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Difficulty: MediumHemodynamic and Coagulation Processes

A 32-year-old man is hospitalized for a newly diagnosed unprovoked deep vein thrombosis of the left popliteal vein. His past medical history is unremarkable, and he takes no medications. Vital signs are within normal limits. Baseline laboratory studies show a normal complete blood count, normal prothrombin time, and an activated partial thromboplastin time (aPTT) of 28 seconds (reference range: 25-35 seconds). He is started on a continuous intravenous infusion of unfractionated heparin. Twelve hours later, despite receiving two weight-based boluses and a significant increase in the continuous infusion rate, his aPTT remains at 34 seconds. Which of the following is the most likely mechanism responsible for this patient's subtherapeutic laboratory response?

  1. A
    Point mutation preventing factor Va inactivation
  2. B
    Formation of autoantibodies against phospholipid-protein complexes
  3. Decreased baseline activity of a serine protease inhibitorAnswer
  4. D
    Autoantibody-mediated clearance of platelet factor 4 complexes
  5. E
    Accelerated renal elimination of the administered anticoagulant

Answer

Decreased baseline activity of a serine protease inhibitor
The correct answer identifies a deficiency in antithrombin, which is a serine protease inhibitor (serpin). Unfractionated heparin has no intrinsic anticoagulant activity; it works solely as a catalyst that binds to antithrombin, inducing a conformational change that increases antithrombin's ability to inactivate thrombin (Factor IIa), Factor Xa, and other coagulation factors by at least 1,000-fold. In a patient with an inherited or acquired antithrombin deficiency, heparin lacks its necessary cofactor. This leads to 'heparin resistance,' classically presenting as a subtherapeutic aPTT despite escalating, highly supratherapeutic doses of an unfractionated heparin infusion. The patient's unprovoked DVT at a young age was the initial manifestation of his underlying antithrombin deficiency.

Step-by-Step Solution

1
Identify the primary clinical problem in the vignette.
The patient has an unprovoked DVT and requires anticoagulation. After initiation of unfractionated heparin, his aPTT fails to rise into the therapeutic range despite escalating, high doses.
Recognizing the failure of the aPTT to prolong is the key to identifying the phenomenon of 'heparin resistance'.
2
Determine the molecular mechanism of action of unfractionated heparin.
Unfractionated heparin works by binding to antithrombin, causing a conformational change that dramatically accelerates antithrombin's ability to inactivate thrombin (Factor IIa) and Factor Xa.
Understanding the pharmacodynamics of heparin is required to understand why it might fail.
3
Correlate the mechanism of action with the clinical presentation of heparin resistance.
Because heparin relies entirely on the presence of antithrombin to function, a patient with an underlying antithrombin deficiency (a serine protease inhibitor) will not exhibit the expected anticoagulant response (prolonged aPTT) to standard or high doses of heparin.
Antithrombin deficiency perfectly explains both the patient's unprovoked DVT (as it is an inherited thrombophilia) and his subsequent heparin resistance.

Key Concept

Heparin resistance is typically caused by antithrombin deficiency. Antithrombin is a serine protease inhibitor that must be present for unfractionated heparin to exert its anticoagulant effect and prolong the aPTT.
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Topics

Hemodynamic and Coagulation Processes

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