Q61 / 110
A patient on ECMO has persistently subtherapeutic anti-Xa levels despite escalating heparin doses. Which mechanism is most likely responsible?
Correct answer: B — Antithrombin deficiency
Heparin exerts its anticoagulant effect through antithrombin. Low antithrombin levels reduce heparin efficacy, leading to apparent heparin resistance. This is common in ECMO because of consumption and dilution. Increasing heparin alone may not correct the problem. Antithrombin supplementation or an alternative anticoagulant strategy may be required.
Board pearl: Antithrombin deficiency is a major cause of heparin resistance on ECMO.
Heparin exerts its anticoagulant effect through antithrombin. Low antithrombin levels reduce heparin efficacy, leading to apparent heparin resistance. This is common in ECMO because of consumption and dilution. Increasing heparin alone may not correct the problem. Antithrombin supplementation or an alternative anticoagulant strategy may be required.
Board pearl: Antithrombin deficiency is a major cause of heparin resistance on ECMO.
Q62 / 110
A patient on VA ECMO develops thrombocytopenia with concern for heparin-induced thrombocytopenia. What is the approximate prevalence of confirmed HIT in this population?
Correct answer: B — About 5%
Published data consistently report confirmed HIT prevalence on VA-ECMO at approximately 3–6%, not less than 1%. Thrombocytopenia itself is very common on ECMO, and many patients with a positive screening 4Ts score or immunoassay are ultimately found not to have functional HIT after confirmatory testing — but true confirmed HIT still occurs in roughly 1 in 20 patients, which is far from rare. This distinction matters because misdiagnosis (over- or under-calling HIT) can lead to unnecessary or dangerously delayed changes in anticoagulation. Clinical scoring (4Ts) plus laboratory confirmation (functional assay) remain essential before committing to a direct thrombin inhibitor.
Board pearl: Confirmed HIT on VA-ECMO occurs in roughly 3–6% of patients — common enough that it must stay on the differential for otherwise-unexplained thrombocytopenia, but still requires functional assay confirmation before switching anticoagulants.
Published data consistently report confirmed HIT prevalence on VA-ECMO at approximately 3–6%, not less than 1%. Thrombocytopenia itself is very common on ECMO, and many patients with a positive screening 4Ts score or immunoassay are ultimately found not to have functional HIT after confirmatory testing — but true confirmed HIT still occurs in roughly 1 in 20 patients, which is far from rare. This distinction matters because misdiagnosis (over- or under-calling HIT) can lead to unnecessary or dangerously delayed changes in anticoagulation. Clinical scoring (4Ts) plus laboratory confirmation (functional assay) remain essential before committing to a direct thrombin inhibitor.
Board pearl: Confirmed HIT on VA-ECMO occurs in roughly 3–6% of patients — common enough that it must stay on the differential for otherwise-unexplained thrombocytopenia, but still requires functional assay confirmation before switching anticoagulants.
Source(s): Lubnow M, Berger J, Schneckenpointner R, et al. Prevalence and Outcomes of Patients Developing Heparin-Induced Thrombocytopenia During Extracorporeal Membrane Oxygenation. PLoS One. 2022;17(8):e0272577. doi:10.1371/journal.pone.0272577. | Lüsebrink E, Scherer C, Binzenhöfer L, et al. Heparin-Induced Thrombocytopenia in Patients Undergoing Venoarterial Extracorporeal Membrane Oxygenation. J Clin Med. 2023;12(1):362. doi:10.3390/jcm12010362. | Jiritano F, Serraino GF, Ten Cate H, et al. Platelets and Extra-Corporeal Membrane Oxygenation in Adult Patients: A Systematic Review and Meta-Analysis. Intensive Care Med. 2020;46(6):1154-1169. doi:10.1007/s00134-020-06031-4.
Q63 / 110
Which of the following best explains why dynamic variables such as pulse pressure variation are superior to static variables such as central venous pressure for predicting fluid responsiveness?
Correct answer: B — They reflect position on the Frank-Starling curve
Dynamic variables such as pulse pressure variation and stroke volume variation estimate how stroke volume changes with cyclic preload changes, which reflects preload responsiveness. Static measures such as central venous pressure do not reliably predict whether stroke volume will increase after fluid administration. They are heavily influenced by ventricular compliance and contractility. Dynamic testing therefore better captures physiology relevant to fluid response.
Board pearl: Dynamic variables assess preload responsiveness by reflecting where the ventricle sits on the Frank-Starling curve.
Dynamic variables such as pulse pressure variation and stroke volume variation estimate how stroke volume changes with cyclic preload changes, which reflects preload responsiveness. Static measures such as central venous pressure do not reliably predict whether stroke volume will increase after fluid administration. They are heavily influenced by ventricular compliance and contractility. Dynamic testing therefore better captures physiology relevant to fluid response.
Board pearl: Dynamic variables assess preload responsiveness by reflecting where the ventricle sits on the Frank-Starling curve.
Q64 / 110
Which condition must be present for pulse pressure variation to reliably predict fluid responsiveness?
Correct answer: C — Controlled mechanical ventilation with sinus rhythm
Dynamic indices such as pulse pressure variation depend on predictable respiratory changes in preload and a regular cardiac rhythm. Spontaneous breathing and arrhythmias disrupt these assumptions and reduce reliability. For this reason, pulse pressure variation is most useful in controlled mechanical ventilation with sinus rhythm. Outside those conditions, other tests should be preferred.
Board pearl: Pulse pressure variation is most reliable in mechanically ventilated patients in sinus rhythm.
Dynamic indices such as pulse pressure variation depend on predictable respiratory changes in preload and a regular cardiac rhythm. Spontaneous breathing and arrhythmias disrupt these assumptions and reduce reliability. For this reason, pulse pressure variation is most useful in controlled mechanical ventilation with sinus rhythm. Outside those conditions, other tests should be preferred.
Board pearl: Pulse pressure variation is most reliable in mechanically ventilated patients in sinus rhythm.
Q65 / 110
A mechanically ventilated patient in sinus rhythm has a pulse pressure variation of 15%. What does this most likely indicate?
Correct answer: B — Fluid responsiveness
In mechanically ventilated patients in sinus rhythm, larger respiratory swings in pulse pressure suggest that stroke volume is preload-dependent. A value above roughly 13% is commonly used as a threshold supporting fluid responsiveness. This does not mean fluid must always be given, but it suggests a hemodynamic response is likely. Clinical context still matters, especially if there are reasons to avoid more volume.
Board pearl: A pulse pressure variation above about 13% suggests the patient is likely to respond to fluid.
In mechanically ventilated patients in sinus rhythm, larger respiratory swings in pulse pressure suggest that stroke volume is preload-dependent. A value above roughly 13% is commonly used as a threshold supporting fluid responsiveness. This does not mean fluid must always be given, but it suggests a hemodynamic response is likely. Clinical context still matters, especially if there are reasons to avoid more volume.
Board pearl: A pulse pressure variation above about 13% suggests the patient is likely to respond to fluid.