Thrombus formation on polytetrafluoroethylene surfaces: the importance of von Willebrand factor

Evaluation of vascular repair by tissue-engineered human acellular vessels or expanded polytetrafluoroethylene grafts in a porcine model of limb ischemia and reperfusion

BACKGROUND

This study evaluated performance of a tissue-engineered human acellular vessel (HAV) in a porcine model of acute vascular injury and ischemia. The HAV is an engineered blood vessel consisted of human vascular extracellular matrix proteins. Limb reperfusion and vascular outcomes of the HAV were compared with those from synthetic expanded polytetrafluoroethylene (ePTFE) grafts.

METHODS

Thirty-six pigs were randomly assigned to four treatment groups, receiving either the HAV or a PTFE graft following a hind limb ischemia period of either 0 or 6 hours. All grafts were 3-cm-long interposition 6-mm diameter grafts implanted within the right iliac artery. Animals were not immunosuppressed and followed for up to 28 days after surgery. Assessments performed preoperatively and postoperatively included evaluation of graft patency, hind limb function, and biochemical markers of tissue ischemia or reperfusion injury. Histological analysis was performed on explants to assess host cell responses.

RESULTS

Postoperative gait assessment and biochemical analysis confirmed that ischemia and reperfusion injury were caused by 6-hour ischemia, regardless of vascular graft type. Hind limb function and tissue damage biomarkers improved in all groups postoperatively. Final patency rates at postoperative day 28 were higher for HAV than for ePTFE graft in both the 0-hour (HAV, 85.7%; ePTFE, 66.7%) and 6-hour (HAV, 100%; ePTFE, 75%) ischemia groups, but these differences were not statistically significant. Histological analyses identified some intimal hyperplasia and host reactivity to the xenogeneic HAV and also to the synthetic ePTFE graft. Positive host integration and vascular cell infiltration were identified in HAV but not ePTFE explants.

CONCLUSION

Based on the functional performance and the histologic profile of explanted HAVs, this study supports further investigation to evaluate long-term performance of the HAV when used to repair traumatic vascular injuries.

Obstacles in haemocompatibility testing

ISO 10993-4 is an international standard describing the methods of testing of medical devices for interactions with blood for regulatory purpose. The complexity of blood responses to biomaterial surfaces and the variability of blood functions in different individuals and species pose difficulties in standardisation. Moreover, in vivo or in vitro testing, as well as the clinical relevance of certain findings, is still matter of debate. This review deals with the major remaining problems, including a brief explanation of surface interactions with blood, the current ISO 10993 requirements for testing, and the role of in vitro test models. The literature is reviewed on anticoagulation, shear rate, blood-air interfaces, incubation time, and the importance of evaluation of the surface area after blood contact. Two test categories deserve further attention: complement and platelet function, including the effects on platelets from adhesion proteins, venipuncture, and animal derived- blood. The material properties, hydrophilicity, and roughness, as well as reference materials, are discussed. Finally this review calls for completing the acceptance criteria in the ISO standard based on a panel of test results.

Diabetes and distal access location are associated with higher wall shear rate in feeding artery of PTFE grafts

BACKGROUND

Surgical creation of permanent vascular access for haemodialysis leads to considerable haemodynamic changes. They could be implicated in the pathogenesis of access complications, which limit access survival, especially in diabetics. Physiologically, the relation between arterial diameter and blood velocity is maintained by wall shear stress (WSS), which is directly related to both blood viscosity and wall shear rate (WSR = blood velocity/internal diameter). Because of methodological difficulties, WSR is used as a measure of WSS. Extremely high values of WSS might induce hypercoagulable states, which might contribute to access thrombosis. We performed a study, which was aimed to (i) describe WSR values in feeding arteries of various polytetrafluoroethylene access types and (ii) prove that diabetic patients have higher WSR than non-diabetics.

METHODS

A linear-array 11 MHz probe of SONOS 5500 (Phillips, USA) was used to obtain blood velocity and internal diameter in the feeding arteries of radial or brachial polytetrafluoroethylene grafts. WSR was calculated as 4 x blood velocity/internal diameter. We compared observed values of WSR according to feeding artery (radial vs brachial artery) and according to diabetic status using unpaired t-test.

RESULTS

We included 106 patients (58 non-diabetic and 48 diabetic) in the study. WSR was significantly higher in radial arteries compared with brachial arteries independent of diabetes status. Diabetic subjects had significantly higher WSR in both radial and brachial arteries.

CONCLUSIONS

Diabetes mellitus and distal vascular access creation are associated with higher WSR in the feeding artery. This could be of relevance in the pathogenesis of access complications, e.g. thrombosis, and thus lower patency rates in diabetic patients.

In vitro verification of antithrombotic effect of recombinant soluble nucleotide triphosphate diphosphohydrolase 1

PURPOSE

Ecto-ADPase (NTPDase1 or CD39) has been identified on endothelial cells and found to be antithrombogenic, with actions resulting from degradation of adenosine diphosphate (ADP), a thrombogenic molecule secreted by activated platelets at sites of vascular injury. Reasoning that the ADPase activity of CD39 might provide clinical use as an antithrombotic agent, the authors investigate the comparative ability of the agent to inhibit platelet and fibrin deposition.

MATERIALS AND METHODS

With use of an in vitro perfusion system, fresh, heparinized human blood was passed over expanded polytetrafluoroethylene grafts at hemodynamic conditions similar to those observed in the human arterial circulation. Three different concentrations of CD39 (30 mcg/mL, 100 mcg/mL, and 300 mcg/mL) were compared with abciximab (4 mcg/mL) and heparin controls. The deposition of radiolabeled platelets and fibrinogen was measured in five perfusions for each treatment group.

RESULTS

The addition of soluble CD39 to heparinized blood inhibited platelet deposition to an extent greater than that of heparin alone (P =.04). Effects were similar to those observed with abciximab. The addition of CD39 to heparinized blood did not result in augmentation of fibrin inhibition beyond that observed with heparin alone. The fibrin inhibitory effects of CD39 appeared to be less significant than the fibrin inhibition observed with abciximab, but this difference did not attain statistical significance.

CONCLUSION

These results suggest that recombinant CD39 holds potential as antithrombotic agent, with the potential to achieve antiplatelet effects similar to that observed with abciximab.

Noninvasive measurement of shear rate in autologous and prosthetic bypass grafts

There is a major difference in thrombogenicity between lower extremity prosthetic and autologous vein bypass grafts, and arterial blood flow shear rate is known to influence thrombus formation. Despite this association, there has been little direct clinical observation of shear rates in bypass grafts. The authors developed a new noninvasive method to quantitate human arterial shear rate and used it in a pilot study to characterize differences in lower extremity bypasses. Shear rates were measured in 10 prosthetic and 14 autologous vein femoropopliteal bypass grafts. With CVI-M-mode color flow ultrasonography in resting supine patients, a velocity profile was recorded from a midgraft longitudinal section in the ultrasound beam direction. Shear rates were calculated by using a mathematical-graphic computer program at the anteromedial (near) and posterolateral (far) graft walls by averaging values immediately before and after peak systolic velocity (PSV). Comparison between prosthetic and autologous graft groups respectively revealed that differences in age (67 +/- 12 [SD] vs 71 +/- 10 yr), male gender (60% vs 43%), prevalence of hypertension (50% vs 71%), diabetes (40% vs 64%), smoking (50% vs 50%), hypercholesterolemia (30% vs 29%), coronary artery disease (60% vs 50%), and critical ischemia (60% vs 86%) did not reach statistical significance (p>0.19). Median PSVs were significantly less in prosthetic than in autologous vein bypasses (37 +/- 13 vs 57 +/- 22 cm/s, p=0.018). Prosthetic and autologous graft diameters were not statistically significantly different (6.3 +/- 1.1 vs 5.6 +/- 1.3 mm, p = 0.18). Shear rates were significantly less in prosthetic than in autologous vein bypasses both at the near wall (382 +/- 146 vs 698 +/- 271 s(-1), p=0.003) and at the far wall (551+/-235 vs 827+/-339 s(-1), p-0.037). This mathematical model can be used to calculate shear rate from observed ultrasound flow patterns. Prosthetic bypass grafts had lower shear rates than autologous vein grafts.

Noninvasive ultrasound measurement of shear rate in leg bypass grafts

Shear has been implicated in the etiology of atherosclerosis, thrombosis and graft stenosis. We measured shear rate noninvasively in infrainguinal bypasses. Velocity profiles were recorded from 35 femoropopliteal and 40 tibial grafts. Flow rate (Q), systolic shear rate (SSR), diameter, and bluntness factor (BF) were measured at midgraft using ultrasound (US). Mean shear rate (MSR) was calculated from flow and diameter. SSR, 671 +/- 260 (SD) vs. 659 +/- 304 s(-1) (p = 0.85), and MSR, 168 +/- 84 vs. 193 +/- 110 s(-1) (p = 0.26), were similar for popliteal and tibial bypasses, but differences in Q, 126 +/- 57 vs. 104 +/- 38 mL/min, were borderline significant (p = 0.058). Popliteal grafts had larger diameters, 5.2 +/- 1.1 mm vs. 4.7 +/- 0.8 mm (p = 0.048), and BF, 3.4 +/- 0.9 vs. 2.8 +/- 0.7 (p = 0.0014). Shear rates were obtained noninvasively in humans. Larger diameters in popliteal vs. tibial bypasses did not result in lower shear rates and were compensated for by larger bluntness factors. Velocity profile bluntness cannot be ignored in shear rate analysis.

The use of glycoprotein IIb/IIIa antagonists in peripheral arterial occlusion

The use of intra-arterial, catheter-directed thrombolytic therapy has been associated with the necessity to infuse the agents over a protracted period of time. The large, randomized studies suggest that most treatments require more than 24 hours of infusion. There are data to suggest that the risk of complications, especially hemorrhagic complications, increases as the duration of administration increases. Moreover, the economic burden of thrombolytic therapy increases in proportion to the length of infusion, specifically with respect to the cost of the agent and the use of hospital resources, such as the intensive care unit. For these reasons, it is desirable to formulate a treatment strategy that results in a more rapid rate of thrombolysis. Thrombolytic dissolution of an occluding thrombus is a dynamic process; new thrombus is laid down as old thrombus is dissolved. Because this process occurs in the high-flow arterial milieu, platelets are of prime importance and contribute directly to the phenomenon of rethrombosis. Platelets have also been implicated in the more chronic problems associated with smooth muscle cell and leukocyte-dependent intimal hyperplasia that culminates in restenosis at sites of arterial injury. Platelet antagonists, such as the glycoprotein IIb/IIIa inhibitors, are attractive as adjuvants to peripheral thrombolysis; the rate of thrombolysis may be increased by inhibiting new thrombus deposition. The agents may also discourage the formation of a restenotic lesion at the site of intervention. Initial studies suggest that this may indeed be the case, and there is much interest in validating the use of platelet antagonists through the performance of well-designed clinical trials.