Rapid determination of maintenance heparin infusion rates with the use of non-steady-state heparin concentrations

A Review of Common Approaches to Heparin Dosing

The use of heparin has remained steady for the last 20 years. Significant advances have been made in the understanding of the pathogenesis of clotting disorders and their prevention. Despite these gains, our understanding of the most safe and effective approach to optimizing the dosing of heparin has lagged. With new insight into improvements in outcome related to rapid achievement of the therapeutic range, it has become apparent that the standard approach to heparin dosing is out of date. Individualized heparin dosing based on weight and computer-assisted dosing methods is superior for bringing patients into the desired therapeutic range more quickly, with less cost and better short- and long-term outcomes. Copyright © 1995 by W.B. Saunders Company

High-Risk Non-ST Elevation Acute Coronary Syndrome Outcomes in Patients Treated with Unfractionated Heparin Monitored Using Anti-Xa Concentrations Versus Activated Partial Thromboplastin Time

BACKGROUND

While the activated partial thromboplastin time (aPTT) is the most widely used assay to monitor unfractionated heparin (UFH), providing a general measure of the extent of anticoagulation, it does not reliably correlate with the blood concentration of heparin or its antithrombotic effect. While cost and availability have limited the widespread use of UFH in hospitals, monitoring UFH with heparin levels has been shown to reduce both the number of monitoring tests and the time to a therapeutic range.

OBJECTIVES

To compare outcomes in patients with non-ST elevation acute coronary syndrome (ACS) treated with weight-based UFH monitored with anti-Xa concentrations versus aPTT.

METHODS

A retrospective chart review was completed in patients admitted with high-risk ACS and compared to the UFH arm of the SYNERGY trial. The primary outcome included the clinical endpoint of all-cause death or non-fatal myocardial infarction until time of hospital discharge. Safety endpoints evaluated included incidence of stroke and major bleeding.

RESULTS

The primary endpoint occurred in 6.3% of patients in the study cohort compared to 6.5% of patients in the heparin arm of the SYNERGY trial at 48 hours (P = .006). Bleeding was reduced in the study cohort with a significant decrease in GUSTO severe bleeding (P = .007). Additionally the study cohort had significantly fewer patients with an absolute drop in hemoglobin or hematocrit. Thrombolysis in Myocardial Infarction (TIMI) major and minor bleeding, rate of transfusion, and platelet counts were similar between groups.

CONCLUSIONS

Outcomes for high-risk ACS patients receiving heparin monitored by anti-Xa concentrations are noninferior to heparin monitored by aPTT.

Heparin pharmacokinetics and pharmacodynamics

Heparin was discovered approximately 75 years ago and has been used extensively for the last 50 years to treat thromboembolic disorders. An endogenous glycosaminoglycan, heparin is found largely in the liver, lung and intestine. It is available for exogenous administration both as unfractionated and low molecular weight heparin. Unfractionated heparin is a heterogenous mixture of polysaccharide chains of varying length resulting in a range of molecular weights from 3000 to 30,000D while low molecular weight heparin ranges from 3000 to 6000D. Heparin produces its antithrombotic effect by binding to antithrombin III and this complex then binds to thrombin. In order to accomplish this a total of 18 to 22 monosaccharide units is necessary including a specific pentasaccharide binding site for antithrombin III. After either subcutaneous or intravenous injection heparin is distributed primarily within the intravascular space. A short distribution phase is seen which is thought to correspond to endothelial cell binding and internalisation. The disposition curve for unfractionated heparin has a unique concave-convex shape which is the result of combined saturable and nonsaturable elimination mechanisms. The nonsaturable elimination mechanism is renal and is the primary route of elimination for low molecular weight heparins. For this reason, the concave-convex pattern is not seen with low molecular weight preparations. Both forms of heparin are useful antithrombotic agents; however, the correlation between the antithrombotic effect and an in vitro laboratory test for either type still needs further clarification.