Elevated gradients across prosthetic valves can occur due to various reasons. But before declaring that prosthetic valve gradients are elevated, the usual gradient across the given type of prosthetic valve should be known. Different types of normally functioning prosthetic heart valves have different gradients. Normal gradient is different between the various valve positions. Transmitral prosthetic valve gradient is lower than transaortic prosthetic valve gradient. If the gradient during post operative review is available, it can be used as a benchmark for comparison.
Some of the important causes for elevated prosthetic valve gradients are prosthetic valve thrombus, pannus, large vegetations, calcific degeneration of bioprosthetic valve, patient prosthesis mismatch, high output state, pressure recovery phenomenon and high velocity central flow in a bileaflet valve .
Clinical scenario should be considered in detail during evaluation of elevated gradients as echocardiographic patterns have their own limitations in the assessment of prosthetic valves. Body surface area should be checked while assessing patient prosthesis mismatch.
Elevated transprosthetic gradient is an important feature of thrombus within the prosthetic valve . In the mitral position, a gradient >8 mm Hg is suggestive of prosthetic valve thrombosis. In case of aortic prosthetic valve, the suggested cut off value is >45 mm Hg . A word of caution is needed here in that gradients may be low in low cardiac output state despite significant obstruction. This has been called “silent Doppler prosthetic valve thrombosis”.
Pannus formation or tissue growth into the prosthetic valve can increase the transvalvular gradients. An in vitro study has shown that when the pannus width was 25% of the valve diameter, gradient can be increased >2.5 times than without pannus .
Large vegetation has been mentioned as a very rare cause of prosthetic valve obstruction . Large vegetations are a feature of fungal endocarditis and staphylococcal endocarditis.
Calcific degeneration can occur in glutaraldehyde preserved bioprosthetic valves . This is yet another cause of increased trans prosthetic valvar gradient.
Patient prosthesis mismatch is important mostly in the case of aortic valve. When the aortic valve is replaced for aortic stenosis, aorta may be small and may take only a relatively smaller prosthesis. Root widening procedures are done sometimes, but can increase the surgical risk. Patient prosthesis mismatch can be assessed by checking the body surface area and the corresponding size of the prosthetic valve. Severe patient prosthesis mismatch is defined as indexed effective orifice area <0.65 cm2/m2 .
As we have seen low gradient with low output state, higher gradient can occur in high output state. This aspect should be considered before considering an elevated prosthetic valve gradient as being due to obstruction of the valve.
Pressure recovery phenomenon can impact the Doppler assessment of prosthetic valve performance . Pressure recovery phenomenon is recovery of aortic pressure beyond the region of the jet across the aortic valve. The lower pressure near the valve is due to the effect of the jet which causes a reduction in the lateral pressure. To get the recovered pressure (settled pressure), catheter has to be pulled back about 2 to 3 cm beyond the sinotubular junction. The lower pressure near the valve gives a falsely elevated transaortic valvular gradient when measured by Doppler echocardiography.
High velocity jet through the relatively narrower central orifice of a bileaflet prosthetic valve can produce a falsely elevated gradient . This was noted on catheter pull back through the central orifice while it was not there on pull back through the side orifices. The mean difference between the central and side orifices was 46%. This value corresponded to the elevation in Doppler gradient . It was an in vitro study in an aortic valve pulsatile model.
Chatterjee D. Raised Prosthetic Valve Gradients: What Should be the Approach? J Indian Acad Echocardiogr Cardiovasc Imaging. 2019;3(3): 156-162.