Cell activation and thrombin generation in heparin bonded cardiopulmonary bypass circuits using a novel in vitro model

Thrombin generation and bleeding in cardiac surgery: a clinical narrative review

This narrative review discusses the role of thrombin generation in coagulation and bleeding in cardiac surgery, the laboratory methods for clinical detection of impaired thrombin generation, and the available hemostatic interventions that can be used to improve thrombin generation. Coagulopathy after cardiopulmonary bypass (CPB) is associated with excessive blood loss and adverse patient outcomes. Thrombin plays a crucial role in primary hemostasis, and impaired thrombin generation can be an important cause of post-CPB coagulopathy. Existing coagulation assays have significant limitations in assessing thrombin generation, but whole-blood assays designed to measure thrombin generation at the bed-side are under development. Until then, clinicians may need to institute therapy empirically for non-surgical bleeding in the setting of normal coagulation measures. Available therapies for impaired thrombin generation include administration of plasma, prothrombin complex concentrate, and bypassing agents (recombinant activated factor VII and factor eight inhibitor bypassing activity). In vitro experiments have explored the relative potency of these therapies, but clinical studies are lacking. The potential incorporation of thrombin generation assays into clinical practice and treatment algorithms for impaired thrombin generation must await further clinical development.

Current Understanding of How Extracorporeal Membrane Oxygenators Activate Haemostasis and Other Blood Components

Extracorporeal membrane oxygenators are used in critical care for the management of severe respiratory and cardiac failure. Activation of the coagulation system is initiated by the exposure of blood to synthetic surfaces and the shear stresses of the circuit, especially from device pumps. Initial fibrinogen deposition and subsequent activation of coagulation factors and complement allow platelets and leucocytes to adhere to oxygenator surfaces and enhance thrombin generation. These changes and others contribute to higher rates of thrombosis seen in these patients. In addition, bleeding rates are also high. Primary haemostasis is impaired by platelet dysfunction and loss of their key adhesive molecules and shear stress causes an acquired von Willebrand defect. In addition, there is also altered fibrinolysis and lastly, administration of systemic anticoagulation is required to maintain circuit patency. Further research is required to fulyl establish the complexities of the haemostatic changes with these devices, and to elucidate the mechanistic changes that are mainly responsible so that plans can be made to reduce their complications and improve management.

The effects of high-dose heparin on inflammatory and coagulation parameters following cardiopulmonary bypass

Systemic inflammation and the activation of the coagulation system following cardiopulmonary bypass (CPB) may contribute to postoperative complications. In vitro studies have demonstrated that heparin possesses anti-inflammatory properties. To ascertain the relative benefits of high versus low heparin doses, we studied the impact of varying heparin doses on the inflammatory response and coagulation system during and following CPB. Forty patients scheduled for elective coronary artery bypass surgery requiring CPB were randomized to either a low dose (300 U/kg) (Group L) or a high dose of unfractionated heparin (600 U/kg) (Group H). To evaluate the inflammatory response, proinflammatory cytokines [tumor necrosis factor-alpha and interleukin-6 (IL-6)] were measured at four different times: before CPB (T0), 30 min after the institution of CPB (T1), 30 min after cross-clamp release (T2), and 4 h after the end of CPB (T3). Thrombin-antithrombin complex, platelet factor 4 and anti-activated factor X heparin concentrations were also measured. Patients in Group H received greater heparin (44.934 U versus 27.741 U, P<0.001) and protamine (P=0.003) doses. Postoperative blood loss and blood products transfusions were not significantly different in the groups. At T1, mean heparin plasma concentration was higher in Group H (P<0.001). IL-6 was significantly lower in Group H compared with Group L (P=0.01) only at T1. Using a mixed-effects statistical model, tumor necrosis factor-alpha and IL-6 levels were comparable regardless of the heparin dose. Thrombin-antithrombin complex levels were lower in Group H (P=0.04) and platelet factor 4 levels were significantly lower in Group H at T2 (P=0.04). Higher heparin doses were associated with higher heparin concentrations during CPB. A high heparin dose achieved a better preservation of the coagulation system with less thrombin formation and platelet activation. The heparin dose had small influence on proinflammatory cytokines release.

Anticoagulant-free Genius® haemodialysis using low molecular weight heparin-coated circuits

BACKGROUND

Regional citrate anticoagulation or saline flushes are often used in haemodialysis patients at high risk of bleeding. In an alternative approach we evaluated the effects of covalent circuit coating with low molecular weight heparin (LMWH) for intermittent haemodialysis.

METHODS

In vitro, we compared the thrombogenicity of an uncoated polyvinylchloride (PVC) tubing set with LMWH-coated tubing (AOThel) and a reference tubing with end-point attached heparin coating (Carmeda Bioactive surface) under dynamic blood contact. In vivo, five chronic haemodialysis patients were studied using the Genius dialysis system and F60S filters. Each patient underwent three dialysis sessions separated by a standard haemodialysis each: (1) standard dialysis (uncoated circuit and regular dalteparin dosage), (2) dialysis with LMWH-coated circuit and regular dalteparin dosage and (3) dialysis with a completely LMWH-coated circuit without anticoagulant use.

RESULTS

In vitro, both coated tubings showed significantly reduced thrombin-antithrombin (TAT) complex levels compared with PVC. The reference coating (Carmeda) released substantial antifactor Xa (antiXa) activity into the plasma. The LMWH coating (AOThel) released low antiXa activity only during the initial rinsing. In vivo, all dialysis sessions were well tolerated and completed without major clotting. Antithrombin levels and platelet counts were similar in all groups. P-selectin and D-dimer levels increased similarly in all groups. TAT levels were comparable in all groups during the first 3 h and significantly increased in the anticoagulant-free group after the fourth hour.

CONCLUSIONS

LMWH surface coating reduces thrombogenicity in vitro without releasing significant amounts of heparin from the surface. In vivo, anticoagulant-free haemodialysis using a completely LMWH-coated circuit is feasible and safe in stable chronic dialysis patients with normal coagulation.

Reducing Hemostatic Activation During Cardiopulmonary Bypass: A Combined Approach

Interventions such as heparin-coated circuits, epsilon-aminocaproic acid, and reduced shed blood reinfusion have shown mixed results when applied individually for limiting hemostatic activation during cardiopulmonary bypass (CPB). We compared coagulation and fibrinolytic activation during conventional CPB (control) (CTRL) using noncoated circuits, no antifibrinolytics, and open cardiotomy with a combined strategy (HAC) that used heparin-coated circuits, epsilon-aminocaproic acid, and closed cardiotomy. Blood samples were drawn before, during, and after CPB for primary coronary bypass grafting surgery from 9 CTRL patients and 10 HAC patients. Thrombin-antithrombin complex and fibrinopeptide A levels (markers of thrombin and fibrin generation) were reduced in the HAC versus CTRL group after 30 min of CPB (P < 0.05). Average tissue plasminogen activator (tPA) levels were significantly lower in the HAC group by 30 min on CPB (P < 0.05), resulting in preservation of plasminogen activator inhibitor (PAI)-1 during CPB (P < 0.05). D-Dimer, a measure of intravascular fibrin formation and removal, was reduced in the HAC group during and after CPB (P < 0.005). Overall, the combined strategy was associated with a reduction in CPB-induced increases in markers of thrombin generation, fibrin formation, tPA release, and fibrin degradation and better preservation of PAI-1.

IMPLICATIONS

A combined approach during cardiopulmonary bypass (CPB) that uses heparin-coated circuits, epsilon-aminocaproic acid, and limited reinfusion of shed pericardial blood is associated with reduced activation of the coagulation and fibrinolytic systems that typically occurs during conventional CPB.

Management of anticoagulation and its reversal during paediatric cardiopulmonary bypass: a review of current UK practice

Protocols for management of heparin and protamine administration in patients undergoing open-heart surgery have been developed from experience gained mainly in adult practice. However, it has been demonstrated that there are marked differences between paediatric and adult patients in their response to systemic anticoagulation and its reversal. The aim of this study was to obtain an overview of current practice of management of anticoagulation and its reversal from paediatric cardiac surgical units of Great Britain and Ireland. All centres performing paediatric cardiac surgery agreed to participate in the survey (n = 16). Telephone interviews were carried out with the chief or a senior perfusionist from all participating institutions, which were based on a structured questionnaire compiled specifically for the purpose. The answers were anonymised. At present, in the UK and Ireland, unfractionated heparin is the anticoagulant of choice in all units, with a slight prevalence of porcine mucosal (9/16, 56.5%) versus bovine lung preparation (7/16, 44.0%). The policy for administration of heparin to the patient is uniform, with a dose of 300 IU/kg. However, there is great variability in the amount of heparin added to the prime and to the volume infused during cardiopulmonary bypass (CPB). Monitoring of anticoagulation is achieved by activated coagulation time alone in all but one centre, with lower limits varying between 400 and 750 s. Use of aprotinin is widely accepted, but clinical indications are highly variable. No centre adopts heparin-bonded or heparin-coated circuitry for CPB. Calculation of initial and additional protamine doses followed a variety of criteria, resulting in a very wide distribution of doses. The data obtained highlighted the lack of uniformity among paediatric cardiac surgical units of Great Britain and Ireland with regard to most of the issues related to the management of anticoagulation and its reversal. The striking heterogeneity of our cross-sectional observations clearly underlines the need for prospective, multicentre studies on a national basis to relate different clinical practices to outcome measures.

A prospective randomized trial of Duraflo II heparin-coated circuits in cardiac reoperations

BACKGROUND

Heparin-coated circuits in cardiopulmonary bypass have been shown to decrease the systemic inflammatory responses associated with cardiopulmonary bypass. Previous clinical studies on low-risk patients who had coronary artery bypass grafting (CABG) and received full-dose systemic heparin did not have clearly improved clinical outcomes. We hypothesized that the beneficial effects of heparin-coated circuits might be seen in patients who had cardiac reoperations.

METHODS

Three hundred fifty patients who had reoperation with CABG only (58%), or with valve operations (42%) were randomly assigned to receive either a heparin-coated (Duraflo II; study group) or uncoated (control group) circuit. Clinical outcomes were compared and the variables were analyzed using the following three groups: entire populations of study group and control group, subgroup of patients who had CABG reoperation only, and a subgroup who had valve reoperation or combined valve and CABG reoperation.

RESULTS

Preoperative variables were the same in both groups. No difference in clinical outcomes could be demonstrated except that the percentage of patients with major bleeding episodes was significantly lower in the study group (1.2% versus 5.4%, p = 0.035). In the subgroup analysis of patients who had valve reoperations, lower blood transfusion requirements in the intensive care unit (p = 0.013) were found in the study group. When the subgroup of patients who had CABG reoperations was analyzed separately, there was a trend toward less reoperation for bleeding in the study group (0% versus 4.0%, p = 0.058).

CONCLUSIONS

We conclude that the use of heparin-coated circuits was safe and imparted protection from reoperations for bleeding and major bleeding episodes. Material-independent blood activation (eg, blood-air interface and cardiotomy suction) blunted the total effect of the heparin-coated surface.

Aprotinin complements heparin bonding in an in vitro model of cardiopulmonary bypass

The relative contribution of full-dose aprotinin, used with heparin-bonded surfaces, to contact activation during cardiopulmonary bypass was examined. In vitro Carmeda-bonded cardiopulmonary bypass circuits were perfused with whole blood anticoagulated with heparin (3.3 U/ml). Aprotinin (300 kIU/ml) was added to the circuits of one set of experiments. Samples were taken prior to perfusion and at 30, 60, 120 and 360 min. The activated coagulation time was extended in the aprotinin experiments, significantly at 30 min (P=0.003) and 120 min (P=0.001). Thrombin-antithrombin complexes and prothrombin fragment F1+2 were both higher in the non-aprotinin experiments at 120 min (P=0.02 each) and 360 min (P=0.005 and 0.001, respectively). Plasma leucocyte elastase was raised in the non-aprotinin experiments in comparison to the aprotinin experiments at each timepoint (30 min, P=0.04; 60 min, P=0.006; 120 min, P=0.001; 360 min, P=0.0001), as was interleukin-8 at 120 min (P=0.05) and 360 min (P=0.0001). No differences were found for the platelet activation marker P-selectin. Platelet and white blood cell counts fell significantly in the non-aprotinin experiments compared with the aprotinin experiments at 360 min (P=0.05 and 0.03, respectively). It would appear that the use of aprotinin has additional haemostatic beneficial effects to those found with heparin-bonded circuits in terms of effects on contact activation and inflammation.

Deep hypothermia and circulatory arrest for surgery of complex intracranial aneurysms

OBJECTIVE

Some intracranial aneurysms may not be operable by conventional neurosurgery due to their location or morphology. Cardiopulmonary bypass (CPB) and deep hypothermic circulatory arrest renders surgery of these complex aneurysms possible. Brain temperatures can be measured directly in this setting.

METHODS

Eight patients with complex intracranial aneurysms were operated on with the aid of CPB. Femoro-femoral bypass with heparin-coated circuit components was used in all cases. Venous drainage was augmented by a centrifugal pump in six patients and by a newly developed vacuum technique in two patients. Temperatures were monitored by probes in brain, tympanum, nasopharynx, bladder, rectum, arterial and venous blood. These measurements were recorded on-line together with those of cerebral oxygen saturation, AP, CVP and PAP. Blood gas analyses and an EEG were also performed continuously.

RESULTS

Outcome was excellent in seven patients, in one patient moderate neurological disability occurred. Mean time on cardiopulmonary bypass was 160 (117-215) min, for cooling to a brain temperature of 18 degrees C 33 (20-47) min, and for total circulatory arrest 27 (15-45) min. Additionally, terminal brain arteries were clamped for up to 68 min in four patients. No cardiac complications were observed. Actual brain temperatures were best reflected by the tympanum probes (max. deviation 2 degrees C), whereas temperatures measured in bladder or rectum exhibited deviations of up to 10 degrees C. EEG activities were arrested between brain temperatures of 19 and 26 degrees C.

CONCLUSIONS

Complex intracranial aneurysms can be treated successfully using deep hypothermic circulatory arrest. Extensive monitoring adds to the speed and safety of the procedure. The resulting comparative measurements of temperatures at different body sites including brain, EEG, and other variables may be of general relevance for operations employing deep hypothermia and circulatory arrest.