Thoracic Aorta Occlusion-Reperfusion Decreases Hemostasis as Assessed by Thromboelastography in Rabbits

Assessing the Methodology for Calculating Platelet Contribution to Clot Strength (Platelet Component) in Thromboelastometry and Thrombelastography

The viscoelastic properties of blood clot have been studied most commonly using thrombelastography (TEG) and thromboelastometry (ROTEM). ROTEM-based bleeding treatment algorithms recommend administering platelets to patients with low EXTEM clot strength (e.g., clot amplitude at 10 minutes [A10] <40 mm) once clot strength of the ROTEM® fibrin-based test (FIBTEM) is corrected. Algorithms based on TEG typically use a low value of maximum amplitude (e.g., <50 mm) as a trigger for administering platelets. However, this parameter reflects the contributions of various blood components to the clot, including platelets and fibrin/fibrinogen. The platelet component of clot strength may provide a more sensitive indication of platelet deficiency than clot amplitude from a whole blood TEG or ROTEM® assay. The platelet component of the formed clot is derived from the results of TEG/ROTEM® tests performed with and without platelet inhibition. In this article, we review the basis for why this calculation should be based on clot elasticity (e.g., the E parameter with TEG and the CE parameter with ROTEM®) as opposed to clot amplitude (e.g., the A parameter with TEG or ROTEM®). This is because clot elasticity, unlike clot amplitude, reflects the force with which the blood clot resists rotation within the device, and the relationship between clot amplitude (variable X) and clot elasticity (variable Y) is nonlinear. A specific increment of X (ΔX) will be associated with different increments of Y (ΔY), depending on the initial value of X. When calculated correctly, using clot elasticity data, the platelet component of the clot can provide a valuable insight into platelet deficiency in emergency bleeding.

Different effects of abciximab and cytochalasin D on clot strength in thrombelastography

Maximum amplitude (MA) in thrombelastography (TEG) consists of a plasmatic and a platelet component. To assess the magnitude of the plasmatic component, pharmacological approaches have been proposed to eliminate the platelet component. We evaluated the individual and combined effects of abciximab and cytochalasin D on the MA of TEG. Whole blood, platelet-rich plasma (PRP) and homologous platelet-poor plasma (PPP) from 20 healthy volunteers were spiked with abciximab or cytochalasin D or a combination of both and TEGs performed. Abciximab and cytochalasin D decreased MA in all samples. MA of whole blood (18.6 +/- 3.1 mm) and PRP (33.7 +/- 3.5 mm) spiked with abciximab or cytochalasin D alone (15.0 +/- 2.9 mm and 25.0 +/- 4.0 mm) were significantly higher when compared with abciximab and cytochalasin D combined (10.4 +/- 3.0 and 20.2 +/- 3.5 mm). While MA of PRP and homologous PPP were significantly (P < 0.001) different after individual administration of abciximab and cytochalasin D, combination of both abolished this difference (20.2 +/- 3.5 mm and 20.4 +/- 3.7 mm, P = 0.372). In whole blood of critically ill patients or patients undergoing major surgery there was also a significant difference of MA between abciximab alone and in combination with cytochalasin D (16.5 +/- 11.3 mm and 11.3 +/- 7.7 mm, P < 0.001). This indicates that in contrast to individual administration of abciximab or cytochalasin D, a combination of both compounds eliminates the platelet-specific effect on MA of TEG tracings.

The effect of tourniquet application, tranexamic acid, and desmopressin on the procoagulant and fibrinolytic systems during total knee replacement

STUDY OBJECTIVE

To assess the influence of tourniquet inflation-deflation as well as desmopressin and tranexamic acid (TA) administration on prothrombin fragment 1.2, fibrinogen, plasmin antiplasmin complex, and D-dimer concentrations during total knee replacement.

DESIGN

Randomized, placebo-controlled study.

SETTING

Large referral hospital.

PATIENTS

30 ASA physical status I, II, and III patients undergoing total knee replacement.

INTERVENTIONS

Patients were randomized to one of three treatment groups. Patients received either tranexamic acid, desmopressin, or an equal volume of saline, intravenously.

MEASUREMENTS AND MAIN RESULTS

Cubital blood was drawn immediately before induction of anesthesia, 1 hour after tourniquet application, and 2 and 15 minutes after tourniquet deflation. Fibrinogen and D-dimer levels were measured using the Clauss Method and latex agglutination, respectively. Plasmin antiplasmin complex and prothrombin fragment 1.2 levels were measured by enzyme-linked immunosorbent assay (ELISA). All assays were performed in duplicate, and intra-assay variability was documented. No statistically significant difference in fibrinogen, D-dimer, plasmin antiplasmin complex, or prothrombin fragment 1.2 levels was demonstrated among the groups. Similarly, within each group there were no statistically significant differences in the variables studied. However, despite the lack of statistical significance, when compared with their levels during tourniquet application, an increase in D-dimer and plasmin antiplasmin complex levels was observed in all three groups at 2 and 15 minutes after tourniquet release. In contrast, no increase in prothrombin fragment 1.2 generation was noted. Significantly more allogeneic blood was transfused in the Control and Desmopressin Groups when compared with the tranexamic acid group (p< 0.02).

CONCLUSIONS

No evidence of tourniquet-induced fibrinolysis or thrombin generation was demonstrated in the systemic circulation. Desmopressin and tranexamic acid had no significant effect on the variables measured.

A comparative study of the postoperative allogeneic blood-sparing effects of tranexamic acid and of desmopressin after total knee replacement

BACKGROUND

Tissue hypoxia and reperfusion induce abnormal hemostatic function. Therefore, bleeding after total knee replacement (TKR) may be a result of a tourniquet-induced imbalance of the procoagulant and fibrinolytic systems. Because laboratory confirmation of tourniquet-induced abnormal hemostasis is difficult to obtain, indirect evidence must be sought.

STUDY DESIGN AND METHODS

A prospective, single-blind study of 40 patients undergoing TKR was performed. In the tranexamic acid (TA) group, in the 30 minutes before the limb tourniquet was deflated, an IV bolus dose of TA (15 mg/kg) was administered. Thereafter, a constant IV infusion of 10 mg per kg per hour was administered until 12 hours after tourniquet deflation. In the desmopressin group, desmopressin (0.3 mg/kg) and saline were administered by a similar protocol. No blood was administered intraoperatively. A postoperative Hct <27 percent constituted the postoperative transfusion trigger. Patients were examined daily for signs of lower-limb deep vein thrombosis, and they underwent lower-limb Doppler ultrasound on postoperative Day 5. Three months after surgery, the incidence of delayed thromboembolic events was assessed.

RESULTS

During the first 12 postoperative hours, blood accumulation in the surgical drain was significantly (p<0.05) lower in the TA group (162 mL +/- 129) than in the desmopressin group (342 mL +/- 169). From the sixth postoperative hour until 3 days postoperatively, Hct levels were significantly lower in the desmopressin group than in the TA group. Significantly more allogeneic blood was transfused in the desmopressin group (11 patients received 16 units) than in the TA group (3 patients each received 1 unit) (p<0.02). There were no clinical signs of deep vein thrombosis or abnormal Doppler ultrasound studies. Three months postoperatively, there were no thromboembolic events among the 37 patients interviewed.

CONCLUSION

TA induces better blood sparing than desmopressin. Therefore, a tourniquet-induced increase in fibrinolysis is the likely cause of delayed bleeding after TKR surgery. However, before routine administration, the effect of TA on the incidence of thromboembolic events requires further investigation.

Pentalyte does not decrease heparinoid release but does decrease circulating thrombotic mediator activity associated with aortic occlusion-reperfusion in rabbits

Hemorrhage and thrombosis are associated with major vascular and trauma surgery. Release of heparinoids and thrombotic mediators may contribute to these complications and have been described in rabbits after aortic occlusion-reperfusion. We hypothesized that the resuscitative fluid used could reduce heparinoid and thrombotic mediator release after aortic occlusion-reperfusion in rabbits as assessed by thromboelastographic variables (R, reaction time; alpha, angle; and G, a measure of clot strength). Anesthetized rabbits were administered lactated Ringer's solution (n = 8) or PentaLyte (n = 8) at reperfusion after 30 min of ischemia. Blood was obtained before ischemia and after 30 min of reperfusion for thromboelastography under four conditions: 1) unmodified sample, 2) platelet inhibition, 3) heparinase, and 4) platelet inhibition and heparinase. During reperfusion, unmodified samples demonstrated a significant increase in R and decrease in alpha and G that was not affected by PentaLyte. In the presence of heparinase, no significant fluid-specific thromboelastographic differences were noted. However, thrombotic mediator release (discerned by a decrease in R and an increase in alpha) during reperfusion in samples with platelet inhibition and heparinase was significantly attenuated by PentaLyte. PentaLyte administration does not decrease heparinoid release but does decrease thrombotic mediator release after aortic occlusion-reperfusion.

Hepatoenteric ischemia-reperfusion increases circulating heparinoid activity in rabbits

PURPOSE

The purpose of this study was to determine if an increase in circulating heparinoid activity contributes to the hemostatic abnormalities associated with hepatoenteric ischemia-reperfusion.

MATERIALS AND METHODS

Anesthetized rabbit (n = 18) underwent thoracic aorta occlusion for 30 minutes with a balloon catheter, followed by 30 minutes of reperfusion. Blood samples were obtained after 30 minutes of equilibration and 30 minutes of reperfusion. Hemostatic function was assessed by changes in the thrombelastographic variables R (reaction time), alpha (a measure of the speed of clot formation), and G (a measure of clot strength). Thrombelastography was performed on blood without platelet inhibition in the presence or absence of heparinase (n = 9 rabbits). Additional samples (n = 9) were exposed to cytochalasin D (platelet inhibitor) with or without heparinase.

RESULTS

Compared with preischemic values, blood samples with intact platelet function obtained during reperfusion demonstrated a decrease in hemostatic function evidenced by a significant (P<.05) increase in R, decrease in alpha, and decrease in G. R, alpha, and G values of samples without platelet inhibition exposed to heparinase did not significantly change after ischemia. Blood samples exposed to cytochalasin D displayed a similar pattern.

CONCLUSION

An increase in circulating heparinoid activity significantly contributes to the hemostatic disorder associated with hepatoenteric ischemia-reperfusion in rabbits.