Evaluation of Prosthetic Aortic Valve Obstruction

Evaluation of Prosthetic Aortic Valve Obstruction

This discussion will focus beyond the basic clinical evaluation, ECG, chest X-ray and hematological workup. Hematological workup is important in aortic valve obstruction because of likelihood of associated hemolytic anemia and acquired von Willebrand Syndrome. Former is due to destruction of red blood cells across the narrowed aortic valve, and latter due to loss of the largest multimers of von Willebrand factor. High shear stress across the narrowed aortic valve exposes a region of the von Willebrand factor which is susceptible to a specific von Willebrand protease. This can lead on to gastrointestinal angiodysplasia (Heyde’s syndrome) and bleeding complications.

Obstruction of prosthetic aortic valve could be due to thrombosis or a fibrotic tissue overgrowth known as pannus formation. Another reason was “stent creep” in certain types of bioprosthesis due to inward bending of stent struts. Stent creep can cause increase in transvalvar gradient and the damage may not be evident on two dimensional echocardiography. Stent creep was due to ultrastructural deformity of polypropylene used in the valve stent. Newer stent materials resistant to such deformity were developed after this was noted in explanted Hancock bioprosthesis.

Most commonly used methods for evaluation of prosthetic valve are echocardiography and fluoroscopy. But they may not identify morphological substrate or the extent of prosthetic valve pathology. In such situations, cardiac computed tomography and magnetic resonance imaging are other potential imaging modalities [1].

While planning echocardiography for suspected prosthetic aortic valve obstruction, date of surgery, type and size of the prosthetic valve used, blood pressure, heart rate, height, weight and body surface are are to be checked [2]. Body surface area is needed to identify potential patient prosthesis mismatch, which is most likely in case of aortic valve replacement than in case of other valves. To put it simply, a prosthetic valve too small for the patient’s body surface area produces this situation. It produces an increase in transvalvar gradient, but imaging studies like echocardiogram, fluoroscopy or computed tomography will show absence of thrombotic masses and pannus. Normal valve opening angles will be documented. Measurement of the orifice area and calculating the indexed area with respect to body surface area will differentiate it from other causes of prosthetic valve dysfunction which can also increase the gradient.

Details regarding postoperative echocardiogram if available, will be quite useful for comparison. It has been suggested that periodic documentation of reports are needed for future comparison. First evaluation should be within two to four weeks of aortic valve replacement. A repeat transthoracic echocardiogram is needed when there is clinical symptoms or signs suggesting prosthetic valve dysfunction. Transesophageal echocardiography may also be needed in selected cases. Most often transthoracic echocardiograms are enough for the evaluation of prosthetic aortic valve.

Normal tissue valves appear thin with unrestricted motion. Reduction in movement of valve discs, ball and leaflets should be looked for both by echocardiography and fluoroscopy in case of radio opaque valves. Reduced movement could be due to thrombus or pannus. Some valve discs are made of non-radio opaque material, limiting the role of fluoroscopy in such cases. Calcification, thickening and reduced mobility of leaflets may be noted in case of degenerated bioprosthesis.

Quantitative parameters measured are the transprosthetic velocity and gradient along with effective orifice area. It has been mentioned that occasionally an abnormally high gradient may be noted across the smaller central orifice of a bileaflet mechanical prosthesis leading to overestimation of gradient. Contour of the jet velocity across the prosthetic aortic valve is also important. Normal velocity contour is triangular, with early peaking. Rounded symmetrical contour is suggestive of significant obstruction [2].

Doppler velocity index is another important measure. It is the ratio of the velocity time integral of the left ventricular tract to that of the transprosthetic flow. Doppler velocity index of 0.3 or less is indicative of prosthetic aortic valve stenosis. Effective orifice area is calculated using the continuity equation, using the velocity time integrals of left ventricular outflow tract and prosthetic valve as well as the cross sectional area of the LVOT near the prosthetic valve. VTI across the valve is measured using continous wave Doppler as it is a high velocity jet. VTI across the LVOT is measured using pulsed Doppler. Effective orifice area of prosthetic aortic valve less than 0.6 sq. cm per sq. m BSA would indicate a patient prosthesis mismatch  in the presence of normal prosthetic valve function as indicated by other parameters.

Computed tomography may complement echocardiography by differentiating between thrombus and pannus as the cause of obstruction. Calcification of bioprosthetic valves can be documented better with CT. MRI can show abnormal asymmetrical flow patterns in prosthetic valve obstruction. But presence of metal artefacts limit direct evaluation of prosthetic valve structure [1].

References

1. Suchá D, Symersky P, Tanis W, Mali WP, Leiner T, van Herwerden LA, Budde RP. Multimodality Imaging Assessment of Prosthetic Heart Valves. Circ Cardiovasc Imaging. 2015 Sep;8(9):e003703. doi: 10.1161/CIRCIMAGING.115.003703. PMID: 26353926.
2. Sordelli C, Severino S, Ascione L, Coppolino P, Caso P. Echocardiographic Assessment of Heart Valve Prostheses. J Cardiovasc Echogr. 2014 Oct-Dec;24(4):103-113. doi: 10.4103/2211-4122.147201. PMID: 28465917; PMCID: PMC5353566.