Page 117 - Biomedical Engineering and Design Handbook Volume 2, Applications

96 MEDICAL DEVICE DESIGN

randomized controlled trial (RCT) is recruiting in the United Kingdom to evaluate the cost-
effectiveness and clinical benefit of modern ECMO technique in this population (Peek et al., 2006).
The RCT will be one of the first RCTs performed in adults (Peek et al., 2006) since the benchmark
NIH trial during the 1970s (Zapol et al., 1979) and could reveal if technical and clinical advance-
ments improve outcomes.

3.9.3 Device Design
Although the first oxygenators were described in the late 1800s, it would not be until 1951 that total
cardiopulmonary bypass would be performed on a human patient (Stammers, l997). Oxygenators, or
artificial lungs, have undergone a dramatic evolution in the five decades since the first total CPB
operation. The initial clinical units are described as film oxygenators because a rotating cylinder was
used to generate a large, thin film of blood on the cylinder surface where it contacted the exchange
gas (Wegner, 1997). Although effective, these early film oxygenators suffered from a number of fail-
ings that eventually led to their replacement. The direct gas-blood interface allowed for adequate gas
exchange but extensive cellular damage and protein denaturation resulted from the blood-gas inter-
face (Wegner, 1997). The large blood-priming volume and time-consuming, complicated mainte-
nance and use procedures characteristic of film oxygenators were addressed through the advent of
bubble oxygenators (Stammers, l997). Direct gas-blood contact remained in bubble oxygenators, but
the large surface area of the dispersed oxygen bubbles resulted in greater mass transfer and a reduc-
tion in priming volume (Wegner, 1997). In addition, these devices were simple and disposable, con-
sisting of a bubbling chamber, defoaming unit, and a return arterial reservoir (Stammers, 1997;
Wegner, 1997). However, the blood damage seen with film oxygenators was not corrected with the
new bubbling technology, and concerns regarding blood trauma during longer perfusions contributed
to the movement toward membrane oxygenators (Stammers, 1997; Wegner, 1997). The use of a
semipermeable membrane to separate the blood and gas phases characterizes all membrane
oxygenator designs. Membrane oxygenators can be further divided into flat sheet/spiral wound and
hollow fiber models. The flat sheet designs restrict blood flow to a conduit formed between
two membranes with gas flowing on the membrane exterior; these systems were the first membrane
oxygenators to enter use (Wegner, l997). Spiral wound oxygenators use membrane sheets as well but
are arranged in a roll rather than the sandwich formation of the original flat sheet assemblies.
Polymers such as polyethylene, cellulose (Clowes et al., 1956), and polytetrafluoroethylene (Clowes
and Neville, 1957) were used for membranes in these early designs as investigators searched for a
material with high permeability to oxygen and carbon dioxide but that elicited mild responses when
in contact with blood. The introduction of polysiloxane as an artificial lung membrane material in the
1960s provided a significant leap in gas transfer efficiency, particularly for carbon dioxide (Galletti
and Mora, 1995). These membranes remain in use today for long-term neonatal ECMO support.
Development of the microporous hollow fiber led to the next evolution in lung design, the hol-
low fiber membrane oxygenator (Stammers, 1997). Increased carbon dioxide permeability compared
to solid membranes, coupled with improved structural stability, has secured the standing of these
devices as the market leader (Stammers, 1997; Wegner, 1997). The current, standard artificial lung
is constructed of hollow microporous polypropylene fibers housed in a plastic shell. An extralumi-
nal crossflow design is used for most models and is characterized by blood flow on the exterior of
the fibers with gas constrained to the the fiber interior. Intraluminal flow designs utilize the reverse
blood-gas arrangement, with blood constrained to the fiber interior. The laminar conditions experi-
enced by blood flowing inside the fibers result in the development of a relatively thick boundary
layer that limits gas transfer. Extraluminal flow devices are less susceptible to this phenomena, and
investigators have used barriers and geometric arrangements to passively disrupt the boundary layer,
resulting in large gains in mass transfer efficiency (Drinker and Lehr, 1978; Galletti, 1993).
Extraluminal flow hollow fiber membrane oxygenators have come to dominate the market because
of their improved mass transfer rates and decreased flow resistance, the latter of which minimizes
blood damage (Wegner, 1997). Figure 3.20 presents a collection of commercial extraluminal flow
membrane oxygenators demonstrating a diversity of design arrangements.