Biomedical Engineering Reference
In-Depth Information
commonly only once in their lifetime and in most cases for only an hour or two.
This is often done in the case of an immediately life-threatening condition, or at
least this comparatively short one-off treatment is performed to cure a probably
fatal disease. Additionally, the major surgical insult of a coronary artery bypass
operation or valve replacement with opening of the chest and exposition of the
heart undoubtedly is bio-incompatible to a degree that the biocompatibility of
the membrane becomes unimportant.
Microporous membranes that are capable of providing a sufficient gas
exchange performance need to be hydrophobic enough not to allow any plasma
entrance into the pores and plasma breakthrough to the gas side. This
hydrophobicity inevitably brings about contact phase activation and will lead
to cell adhesion to the membrane. This has been shown in numerous investiga-
tions:
20,21
main concerns with the use of oxygenators are the activation of the
coagulation cascade, thrombus formation and drops in cell counts. Manufac-
turers deal with this problem by coating the whole circuit tip-to-tip with bio-
compatible coatings such as heparin. Manufacturers have their own proprietary
technologies. None of them has yet shown a significant clinical benefit, e.g. in
terms of morbidity or mortality, but all coatings provide reduced cell adhesion,
reduced coagulation and contact phase activation, many studies show reduced
use of donor blood and some show reduced length of stay in the ICU and/or in
the hospital. The drawback in most of the studies are the many confounders like
the surgical insult as such, the return of suction blood, a great number of
comorbidities, the exact conduct of the extracorporeal circulation (temperature,
anticoagulation, blood dilution and haematocrit, blood flows, cannulation sites,
antegrade or retrograde brain perfusion, unphysiological flows in the whole
extracorporeal circuit, non-pulsatility of flow, etc.).
Numerous studies have compared microporous membrane oxygenators of
different builds and with different coatings. Most of them have been done on a
very small number of patients and none could show a significant superiority of
one product over the other. Although it needs to be acknowledged that a hydro-
phobic microporous membrane is undoubtedly not biocompatible, the other
contributors to cell and immune activation obviously significantly override the
membrane and oxygenator build effects.
Silicone membranes by nature are more biocompatible than, for example,
membranes made from PP because they are less hydrophobic. The downside is
that owing to the lower gas exchange performance much higher surfaces are
needed, which means higher priming volume, greater blood dilution and a larger
foreign surface contact that triggers immune response. An additional advantage
over microporous membranes is that blood does not come into direct contact
with oxygen or air in the oxygenator. However, the blood±air contact at the
direct surgical site seems to override this benefit. Owing to the large priming
volume and inferior performance of silicone oxygenators, these do not play a
significant role in the cardiac surgery market, anyway.
