Unfractionated Heparin — Raschke Nomogram
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Unfractionated heparin (UFH) is a parenterally administered anticoagulant derived from porcine intestinal mucosa or bovine lung, consisting of a heterogeneous mixture of glycosaminoglycan chains. It exerts its anticoagulant effect primarily by binding to antithrombin III (AT-III), markedly accelerating AT-III's inhibition of thrombin (factor IIa) and factor Xa, as well as lesser effects on factors IXa, XIa, and XIIa. Because of its heterogeneous molecular weight, variable bioavailability, and non-linear pharmacokinetics — including binding to plasma proteins, endothelial cells, and macrophages — UFH requires laboratory monitoring with the activated partial thromboplastin time (aPTT). The weight-based Raschke nomogram, published in 1993, provides the most evidence-based approach to UFH dosing: an initial bolus of 80 units/kg IV followed by a continuous infusion of 18 units/kg/hour. The aPTT target range of 60–100 seconds (corresponding to a heparin level of approximately 0.3–0.7 units/mL by anti-Xa assay) is the standard therapeutic range for venous thromboembolism (VTE). aPTT should be measured 6 hours after initiating therapy and 6 hours after each dose adjustment, with dose changes guided by a sliding scale. Despite the availability of low molecular weight heparins (LMWHs) and direct oral anticoagulants (DOACs), UFH retains important advantages: it is fully reversible with protamine sulphate, it can be used in renal failure without dose adjustment, and it has a short half-life (~1.5 hours) allowing rapid cessation — making it the anticoagulant of choice in high-bleeding-risk settings such as the peri-operative period or active heparin infusion in the ICU.
IV Bolus: 80 units/kg (max 10,000 units) Initial Infusion: 18 units/kg/hour aPTT target: 60–100 seconds Check aPTT 6 hours after each dose change Raschke Nomogram dose adjustments based on aPTT: <35 s: Bolus 80 u/kg + increase by 4 u/kg/h 35–45 s: Bolus 40 u/kg + increase by 2 u/kg/h 46–70 s: No change 71–90 s: Reduce by 2 u/kg/h >90 s: Hold 1 hour + reduce by 3 u/kg/h
- 1Confirm indication (VTE treatment, ACS, PE, DVT, or other) and absence of absolute contraindications (active major bleeding, recent neurosurgery, HIT history).
- 2Obtain baseline aPTT, PT/INR, full blood count, and renal function before initiating heparin.
- 3Calculate and administer the weight-based IV bolus: 80 units/kg (maximum 10,000 units).
- 4Start continuous infusion at 18 units/kg/hour via a programmable syringe pump or infusion pump.
- 5Check aPTT 6 hours after starting the infusion and adjust dose per the Raschke nomogram or local institutional protocol.
- 6Continue 6-hourly aPTT monitoring until two consecutive results are in the therapeutic range (60–100 s); thereafter daily monitoring is acceptable.
- 7Monitor platelet count every 2 days from day 4 to detect heparin-induced thrombocytopenia (HIT), defined as a >50% platelet fall with thrombosis.
Check aPTT at 6 hours; target 60–100 seconds
The weight-based nomogram provides a therapeutic aPTT in the majority of patients without supplemental boluses. At 6 hours, if aPTT is sub-therapeutic, an additional bolus and rate increase are given per the sliding scale.
aPTT 35–45 s → bolus 40 u/kg + increase infusion 2 u/kg/h
A sub-therapeutic aPTT increases the risk of recurrent VTE. The Raschke nomogram mandates a supplemental bolus AND an infusion rate increase to quickly re-establish therapeutic anticoagulation. Failing to give the bolus risks a prolonged sub-therapeutic trough.
aPTT >90 s → stop 1 hour then reduce infusion rate
A markedly elevated aPTT significantly increases bleeding risk. The infusion is paused to allow partial heparin clearance (half-life ~1.5h), then restarted at a lower rate. Clinical bleeding assessment should accompany the laboratory check.
Max bolus cap 10,000 units; use adjusted body weight, not actual weight, in morbid obesity
Heparin does not distribute significantly into adipose tissue. Using actual weight in morbid obesity risks over-anticoagulation. Adjusted body weight corrects for this while still accounting for some excess weight. Local haematology guidance on weight-based dosing in obesity should be followed.
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Extreme input values
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in heparin dose calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Assumption violations
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in heparin dose calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Rounding and precision effects
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in heparin dose calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
| aPTT (seconds) | Dose Adjustment | Additional Bolus |
|---|---|---|
| <35 | Increase infusion by 4 u/kg/h | Bolus 80 u/kg |
| 35–45 | Increase infusion by 2 u/kg/h | Bolus 40 u/kg |
| 46–70 (therapeutic) | No change | None |
| 71–90 | Reduce infusion by 2 u/kg/h | None |
| >90 | Stop 1 hour; reduce by 3 u/kg/h | None; hold and recheck in 6h |
Why is aPTT used to monitor UFH rather than anti-Xa?
aPTT is the traditional monitoring parameter and is universally available. However, anti-Xa levels (target 0.3–0.7 units/mL) are more specific for heparin effect and are preferred when aPTT results are unreliable — such as in patients with lupus anticoagulant, elevated factor VIII, or other causes of aPTT prolongation unrelated to heparin.
What is heparin-induced thrombocytopenia (HIT) and how is it detected?
HIT is an immune-mediated prothrombotic complication caused by antibodies against the heparin-platelet factor 4 (PF4) complex. It presents with a >50% platelet count drop, typically between days 5–10 of heparin exposure, often with new arterial or venous thrombosis. The 4T score is the validated clinical pre-test probability tool. HIT must be treated by stopping all heparin and switching to a non-heparin anticoagulant (argatroban, danaparoid, or fondaparinux).
Can UFH be used in renal failure?
In the context of Heparin Dose, this depends on the specific inputs, assumptions, and goals of the user. The underlying formula provides a deterministic relationship between inputs and output, but real-world application requires interpreting the result within the broader context of health and medical practice. Professionals typically cross-reference calculator output with industry benchmarks, historical data, and regulatory requirements. For the most reliable results, ensure inputs are sourced from verified data, understand which assumptions the formula makes, and consider running multiple scenarios to bracket the range of likely outcomes.
How is heparin reversed in bleeding?
Protamine sulphate reverses UFH at a dose of 1 mg per 100 units of heparin given in the last 2–3 hours (maximum 50 mg IV slow infusion). Protamine binds heparin ionically, forming a stable complex with no anticoagulant activity. Over-dosing protamine itself has anticoagulant effects, so precise calculation is important.
What is the difference between therapeutic and prophylactic heparin?
Heparin Dose is a specialized calculation tool designed to help users compute and analyze key metrics in the health and medical domain. It takes specific numeric inputs — typically drawn from real-world data such as measurements, rates, or quantities — and applies a validated mathematical formula to produce actionable results. The tool is valuable because it eliminates manual calculation errors, provides instant feedback when exploring different scenarios, and serves as both a decision-support instrument for professionals and a learning aid for students studying the underlying principles.
How long does it take UFH to achieve therapeutic aPTT?
In the context of Heparin Dose, this depends on the specific inputs, assumptions, and goals of the user. The underlying formula provides a deterministic relationship between inputs and output, but real-world application requires interpreting the result within the broader context of health and medical practice. Professionals typically cross-reference calculator output with industry benchmarks, historical data, and regulatory requirements. For the most reliable results, ensure inputs are sourced from verified data, understand which assumptions the formula makes, and consider running multiple scenarios to bracket the range of likely outcomes.
When should bridging heparin be used perioperatively?
Bridging UFH or LMWH is indicated when a patient on warfarin or vitamin K antagonist requires surgery and has high thromboembolic risk (mechanical heart valve, recent VTE within 3 months, AF with high CHADS2-VASc score). Low and moderate risk patients can usually have anticoagulation stopped without bridging, per current BRIDGE trial data.
Can heparin be given subcutaneously?
Yes. UFH can be given subcutaneously (SC) for prophylaxis at 5,000 units every 8–12 hours. SC UFH is also occasionally used for therapeutic anticoagulation during pregnancy (as it does not cross the placenta) but requires larger doses and anti-Xa monitoring due to variable absorption. LMWH SC is generally preferred for therapeutic use in pregnancy.
专业提示
When starting heparin in a patient whose baseline aPTT is already prolonged (due to lupus anticoagulant or factor deficiency), the aPTT cannot be reliably used for monitoring. Switch to anti-Xa monitoring (target 0.3–0.7 units/mL) to avoid over- or under-anticoagulation based on a misleading aPTT.
你知道吗？
Heparin was accidentally discovered in 1916 by Jay McLean, a second-year medical student at Johns Hopkins, who was investigating procoagulant substances in canine liver. He found the opposite — a powerful anticoagulant. The substance was later named 'heparin' from the Greek 'hepar' meaning liver. It remains one of the oldest medications still in widespread clinical use.
参考资料
- ›Raschke RA et al. The weight-based heparin dosing nomogram compared with a standard care nomogram. Ann Intern Med. 1993;119(9):874-881.
- ›Kearon C et al. Antithrombotic Therapy for VTE Disease: CHEST Guideline. Chest. 2016;149(2):315-352.
- ›Linkins LA et al. Treatment and prevention of heparin-induced thrombocytopenia. Chest. 2012;141(2 Suppl):e495S-530S.
- ›Garcia DA et al. Parenteral anticoagulants: CHEST guideline and expert panel report. Chest. 2012;141(2 Suppl):e24S-43S.