Unfractionated heparin is used during extracorporeal membrane oxygenation (ECMO)1,2 to prevent activation of coagulation caused by blood contact with foreign surfaces.3 Clinical effectiveness depends on attaining adequate anticoagulation, commonly defined as activated partial thromboplastin time (aPTT; individual-to-reference) ratio within desired therapeutic ranges.4,5 Insufficient or excessive anticoagulation may increase the risk of thrombosis or hemorrhage, two frequent and serious complications of ECMO.6–8
Heparin works by potentiating antithrombin, a circulating protein that inactivates thrombin and other coagulation factors.9 Therefore, it comes as no surprise that low antithrombin activity decreases, and antithrombin supplementation preserves or restores, the anticoagulant effect of heparin during cardiac surgery.10–12 The Extracorporeal Life Support Organization suggests considering correction of antithrombin deficiency in case of escalating heparin requirements or clinically subtherapeutic anticoagulation during ECMO.13 However, the impact of antithrombin deficiency or replacement during ECMO is unclear in neonates and children and virtually unknown in adults.14–17 This is one reason why some ECMO-centers routinely measure antithrombin activity and administer antithrombin concentrate, whereas others do so only rarely, if ever.1,2,8 Importantly, the off-label use of antithrombin concentrate during ECMO is increasing although it produces uncertain benefits, carries some risks, and can be expensive.18,19 For example, among 43 US children’s hospitals, the proportion of subjects receiving at least one dose of exogenous antithrombin while on ECMO rose from 2.4% (21/875) in 2005 to 51.9% (428/824) in 2012.18
At our institution, adults on veno-venous ECMO receive unfractionated heparin to maintain their aPTT ratio at 1.5–2.0. Compared with the activated clotting time, the aPTT may be more sensitive to the relatively low doses (0.1–1.0 IU/ml) of heparin used during ECMO.20,21 Compared with the aPTT in seconds, the aPTT ratio is less dependent on instruments and reagents employed for testing; it allows better standardization of the management of anticoagulation between different laboratories. We measure antithrombin activity—routinely every other day—and, if this is ≤70%, we administer antithrombin concentrate regardless of the amount of heparin being given and the corresponding aPTT ratio. This practice is based on the (pre)concept that managing anticoagulation is easier if antithrombin level is normal, not only during cardiac surgery10–12 but also during veno-venous ECMO.
The aim of this study is twofold. First, to survey the use of antithrombin supplementation in 16 Italian ECMO-Centers. Second, to start exploring the relationship between low antithrombin activity, use of antithrombin concentrate, and aPTT ratio in adults treated with ECMO for acute respiratory failure.
Materials and Methods
The institutional ethical review committee approved the study, with waiver of written informed consent.
In October 2017, we distributed a survey to the 16 ECMO-program coordinators or directors of the Italian network for respiratory support (“Rete specializzata nell’insufficienza respiratoria acuta”—ReSpIRA) using a web-based instrument (SurveyMonkey.com). The survey, that is fully reported as supplemental content (see Supplemental Digital Content, http://links.lww.com/ASAIO/A270), comprised 10 questions related to routine management of anticoagulation and antithrombin deficiency during veno-venous ECMO in adults.
We reviewed the digital records of 106 consecutive adults treated with veno-venous ECMO at our institution, from November 2011 to November 2016. The extracorporeal systems were the PLS System or the HLS Set Advanced (Maquet GmbH, Hirrlingen, Germany). At the time of commencing ECMO, all subjects received a bolus (70 IU/kg of actual body weight) followed by a continuous infusion of heparin with an aPTT ratio target of 1.5–2.0, unless otherwise indicated. If the aPTT ratio was not in target, the infusion of heparin was modified at the discretion of the attending physician (until 2013) or in accordance with a standardized protocol (since 2013), described elsewhere.22 If antithrombin activity (HemosIL Liquid Antithrombin, Instrumentation Laboratory, Werfen Group, Bedford, MA) was ≤70%, human plasma-derived antithrombin (Kybernin P, CSL Behring GmbH, Marburg, Germany; 15–30 IU/kg/day) was administered through a central venous catheter with an antithrombin activity target of >70%. According to the manufacturer, each 1,000 IU of antithrombin concentrate (that at our institution costs € 240) was diluted in 20 ml and infused in 8 h.
This retrospective analysis consisted of two parts. First, we investigated the impact of low (≤70%) antithrombin activity on the anticoagulant effect of heparin. We collected simultaneous antithrombin activity, heparin dose, and aPTT ratio at 24–72 h from the beginning of ECMO (and the initial heparin bolus), selecting the earliest available complete data set. Circulating levels of C-reactive protein and fibrinogen, that when elevated predict hypercoagulability,23 were also noted. Subjects with pre-existing coagulopathy, those who received antithrombin concentrate or more than 1 L of fresh frozen plasma (FFP) during those same 24–72 h and those with aPTT ratio target <1.5, were excluded. Second, we studied the impact of antithrombin replacement on the anticoagulant effect of heparin. Subjects with initially low antithrombin activity were re-evaluated as above, after 24–48 h of infusion of antithrombin concentrate. Adults who changed or removed the extracorporeal circuit, those who received more than one liter of FFP, and those with aPTT ratio target <1.5 were excluded. The anticoagulant effect of heparin was primarily assessed as the proportion of subjects with aPTT ratio ≥1.5.
Continuous variables, reported as mean (± SD), were analyzed with Student’s t-test or Wilcoxon rank sum test as appropriate. Proportions were analyzed with χ2 test or Fisher’s exact test. Repeated measures were analyzed with paired t-test, signed rank sum test, or McNemar’s test. A p < 0.05 indicated statistical significance (SigmaPlot 11.0; Jandel Scientific Software, San Jose, CA).
Response rate was 100%. Unfractionated heparin is the drug of choice for anticoagulation in all Centers; its effect is monitored with the aPTT ratio in seven (44%) Centers. Antithrombin activity is monitored in all Centers, usually every day. Antithrombin is routinely supplemented if its circulating activity is <70% in four (25%) Centers or ≤70–100% in nine (56%) Centers. In the remaining three (19%) Centers, antithrombin supplementation is based on individual response to heparin. Other results are reported as supplemental content (see Supplemental Digital Content, http://links.lww.com/ASAIO/A270).
Sixty-six subjects were included in the first part of this analysis, at 42 ± 17 h from the beginning of ECMO. Their main characteristics are reported in Table 1. Reasons for excluding 40 other subjects are reported in Figure 1.
Impact of low antithrombin activity on the anticoagulant effect of heparin.
Antithrombin activity was low (53 ± 10%) in 47 (71%) subjects and normal (87 ± 19%) in 19 (29%) subjects. In Table 2 and Figures 2–4, we describe these two groups. Low, compared with normal, antithrombin activity was associated with higher heparin dose (19 ± 7 vs. 15 ± 5 IU/kg/h; p = 0.032), equal aPTT ratio (1.64 ± 0.28 vs. 1.64 ± 0.20; p = 0.995), and similar proportion of subjects with aPTT ratio ≥1.5 (31/47 [66%] vs. 15/19 [79%]; p = 0.383).
In Table 3, these same subjects are classified based on their aPTT ratio, instead of antithrombin activity. Failure to reach the therapeutic aPTT ratio, despite increasing heparin dose, was more common among young subjects, with acute respiratory distress syndrome or exacerbated end-stage lung disease (mainly cystic fibrosis) and with high circulating levels of C-reactive protein and fibrinogen. Antithrombin activity (61 ± 17 vs. 63 ± 22; p = 0.983) and the proportion of subjects with low antithrombin activity (16/20 [80%] vs. 31/46 [67%]; p = 0.383) did not significantly differ between the two groups.
Impact of antithrombin replacement on the anticoagulant effect of heparin.
Thirty-four subjects were included in this second part of the analysis, after having received 2621 ± 1034 IU of antithrombin concentrate. Reasons for excluding the others are reported in Figure 1.
Overall, the administration of antithrombin concentrate increased antithrombin activity (from 54 ± 9 to 84 ± 13%; p < 0.001), the aPTT ratio (from 1.62 ± 0.27 to 1.73 ± 0.23; p = 0.056), and the proportion of subjects with aPTT ratio ≥1.5 (from 21/34 [62%] to 31/34 [91%]; p = 0.004) without affecting heparin dose (from 19 ± 7 to 19 ± 6 IU/kg/h; p = 0.543). Antithrombin replacement was also associated with lower circulating levels of C-reactive protein (from 17 ± 10 to 13 ± 9 mg/dl; p = 0.013) but not of fibrinogen (from 496 ± 191 to 489 ± 193 mg/dl; p = 0.759).
In Figures 5 and 6, we describe the effects of antithrombin concentrate in subjects with an aPTT ratio ≥1.5 or <1.5 at the start of the infusion. Antithrombin concentrate increased circulating antithrombin activity in both groups. In 21 subjects with initial aPTT ratio ≥1.5, with lower C-reactive protein (12 ± 8 mg/dl) and fibrinogen (391 ± 99 mg/dl) levels, this was not associated with any significant decrease in heparin dose or increase in aPTT ratio (Figure 5). By contrast, in 13 subjects with initial aPTT ratio <1.5, with higher C-reactive protein (24 ± 9 mg/dl) and fibrinogen (667 ± 183 mg/dl) levels, the administration of antithrombin concentrate was associated with more subjects with aPTT ratio ≥1.5 (from 0/13 [0%] to 10/13 [77%]; p = 0.004) but also with use of more heparin and decreasing C-reactive protein (Figure 6) and fibrinogen (down to 593 ± 209 mg/dl; p = 0.140 when compared with initial values within the same group).
To our knowledge, this is the first study investigating the role of antithrombin in the management of anticoagulation of adults on veno-venous ECMO. Our nation-wide, web-based survey shows that antithrombin is routinely supplemented if its circulating activity falls below a critical value in many Italian adult ECMO-Centers. The retrospective analysis shows that 1) antithrombin deficiency is common but not always associated with reduced heparin responsiveness (the ability of heparin to prolong the aPTT ratio); 2) antithrombin supplementation improves heparin responsiveness in some subjects but not in others; and 3) reduced heparin responsiveness is associated with signs of inflammation and hypercoagulability.
Our purpose was not to clarify whether the anticoagulant effect of heparin should be monitored with the aPTT ratio or with another test, and whether low response to heparin should be treated with exogenous antithrombin or more heparin. We simply aimed at verifying the impact of low antithrombin activity and its correction on achieving a predefined anticoagulation target. The way we use antithrombin concentrate may be questioned but is quite common,1,2 as confirmed by our national survey. Antithrombin activity was initially ≤70% in almost three quarters of the subjects included in the analysis. Unexpectedly, low antithrombin activity was usually associated with aPTT ratio ≥1.5 with quite ordinary heparin dose. Antithrombin concentrate was generally more effective in rising antithrombin activity than in increasing the (already normal) anticoagulant effect of heparin.
Subjects with aPTT ratio <1.5 accounted for one third of the study population. The association between low response to heparin and higher levels of C-reactive protein and fibrinogen can have several explanations. First, C-reactive protein and fibrinogen typically increase in parallel with other pro-coagulant factors, such as factor VIII (not measured in this study).24 Fibrinogen itself promotes thrombosis.25 In turn, hypercoagulability can shorten the aPTT ratio.26–29 Second, fibrin(ogen) can hide thrombin from the heparin-antithrombin complex and thus attenuates the activity of heparin.30,31 Third, several plasma proteins other than antithrombin can bind to and neutralize heparin. They include acute phase-reactants (associated with increased C-reactive protein level) such as vitronectin,32 lactoferrin,33 and fibrinogen.34 As a result, the amount of heparin free to bind to antithrombin decreases.35
Experimental and clinical evidence suggests that an isolated defect of antithrombin activity, down to 30–50%, does not alter the anticoagulant effect of heparin.36,37 In line with this premise, subjects with low antithrombin activity (50–70%) but nonelevated circulating levels of C-reactive protein and fibrinogen, usually reached the therapeutic aPTT ratio with normal drug dose. In this subgroup, the administration of antithrombin concentrate neither decreased the dose of heparin nor prolonged the aPTT ratio. Diversely, subjects with low antithrombin activity and elevated circulating levels of C-reactive protein and fibrinogen were less sensitive to heparin and apparently benefited most from the administration of antithrombin concentrate. Because heparin dose was increased and markers of inflammation and hypercoagulability tended to decrease at the same time, the observed changes in this latter group may be explained by three, nonmutually exclusive, events. First, in the presence of pro-coagulant imbalance, even a nonsevere antithrombin deficit may become clinically relevant. Antithrombin concentrate possibly improved the response to heparin by attenuating this imbalance. Second, by giving more heparin, the amount of drug free to bind to antithrombin may have risen enough to produce the desired anticoagulant effect. Third, attenuation of severe inflammation and hypercoagulability, probably because of initial resolution of the underlying disease and perhaps even to a direct effect of antithrombin supplementation,38 may have contributed at increasing the activity of heparin.
Our work has several limitations. It was a single-center retrospective study, data were not collected at a uniform time, activated clotting time was not measured, and some analyses were run on very small cohorts. Mechanisms responsible for low antithrombin activity were not evaluated; the amount of circulating protein was not measured and therefore we could not discriminate between acquired quantitative and qualitative antithrombin defects. Other important players in the hemostatic balance, such as factor VIII activity and thrombin generation potential, were not investigated. Effects of antithrombin supplementation on thrombotic and hemorrhagic events could not be assessed because of the lack of a control group (subjects with low antithrombin activity not treated with antithrombin concentrate). For all these reasons, our results should be regarded as hypothesis generating and need to be prospectively validated.
In conclusion, antithrombin supplementation is common practice during veno-venous ECMO in adults. However, our retrospective analysis questions the usefulness of routinely measuring antithrombin activity and administering antithrombin concentrate in subjects with an apparently normal response to heparin. Benefits, if any, may be restricted to subjects who do not reach the therapeutic aPTT ratio despite increasing heparin dose, with signs of severe inflammation and hypercoagulability.
The authors are grateful to the ECMO-program coordinators or directors of the other 15 Centers of the Italian network for treatment of acute respiratory failure (“Rete specializzata nell’insufficienza respiratoria acuta”—ReSpIRA), who took part to the survey reported as Supplemental Material (see Supplemental Digital Content, http://links.lww.com/ASAIO/A270). In alphabetical order, they are Alessandri Francesco (Policlinico Umberto I, Roma); Antonelli Massimo (Policlinico Gemelli, Roma); Arcadipane Antonio (IStituto MEditerraneo per i Trapianti e Terapie ad alta specializzazione [ISMETT], Palermo); Brazzi Luca (Azienda Ospedaliera Città della Salute e della Scienza, Torino); Di Nardo Matteo (Ospedale Pediatrico Bambino Gesù, Roma); Foti Giuseppe (Ospedale San Gerardo, Monza); Frascaroli Guido (Policlinico S. Orsola-Malpighi, Bologna); Grasso Salvatore (Ospedale Policlinico, Bari); Iotti Giorgio (Policlinico San Matteo, Pavia); Lorini Ferdinando Luca (Ospedale Papa Giovanni XXIII, Bergamo); Ori Carlo (Azienda Ospedaliera di Padova, Padova); Peris Adriano (Azienda Ospedaliera Universitaria Careggi, Firenze); Servillo Giuseppe (Azienda Ospedaliera Universitaria Federico II, Napoli); Terragni Pierpaolo (Azienda Ospedaliera Universitaria di Sassari, Sassari); and Zangrillo Alberto (Ospedale San Raffaele, Milano).
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