occluders (which account for the variable visualization of occluder elements above and below the ring)—often are underappreciated.

Bioprostheses are just as variable in design: some have no struts or stents (the stentless aortic root models); some have wire, plastic sewing rings, or struts; some have the struts under and some have the struts over the leaflets; some use actual porcine or cadaveric aortic valves; and some have constructed bovine pericardial leaflets.

Reporting Issues

Know (and record) the type, model, size, and year of the valve you are scanning.

The effect of valve type (P = 0.0003)¹ and size (P = 0.01)¹ on gradient and area is considerable; therefore, it is necessary to know the type and size of the prosthesis. This will facilitate interpretation and imaging recognition of abnormalities.

Clearly state whether the recorded gradient is expected or greater than expected for that type of prosthesis.

Clearly state whether any insufficiency present is within the expected amount or of either pathologic quantity or pathologic origin (paravalvar is always pathologic).

Terminology

If there is a severe gradient across a mechanical MVR, use the term obstruction.

Leakage across the prosthesis is transvalvar.

Leakage beside the prosthesis is paravalvar.

If there is ≥3+ paravalvar insufficiency, and rocking, use the term dehiscence.

The moving mechanical elements of prostheses are occluders or hemi-disk occluders, depending on their geometry.

The implanted ring is the sewing ring.

Soft tissue on the sewing ring may be thrombus, pannus, or vegetations—or a combination. There is little ability case-by-case to accurately resolve (by echocardiography alone) which it is, or whether multiple causes are present.

Flail bioprosthetic leaflets may be reasonably referred to as torn.

Use of the term retroverted leaflet is questionable, because it is difficult to be certain, short of surgical inspection.

Do not state “normal appearance” for a mechanical prosthesis, because mechanical prostheses are so poorly visualized that such a phrase says little, especially for bileaflet occluders, and may easily be wrong. Most of the “appearance” of mechanical prostheses is artifact.

When describing by transesophageal echocardiography (TEE) the position(s) of paravalvar leaks, use a clock-face numeric system, with 12 o’clock at the top, 6 o’clock at the bottom, and the left atrial appendage at about 10 o’clock.

Describe the location of all paravalvar leaks, not only the largest. If the patient is submitted to surgical repair, all leaks will need to be addressed.

Gradient Issues

Important issues to understand include the following:

Correlation versus accuracy

Pressure recovery

Localized gradient

Net (peak or mean) gradient

Potential and kinetic energy

Spatial distribution of energy loss and gradient

Correlation of mean gradient is generally good for simultaneous comparisons of the following:

Bioprosthesis: r = 0.93; standard error of estimate (SEE) = 3 mm Hg²

Mechanical: r = 0.93; SEE = 3 mm Hg² or better ^2,³

Mitral prostheses: r = 0.93; SEE = 3 mm Hg²

Aortic prostheses: r = 0.94; SEE = 3 mm Hg²

However, overestimation may occur while the correlation is good, especially for the following:

Bileaflet occluder mechanical prostheses ³

Ball-in-cage mechanical prostheses ³

Older bioprostheses: Hancock and Ionescu-Shiley ¹

Nonsimultaneous correlation of mean gradient is less, as would be expected:

Bioprosthesis: r = 0.85 SEE or greater

Mechanical: r = 0.87 SEE or greater

Mechanical prosthetic valve gradients are flow dependent. In the case of very small prostheses in the aortic position, gradients rise very rapidly and conspicuously as flow increases beyond resting state flow (90 mm Hg peak for SJM and Hancock valves).³ Therefore, be attentive to anything likely to be causing increased cardiac output (e.g., anemia, fever, pregnancy, thyrotoxicosis).

Recall the significant 5-, 10-, and 15-year incidence of structural valve failure for older designs of bioprostheses (Table 8-2).⁴

Pressure Recovery Phenomenon

The pressure recovery phenomenon is a fascinating occurrence of hydrodynamics in which a localized gradient develops within a smooth-walled restrictive orifice. Bileaflet occluders are the most common cause of localized gradients and pressure recovery.

The pressure recovery phenomenon is well described for bileaflet occluder prostheses in the aortic position, and for the ball-in-cage Starr-Edwards prosthesis. The phenomenon is described in vitro, but probably does account for some patient discrepancies with catheterization. The pressure recovery is greatest for the central orifice. The pressure loss coefficient for the continuity relation has been shown in vitro to be K = 0.64. As much as 40% of the initial pressure loss is recovered by the end of the leaflets in the central orifice, and another 20% within another 5 cm. The pressure recovery of the side orifices is less (30%), and occurs further downstream. Side orifice velocities are 85 ± 4% of the center orifice velocity.⁵

Overestimation of peak and mean trans–aortic valve replacement (AVR) gradient is described for the St. Jude prostheses and for the Starr-Edwards prosthesis (with gradient differences as great as 44 mm Hg), but not for the Medtronic Hall tilting disk prosthesis.³

The pressure recovery phenomenon also may occur for mitral prostheses.⁵

A smooth-walled flaring restrictive orifice may establish pressure recovery: some of the kinetic energy recovers to potential energy (pressure).⁵ The pressure recovery phenomenon is greater for small valve prostheses (26-mm valve: 167 ± 52% vs. 31-mm valve: 123 ± 41%), and for the centerline gradients than the side orifice gradients (13 ± 12 mm Hg vs. 6 ± 4 mm Hg).⁵ Although there is good correlation of valve gradient by Doppler and catheterization, the Doppler estimates experimentally are significantly higher. A total pressure loss coefficient² for bileaflet occluder devices of 0.64 (±0.04) can be used.⁵

Within a nonplanar, smooth-walled orifice such as the central (minor) orifice of a bileaflet occluder valve prosthesis, a localized gradient can occur within the length of the smooth-walled orifice (“early” or “valvular” recovery). There is a small amount of subsequent recovery of pressure (“late” or “post-valvular” recovery). Within a few centimeters of the tip of the occluders, the total pressure recovery has occurred.

The magnitude of the pressure-recovery phenomenon is greater within the central orifice (15%) than the side orifices.

The pressure-recovery phenomenon is seen with both mechanical AVRs and MVRs. The late or post-valvular pressure recovery is greater in the case of AVRs, probably due to aortic root effects. (A narrow sinotubular junction, <30 mm diameter, appears to facilitate pressure recovery in vivo.)

Patient–Prosthesis Mismatch

Cardiac output and stroke volume are related to body size. Therefore, larger patients need larger valves and larger valve prostheses, or the larger flow in a larger patient will generate a high gradient across an undersized prosthesis. However, the aortic root in particular may have variable size with regard to body size, and the annular size ultimately determines the largest prosthesis size that can be inserted.

When a prosthesis is so small (i.e., its EOA is so small) that it is conferring a large gradient that may fall within the severe range, patient prosthesis mismatch (PPM) is said to exist. PPM is likely to occur with an EOA of <0.9 cm²/m².⁶ Surgeons insert the largest prosthesis than can be fitted into the annulus, to minimize the frequency of this complication, but ultimately, the annulus size may be small (mismatched) for the size of the patient.

Smaller prostheses produce higher gradients ^3,⁶ in the resting state, and especially with exercise or any context of increased flow. With exercise gradients not only rise, but rise very steeply.³ For example a small St. Jude prosthesis in the aortic position may produce a 90-mm Hg peak gradient with exercise levels of flow.³ Therefore, when interrogating a prosthesis, it is imperative to know the size of the prosthesis and type, and to anticipate its gradients. For smaller prostheses, where there are symptoms and a somewhat elevated gradient, mild exercise may bring out the degree of gradient and pulmonary hypertension in such states.

Assumptions Inherent in the Modified Bernoulli Equation

That a blood viscosity effect is negligible (as boundary layer formation is negligible other than against the walls to flow)

That inertial issues are negligible for an orifice restrictive to flow (as the amount of weight of blood involved is very small)

That most energy proximal to the orifice is potential, and can be omitted

That V₁ can be omitted

That there is no pressure recovery (i.e., all kinetic energy developed is lost to friction, heat, and vortices).⁵

A subvalvar gradient may be present in some cases of (native valve) AS cases ⁷; therefore, the omission of V1 from the modified Bernouilli equation may be a relevant problem in a clinically meaningful number of cases.