Cardiac valve prostheses are subject to both transvalvular and paravalvular leakage, particularly devices implanted by transcatheter techniques (transcatheter heart valves).
Components of valve leakage can be defined as follows:
Transvalvular Leakage – The leakage through the closed valve during the closed phase of a valve cycle
Paravalvular Leakage – The leakage around the closed valve during the closed phase of a valve cycle
The sum of the transvalvular leakage and paravalvular leakage results in the Total Leakage through the valve during the closed phase of a valve cycle. See Figure 1 below. Note that this does not include the closing volume of a valve, which is the volume of reverse flow associated with the closing phase of a valve cycle.
Total Leakage = Transvalvular Leakage + Paravalvular Leakage
Figure 1 – Source: jcecho.org
As suggested in ISO 5840-3:2013 section N.4.5, the paravalvular leakage of a valve can be assessed by testing in a cardiovascular hydrodynamic test system with the method given below. However, other methods may be employed and justified by the study director as required.
The value measured is the Total Regurgitant Fraction (Total Leakage) since there will be leakage through the valve (Transvalvular Leakage) and around the valve (Paravalvular Leakage).
a. Mount the test valve in a silicone holder with paravalvular sealing as follows:a.Fill a fine tipped plastic syringe with flowable RTV sealant. Dow Corning 734 is recommended.
b. Deploy the valve in the desired holder. This should be the same holder used for the test performed in step 2 above.
c. Place the tip of the syringe between the valve holder and the outside of the valve, midway between the inflow and outflow of the valve.
d. Inject silicone until it starts to appear at the inflow and outflow edges of the valve. Start at post 1 and move the syringe around the circumference of the valve evenly depositing silicone to ensure full coverage around the valve. NOTE: For mechanical valves, ensure silicone does not contact leaflets or hinges of the device.
e. Carefully remove any excess sealant.
f. Place the valve assembly in saline to allow the sealant to cure while preventing damage to the valve leaflet material, if the material is subject to drying out in air. The sealant listed in step 3a is able to cure while submerged in saline. Depending on the sealant used, full hardness may not be achieved until 24 hours have elapsed.
Paravalvular Leakage = Total Regurgitant Fraction – Transvalvular Regurgitant Fraction