Biomedical Engineering Reference
In-Depth Information
Figure 18.5
(a) Thoratec paracorporeal VAD and (b) HeartMate device.
actuated (IP) device (Frazier, 1994). The unit is made from sintered titanium and houses a flexible,
textured polyurethane diaphragm and operated by a pusher plate mechanism. The newer electrically
vented model is HeartMate VE (Figure 18.6) and which was introduced in 1991 (Frazier et al.,
2001). This latter unit is composed of textured titanium, incorporating titanium microspheres; the
polyurethane diaphragm specially treated with textured polyurethane, encouraging deposition of
fibrin-collagen matrix, and is more biocompatible with reduced need for anticoagulation (Morgan
et al., 2004; Rose et al., 2001). Novacor LVAS (World Heart Corp., Ottawa, Canada) incorporates a
dual pusher-plate sac type pump, with a smooth blood contacting surface (Di Bella et al., 2000;
Robbins et al., 2001). The system uses a high efficiency linear motor, with a pulsed solenoid energy
converter with a two armature assembly (Figure 18.7). This system requires no gears, cams, or
intermediate hydraulic conversion; theoretically, it has advantage of low mechanical failure. The
advantage of the LionHeart LVAS (Arrow International, Reading, Pennsylvania) is that it is
completely implantable; it has a brushless motor which actuates a pusher plate using a roller and
screw mechanism (El-Banayosy et al., 2003; Mehta et al., 2001). Energy transfer through the chest
wall is achieved using radiofrequency induction (Figure 18.8).
18.7.4
Newer Rotary Axial Pumps
In order to overcome the disadvantages offered by the various paracorporeal and intracorporeal
devices such as external drivelines and the large consoles, the radial axial flow devices were
devised (Kung and Hart, 1997; Nose, 1998; Okada et al., 1997; Wu et al., 1999). These devices
have only one moving part and hence are smaller in size with less need for energy (Mesana, 2004;
Nose et al., 2000). There are, however, some inherent problems associated with the simplicity of the
design. For example, the absence of a valve can result in significant backflow with device failure,
which can be detrimental in an already diseased heart. There are also concerns about the very high
speeds at which axial pumps rotate and some aspects regarding bio-compatibility are still open to
question (Mesana, 2004). The three systems which are clinically used include: (1) DeBakey VAD
(MicroMed Technology, Inc, Houston, Texas), a small 30-mm diameter device, 76 mm in length,
which weighs 95 gm (Figure 18.9) and is made of titanium (Goldstein, 2003; Noon et al., 2001);
(2) The Flowmaker (Jarvik 2000), (Jarvik Heart, Inc., New York, NY) titanium based pump, which
is 25 mm in diameter, 51 mm in length and weighs 90 gm (Frazier et al., 2004; Kaplon et al., 1996);
and (3) HeartMate II (Thoratec Corp., Pleasanton, CA), which has a diameter of 40 mm, length of
70 mm and weighs 176 gm (Figure 18.10) (Burke et al., 2001; Griffith et al., 2001).
All these devices work on similar principle and are composed of an impeller and inducer
assembly with blood lubricated bearings. In spite of their perceived shortcomings, these devices
offer the advantages of small size and potential for a destination therapy for heart failure.
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