The Effects of Thrombocytopenia on the Activated Coagulation Time

Unfractionated heparin and the activated clotting time in non-cardiac arterial procedures

INTRODUCTION

Unfractionated heparin is administered during non-cardiac arterial procedures (NCAP) to prevent thromboembolic complications. In order to achieve a safe level of anticoagulation, the effect of heparin can be measured. The aim of this review was to provide an overview on what is known about heparin, suggested tests to monitor the effect of heparin, including the activated clotting time (ACT), and the factors that could influence that ACT.

EVIDENCE ACQUISITION

A literature search in PubMed was performed. Articles reporting on heparin, clotting time tests (including thrombin time, activated partial thromboplastin time, anti-activated factor X and ACT), and ACT measurement devices were selected.

EVIDENCE SYNTHESIS

Heparin has a non-predictable effect in the individual patient, which could be measured using the ACT. However, ACT values can be influenced by many factors, such as hemodilution, hypothermia and thrombocytopenia. In addition, a high variation in ACT outcomes is found between measurement devices of different brands. In the sparse literature on the role of ACT during NCAP, no consensus has been reached on optimal target ACT values. An ACT >250 seconds leads to more bleeding complications. Females have a longer ACT after heparin administration, with a higher risk of bleeding complications.

CONCLUSIONS

The effect of heparin is unpredictable. ACT can be used to monitor the effect of heparin and achieve individualized anticoagulation, tailored to the patient and the specifics of the operative procedure. However, the ACT itself can be affected by several factors and caution must be present, as measured ACT values differ between measurement devices.

Hemostatic Management in Extracorporeal Membrane Oxygenation

The use of extracorporeal membrane oxygenation (ECMO) for acute cardiac and/or respiratory failure has grown exponentially in the past several decades. Systemic anticoagulation is a fundamental element of caring for ECMO patients. Hemostatic management during ECMO walks a fine line to balance the risk of safe and effective anticoagulant delivery to mitigate thromboembolic complications and minimizing hemorrhagic sequelae. This review discusses the pharmacology, monitoring parameters, and special considerations for anticoagulation in patients requiring ECMO.

Clinical controversies in anticoagulation monitoring and antithrombin supplementation for ECMO

During extracorporeal membrane oxygenation (ECMO), a delicate balance is required to titrate systemic anticoagulation to prevent thrombotic complications within the circuit and prevent bleeding in the patient. Despite focused efforts to achieve this balance, the frequency of both thrombotic and bleeding events remains high. Anticoagulation is complicated to manage in this population due to the complexities of the hemostatic system that are compounded by age-related developmental hemostatic changes, variable effects of the etiology of critical illness on hemostasis, and blood-circuit interaction. Lack of high-quality data to guide anticoagulation management in ECMO patients results in marked practice variability among centers. One aspect of anticoagulation therapy that is particularly challenging is the use of antithrombin (AT) supplementation for heparin resistance. This is especially controversial in the neonatal and pediatric population due to the baseline higher risk of bleeding in this cohort. The indication for AT supplementation is further compounded by the potential inaccuracy of the diagnosis of heparin resistance based on the standard laboratory parameters used to assess heparin effect. With concerns regarding the adverse impact of bleeding and thrombosis, clinicians and institutions are faced with making difficult, real-time decisions aimed at optimizing anticoagulation in this setting. In this clinically focused review, the authors discuss the complexities of anticoagulation monitoring and therapeutic intervention for patients on ECMO and examine the challenges surrounding AT supplementation given both the historical and current perspectives summarized in the literature on these topics.

Reversal of heparin after cardiac surgery: protamine titration using a statistical model

OBJECTIVE

To establish a statistical model for determination of protamine dose in conjunction with cardiopulmonary bypass.

DESIGN

Prospective.

SETTING

University hospital.

PARTICIPANTS

Ninety consecutive cardiac surgical patients.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

A series of clinically oriented variables were introduced into a statistical model for projection of the protamine dose after cardiopulmonary bypass. The following significant predictors were identified using multivariable regression analysis: The patient's body surface area, the administered dose of heparin, heparin clearance, and the preoperative platelet count. The statistical model projected the protamine dose within 3±23 mg of the point-of-care test used as reference.

CONCLUSION

Protamine dosing based on statistical modeling represents an alternative to point-of-care tests.

Review article: heparin sensitivity and resistance: management during cardiopulmonary bypass

Heparin resistance during cardiac surgery is defined as the inability of an adequate heparin dose to increase the activated clotting time (ACT) to the desired level. Failure to attain the target ACT raises concerns that the patient is not fully anticoagulated and initiating cardiopulmonary bypass may result in excessive activation of the hemostatic system. Although antithrombin deficiency has generally been thought to be the primary mechanism of heparin resistance, the reasons for heparin resistance are both complex and multifactorial. Furthermore, the ACT is not specific to heparin's anticoagulant effect and is affected by multiple variables that are commonly present during cardiac surgery. Due to these many variables, it remains unclear whether decreased heparin responsiveness as measured by the ACT represents inadequate anticoagulation. Nevertheless, many clinicians choose a target ACT to assess anticoagulation, and interventions aimed at achieving the target ACT are routinely performed in the setting of heparin resistance. Treatments for heparin resistance/alterations in heparin responsiveness include additional heparin or antithrombin supplementation. In this review, we discuss the variability of heparin potency, heparin responsiveness as measured by the ACT, and the current management of heparin resistance.

The relationship between heparin level and activated clotting time in the adult cardiac surgery population

The purpose of this descriptive study was to examine the relationship between heparin levels (HL) determined by heparin protamine titration (HPT) and activated clotting time (ACT) for cardiopulmonary bypass (CPB) in an adult cardiac surgery population. We examined institutional databases for all patients who underwent CPB at a single US academic institution from February 2005 until July 2007. Baseline ACT, predicted and actual heparin dose response (HDR), target and actual ACT, heparin concentration and heparin bolus dose were recorded. We examined the ACT and HL after the initial heparin bolus dose (Post-Hep) and 10 minutes after the initiation of CPB (CPB+10). The Post-Hep and CPB+10 ACT and HL are reported for 3802 patients. The distribution of ACTs for HL of 0.7, 1.4, 2.0, 2.7 and 3.4 units heparin/mL blood at both time points are reported. Additional analysis of the relationship of HL to ACTs of 300, 350, 400 and 480 seconds is also presented.

Monitoring anticoagulation and hemostasis in cardiac surgery

The need to monitor anticoagulation and hemostasis during and after cardiac surgery has led to recognition of the importance of evaluation and use of hemostasis monitors in this setting. Consequently, rapid and accurate identification of abnormal hemostasis has been the major impetus for the development of point-of-care tests and their use in transfusion algorithms for cardiac surgical and other critically ill patients.

Inhibition of platelet aggregation by abciximab but not by aspirin can be detected by a new point-of-care test, the hemostatus

We have investigated the type of platelet defect that can be detected with the Hemostatus test performed with the Hepcon/HMS instrument (Medtronic) designed to investigate platelet function during and after surgery. This assay is based on the comparison of the activated clotting time of whole blood measured in cartridges containing kaolin or kaolin and platelet-activating factor in different concentrations. Addition of platelet-activating factor shortened the blood activated clotting time when the platelet counts were normal. However, when platelet counts were below 70000/microL, the activated clotting time was prolonged in all channels including those without platelet-activating factor showing the influence of platelets in the formation of the clot under the conditions tested. Inhibition of platelet aggregation with c7E3 (abciximab, ReoPro) also induced a much-prolonged activated clotting time, and a similar finding was seen with blood from a patient with Glanzmann's thrombasthenia confirming the need for platelet aggregation and/or the glycoprotein (GP) IIb-IIIa complex. In contrast, the interaction of GP Ib with von Willebrand Factor was not of major importance, since inhibition of this interaction with the anti-GP Ib murine monoclonal antibody, ALMA-12, did not modify the activated clotting time. Furthermore, the activated clotting time measured for patients with an acquired defect in von Willebrand Factor activity were unchanged. Finally, inhibition of thromboxane A2 formation by aspirin did not influence the results of this test. Globally, the Hemostatus test was able to detect major abnormalities of GP IIb-IIIa function in the presence or absence of platelet-activating factor.

Effect of heparin-bonded pulmonary artery catheters on the activated coagulation time

OBJECTIVE

This study evaluated the effect of a heparin-bonded pulmonary artery catheter (PAC) on the activated coagulation time (ACT).

DESIGN

A prospective, controlled comparison.

SETTING

A tertiary care university hospital.

PARTICIPANTS

Adult cardiac surgery patients.

INTERVENTIONS

Celite ACTs were measured from arterial and central venous blood samples before and after the insertion of a heparin-bonded PAC. Thromboelastograms were also obtained from central venous blood samples before and 2 minutes after PAC insertion.

MEASUREMENTS AND MAIN RESULTS

There was no significant difference between the sample sites before PAC insertion. After PAC insertion, the central venous ACTs were significantly increased compared with the corresponding arterial measurements at 2, 5, 10, and 20 minutes (p < 0.005, analysis of variance [ANOVA] for repeated measures, Fisher's protected least significant difference [PLSD]). The 2-minute post-PAC reaction time from the central venous blood sample was greater than 60 minutes in all cases.

CONCLUSION

The heparin-bonded PAC was associated with a localized, time-dependent alteration in the ACT. Whenever possible, blood samples for baseline ACT measurements should be obtained from an arterial catheter to minimize the anticoagulant effects from the PAC.