In this experimental study all six tested catheter material from commonly used central venous accesses activated coagulation, the complement system, and inflammation to some extent, but there were significant differences between the devices. The polyurethane catheter coated with chlorohexidine and silver sulfadiazine exhibited reduced blood compatibility including increased hemolysis and inflammation, compared to the other catheters. The silicone catheter showed the greatest variation in blood compatibility test results. Poor blood compatibility could cause inflammation and facilitate the development of catheter-related thrombosis in patients receiving these central venous catheters, but given the experimental design of the current study, clinical significance has to be studied further.
Blood coagulation and inflammation in the presence of foreign materials
Entry of a foreign CVC material into the bloodstream triggers hemostatic processes (Fig. 1). Surface-related events depend on the structure and/or chemistry of the surface in question, and they initially involve adsorption and the activation of plasma proteins (see Fig. 1). FXII is the first protein to be activated in the contact activation system. The resulting FXIIa initiates contact coagulation activation, generation of thrombin (measured as TAT level), and fibrin clot formation. Complement is also activated by other mechanisms, such as the adsorption of immunoglobulins, Ficolin-2, and C3 (leading to increased C3a and sC5b-9). FXIIa can also trigger complement activation via C1s. Given the poor ex vivo blood compatibility demonstrated in the present study it is surprising how well the studied catheter materials seem to be tolerated in vivo8,12,13. However, it should be noted that any symptoms originating from bad blood compatibility from the catheter material may be masked by the patient’s critical condition given the need of a central venous access.
Differences between the tested materials
Compared to the control loop, all the materials assessed in this study to some extent triggered either the blood cell system or the molecular innate immune system when tested in the Chandler model (Fig. 1 and Tables 2 and 3).
Among the uncoated catheter materials, silicone (Si-1) demonstrated the lowest blood compatibility and also the greatest variation in such compatibility between test runs. Silicone seemed to trigger contact coagulation, as indicated by increased levels of FXIIa, prothrombin fragment F1 + 2, and TAT, leading to platelet activation and aggregation with the release of P-selectin MPs and VEGF. Activation of both the complement system and inflammation was also demonstrated by increased levels of C3a, sC5b-9, and IL-8. The activation of contact coagulation and thrombocytes by silicone materials has been observed in previous studies in which modifications of the silicone surface were found to result in improved blood compatibility14,15. A potential mechanism for this effect is that the negative charge of the silicone surface activated not only FXII, but also thrombocytes, via the intrinsic contact coagulation pathway16,17. The pronounced variation in surface properties between test runs might be explained by different amounts of silicone oil slowly exuding to the surface of the material during storage, thereby renewing its negative charges. This problem has been observed in studies of silicone breast prostheses18. Such exudation of silicone oil is also potentially toxic to thrombocytes, representing the same type of negative effect14. In animal studies, compared to polyurethane catheters, silicone catheters have been found to show increased susceptibility to staphylococcal infections and a stronger local inflammatory response19,20. These effects have been demonstrated to be secondary to increased complement activation via the alternative pathway, thus leading to reduced opsonizing ability and increased risk of infection21. Our results corroborate those findings but also further describe the different mechanisms that are activated when silicone comes in contact with blood, Tables 2 and 3.
The two polyurethanes PU-1 and PU-2 differed significantly in blood compatibility, indicating variation in the surface composition of polyurethanes from different manufacturers. PU-2 exhibited good blood compatibility with only a slight increase in P-selectin MPs, F1 + 2, TAT, and complement activation compared to the control loop (Tables 2 and 3). PU-1, on the other hand, induced contact coagulation (increased FXIIa and TAT), which can theoretically increase the risk of catheter-related thrombosis13,22. Differences in the composition of the two polyurethane catheters may include the presence of sulphate groups, which would give the surface a negative charge and thereby trigger FXII and activation of contact coagulation (i.e. increased TAT is seen)23.
It is also possible that the surface of a catheter coated with an anti-infective substance may impair the blood compatibility of the device. In the present study, this particularly applies to the polyurethane catheter coated with chlorhexidine and silver sulfadiazine (PU-2 + CHSS), which demonstrated the most unfavorable blood compatibility in this ex vivo laboratory study by inducing substantial hemolysis, platelet activation, TAT, F1 + 2 generation, complement activation, and release of proinflammatory cytokines. The effectiveness of the PU-2 + CHSS catheter in reducing CRBSI has been extensively investigated9,12,24, and international guidelines recommend the use of this catheter to reduce CRBSIs in critically ill patients25. To the best of our knowledge, thrombotic or hemolytic complications associated with the use of the PU-2 + CHSS catheters have not been observed in clinical studies. Animal experiments have demonstrated that chlorhexidine diacetate in a range of concentrations is damaging to rabbit erythrocytes26 as it binds to phospholipids in the cell membrane causing rupture of the cell and release of fractions of cell membrane and heme into the blood. Accordingly, as seen in our study, fragments of disrupted erythrocytes and leukocytes interfered with the platelet measurements, giving rise to falsely higher platelets counts of 110%. Further, our results indicate that the hemolysis in the PU-2 + CHSS loop (29%) was due to a toxic effect of the coating rather than primary complement activation, as the hemolysis occurred after complement inhibition with eculizumab or calcium. The strong complement activation seen with PU-2 + CHSS catheters might be explained by massive leakage of heme into plasma, which is known to activate the complement system27.
Chlorhexidine is a commonly used synthetic antiseptic and disinfectant that was introduced in the 1950s, and it affects both gram-negative and gram-positive bacteria, and also Candida albicans and some viruses28. The use of chlorhexidine in healthcare settings has increased substantially in recent years, and the results of large studies have indicated that this might lower the rates of healthcare-associated infections in different settings29,30. Case reports have described allergic reactions associated with use of chlorhexidine-impregnated CVCs in several countries31,32,33,34. It appears that the prevalence of allergic reactions is greater when chlorhexidine comes in contact with mucosal membranes or blood as compared to the skin, especially in persons of Japanese descent29,30,35,36, although more recent case reports describe allergic reactions in other ethnic groups as well33. Few investigations have focused on the systemic effects of chlorhexidine, but there are some case reports describing chlorhexidine as highly toxic and causing acute respiratory distress syndrome and shock35,37.
The release of proinflammatory cytokines has been associated with an increased risk of venous thrombosis, but the contribution of each cytokine remains to be elucidated38,39. Both silver sulfadiazine and chlorhexidine are known allergens36,40, which is consistent with the activation of pro-inflammatory markers seen in our study. Interestingly, Staphylococcal species, the most common catheter-infecting organisms, show receptor-mediated binding with fibronectin, fibrin, and other components of the fibrin sheath3,41. Hence, it is possible that impaired blood compatibility and limited duration of the coating (< 15 days in laboratory studies)42 can partly explain why it appears that antimicrobial CVCs do not clearly reduce clinically diagnosed sepsis or mortality9.
The second coated CVC we tested, PU-3 + BZC-H, was coated with both the cationic surfactant benzalkonium chloride and a hydrophilic hydromer, the latter of which is intended to improve the blood compatibility of the surface. All blood compatibility parameters except L-selectin were found at low levels. It is possible that the release of L-selectin from leukocytes can be triggered by a toxic effect of benzalkonium chloride, as has been suggested by other researchers, and this may potentially have a negative impact on the patients’ immune defense43.
It should also be noted that, compared to the control loop, the CVC coated with a noble metal alloy, PU-4 + NbMC, led to a slight increase in P-selectin MPs, F1 + 2, TAT, and complement activation. This noble metal coating has previously been evaluated in an in vitro blood compatibility study10, although the reporting authors described fewer blood compatibility parameters and values differing from those we noted (a TAT level of ~ 300 µg/L compared to ~ 200 µg/L in our study). Those investigators also described similar behavior in contact with blood as observed in our study.
Limitations of an experimental model and possible application to an in vivo situation
We recognize the limitations of the present study given the experimental design. However, the method we used is well established and recognized for testing of materials used in health care contexts and can elucidate relative differences in blood compatibility between CVC materials. It should be noted that the experimental set up in the present study may facilitate the accumulation of any released chlorohexidine from the coated surface which may have affected the results of the blood combability tests which may not be as obvious in the clinical setting where released chlorohexidine is diluted in a large blood volume. Nevertheless, blood combability may be an issue with the PU-2 + CHSS. In this experimental study the exposure of material surface to blood volume was 140 times higher per hour compared to an in vivo situation. In about 6 days the exposure in vivo would be the same as in the experiment. Prior data show that patients have their CVC about a week on average44. Accordingly, the clinical significance remains to be explored and may also vary for different patient groups depending on their diseases, comorbidities and medication with anti-inflammatory or anticoagulant therapy. Our investigation was conducted using a limited number of catheters from one or two different product lots, and possible variability between lots has not been evaluated.
