Early Heparin Administration Attenuates Tissue Factor-Mediated Thrombin Generation During Simulated Cardiopulmonary Bypass

Effect of cardiopulmonary bypass on coagulation factors II, VII and X in a primate model: an exploratory pilot study

OBJECTIVES

The use of cardiopulmonary bypass (CPB) in cardiac surgery is a major risk factor for postoperative bleeding. We hypothesized that consumptive coagulopathy and haemodilution influence the coagulation factors; therefore, we aimed to estimate the activity profiles of coagulation factors II, VII and X during CPB circulation.

METHODS

A 120-min bypass was surgically established in cynomolgus monkeys (n = 7). Activities of coagulation factors II, VII and X were measured at 6 time points during the experiment (baseline, 0, 30, 60, 120 min of bypass and 60 min after bypass). To assess the influence of consumptive coagulopathy, the values were adjusted for haemodilution using the haematocrit values. Data were expressed as mean (standard deviation).

RESULTS

Activities of coagulation factors decreased during the experiment. In particular, the activities for II, VII and X were decreased the most by 44.2% (5.0), 61.4% (4.3) and 49.0% (3.7) at 30 min following CPB initiation (P < 0.001, P < 0.001 and P < 0.001, respectively). Following adjustments for haemodilution, change magnitudes lessened but remained significant for factor VII. The adjusted concentration of factor VII was observed to decrease from the baseline to the initiation of bypass circulation.

CONCLUSIONS

In conclusion, coagulation factor II, VII and X concentrations decreased during CPB. Following adjustment for haemodilution, a decrease in concentration was observed with factor VII.

Plasma Proteolytic Cascade Activation during Neonatal Cardiopulmonary Bypass Surgery

BACKGROUND

 Neonates undergoing cardiopulmonary bypass (CPB) surgery to correct congenital heart defects often experience excessive bleeding. Exposure of blood to artificial materials during CPB may activate coagulation, complement and inflammatory pathways. In addition, the surgical stress placed on the haemostatic system may result in cross-activation of other plasma proteolytic cascades, which could further complicate physiological responses to the surgical procedure and post-operative recovery. Plasma protease inhibitors undergo distinct conformational changes upon interaction with proteases, and, thereby, can serve as endogenous biosensors to identify activation of the different proteolytic cascades. We tested the hypothesis that changes in the concentration and conformation of protease inhibitors regulating plasma proteolytic cascades during neonatal CPB are associated with post-operative bleeding.

PATIENTS AND METHODS

 Plasma samples from 44 neonates were obtained at four time points across the surgical procedure. Anti-thrombin, antitrypsin, anti-chymotrypsin, anti-plasmin, C1-inhibitor and tissue factor pathway inhibitor (TFPI) concentrations and conformations were evaluated by enzyme-linked immunosorbent assay, transverse urea gradient gel electrophoresis and sodium dodecyl sulphate-polyacrylamide gel electrophoresis.

RESULTS/CONCLUSION

 The most striking changes were observed following heparin administration and were associated with the appearance of inactive forms of anti-thrombin and an increase in the plasma concentration of TFPI. Changes in anti-thrombin and TFPI remained evident throughout surgery and into the post-operative period but were not different between patients with or without post-operative bleeding. The concentration of antitrypsin decreased across surgery, but there was no significant accumulation of inactive conformations of any inhibitor besides anti-thrombin, indicating that widespread cross-activation of other plasma proteolytic cascades by coagulation proteases did not occur.

A reduction of prothrombin conversion by cardiac surgery with cardiopulmonary bypass shifts the haemostatic balance towards bleeding

Cardiac surgery with cardiopulmonary bypass (CPB) is associated with blood loss and post-surgery thrombotic complications. The process of thrombin generation is disturbed during surgery with CPB because of haemodilution, coagulation factor consumption and heparin administration. We aimed to investigate the changes in thrombin generation during cardiac surgery and its underlying pro- and anticoagulant processes, and to explore the clinical consequences of these changes using in silico experimentation. Plasma was obtained from 29 patients undergoing surgery with CPB before heparinisation, after heparinisation, after haemodilution, and after protamine administration. Thrombin generation was measured and prothrombin conversion and thrombin inactivation were quantified. In silico experimentation was used to investigate the reaction of patients to the administration of procoagulant factors and/or anticoagulant factors. Surgery with CPB causes significant coagulation factor consumption and a reduction of thrombin generation. The total amount of prothrombin converted and the rate of prothrombin conversion decreased during surgery. As the surgery progressed, the relative contribution of α2-macroglobulin-dependent thrombin inhibition increased, at the expense of antithrombin-dependent inhibition. In silico restoration of post-surgical prothrombin conversion to pre-surgical levels increased thrombin generation excessively, whereas co-administration of antithrombin resulted in the normalisation of post-surgical thrombin generation. Thrombin generation is reduced during surgery with cardiopulmonary bypass because of a balance shift between prothrombin conversion and thrombin inactivation. According to in silico predictions of thrombin generation, this new balance increases the risk of thrombotic complications with prothrombin complex concentrate administration, but not if antithrombin is co-administered.

Ex vivo simulation of cardiopulmonary bypass with human blood for hemocompatibility testing

OBJECT

Experimental circuits for biomaterial surface testing are frequently limited by the tested blood volume, composition of the circuit, flow conditions and the use of animal blood. This report describes an ex vivo set-up for simulated cardiopulmonary bypass with human blood perfusion. We investigated the clinical generalizability of the observed effects on hematological and metabolic parameters and the hemocompatibility of the system.

METHODS

The simulated cardiopulmonary bypass circuit consisted of a heparin-coated tubing system connected to an oxygenator and a venous reservoir. Normothermic flow of blood obtained from healthy donors was maintained at 2.4 L/min/m(2) by a roller pump. Heparin was dosed to obtain a target activated clotting time (ACT) ⩾500 s. Blood was drawn at baseline and 0, 10, 60 and 120 minutes following the initiation of blood flow to determine hematological and metabolic parameters and the hemocompatibility of the extracorporeal system. Data were analyzed using repeated measures ANOVA.

RESULTS

Two hours of blood perfusion resulted in a small, but clinically unimportant reduction in hematocrit, whereas hemoglobin levels and red blood cell, platelet and leukocyte counts remained stable. There was a significant increase in ACT throughout the experiment. While pO2 levels and the pH remained unaltered during the experiment, pCO2 values decreased from 51 ± 6 mmHg at T0 to 41 ± 3 mmHg at T120 (p<0.001). Simulated cardiopulmonary bypass induced a two-fold increase in C3a (p=0.001) while tissue factor was decreased from 44 ± 14 pg/mL at T0 to 38 ± 13 pg/mL at T120 (p=0.009). Levels of CD40L, prothrombin fragment 1+2, β-thromboglobulin and factor VIIa remained stable over time.

CONCLUSION

The ex vivo set-up for simulated cardiopulmonary bypass mimicked the clinical cardiosurgical setting. Exposure of fresh donor blood to the extracorporeal circuit showed a good hemocompatibility, indicated by maintained hematological parameters and a mild immune response.