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Aortic Prosthetic Valves


Acknowledgment

We thank Dr. Damian Roper for his contributions to the earlier edition of this chapter.

Transthoracic echocardiography (TTE) is established as the reference imaging modality for the routine evaluation of prosthetic valves because of the comprehensive anatomic, functional, and hemodynamic data obtained combined with its excellent safety profile. However, the unique acoustic properties of prosthetic valves can impair the echocardiographic examination, resulting in reduced diagnostic accuracy. To reduce errors in interpretation, an understanding of the distinctive structural and hemodynamic features of prosthetic valves is required combined with a standardized TTE examination protocol that includes integration of all data obtained from two-dimensional (2D), spectral, and color-flow Doppler modalities. In addition, knowledge of the limitations of TTE will enable the appropriate selection of cases in which additional imaging procedures, such as transesophageal echocardiography (TEE) and cine-fluoroscopy (CF), may be required.

Standard Transthoracic Echocardiography Assessment of Aortic Prosthetic Valve Function

Two-Dimensional Imaging

Before imaging, the type and size of the prosthetic valve should be established. The appearance and motion of the leaflets or occluder can then be assessed. Careful attention to specific imaging planes will reduce the problems caused by acoustic shadowing and improve diagnostic yield. If the ultrasound beam can be orientated parallel to the direction of occluder opening, acoustic shadowing across the plane of the valve is reduced with resultant improvement in visualization of occluder motion. This is particularly relevant in the assessment of aortic prostheses in which the apical five-chamber view can provide an improved view of the leaflets or occluder ( ).

Video 110.1. Apical five-chamber view allowing orientation of the ultrasound beam parallel to the direction of the occluder opening resulting in improved visualization of occluder motion.

Doppler Parameters Used to Assess Aortic Prosthetic Valve Function

All prosthetic valve designs cause variable degrees of obstruction to flow, resulting in measurable gradients. Doppler assessment allows the noninvasive measurement of flow velocities across prosthetic valves from which gradients are calculated. As with native aortic valve stenosis, Doppler interrogation of the aortic prosthesis should be performed from multiple acoustic windows to ensure the highest peak velocity and derived gradients have been obtained. Importantly, valve gradients are determined by both volumetric flow and the prosthetic valve area. Thus, in addition to valve gradients, the derivation of flow-independent parameters of prosthetic valve function such as effective orifice area (EOA) and dimensionless velocity index (DVI) is essential ( Box 110.1 ).

BOX 110.1
Measured and Derived Parameters Used When Assessing Aortic Valve Replacement (AVR) Function
AT , Acceleration time; CSA , cross-sectional area; CW , continuous-wave; DVI, Doppler velocity index; EOA , effective orifice area; ET , ejection time; LVOT , left ventricular outflow tract; LVOTd , left ventricular outflow tract diameter; PW , pulsed-wave; VTI , velocity time integral.

AVR Velocity (CW Doppler)

  • Peak velocity (V 2 ) cm/s

  • Maximum gradient (4V 2 2 ) mm Hg

  • Mean gradient mm Hg

  • AVR VTI cm

  • AT/ET (AT ms, ET ms)

LVOT Velocity (PW Doppler)

  • Peak velocity (V 1 ) cm/s

  • LVOT VTI cm

  • LVOTd cm

DVI = V 1 ÷ V 2

EOA (cm 2 ) = (LVOT VTI × (LVOTd 2 × 0.785)/AVR VTI) = (LVOT VTI × LVOT CSA/AVR VTI)

Dimensionless Velocity Index

The DVI is a useful flow independent parameter of aortic prosthetic valve function that is easily derived (V 1 /V 2 ) and does not require the measurement of a dimension such as the left ventricular outflow tract diameter (LVOTd), which can be technically difficult. In an individual patient, a baseline DVI value obtained in the early postoperative period can serve as the control value or “valve fingerprint” for future examinations. Providing prosthetic valve function remains normal, the DVI will remain constant even with changes in stroke volume.

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