Understanding
the role of Doppler for detection of abnormal flows through prosthetic
valves begins with appreciation of the concept of flow masking.
Once emitted from the transducer into the tissues, ultrasound is
either reflected, attenuated (absorbed) or continues on to another
tissue interface where the process is repeated. All prosthetic valves
contain some degree of nonbiologic material which can be plastic,
metal or cloth. Each of these materials may have highly reflective
or attenuative properties that may not allow the ultrasound to penetrate
and pass through the nonbiologic portion of the valve.
The nonbiologic material can interfere with the transmission of
sound waves to such a degree that it may be impossible to detect
some valvular regurgitation. Six commonly used heart valves are
shown in Figure 2-A to demonstrate the varied appearance and materials
used in fabrication of these devices: Starr-Edwards silastic ball,
Starr-Edwards stellite ball, Bjork-Shiley, St. Jude's, Hall-Kastor,
and a Carpentier-Edwards porcine bioprosthesis.
 |
| Fig.2.38 |
It is remarkable that all of the cardiac prosthetic valves cast
a characteristic shadow, or mask, on the chamber behind them that
obscured proper flow detection by Doppler. Figure 2.38 shows idealized
masks from two valves with the largest and smallest flow-masked
areas.
The largest masked area emanates from behind the Starr-Edwards silastic
ball, and the smallest is the Carpentier-Edwards porcine bioprosthesis
where flow masking is limited only to the sewing ring of the prosthesis.
All valve sewing rings prevent proper transmission of ultrasound,
and without the central occluding objects the area of masking would
resemble that from the Carpentier-Edwards porcine valve. With the
central occluding objects in place and seated in the close position,
each of the valves depicted earlier has differential central transmission
characteristics. The worst penetration is clearly encountered with
the two Starr-Edwards prostheses, each casting a large mask field
behind. Very little ultrasound passes the Bjork-Shiley or the St.
Jude's valves. Only slightly more penetrates the Hall-Kastor. Note
that this valve's occluding disc has a hole in the center to allow
the disc to interact with the central pivot arm. The Carpentier-Edwards
bioprosthesis masks sound only around the valve sewing ring. The
central portions of this valve assembly, made up of preserved biologic
tissue, allow the sound to penetrate readily.
 |
| Fig.2.39 |
This differential penetration is illustrated in Figure 2.39 where
a CW Doppler system was used to attempt simulated flow behind these
various valves placed on a stage. In a large water tank, a large
flow phantom was positioned beneath the stage. Each valve was successively
placed on the stage, before and after control (con) periods with
no prosthesis in place. A strong signal was obtained from the flow
during the control periods, which are shown on either end of the
figure. Resultant spectral displays from all the valves are illustrated.
Although system gain is held constant, there is a significant reduction
in the flow signal resulting from the interposition of the prosthetic
valves. Panel A demonstrates no flow detection through a Starr-Edwards
valve with stellite poppit, panel C is through a Bjork-Shiley valve,
panel D through a St. Jude's valve, panel E through a Hall-Kastor
and panel F through a Carpentier-Edwards porcine valve.
2004