A randomized trial of antithrombin concentrate for treatment of heparin resistance

Antithrombin Levels and Heparin Responsiveness during Venoarterial Extracorporeal Membrane Oxygenation: A Prospective Single-center Cohort Study

BACKGROUND

Unfractionated heparin, administered during venoarterial extracorporeal membrane oxygenation to prevent thromboembolic events, largely depends on plasma antithrombin for its antithrombotic effects. Decreased heparin responsiveness seems frequent on extracorporeal membrane oxygenation; however, its association with acquired antithrombin deficiency is poorly understood. The objective of this study was to describe longitudinal changes in plasma antithrombin levels during extracorporeal membrane oxygenation support and evaluate the association between antithrombin levels and heparin responsiveness. The hypothesis was that extracorporeal membrane oxygenation support would be associated with acquired antithrombin deficiency and related decreased heparin responsiveness.

METHODS

Adults receiving venoarterial extracorporeal membrane oxygenation were prospectively included. All patients received continuous intravenous unfractionated heparin using a standardized protocol (target anti-Xa 0.3 to 0.5 IU/ml). For each patient, arterial blood was withdrawn into citrate-containing tubes at 11 time points (from hour 0 up to day 7). Anti-Xa (without dextran or antithrombin added) and antithrombin levels were measured. The primary outcome was the antithrombin plasma level. In the absence of consensus, antithrombin deficiency was defined as a time-weighted average of antithrombin less than or equal to 70%. Data regarding clinical management and heparin dosage were collected.

RESULTS

Fifty patients, including 42% postcardiotomy, were included between April 2020 and May 2021, with a total of 447 samples. Median extracorporeal membrane oxygenation duration was 7 (interquartile range, 4 to 12) days. Median antithrombin level was 48% (37 to 60%) at baseline. Antithrombin levels significantly increased throughout the follow-up. Time-weighted average of antithrombin levels was 63% (57 to 73%) and was less than or equal to 70% in 32 (64%) of patients. Overall, 45 (90%) patients had at least one antithrombin value less than 70%, and 35 (70%) had at least one antithrombin value less than 50%. Antithrombin levels were not significantly associated with heparin responsiveness evaluated by anti-Xa assay or heparin dosage.

CONCLUSIONS

Venoarterial extracorporeal membrane oxygenation support was associated with a moderate acquired antithrombin deficiency, mainly during the first 72 h, that did not correlate with heparin responsiveness.

EDITOR’S PERSPECTIVE

Finding a common definition of heparin resistance in adult cardiac surgery: communication from the ISTH SSC subcommittee on perioperative and critical care thrombosis and hemostasis

Ensuring adequate anticoagulation for patients requiring cardiac surgery and cardiopulmonary bypass (CPB) is important due to the adverse consequences of inadequate anticoagulation with respect to bleeding and thrombosis. When target anticoagulation is not achieved with typical doses, the term heparin resistance is routinely used despite the lack of uniform diagnostic criteria. Prior reports and guidance documents that define heparin resistance in patients requiring CPB and guidance documents remain variable based on the lack of standardized criteria. As a result, we conducted a review of clinical trials and reports to evaluate the various heparin resistance definitions employed in this clinical setting and to identify potential standards for future clinical trials and clinical management. In addition, we also aimed to characterize the differences in the reported incidence of heparin resistance in the adult cardiac surgical literature based on the variability of both target-activated clotting (ACT) values and unfractionated heparin doses. Our findings suggest that the most extensively reported ACT target for CPB is 480 seconds or higher. Although most publications define heparin resistance as a failure to achieve this target after a weight-based dose of either 400 U/kg or 500 U/kg of heparin, a standardized definition would be useful to guide future clinical trials and help improve clinical management. We propose the inability to obtain an ACT target for CPB of 480 seconds or more after 500 U/kg as a standardized definition for heparin resistance in this setting.

Investigation of real-world heparin resistance and anticoagulation management prior to cardiopulmonary bypass: report from a nationwide survey by the Japanese Association for Thoracic Surgery heparin resistance working group

OBJECTIVE

Heparin resistance is often encountered during cardiopulmonary bypass. Heparin dose and activated clotting time target values for the initiation of cardiopulmonary bypass are not yet universally standardized; further no consensus exists on the management of heparin resistance. This study aimed to investigate the current real-world practice on heparin management and anticoagulant treatment for heparin resistance in Japan.

METHODS

A questionnaire survey was conducted at medical institutions nationwide with which The Japanese Society of Extra-Corporeal Technology in Medicine members are affiliated, targeting surgical cases with cardiopulmonary bypass performed from January 2019 through December 2019.

RESULTS

Among 69% (230/332) of the participating institutions, the criterion for heparin resistance was defined as "the target activated clotting time value not reached even with an additional dose of heparin administration". Cases of heparin resistance were reported in 89.8% (202/225) of the responded institutions. Of note, 75% (106/141) of the responded institutions reported heparin resistance associated with antithrombin activity ≥ 80%. Antithrombin concentrate was used in 38.4% (238/619 responses) or third dose of heparin in 37.8% (234/619 responses) for advanced heparin resistance treatment. Antithrombin concentrate was found to be effective in resolving heparin resistance in patients having normal, as well as lower antithrombin activity.

CONCLUSION

Heparin resistance has occurred in many cardiovascular centers, even among patients with normal antithrombin activities. Interestingly, the administration of antithrombin concentrate resolved heparin resistance, regardless of the baseline antithrombin activity value.

Heparin Resistance During Cardiopulmonary Bypass in Adult Cardiac Surgery

The use of heparin for anticoagulation has changed the face of cardiac surgery by allowing a bloodless and motionless surgical field throughout the introduction of cardiopulmonary bypass (CPB). However, heparin is a drug with complex pharmacologic properties that can cause significant interpatient differences in terms of responsiveness. Heparin resistance during CPB is a weighty issue due to the catastrophic consequences stemming from inadequate anticoagulation, and the treatment of it necessitates a rationalized stepwise approach due to the multifactorial contributions toward this entity. The widespread use of activated clotting time (ACT) as a measurement of anticoagulation during CPB is examined, as it may be a false indicator of heparin resistance. Heparin resistance also has been repeatedly reported in patients infected with COVID-19, which deserves further exploration in this pandemic era. This review aims to examine the variability in heparin potency, underlying mechanisms, and limitations of using ACT for monitoring, as well as provide a framework towards the current management of heparin resistance.

Heparin Resistance Is Common in Patients Undergoing Extracorporeal Membrane Oxygenation but Is Not Associated with Worse Clinical Outcomes

Extracorporeal membrane oxygenation (ECMO) protocols generally require systemic anticoagulation with heparin to prevent circuit thrombosis. The prevalence, risk factors, and outcomes of heparin resistance in this setting are ill-defined. To better understand the prevalence and clinical consequences of heparin resistance in this population, we conducted a retrospective analysis of all patients treated with ECMO at a single academic medical center between 2016 and 2019. Univariate and multivariate analyses were used to evaluate predictors and outcomes of heparin resistance. Of 67 patients in our study, 50.7% met the threshold for heparin resistance for at least 1 day, which was managed in all cases with increases in heparin dose. Patients with heparin resistance were more likely to be male (82.4% vs. 48.5%, p = 0.005) and to have a higher mean platelet count (132 vs. 104 × 103/mL, p = 0.027) compared with those without heparin resistance. Multivariate logistic regression found no significant association between the development of heparin resistance and rates of thrombosis, hemorrhage, or overall survival. Additional prospective studies are required to clarify the clinical implications of heparin resistance in this population.

Understanding the andromeda strain - The role of cytokine release, coagulopathy and antithrombin III in SARS-CoV2 critical illness

As the current coronavirus pandemic continues and cases of COVID-19 critical illness rise, physicians and scientists across the globe are working to understand and study its pathophysiology. Part of the pathology of this illness may result from its prothrombotic potential as witnessed from derangements in coagulation and thrombotic complications reported in observational studies performed in China and Europe to findings of microthrombosis upon autopsy analysis of patients who succumbed to COVID-19. Multiple organizations, including the American Society of Hematology (ASH), recommend the routine use of prophylactic heparin to temper the thrombotic complications of this illness given its mortality benefit in severe COVID-19 infections. Reductions in circulating levels of Antithrombin III (AT), the primary mediator of heparin's action, is present in cases of coronavirus related critical illness. AT's use as a prognostic marker, an important effector of heparin resistance, and a potential therapeutic target for COVID-19 remains to be explored.

Impact of exogenous antithrombin on low molecular weight heparin anti-Xa activity assays in a pediatric and young adult leukemia and lymphoma cohort with variable antithrombin levels

BACKGROUND

Low molecular weight heparin (LMWH) remains the most commonly prescribed pediatric anticoagulant. There is debate whether LMWH anti-Xa assays with or without exogenous antithrombin (AT) best reflect anticoagulation effect, and how much discrepancy exists between assay types.

OBJECTIVES

We assessed the effect of variable AT activity on LMWH anti-Xa levels in plasma samples from anticoagulated pediatric and young adult acute lymphoblastic leukemia and lymphoma (ALL/L) patients, using two instruments and their commercial kits with and without exogenous AT (ie, four platforms).

METHODS

We analyzed LMWH anti-Xa levels on 60 plasma samples with known AT activity from 12 enoxaparin-treated ALL/L patients, using four commercial kits from Siemens and Stago containing AT or not, on Siemens BCS and Stago STA R Max, respectively.

RESULTS

Of 236/240 samples with interpretable results, mean AT activity was 80% (46-138%). Correlation was acceptable for published kit ranges of LMWH anti-Xa levels when comparing kits containing AT (r = 0.82, P < .0001), or not (r = 0.93, P < .0001), and within a manufacturer (Berichrom to Innovance, r = 0.92, P < .0001; Stachrom to STA-Liquid Anti-Xa r = 0.98, P < .0001). LMWH anti-Xa levels were lower in platforms without added AT (P < .0001). For Stago kits, this effect increased when AT < 70% (P = .001, n = 19, mean 56%). Assay variability, measured as mean percent difference, was less pronounced with Stago kits (14.7%, n = 49) than Siemens (41.9%, n = 50).

CONCLUSIONS

Although LMWH levels from anti-Xa assays with added AT trend higher than in those without, correlation was fairly good between platforms in pediatric ALL/L plasmas, even when AT activity was <70%.

How I treat patients with hereditary antithrombin deficiency

Genetic predispositions to venous thromboembolism (VTE) are relatively frequent in the general population and comprise a heterogeneous group of disorders. Whereas the most frequent congenital risk factors for thrombosis only moderately increase the risk, a deficiency in antithrombin (AT), one of the most important natural inhibitors of blood coagulation, carries a higher risk. Congenital AT deficiency is an infrequently encountered genetic risk factor for VTE, and different subtypes vary with regard to their thrombotic risk. Patients with congenital AT deficiency, especially those with quantitative deficiency (type 1), may develop thrombosis early in life and often have a conspicuous family history of first- and second-degree relatives with VTE. Women are particularly affected because of the risk potentiation by combined estrogen/progestogen oral contraceptive use or pregnancy. The lack of controlled trials or even observational studies of large cohorts does not allow therapeutic decisions to be based on scientific evidence. In this review, we will discuss cases with thrombotic manifestations and the tailored management of patients with this congenital thrombosis risk factor.

Management of antithrombin deficiency: an update for clinicians

Introduction. Antithrombin is a serpin that inhibits multiple procoagulant serine proteases and acts as an endogenous anticoagulant. Thus, congenital antithrombin deficiency constitutes a major thrombophilic state, the most severe so far. Areas covered. In the present work, we globally review the biology, genetics, diagnosis, and management of congenital antithrombin deficiency, and also discuss puzzling questions and future perspectives regarding this severe inherited thrombophilia. Expert opinion. Although this disorder exerts high clinical heterogeneity, many carriers will need careful and long-term anticoagulation and/or thromboprophylaxis, especially in high-risk situations, such as surgery and pregnancy. Notably, antithrombin concentrates constitute a considerable arsenal for both treatment and prevention of acute venous thrombosis in subjects with antithrombin deficiency. Current evidences are based almost exclusively on retrospective case series, so an integrated functional, biochemical and molecular characterization will be of clinical relevance and guide hematologists' personalized decisions.

Complete antithrombin replacement for anticoagulation for cardiopulmonary bypass to repair severe infective mitral valve endocarditis

: We present a case of a 26-year-old patient with severe infective endocarditis complicated with cerebral septic emboli that required essentially complete replacement of his circulating antithrombin activity to achieve an activated coagulation time near 480 s. The need for this degree of antithrombin administration may have been secondary to ongoing systemic inflammation and consequent thrombin generation despite blood culture results demonstrating no bacteremia. In sum, ongoing loss of endogenous antithrombin activity secondary to inflammation and the need for more than 80% normal activity to conduct safe cardiopulmonary bypass may require extraordinary administration of exogenous antithrombin in similar settings.

Antithrombin Administration During Intravenous Heparin Anticoagulation in the Intensive Care Unit: A Single-Center Matched Retrospective Cohort Study

Unfractionated heparin (UFH) is a frequently utilized indirect anticoagulant that induces therapeutic effect by enhancing antithrombin (AT)-mediated procoagulant enzyme inhibition. In suspected heparin resistance (HR) during cardiopulmonary bypass, AT activity may be decreased and AT supplementation helps restore UFH responsiveness. The benefit of AT supplementation in HR over longer durations of UFH therapy is unclear. The objective of this study was to describe and evaluate the use of AT III concentrate in the intensive care units (ICUs) at our institution for improving UFH therapy response over 72 hours. A total of 44 critically ill patients were included in the analysis-22 patients received at least 1 dose of AT and 22 patients received no AT. Thirty (68.2%) of the 44 patients were receiving mechanical circulatory support. Baseline characteristics were similar between groups. The average AT activity prior to AT supplementation was 57.9% in the treatment group, and the median cumulative dose of AT was 786.5 U (9.26 U/kg) per patient. There were no significant differences observed in proportion of time spent in therapeutic range (31.9% vs 35.2%, P = .65), time to therapeutic goal (16.5 vs 15.5 hours, P = .97), or patients who experienced a bleeding event (5 vs 5, P = .99) between groups. In conclusion, AT supplementation had minimal impact on anticoagulant response in this cohort of ICU patients with mild to moderate HR receiving a prolonged UFH infusion. Additional research is needed to define AT activity targets and to standardize AT supplementation practices in patients receiving prolonged heparin infusion.

Antithrombin: anti-inflammatory properties and clinical applications

Many humoral and cellular components participate in bidirectional communication between the coagulation and inflammation pathways. Natural anticoagulant proteins, including antithrombin (AT), tissue factor pathway inhibitor, and protein C, suppress proinflammatory mediators. Conversely, inflammation blunts anticoagulant activity and, when uncontrolled, promotes systemic inflammation-induced coagulation, such as those that occur in disseminated intravascular coagulation and severe sepsis. This review discusses the mechanisms of action and clinical use of AT concentrate in critically ill patients and in the settings of perioperative anticoagulation management for surgery and obstetrics. AT is a serine protease inhibitor with broad anticoagulant activity and potent anti-inflammatory properties. In clinical conditions associated with hereditary or acquired AT deficiency, administration of AT concentrate has been shown to restore proper haemostasis and attenuate inflammation. Of note, AT modulates inflammatory responses not only by inhibiting thrombin and other clotting factors that induce cytokine activity and leukocyte-endothelial cell interaction, but also by coagulation-independent effects, including direct interaction with cellular mediators of inflammation. An increasing body of evidence suggests that AT concentrate may be a potential therapeutic agent in certain clinical settings associated with inflammation. In addition to the well-known anticoagulation properties of AT for the treatment of hereditary AT deficiency, AT also possesses noteworthy anti-inflammatory properties that could be valuable in treating acquired AT deficiency, which often result in thrombotic states associated with an inflammatory component.

Antithrombin Concentrate Use in Children Receiving Unfractionated Heparin for Acute Thrombosis

OBJECTIVE

To characterize features of antithrombin concentrate (ATC) use in children receiving unfractionated heparin (UFH) therapy for acute thrombosis.

STUDY DESIGN

All pediatric patients at Texas Children's Hospital who received ATC in the context of UFH therapy for acute thrombosis during February 2011 to May 2013 were analyzed.

RESULTS

Fifty-one children received ATC during UFH therapy for acute thrombosis. Median age was 3 months (IQR 1 to 18 months). Clinical indications included venous (53%), arterial (37%), venous and arterial (6%), and intracardiac (4%) thrombosis. Median baseline antithrombin (AT) level was 61% and UFH dose was 26 U/kg/h. The median dose of ATC was 49.9 IU/kg (IQR 32.6 to 50.0 IU/kg). Although most patients (86%) did not undergo a change in UFH dose, there was a significant increase in both AT and anti-factor Xa level after the first dose of ATC (P < .001 for both). There was no correlation between ATC dose or increment in AT level above baseline and the achievement of targeted anticoagulation by anti-factor X activity level. Adverse bleeding events occurred in 10% of patients.

CONCLUSIONS

There was a significant change in AT and anti-factor Xa activity level after a single dose of ATC despite little to no change in dose of UFH. ATC appears to facilitate anticoagulation with UFH in some children with acute thrombosis but the degree of response is variable and dependent on factors identified in this study. Bleeding and other theoretical risks must be carefully considered.

Bivalirudin as an adjunctive anticoagulant to heparin in the treatment of heparin resistance during cardiopulmonary bypass-assisted cardiac surgery

Heparin resistance (unresponsiveness to heparin) is characterized by the inability to reach acceptable activated clotting time values following a calculated dose of heparin. Up to 20% of the patients undergoing cardiothoracic surgery with cardiopulmonary bypass using unfractionated heparin (UFH) for anticoagulation experience heparin resistance. Although UFH has been the "gold standard" for anticoagulation, it is not without its limitations. It is contraindicated in patients with confirmed heparin-induced thrombocytopenia (HIT) and heparin or protamine allergy. The safety and efficacy of the use of the direct thrombin inhibitor bivalirudin for anticoagulation during cardiac surgery has been reported. However, there have been no reports on the treatment of heparin resistance with bivalirudin during CPB. In this review, we report the favorable outcome of our single-center experience with the alternative use of bivalirudin in the management of anticoagulation of heparin unresponsive patients undergoing coronary artery bypass graft surgery.

Anticoagulation management associated with extracorporeal circulation

The use of extracorporeal circulation requires anticoagulation to maintain blood fluidity throughout the circuit, and to prevent thrombotic complications. Additionally, adequate suppression of hemostatic activation avoids the unnecessary consumption of coagulation factors caused by the contact of blood with foreign surfaces. Cardiopulmonary bypass represents the greatest challenge in this regard, necessitating profound levels of anticoagulation during its conduct, but also quick, efficient reversal of this state once the surgical procedure is completed. Although extracorporeal circulation has been around for more than half a century, many questions remain regarding how to best achieve anticoagulation for it. Although unfractionated heparin is the predominant agent used for cardiopulmonary bypass, the amount required and how best to monitor its effects are still unresolved. This review discusses the use of heparin, novel anticoagulants, and the monitoring of anticoagulation during the conduct of cardiopulmonary bypass.

Achievement of therapeutic anti-Xa levels in a proven heparin-resistant patient through the use of nontraditional high-dose enoxaparin

OBJECTIVE

To describe the successful use of high-dose enoxaparin therapy (1.5 mg/kg subcutaneously twice daily) to attain a therapeutic anti-factor Xa (anti-Xa) level in a cancer patient with heparin resistance.

CASE SUMMARY

A proven heparin-resistant patient with venous thromboembolism (VTE) and lung cancer who required approximately 66 000 units of unfractionated heparin daily was successfully transitioned to an off-label high-dose enoxaparin (OLHDE) 1.5 mg/kg subcutaneously twice daily. The patient was maintained on this same dose, and therapeutic levels were confirmed via use of the anti-Xa monitoring test. The patient was able to be discharged from the medical floor on this same dose with no further complications of VTE noted. No adverse events from this dosing were observed during the duration of therapy.

DISCUSSION

Options for overcoming heparin resistance are limited to case reports and small studies. The best course of treatment in the cancer patient is unclear. OLHDE allowed for the transition from intravenous to subcutaneous medication and transition off the medical floor. This case supports the use of OLHDE as a therapeutic option in heparin-resistant patients with cancer. Further study is needed to confirm the efficacy of OLHDE in this patient population.

CONCLUSION

High-dose enoxaparin may be an option to treat cancer patients with confirmed heparin resistance and venous thromboembolism.

Antithrombin III supplementation on extracorporeal membrane oxygenation: impact on heparin dose and circuit life

Antithrombin III (ATIII) is used during extracorporeal membrane oxygenation (ECMO) based on physiologic rationale and studies during cardiopulmonary bypass. In February 2008, our institution began using ATIII as replacement for low ATIII activity (<70%) in patients supported with ECMO. We hypothesized that ATIII supplementation would reduce heparin infusion rates, increase unfractionated heparin anti-Xa levels, and prolong ECMO circuit life. Data from 40 consecutive patients (45 deployments) requiring ECMO support for >72 hours with venoarterial ECMO from January 1, 2007, through December 31, 2008, were collected. Antithrombin III concentrate was administered for ATIII activity <70% at the discretion of the attending physician. The primary outcome was whether the heparin infusion rate was reduced by 10% or more as a result of ATIII administration. No difference in heparin infusion rate (p = 0.245) as a result of ATIII administration was observed. Anti-Xa levels were lower before ATIII administration (p< 0.001) and were increased after ATIII administration (p < 0.001). There was an increased frequency of circuit failure in ATIII treatment group compared with nontreatment group (p = 0.018). Neither heparin responsiveness nor circuit life was enhanced by daily ATIII supplementation for activity <70%. Future studies are warranted to evaluate the effectiveness of antithrombin replacement.

Overcoming Challenges in the Management of Critical Events During Cardiopulmonary Bypass

Critical events during cardiopulmonary bypass (CPB) can challenge the most experienced perfusionists, anesthesiologists, and surgeons and can potentially lead to devastating outcomes. Much of the challenge of troubleshooting these events requires a key understanding of these situations and a well-defined strategy for early recognition and treatment. Adverse situations may be anticipated prior to going on CPB. Atherosclerosis is pervasive, and a high plaque burden may have implications in surgical technique modification and planning of CPB. Hematologic abnormalities such as cold agglutinins, antithrombin III deficiency, and hemoglobin S have been discussed with emphasis on managing complications arising from their altered pathophysiology. Jehovah’s witness patients require appropriate techniques for cell salvage to minimize blood loss. During initiation of CPB, devastating situations leading to acute hypoperfusion and multiorgan failure may be encountered in patients undergoing surgery for aortic dissection. Massive air emboli during CPB, though rare, necessitate an urgent diagnosis to detect the source and prompt management to contain catastrophic outcomes. Gaseous microemboli remain ubiquitous and continue to be a major concern for neurocognitive impairment despite our best efforts to improve techniques and refine the CPB circuit. During maintenance of CPB, adverse events reflect inability to provide optimal perfusion and can be ascribed to CPB machine malfunction or physiological aberrations. We also discuss critical events that can occur during perfusion and the need to monitor for organ perfusion in altered physiologic states emanating from hemodilution, hypothermia, and acid–base alterations.

Antithrombin concentrate use in children: a multicenter cohort study

OBJECTIVE

To describe the off-label use of antithrombin concentrate in tertiary care pediatric hospitals across the US.

STUDY DESIGN

This is a retrospective, multicenter, cohort study of 4210 admissions of children younger than 18 years of age who received antithrombin concentrate between 2002 and 2011 within the Pediatric Health Information System administrative database. An on-label admission was defined as an admission with an International Classification of Diseases diagnostic code for a primary hypercoagulable state; admissions without this code were classified as off-label.

RESULTS

During the 10-year study period, off-label use of antithrombin concentrate increased 5-fold. Overall, 97% of study subjects received antithrombin off-label. Neonates younger than 30 days of age comprised the largest age group (45.7%) of use; 87% of patients had at least one complex chronic condition, with congenital heart/lung defects being the most prevalent primary diagnosis (36.3%). Extracorporeal membrane oxygenation was the most common procedure associated with antithrombin use (43.7%).

CONCLUSIONS

The off-label use of antithrombin concentrate is increasing rapidly, particularly in critically ill children receiving extracorporeal membrane oxygenation, with few parallel studies to substantiate its safety or efficacy. Further preclinical and controlled clinical studies are critical to expanding our knowledge of this drug. In the meantime, antithrombin concentrate should be used judiciously by clinicians and following guidelines instated by hospitals.

Is there evidence that fresh frozen plasma is superior to antithrombin administration to treat heparin resistance in cardiac surgery?

A best evidence topic in cardiac surgery was written according to a structured protocol. The question addressed was, 'in [patients with heparin resistance] is [treatment with FFP] superior [to antithrombin administration] in [achieving adequate anticoagulation to facilitate safe cardiopulmonary bypass]?' More than 29 papers were found using the reported search, of which six represented the best evidence to answer the clinical question. The authors, journal, date and country of publication, patient group studied, study type, relevant outcomes and results of these papers are tabulated. Antithrombin (AT) binds to heparin and increases the rate at which it binds to thrombin. The levels of antithrombin in the blood are an important aspect of the heparin dose-response curve. When the activated clotting time (ACT) fails to reach a target >480, this is commonly defined as heparin resistance (HR). Heparin resistance is usually treated with a combination of supplementary heparin, fresh frozen plasma (FFP) or antithrombin III concentrate. There is a paucity of evidence on the treatment of heparin resistance with FFP, with only five studies identified, including one retrospective study, one in vitro trial and three case reports. AT has been studied more extensively with multiple studies, including a crossover trial comparing AT to supplemental heparin and a multicentre, randomized, double blind, placebo-controlled trial. Antithrombin (AT) concentrate is a safe and efficient treatment for heparin resistance to elevate the activated clotting time (ACT). It avoids the risk of transfusion-related acute lung injury (TRALI), volume overload, intraoperative time delay and viral or vCJD transmission. Antithrombin concentrates are more expensive than fresh frozen plasma and may put patients at risk of heparin rebound in the early postoperative period. Patients treated with AT have a lower risk of further FFP transfusions during their stay in hospital. We conclude that the treatment of HR with FFP may not restore the ACT to therapeutic levels with adequate heparinization, but AT is efficient with benefits including lower volume administration, less risk of TRALI and lower risk of transfusion-related infections.

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.

2011 update to the Society of Thoracic Surgeons and the Society of Cardiovascular Anesthesiologists blood conservation clinical practice guidelines

BACKGROUND

Practice guidelines reflect published literature. Because of the ever changing literature base, it is necessary to update and revise guideline recommendations from time to time. The Society of Thoracic Surgeons recommends review and possible update of previously published guidelines at least every three years. This summary is an update of the blood conservation guideline published in 2007.

METHODS

The search methods used in the current version differ compared to the previously published guideline. Literature searches were conducted using standardized MeSH terms from the National Library of Medicine PUBMED database list of search terms. The following terms comprised the standard baseline search terms for all topics and were connected with the logical 'OR' connector--Extracorporeal circulation (MeSH number E04.292), cardiovascular surgical procedures (MeSH number E04.100), and vascular diseases (MeSH number C14.907). Use of these broad search terms allowed specific topics to be added to the search with the logical 'AND' connector.

RESULTS

In this 2011 guideline update, areas of major revision include: 1) management of dual anti-platelet therapy before operation, 2) use of drugs that augment red blood cell volume or limit blood loss, 3) use of blood derivatives including fresh frozen plasma, Factor XIII, leukoreduced red blood cells, platelet plasmapheresis, recombinant Factor VII, antithrombin III, and Factor IX concentrates, 4) changes in management of blood salvage, 5) use of minimally invasive procedures to limit perioperative bleeding and blood transfusion, 6) recommendations for blood conservation related to extracorporeal membrane oxygenation and cardiopulmonary perfusion, 7) use of topical hemostatic agents, and 8) new insights into the value of team interventions in blood management.

CONCLUSIONS

Much has changed since the previously published 2007 STS blood management guidelines and this document contains new and revised recommendations.

Comparison of two infusion rates of antithrombin concentrate in cardiopulmonary bypass surgery

Background: Antithrombin concentrate (AT) is used to treat heparin resistance (HR) in cardiac surgery. It is usually given slowly due to the fear of anaphylaxis. This may delay cardiac catheterisation and the start of cardiopulmonary bypass (CPB). HR is often defined as the failure to reach or maintain a target activated clotting time (ACT) despite a standard dose of heparin. It is not generally possible to predict which patients will display HR, although there are known risk factors. Routine early administration of AT before heparinisation is probably not cost-effective. Infusing AT relatively quickly after demonstrating HR may be more cost-effective, while not delaying surgery. The aim of this study is to investigate the safety and side effects of a faster infusion of AT.

Methods: Forty patients undergoing elective heart surgery were included and randomised to two groups in a double-blind fashion. Each group received 1000 IU of AT intravenously (IV). One group received a slow infusion (100 IU/min) before full-dose heparinisation. The other group received a fast infusion (250 IU/min). Haemodynamic and respiratory data were recorded. Any adverse effects were noted. Thrombin-antithrombin, anti-Xa and antithrombin levels in plasma were measured.

Results: No anaphylaxis occurred in either group. No differences were found regarding haemodynamics, respiration or laboratory results. Two patients experienced major haemorrhage and recovered; there were two deaths, thought to be unrelated to the study drugs.

Conclusion: AT can be infused at a rate of 250 IU/min. This is faster than the current recommendation of 100 IU/min. This rate of infusion allows restricting AT infusion to those patients who display HR, without delaying surgery. Optimal anticoagulant therapy for CPB probably includes point-of-care measurement of ACT and plasma AT and small, but rapid, infusions of AT in heparin-resistant patients.

Heparin dose response is independent of preoperative antithrombin activity in patients undergoing coronary artery bypass graft surgery using low heparin concentrations

BACKGROUND

Unfractionated heparin's primary mechanism of action is to enhance the enzymatic activity of antithrombin (AT). We hypothesized that there would be a direct association between preoperative AT activity and both heparin dose response (HDR) and heparin sensitivity index (HSI) in patients undergoing coronary artery bypass graft surgery.

METHODS

Demographic and perioperative data were collected from 304 patients undergoing primary coronary artery bypass graft surgery. AT activity was measured after induction of general anesthesia using a colorimetric method (Siemens Healthcare Diagnostics, Tarrytown, NY). Activated coagulation time (ACT), HDR, and HSI were measured using the Hepcon HMS Plus system (Medtronic, Minneapolis, MN). Heparin dose was calculated for a target ACT using measured HDR by the same system. Multivariate linear regression was performed to identify independent predictors of HDR. Subgroup analysis of patients with low AT activity (<80% normal; <0.813 U/mL) who may be at risk for heparin resistance was also performed.

RESULTS

Mean baseline ACT was 135 ± 18 seconds. Mean calculated HDR was 98 ± 21 s/U/mL. Mean baseline AT activity was 0.93 ± 0.13 U/mL. Baseline AT activity was not significantly associated with baseline or postheparin ACT, HDR, or HSI. Addition of AT activity to multivariable linear regression models of both HDR and HSI did not significantly improve model performance. Subgroup analysis of 49 patients with baseline AT <80% of normal levels did not reveal a relationship between low AT activity and HDR or HSI. Preoperative AT activity, HDR, and HSI were not associated with cardiac troponin I levels on the first postoperative day, intensive care unit duration, or hospital length of stay.

CONCLUSION

Although enhancing AT activity is the primary mechanism by which heparin facilitates cardiopulmonary bypass anticoagulation, low preoperative AT activity is not associated with impaired response to heparin or to clinical outcomes when using target ACTs of 300 to 350 seconds.

Postoperative activity, but not preoperative activity, of antithrombin is associated with major adverse cardiac events after coronary artery bypass graft surgery

BACKGROUND

Low levels of antithrombin (AT) have been independently associated with prolonged intensive care unit stay and an increased incidence of neurologic and thromboembolic events after cardiac surgery. We hypothesized that perioperative AT activity is independently associated with postoperative major adverse cardiac events (MACEs) in patients undergoing coronary artery bypass graft (CABG) surgery.

METHODS

We prospectively studied 1403 patients undergoing primary CABG surgery with cardiopulmonary bypass (CPB) (http://clinicaltrials.gov/show/NCT00281164). The primary clinical end point was occurrence of MACE, defined as a composite outcome of any one or more of the following: postoperative death, reoperation for coronary graft occlusion, myocardial infarction, stroke, pulmonary embolism, or cardiac arrest until first hospital discharge. Plasma AT activity was measured before surgery, after post-CPB protamine, and on postoperative days (PODs) 1-5. Multivariate logistic regression modeling was performed to estimate the independent effect of perioperative AT activity upon MACE.

RESULTS

MACE occurred in 146 patients (10.4%), consisting of postoperative mortality (n = 12), myocardial infarction (n = 108), stroke (n = 17), pulmonary embolism (n = 8), cardiac arrest (n = 16), or a subsequent postoperative or catheter-based treatment for graft occlusion (n = 6). AT activity at baseline did not differ between patients with (0.91 ± 0.13 IU/mL; n = 146) and without (0.92 ± 0.13 IU/mL; n = 1257) (P = 0.18) MACE. AT activity in both groups was markedly reduced immediately after CPB and recovered to baseline values over the ensuing 5 PODs. Postoperative AT activity was significantly lower in patients with MACE than those without MACE. After adjustment for clinical predictors of MACE, AT activity on PODs 2 and 3 was associated with MACE.

CONCLUSIONS

Preoperative AT activity is not associated with MACE after CABG surgery. MACE is independently associated with postoperative AT activity but only at time points occurring predominantly after the MACE.

[Heparin resistance and antithrombin deficiency]

The phenomenon of heparin resistance (HR) is characterized by high doses of unfractionated heparin (UFH) being required to bring activated partial thromboplastin time (aPTT) and activated coagulation time (ACT) within therapeutically desired ranges, or by the impossibility of reaching these ranges. At UFH dosages > 35,000 IU/d, HR should be considered a factor. The most frequent cause of HR is deficiency of antithrombin (AT), the presence of which is essential for UFH to exert its anticoagulatory effect. AT in concentrate form may be applied to overcome AT-dependent HR. The main clinically relevant situations in which AT-dependent HR occurs, with possible indication of AT substitution, are congenital AT deficiency, asparaginase therapy, disseminated intravascular coagulation (DIC) and administration of high doses of heparin during extracorporeal circulation, where it is significant, due to the need to maintain a very high ACT (> 400 s), that use of heart-lung machines is associated with an HR incidence of approximately 20%. The following procedure is recommended when there is no DIC and when extracorporeal circulation is not used: if HR is suspected and AT activity is < or = 60%, UFH should first be reduced to 500 IU/h (to prevent bleeding complications), before AT is substituted. AT activity should then exceed 80%. Under normalized and stable AT activity, the UFH dose should be adjusted such that aPTT is within a range of 60-100 s. If anticoagulation over a longer term is indicated, then overlapping anticoagulation with a vitamin K antagonist should be started as quickly as possible.