Laboratory heparin resistance in burn injury complicated by venous thrombosis

Heparin resistance in severe thermal injury: A prospective cohort study

BACKGROUND

Low molecular-weight heparin (LMWH) is routinely administered to burn patients for thromboprophylaxis. Some studies have reported heparin resistance, yet the mechanism(s) and prevalence have not been systematically studied. We hypothesized that nucleosomes, composed of histone structures with associated DNA released from injured tissue and activated immune cells in the form of neutrophil extracellular traps (NETs or NETosis), neutralize LMWH resulting in suboptimal anticoagulation, assessed by reduction in anti-factor Xa activity.

METHODS

Blood was sampled from >15% total body surface area (TBSA) burn patients receiving LMWH on days 5, 10 and 14. Peak anti-factor Xa (AFXa) activity, anti-thrombin (ATIII) activity, cell-free DNA (cfDNA) levels and nucleosome levels were measured. Mixed effects regression was adjusted for multiple confounders, including injury severity and ATIII activity, and was used to test the association between nucleosomes and AFXa.

RESULTS

A total of 30 patients with severe burns were included. Mean TBSA 43% (SD 17). Twenty-three (77%) patients were affected by heparin resistance (defined by AFXa activity <0.2 IU/mL). Mean peak AFXa activity across samples was 0.18 IU/mL (SD 0.11). Mean ATIII was 81.9% activity (SD 20.4). Samples taken at higher LWMH doses were found to have significantly increased AFXa activity, though the effect was not observed at all doses, at 8000 IU no samples were heparin resistant. Nucleosome levels were negatively correlated with AFXa (r = -0.29, p = 0.050) consistent with the hypothesis. The final model, with peak AFXa as the response variable, was adjusted for nucleosome levels (p = 0.0453), ATIII activity (p = 0.0053), LMWH dose pre-sample (p = 0.0049), drug given (enoxaparin or tinzaparin) (p = 0.03), and other confounders including severity of injury, age, gender, time point of sample.

CONCLUSIONS

Heparin resistance is a prevalent issue in severe burns. Nucleosome levels were increased post-burn, and showed an inverse association with AFXa consistent with the hypothesis that they may interfere with the anticoagulant effect of heparin in vivo and contribute to heparin resistance. Accurate monitoring of AFXa activity with appropriate therapy escalation plans are recommended with dose adjustment following severe burn injury.

A Severely Burned Hemophiliac

Patients suffering from moderate or severe hemophilia A are particularly vulnerable to trauma injury, being on high risk of immediate exsanguination. Due to a rareness of this disease, there are very few reports about the management of severe injuries of the affected patients. No guidelines for the management of burn trauma of hemophiliac patients have been yet established. Since, to our knowledge, this is the first case report about a successful treatment of a severely burned hemophiliac, requiring intensive care, long-time intubation, and multiple epifascial necrosectomies of third grade wounds, the authors are proposing the approach to this condition. The factor VIII has been substituted, enhancing its activity to over 70% during the planned interventions and keeping it above 30% in the intervals between the operations. To minimize the risk of surgical bleeding, our therapeutic strategy consisted of multiple short radical operations. The patient was healed and was dismissed after 68 days. The surgical blood loss was comparable with noncoagulopathic patients with similar burn extension. In case of a vast burn injury of the moderate or severe hemophiliacs, where the trauma-related consumption coagulopathy is expected and deep necrosectomies are required, the authors suggest starting a substitution of the factor VIII immediately after the admission, followed by a structured operative planning.

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 use and limitations of unfractionated heparin

Despite the development of newer anticoagulants, unfractionated heparin remains an indispensible agent in the treatment of thrombotic disorders. Heparin exerts its major effect via antithrombin, converting antithrombin to a more efficient inhibitor of circulating thrombin (factor IIa), factor Xa, factor IXa, factor XIIa, and kallikrein. However, due to the multiple anticoagulant mechanisms of heparin, differential molecular weight-based clearance, issues of heparin resistance, and patient-specific characteristics (age, weight, gender, and tobacco), attaining therapeutic anticoagulation is complicated. As a result, a minority of patients in major clinical trials achieve an activated partial thromboplastin time within the target window in an appropriate time-frame despite the use of weight-based titration nomograms. The resultant under- or over-therapeutic anticoagulation is associated with increased risks of ischemic and bleeding complications, suggesting the importance of maintaining heparin anticoagulation within a relatively narrow therapeutic range. In this review we discuss the mechanisms of heparin action, clinical ramifications of incorrect dosing in major trials, and attempts to improve the achievement of therapeutic anticoagulation.

Antifactor Xa Levels in Four Patients with Burn Injuries Who Received Enoxaparin to Prevent Venous Thromboembolism

Four patients with severe burn injuries received enoxaparin 40 mg twice/day subcutaneously for the prophylaxis of venous thromboembolism (VTE). Peak antifactor Xa levels were measured 4 hours after administration of a dose, and trough antifactor Xa levels were measured 30 minutes before the next scheduled dose. Ultrasonography was performed once/week to assess the presence of VTE. Any occurrence of major bleeding was documented in the patients' charts. All patients had trough antifactor Xa levels below 0.1 U/ml. Enoxaparin dosages were subsequently adjusted to achieve trough antifactor Xa levels of 0.1-0.2 U/ml. This required dosages higher than those typically recommended for VTE prophylaxis (40 mg every 24 hrs or 30 mg every 12 hrs). One patient needed more than 60 mg every 12 hours. No patient had a venous thromboembolic event or major bleeding. The low antifactor Xa levels that were observed suggest that a reduced dose-response relationship may exist between subcutaneously administered enoxaparin and antifactor Xa activity in patients with severe burn injuries. Prospective studies should be performed to further investigate this relationship.