Before assessing prosthetic valves you need to know:
- Type of prosthetic valve – biological/mechanical – and specific name.
- Size of prosthetic valve.
- Date of implantation.
- BSE app is a good reference for normal pressure/velocity values for each type of valve.
Mechanical valve structure includes a sewing ring (like the annulus), an occluder mechanism (opens and closes) and the retainer (holds occluder mechanism in place).
Also may consist of major and minor orifices due to mechanism which needs to be considered – e.g. bileaflet disc valve may have a high gradient in the inner minor orifice but lower gradient in the two major orifices.
Main advantage to mechanical valves is long term durability. Main drawback is requirement for anticoagulation.
3 main types:
- Ball and cage valves – used less commonly nowadays.
- Tilting disc valves – single disc occluder tilts within its occluder.
- Bileaflet valves – two semicircular disc occluders open and close on hinges – most common mechanical valve used nowadays.
Assessment can be challenging due to reverberation artefact obscuring parts of the valve.
Mitral prostheses best assessed from apex. May need TOE.
Aortic prostheses best assessed from parasternal view.
- Is valve well seated or is it ‘rocking’ suggesting dehiscence? Look for paravalvular regurgitation.
- Is there a normal range of movement for the valve occluder? Can be obstructed by pannus/thrombus/vegetation.
- Assess for stenosis or regurgitation. Care – some valves have a recognised pattern of regurgitation.
- Microbubbles can be a normal finding – cavitation.
Forward flow – assess gradient (peak and mean), pressure 1/2 time (for mitral prostheses) and effective orifice area (EOA). Acceleration time also useful – will be prolonged if mechanical obstruction.
EOA is assessed with continuity equation for both aortic and mitral valves. For mitral valves it is only accurate if there is no associated aortic/mitral regurgitation.
Careful measurements of LVOT and placement of PW doppler 0.5-1cm proximal to the valve to avoid the subvalvular flow acceleration.
High forward flow velocities may be seen in a patient prosthesis mismatch (small for size) and does not necessarily indicate valvular obstruction. High output states (sepsis, pregnancy, thyrotoxicosis etc) will also increase velocities.
Calculate the projected indexed EOA – based upon normal reference value for type and size of valve (need to look it up). Index projected EOA based upon patient body surface area.
For AV prostheses – indexed EOA <0.85cm2/m2 suggests patient-prosthesis mismatch, <0.65cm2/m2 suggests severe mismatch.
For MV prostheses – <1.2cm2/m2 suggests patient-prosthesis mismatch, <0.9cm2/m2 suggest severe mismatch.
If mismatch does not fully explain high velocity need to carefully interrogate valve.
Measure doppler velocity index (aka dimensionless index):
For aortic prostheses – LVOT flow velocity (PW in LVOT) divided by aortic prosthesis flow velocity (CW across valve). Value <0.3 suggests central jet artefact in bileaflet valve with normal mobility, valve dysfunction or technical error.
For mitral prostheses – VTI of flow across MV divided by VTI of LVOT flow. Value of >2.2 suggests central jet artefact in bileaflet valve with normal mobility, valve dysfunction or technical error.
Normal DVI is consistent with high flow state, LVOT flow acceleration or technical error.
A small amount of regurgitation is a normal finding for mechanical valves and can be particularly difficult to visualise in mitral prostheses.
The direction of regurgitant flow varies but is usually mild.
Key thing to look for is a PISA shell above the valve as this is almost always an abnormal finding. If this is present then consider more detailed assessment (i.e. TOE) as more significant regurgitation is likely present.
Remember to look for both transvalvular and paravalvular regurgitation. Paravalvular regurgitation from >10% of valve should be deemed abnormal.
Also consist of sewing ring from which projects a number of struts (stents) to which the valve leaflets are attached.
These stents can cause a degree of obstruction and stentless valves are available which provide a greater EOA for the size.
- Xenograft – fashioned from porcine or bovine valves or pericardium. Pericardium valves will not fully coapt. Stented porcine bioprosthetic valve (top image) has annular closure line, small muscular shelf with one restricted leaflet, should not have regurgitation and relatively more obstruction. Often cannot see leaflets as they coapt at the annulus and so are obstructed. Stented bovine pericardial valve (bottom image) has closure line at stent tips – coaptation point is closer to the struts so often see some valves. Minor lack of central coaptation with mild central regurgitation. Relatively less obstruction.
- Homograft – human valves are obtained from cadavers. Often completely normal appearance (may have slightly thickened annulus).
- Autograft – patient’s own pulmonary valve is used to replace the aortic valve, and pulmonary valve is replaced with xenograft/homograft. Often look normal.
- Percutaneous aortic valves (corevalve, lotus valve, Edwards-sapiens, direct flow valve) – for TAVI procedures. Often cannot clearly see leaflets – large circumferential struts which support valve obscure view. May also not be fully round due to lack of full deployment and may result in mild transvalvular regurgitation.
Assessment is mostly the same but careful consideration of the leaflets is important.
When a bioprosthetic valve begins to fail it will fail quickly – meaning any obstruction/leaflet abnormality is important to identify early.
Over time the biological valves tend to become fibrosed leading to a degree of obstruction. This will increase the velocity/gradient and reduce the EOA.
- Gradient (peak and mean)
- Pressure half time (for mitral prostheses – but do not use this to calculate EOA as will not be accurate).
- EOA – using continuity equation.
Reference calculations to normal values for each valve.
Up to half of all biological valves have a degree of regurgitation – usually mild. Important to assess for transvalvular and paravalvular regurgitation as may be a marker of fibrotic change or dehiscence.
Pannus vs thrombus:
Thrombus – short duration of symptoms, large mass, less echo-dense, poor anticoagulation.
Pannus – Longer duration of symptoms, small mass, more echo-dense, better anticoagulation.
Often a preferred method, typically using an annuloplasty ring.
Assess for morphology, leaflet mobility and for presence of annuloplasty ring.
Measurements as for native valves.
Remember tethering of annulus will often impact longitudinal function measurements (TAPSE/MAPSE/S’).
Percutaneous balloon mitral valvuloplasty:
Assessment with the Wilkins score which grades the valve’s suitability based on four criteria, each scored out of 4:
- Leaflet mobility (mobile = 1; immobile = 4)
- Valvular thickening (normal (<5 mm thick) = 1; severe thickening (>8–10 mm) = 4)
- Subvalvular thickening (minimal thickening = 1; thickening of all chordal structures = 4)
- Valvular calcification (no bright echoes = 1; extensive brightness = 4).
Total score >8 indicates low probability of successful PBMV. Full assessment with TOE.
Percutaneous mitral valve repair:
Intraoperative TOE guidance for placement of clip followed by assessment of any residual regurgitation assessed – though this may be challenging given there will be two orifices so normal measures cannot be used.
3D is more useful.