Heparin-Induced Thrombocytopenia

Correct answer: C — High probability (6–8 points).

Scoring this patient on the 4Ts:
— Thrombocytopenia: fall from 110,000 to 44,000/µL = 60% fall, nadir ≥20,000 → 2 points
— Timing: POD#1 appears too early for de novo HIT (classic window is day 5–10). However, this patient received UFH one week prior for cardiac catheterization. Prior heparin exposure within 30 days means circulating HIT antibodies may already be present — the platelet count can fall within 24 hours of re-exposure. This is called rapid-onset HIT, and scores 2 points.
— Thrombosis: none → 0 points
— oTher causes: no alternative explanation → 2 points
Total: 6/8 — High probability.

At high probability, clinical action should not wait for confirmatory testing. Stop all heparin sources immediately — including flushes and heparin-bonded catheters. Send anti-PF4 ELISA and SRA. Start bivalirudin if the ELISA is positive and the patient is not bleeding. Avoid platelet transfusion unless there is significant bleeding — HIT is a prothrombotic state and transfusing platelets risks accelerating clot formation.

Correct answer: B — IgG-mediated platelet FcγRIIa activation with massive thrombin generation.

HIT type 2 is immune-mediated: heparin binds PF4 released from platelets, forming an immunogenic complex. IgG antibodies develop within 5–14 days (or within hours with prior exposure) and bind the platelet FcγRIIa receptor, triggering platelet activation and massive thrombin generation. Activated platelets are cleared — hence the falling count — but thrombin generation far outpaces platelet consumption, so thrombosis dominates over bleeding.

HIT type 1 vs type 2:
— Type 1 is a non-immune, direct effect of heparin on platelet membranes. It occurs within the first 1–2 days, causes mild thrombocytopenia (rarely below 100,000/µL), is self-limiting, and carries no thrombotic risk. No treatment is required.
— Type 2 is the immune-mediated form described above — thrombotic risk is 30–50% if untreated.

Correct answer: E — Stop all heparin sources; hold anticoagulation briefly while awaiting ELISA; start bivalirudin promptly once confirmed.

In post-cardiac surgery HIT, the prothrombotic risk of withholding anticoagulation — amplified here by a mechanical valve — must be weighed against the hemorrhagic risk of empiric treatment in a patient who required re-exploration just days ago.

Heparin elimination is non-negotiable and must happen immediately:
— Stop IV UFH infusion
— Replace heparin catheter flushes with normal saline
— Replace heparin-bonded PA catheter as clinically indicated
— Switch CRRT circuit from UFH to citrate anticoagulation

On the anticoagulation decision: ASH guidelines recommend starting a non-heparin anticoagulant empirically at high 4Ts probability. However, in a patient with recent surgical re-exploration and active bleeding risk, it is clinically reasonable to hold anticoagulation briefly while awaiting ELISA confirmation — provided the result is expected within hours and the patient is closely monitored. Once ELISA confirms HIT, bivalirudin should be started promptly. The decision to treat empirically versus wait for confirmation should be made jointly with surgery.

Why bivalirudin over argatroban? This patient has shock liver (ALT 640, bilirubin 4.1) — argatroban is exclusively hepatically cleared and will accumulate unpredictably. Bivalirudin is cleared ~80% by proteolytic degradation and is the preferred agent when hepatic dysfunction is present.

Correct answer: B — Bivalirudin for CPB.

This is the most clinically nuanced HIT scenario in cardiac anesthesia. Management depends on which phase of HIT the patient is in.

The five phases of HIT and their implications for cardiac surgery:

Phase	Platelets	Functional Assay	Immunoassay	CPB Anticoagulation
Suspected HIT	↓	?	?	Alternative agent
Acute HIT	↓	+	+	Alternative agent
Subacute HIT A	Normal	+	+	Alternative agent
Subacute HIT B	Normal	−	+	UFH intraop acceptable
Remote HIT	Normal	−	−	UFH intraop acceptable

If surgery can be delayed, the ideal is to wait until subacute HIT phase B or remote HIT, at which point intraoperative UFH is acceptable. HIT antibodies typically clear within 50–85 days.

This patient is in subacute HIT phase A — platelet count has recovered but SRA remains positive at 15%, just below the formal threshold but not fully negative. UFH re-exposure is acceptable only when HIT antibodies are completely undetectable by both ELISA (OD <0.4) and SRA (<20% serotonin release). Re-exposure risks acute anaphylactoid reaction, rapid platelet consumption, and catastrophic intraoperative thrombosis on CPB.

Bivalirudin for CPB:
— Direct thrombin inhibitor — does not require antithrombin as a cofactor
— Dosing: induction bolus 1–1.5 mg/kg + circuit prime 50 mg + infusion 2.5 mg/kg/hr; target ACT >400 sec
— Circuit management is critical: bivalirudin has a short half-life (~25 min) and is degraded by thrombin — stasis allows clot to form. Minimize vent suction, avoid cardiotomy recirculation, keep pump running
— No reversal agent exists — plan for controlled discontinuation 15–30 minutes before separation from CPB

Options when bivalirudin is unavailable:
— Plasmapheresis + UFH: reduces circulating antibody titers, allowing safer heparin exposure. Rebound IgG synthesis can occur but plasmapheresis has been used successfully in this context
— Platelet anesthesia + UFH: potent platelet blockers (cangrelor, iloprost, abciximab) prevent platelet activation in the presence of heparin. Iloprost is a systemic vasodilator and can cause significant hypotension
— Argatroban or danaparoid are additional alternatives, though with more limited CPB data than bivalirudin

Correct answer: C — Stop all heparin; transition to bivalirudin with cautious titration given hepatic dysfunction.

1. Anticoagulation discontinuation on ECMO carries high risk. Even at flows >3 L/min, circuit surfaces generate thrombin continuously. Stopping anticoagulation risks oxygenator thrombosis, circuit clot, and systemic thromboembolism — particularly dangerous on VA-ECMO where arterial emboli go to the brain and coronaries.

2. Why bivalirudin over argatroban? Argatroban is exclusively hepatically cleared. This patient has congestive hepatopathy (ALT 420, bilirubin 3.2) — argatroban will accumulate, making titration unreliable and bleeding risk high. Bivalirudin is cleared ~80% by proteolytic degradation and ~20% renally — it is the preferred agent when hepatic dysfunction is present.

3. Bivalirudin monitoring on ECMO:
— In post-surgical patients, target aPTT 40–60 sec initially to balance thrombotic and bleeding risk; standard ECMO targets (60–100 sec) can be considered once early postoperative bleeding risk has passed
— Ecarin clotting time (ECT) is the most accurate monitor for direct thrombin inhibitors — less affected by circuit-induced factor consumption than aPTT
— Anti-Xa assay does not measure bivalirudin effect — it measures heparin and LMWH activity only
— ECMO consumes bivalirudin via thrombin-mediated degradation at circuit surfaces — dose requirements are often higher than in non-ECMO patients

4. Why not fondaparinux? Fondaparinux has very limited data for ECMO anticoagulation, cannot be monitored by standard ECMO assays, and has no antidote.