Biomedical Engineering Reference
In-Depth Information
increased TAT concentrations in experiments with uncoated or hirudin-coated endoluminal grafts
relative to that with ATH.
296
In vivo
studies with polyurethane catheters having Chan et al. ATH-linked to polymerized
base coats gave more detailed information about effects of ATH on surface pacifi cation. Investiga-
tions involved either an acute protocol of accelerated coagulation stimulation
303
or chronic catheter
survival.
304
In acute experiments, rabbits were injected with radiolabeled fi brinogen, immediately
followed by insertion of catheters from the upper jugular vein down to the ventricle. Every 5 min,
over a maximum of 4 h, blood would be taken up through the catheter, held in a syringe for 2 min
to initiate activation and then be slowly reinjected, followed by saline fl ush. Time of occlusion was
noted when blood could no longer be withdrawn. Experiments were terminated either at occlusion
or after 4 h and the catheter removed, rinsed, and taken for determination of accreted radiolabeled
fi brin(ogen) per hour of dwell time
in vivo
. While the average occlusion time for uncoated and
commercially heparin-coated catheters, was less than 80 min, on no occasions did any of the ATH-
coated catheters occlude during the experiment.
303
Correspondingly, rate of fi brin(ogen) accretion
was signifi cantly reduced for ATH relative to heparin-coated or uncoated catheters. Further studies
were undertaken to determine mechanisms involved for ATH surface activity. Tests using cath-
eters coated with base coat alone, base coat
PEO-linking agent and catheters with base coats
containing PEO-linked AT all showed greatly reduced occlusion times compared to results with
ATH. Similarly, radioactive fi brin(ogen) accretion rates were increased with these catheters com-
pared to ATH catheters. Thus, the full biocompatibility was exhibited only if AT coatings on the
catheter surface also had a covalently linked heparin chain. Finally, the importance of the highly
active heparin chains in ATH coatings was confi rmed by treatment of ATH surfaces with NaIO
4
(to degrade the pentasaccharide), yielding occlusion times slightly less than that of uncoated con-
trols and fi brin(ogen) accretion rates on par with heparin coatings.
303
Interestingly, NaIO
4
treatment
of commercially coated heparin catheters had little effect on these coagulant parameters. Similar
to
in vitro
tests for stability, the lack of occlusion by ATH catheter surfaces was retained even if the
catheters were pretreated with a general protease.
303
Chronic studies, employing a more clinical regimen, gave similar results to the acute experi-
ments. After insertion of the catheter from the upper jugular vein to the ventricle in the rabbits,
they were allowed to come out of anesthesia and were maintained in cages with free access to food
and water. Twice daily, blood samples were taken through the catheter and then followed by saline
fl ush. Occlusion was noted when fl uid could neither be withdrawn or injected through the catheter.
At occlusion, or after approximately 3 months, catheters were removed and examined. As with the
acute studies, while uncoated and commercial heparin catheters occluded within 8 days, no ATH-
coated catheters ever occluded over the entire experimental period.
304
This fi nding was remarkable
since no anticoagulant was ever given to the rabbits systemically. Another point of note was that the
average occlusion time for heparin-coated catheters (3.64
+
±
0.78 days) was statistically shorter than
even that for uncoated surfaces (7.84
0.96 days). This initially curious result, on closer inspec-
tion, is not without some rationale. Although end-point attachment of heparin chains give better
anticoagulant properties,
309
reports have shown it to be insuffi cient for good thrombin inhibition
35
without signifi cant spacer arms (
±
2000 MW) added to the heparin-end terminus.
35
Even then,
improved base coatings,
309
coimmobilization of heparin with AT,
310
and saturation of heparin-
coated surfaces with free AT
311
have been tried to alleviate heparin-coating problems. Visual and
electron microscopic observation of catheters in the Chan et al. study
ex vivo
revealed the expected
high degree of thrombi associated with uncoated and heparin-coated surfaces. However, ATH coat-
ings appeared essentially free of anything but a fi ne protein-like fi lm. To understand topographic
aspects of the three surfaces prior to
in vivo
implantation, atomic force microscopy was performed.
Surprisingly, although ATH coatings resisted thrombosis, their surfaces were approximately seven
times rougher than that of the other catheters.
304
Thus, it would seem that the activities of ATH
coatings may be more relevant to their biocompatibility than any rheological effects of ATH surface
geometry.
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