Mechanical Circulatory SupportQuestion Bank

Correct answer: B — Anti-Xa better reflects heparin concentration, and low levels are associated with increased circuit thrombosis risk

Anti-Xa and aPTT frequently disagree during ECMO, and each test captures different aspects of anticoagulation. Anti-Xa best correlates with actual heparin concentration, whereas aPTT is influenced by multiple non-heparin factors including fibrinogen levels, factor consumption, and the inflammatory response. The ISHLT/HFSA guideline notes that thrombotic events were reported at low anti-Xa levels, compared with no thrombotic complications at higher levels, despite no difference in median aPTT between groups. However, aPTT may be a better predictor of bleeding risk. The ISTH SSC 2023 communication suggests target anti-Xa of 0.3–0.5 U/mL, aPTT of 50–70 seconds, or ACT of 180–220 seconds. No single assay is sufficient — a multimodal approach is recommended.

Board pearl: Anti-Xa best reflects heparin concentration and predicts circuit thrombosis; aPTT may better predict bleeding — use both.

Correct answer: B — 50–100 U/kg, unless the patient is already anticoagulated

The ISHLT/HFSA guideline recommends an initial UFH bolus of 50–100 U/kg at the time of cannulation, though this may be unwarranted if the patient has previously been anticoagulated. After initiation, heparin is maintained as a continuous infusion titrated to institutional targets. The JACC Scientific Expert Panel recommends monitoring with aPTT (goal 50–75 seconds, or 1.5–2.5× baseline), anti-Xa (goal 0.3–0.7 IU/mL), or ACT (goal 180–220 seconds), with significant center-to-center variation in preferred assay and target range. Lower anticoagulation goals may be considered in patients with high bleeding risk.

Board pearl: The initial heparin bolus is weight-based (50–100 U/kg) and should be omitted or reduced if the patient is already anticoagulated.

Correct answer: B — Antithrombin deficiency is common on ECMO, but the only RCT in VV-ECMO showed no reduction in heparin dose or improvement in bleeding/thrombosis with supplementation

Acquired antithrombin deficiency is extremely common during ECMO — Mansour et al. found that roughly 90% of VA-ECMO patients had at least one low antithrombin value, though levels did not correlate with heparin responsiveness as measured by anti-Xa assay in that cohort. The only RCT of antithrombin supplementation during ECMO (Panigada et al., 48 VV-ECMO patients) found that supplementation to maintain levels of 80–120% did not decrease heparin dose and did not reduce bleeding or thrombosis. This contrasts with the cardiopulmonary bypass literature, where antithrombin supplementation has been effective in restoring heparin responsiveness in several trials. The AHA pediatric guideline recommends antithrombin concentrate for heparin resistance as a Class I recommendation, but this is based on expert consensus rather than ECMO-specific RCT data.

Board pearl: Antithrombin deficiency is nearly universal on ECMO, but the only RCT showed supplementation did not reduce heparin requirements — the CPB literature may not extrapolate.

Correct answer: B — Retrospective and meta-analytic data suggest bivalirudin reduces circuit and patient thrombosis compared with heparin, with comparable or lower bleeding rates, but no definitive RCT exists

No large RCT has compared bivalirudin to heparin during ECMO. However, multiple retrospective studies and meta-analyses consistently show favorable signals. Rivosecchi et al. (295 VV-ECMO patients) found bivalirudin was associated with fewer circuit thrombotic complications and less blood product transfusion. A network meta-analysis by Chen et al. (2,522 patients, 23 studies) found bivalirudin reduced device-related and patient-related thrombosis compared with UFH, and was superior to argatroban for device thrombosis. Seelhammer et al. found reduced mortality in the adult bivalirudin group. A 2024 Cochrane review confirmed that no definitive anticoagulation strategy exists despite over 50 years of innovation. A 2026 retrospective study found higher time in therapeutic range with bivalirudin than heparin.

Board pearl: Bivalirudin shows consistent signals for reduced thrombosis and comparable or lower bleeding across retrospective data, but a definitive RCT is still needed.

Correct answer: B — 0.15–0.30 mg/kg/h, titrated to aPTT or ACT targets

Published bivalirudin protocols for ECMO use initial infusion rates of 0.15–0.30 mg/kg/h without a loading dose in most cases, titrated to aPTT (target 50–80 seconds) or ACT (target 160–220 seconds). A systematic review by Zhong et al. found a mean maintenance rate of approximately 0.27 mg/kg/h with a target ACT of 160–220 seconds. Patients on concurrent continuous renal replacement therapy (CRRT) require lower doses, because bivalirudin is partially cleared by proteolytic cleavage and renal excretion; one study found dosing requirements increased by 75–125% when renal replacement was added to the circuit. Wide variation in dosing underscores the need for careful individualized titration.

Board pearl: Bivalirudin dosing on ECMO starts at 0.15–0.30 mg/kg/h without a loading dose, and must be reduced in patients on CRRT.