The effects of vasoactive agents, platelet agonists and anticoagulation on thrombelastography

Monitoring of Antiplatelet Therapy in Children on Ventricular Assist Device Support: Comparison of Multiplate and Thromboelastography Platelet Mapping

The optimal method for monitoring antiplatelet therapy in children supported with ventricular assist devices (VADs) is unknown. We conducted a retrospective study to compare Thromboelastography Platelet Mapping (TEG/PM) with multiple electrode platelet aggregometry (MEA) on a Multiplate analyzer (Roche Diagnostics, Mannheim, Germany). We analyzed data from 66 paired blood samples from 9 patients <16 years of age on VAD where platelet function was simultaneously measured with TEG/PM and MEA. Antiplatelet dose-response relationships and intraindividual variability during steady state therapy were determined. Agreement in determination of therapeutic antiplatelet therapy was poor (arachidonic acid, κ 0.23; adenosine diphosphate [ADP], κ 0.13). Rate of aspirin and clopidogrel resistance was much higher when determined using TEG/PM than MEA. In patients receiving ≥5 mg/kg/day aspirin, 72% of TEG/PM measurements showed subtherapeutic response compared with 11% of MEA measurements. There was evidence of a dose-response relationship with clopidogrel and MEA ADP-induced aggregation (R2 = 0.56; p < 0.0001); however, there was no association between dose and TEG/PM% ADP inhibition (p = 0.15). Intraindividual variability in platelet reactivity was far greater when measured by TEG/PM during steady state therapy. Multiple electrode platelet aggregometry appears to be more reliable than TEG/PM for monitoring antiplatelet therapy in children supported with VAD.

Impact of levosimendan on platelet function

Levosimendan has been developed for treatment of severe heart failure. The favorable hemodynamic effect of levosimendan is related to its unique dual mechanism of action - increase of the contractile force of the myocardium caused by enhanced sensitivity of myofilaments to calcium combined with vasodilatation caused by the opening of adenosine triphosphate - dependent potassium channels. Due to the structural similarities to phosphodiesterase inhibitors it may partly exert its action via inhibition of phosphodiesterase inhibitors III. Inhibition of the phosphodiesterase inhibitors III leads to an increase of intracellular concentration of cyclic adenosine monophosphate causing an anti-aggregatory effect. There are some contradictory or indirect and inconclusive reports related to the impact of levosimendan on platelet function. The aim of this systematic review was to critically discuss the impact of levosimendan on platelet function according to currently available knowledge based on the findings of experimental as well as observational and randomized clinical studies.

Thromboelastographic Study of Psychophysiological Stress: A Review

Thromboelastography (TEG) is drawing more attention for clinical and laboratory studies of hemostasis. It has been applied to evaluate the effects of both psychological and physiological stress on whole blood coagulation from the onset of the coagulation cascade through clot formation, to the end with fibrinolysis. We conducted a comprehensive review on the applications of TEG for assessment of different stressors, ranging from physical exercise to emotional situations. The methodology is unique in terms of instrumentation, the methods to activate blood coagulation, the type of blood (citrated vs fresh blood), and study settings (in vitro vs in vivo vs clinical trials). Thromboelastography has most often been used to study the effects of physiological stress. The author's own work and future directions are discussed as well. The review would facilitate future development of TEG for evaluating hemostasis and potential pathological pathways in response to various forms of stress.

In vitro effects of recombinant activated factor VII on thrombin generation and coagulation following inhibition of platelet procoagulant activity by prasugrel

INTRODUCTION

Prasugrel is a thienopyridyl P2Y12 antagonist with potent antiplatelet effects. At present, little is known about its effects on thrombin generation or what strategies may emergently reverse its anticoagulant effects. In the current study we evaluated whether recombinant activated factor VII may reverse prasugrel induced effects and increase thrombin generation in an in vitro model.

METHODS

The effect of prasugrel active metabolite, PAM (R-138727), was evaluated on platelet aggregation, thrombin generation, and rotational thromboelastometry parameters using blood from 20 healthy volunteers. Additionally, we evaluated the effects of adenosine diphosphate (ADP) and recombinant activated factor VII on restoring these parameters towards baseline values.

RESULTS

PAM reduced maximum platelet aggregation and led to platelet disaggregation. It also decreased peak thrombin, increased lag time, and increased time to peak thrombin. Treatment with recombinant activated factor VII restored all three parameters of thrombin generation towards baseline. ADP decreased lag time and time to peak thrombin, but had no effect on peak thrombin. When recombinant activated factor VII and ADP were combined they had a greater effect on thrombin parameters than either drug alone. PAM also increased thromboelastometric clotting time and clot formation time, but had no effect on maximum clot firmness. Treatment with either recombinant activated factor VII or ADP restored these values towards baseline.

CONCLUSIONS

Recombinant activated factor VII restores thrombin generation in the presence of PAM. In patients taking prasugrel with life-threatening refractory bleeding it has the potential to be a useful therapeutic approach. Additional clinical studies are needed to validate our findings.

A comparative evaluation of rotation thromboelastometry and standard coagulation tests in hemodilution-induced coagulation changes after cardiac surgery

BACKGROUND

Coagulopathy after cardiopulmonary bypass (CPB) is caused by multiple perturbations in cellular and humoral elements of coagulation. A timely and comprehensive method to evaluate hemostasis would be helpful in the management of bleeding patients after CPB. The assessment of whole blood coagulation using rotation thromboelastometry (ROTEM) was compared to coagulation tests routinely performed during cardiac surgery.

STUDY DESIGN AND METHODS

Blood was obtained from 26 patients undergoing CPB surgery at baseline, at 60 minutes on CPB, at the end of CPB, and on admission to intensive care unit. ROTEM tests (extrinsically activated [EXTEM], intrinsically activated [INTEM], specific clot formation [FIBTEM]), prothrombin time, activated partial thromboplastin time, platelet (PLT) count, fibrinogen, prothrombin level, antithrombin level, and thrombin generation (TG) measurement were performed.

RESULTS

We observed strong correlations between FIBTEM-amplitude at 10 minutes (A10) and fibrinogen level (r=0.87; p<0.001) and between EXTEM/ INTEM-A10 variables and PLT count (r=0.72 and 0.67, respectively; p<0.001). Receiver operating characteristic analysis demonstrated that EXTEM-A10 and INTEM-A10 are predictive of thrombocytopenia below 80×10(9)/L (area under the curve [AUC], 0.83 and 0.82, respectively), and FIBTEM-A10 was highly predictive of fibrinogen level below 200 mg/dL (AUC, 0.96). There were only weak correlations found between TG peak and clot formation time of EXTEM or INTEM (r=0.30 and 0.29, respectively; p<0.05).

CONCLUSION

ROTEM variables demonstrated clinically relevant correlations with PLT counts and fibrinogen levels. In particular, decreasing levels of fibrinogen can be quickly determined (<15-20 min) using FIBTEM.

Coagulopathy during cardiac arrest and resuscitation in a swine model of electrically induced ventricular fibrillation

AIMS

Coagulopathy is often present after resuscitation from cardiac arrest but plays an undefined role in the post cardiac arrest syndrome. The aim of this study was to characterize coagulation changes during cardiac arrest and post-resuscitation care in order to direct further focused study.

METHODS

Ventricular fibrillation (VF) was induced electrically in immature male swine, followed by normothermic American Heart Association Advanced Cardiac Life Support and a uniform post-resuscitation goal-directed resuscitation protocol. PT, aPTT, fibrinogen, Thrombelastography (TEG), platelet contractile force (PCF), clot elastic modulus (CEM), and collagen-induced platelet aggregation were compared at baseline, at 8 min of VF, during the 3rd round of chest compressions (CPR), and at 15, 90, 180, and 360 min after return of circulation using repeated measures ANOVA.

RESULTS

8/18 (44%) animals were resuscitated after 10.9 ± 0.9 min of VF and 7.6 ± 3.4 min of CPR. TEG revealed a significant impairment in clot strength (MA) and clot formation kinetics (K, alpha angle) arising during CPR, followed by a brief prolongation of clot onset times (R) after return of circulation. Both PCF and CEM fell significantly during CPR (PCF by 50%, CEM by 47% of baseline) and platelet aggregation was significantly decreased during CPR. Coagulation changes were partially recovered by 3h of post-resuscitation care.

CONCLUSION

Whole blood coagulation was rapidly impaired during CPR after electrically induced VF in this swine model by impaired platelet aggregation/contractile function and clotting kinetics. Further platelet-specific study is indicated.

Hypothermic cardiopulmonary bypass as a determinant of late thrombocytopenia following cardiac operations in pediatric patients

BACKGROUND

Thrombocytopenia after cardiac operations is a common event in both adult and pediatric patients. Late thrombocytopenia (LTCP) is a less common event that is still without a well-recognized cause. This study explores the role of heparin-induced thrombocytopenia (HIT) and other factors (complexity of the operation, temperature management, and drug use) in determining LTCP.

METHODS

We conducted an observational study of 63 consecutive patients aged <36 months operated with or without cardiopulmonary bypass (CPB). LTCP was defined as a platelet count <100,000 cells/microl or <50% of the pre-operative count at any point in time between post-operative days 5 and 10. A diagnostic test for heparin-platelet factor 4 (PF4) antibodies was performed in patients with LTCP. Other pre- and post-operative factors were investigated for their association with LTCP.

RESULTS

LTCP occurred in 15 (24%) patients. No patient had positive heparin-PF4 antibodies. The lowest temperature on CPB was an independent predictor of LTCP, with a cut-off value at 29 degrees C (sensitivity 80%, specificity 70%). Other factors associated with LTCP were prolonged post-operative use of unfractionated heparin and milrinone. LTCP was associated with increased post-operative morbidity.

CONCLUSION

LTCP was related to a combination of factors (operation severity, degree of hypothermia during CPB, prolonged use of unfractionated heparin, and milrinone). The individual contribution of each factor seems difficult to establish. However, the degree of hypothermia during CPB and drug-associated effects were identified. HIT could be excluded in all cases.

The effect of milrinone on platelet activation as determined by TEG platelet mapping

BACKGROUND

Milrinone is a phosphodiesterase III inhibitor that increases intracellular cyclic adenosine monophosphate resulting in improved ventricular function and vasodilation. Increased intracellular levels of cyclic adenosine monophosphate also inhibit adenosine diphosphate (ADP) and arachidonic acid (AA)-induced platelet aggregation. We hypothesized that inhibition of ADP and AA-induced platelet activation by therapeutic blood concentrations of milrinone could be quantified using TEG Platelet Mapping.

METHODS

Blood was taken from 15 healthy adults who had not been taking antiplatelet medications. Milrinone was added to whole blood in three clinically relevant concentrations (30, 100, and 300 ng/mL). Conventional thromboelastography (TEG) and TEG Platelet Mapping were performed on whole blood without milrinone and at each of these three concentrations.

RESULTS

Increased blood concentrations of milrinone were associated with increased inhibition of ADP and AA-induced platelet activation (P < 0.0001). Milrinone at a blood concentration of 300 ng/mL markedly impaired the platelet activation response to ADP and AA.

CONCLUSIONS

Therapeutic blood concentrations of milrinone exhibit a significant inhibitory effect on ADP and AA-induced platelet activation as determined by TEG Platelet Mapping, without affecting the conventional kaolin-activated TEG. We suggest that TEG Platelet Mapping results be interpreted with caution in patients being treated with milrinone, and other drugs that modify platelet cyclic nucleotide concentrations.