Heparin Dosing Calculator

Unfractionated heparin has no intrinsic anticoagulant activity. A specific pentasaccharide sequence within the polysaccharide chain binds antithrombin and induces a conformational change that accelerates its inhibition of thrombin and factor Xa by around a thousandfold. Inhibiting thrombin additionally requires the chain to bridge both proteins, which needs at least eighteen saccharide units, and this length requirement is the structural reason unfractionated heparin acts on both targets while the shorter low molecular weight fractions act mainly on factor Xa. Chain lengths in a commercial preparation range from roughly 3,000 to 30,000 daltons, so activity is expressed in units of biological effect rather than milligrams. Clearance is partly by a saturable cellular mechanism and partly renal, giving dose-dependent kinetics and an effective half-life of about 60 to 90 minutes at therapeutic infusion rates. Dosing is therefore anchored to weight and then titrated to effect. A common venous thromboembolism protocol gives an 80 units/kg bolus followed by 18 units/kg/hour, while acute coronary syndrome protocols use lower figures such as 60 and 12 because of concurrent antiplatelet therapy. The pump rate in mL/hour follows from the bag concentration, typically 25,000 units in 500 mL giving 50 units/mL. Monitoring targets an aPTT of roughly 1.5 to 2.5 times control, often 45 to 70 seconds, though reagent sensitivity varies enough between laboratories that each institution should calibrate its range against an anti-Xa level of 0.3 to 0.7 units/mL. Platelet counts every two to three days screen for heparin-induced thrombocytopenia, which typically appears between days 5 and 14 and causes thrombosis rather than bleeding.

Heparin was discovered in 1916 by Jay McLean, a second-year medical student in William Henry Howell's physiology laboratory at Johns Hopkins, who was searching for procoagulant phosphatides and instead found a liver extract that prevented blood from clotting. Howell and Emmett Holt continued the work and gave the substance its name in 1918, from the Greek for liver. Clinical use waited nearly two decades for purification: Charles Best, Arthur Charles and David Scott at the University of Toronto and Erik Jorpes at the Karolinska Institute independently produced preparations pure enough for injection in the 1930s, and the Swedish surgeon Clarence Crafoord administered it to postoperative patients to prevent thrombosis in the mid-1930s. The mechanism was only clarified in the 1970s, when antithrombin was identified as the required cofactor and the critical pentasaccharide binding sequence was characterised, work that later made possible both low molecular weight heparins and the synthetic pentasaccharide fondaparinux. Dosing practice changed in 1993, when Robert Raschke and colleagues published a randomised comparison in the Annals of Internal Medicine showing that a weight-based nomogram brought far more patients into the therapeutic aPTT range within 24 hours than physician-directed fixed dosing, with fewer recurrent thromboembolic events and no excess bleeding. That trial established the 80 and 18 units/kg figures still in use. Heparin remains preferred over low molecular weight agents when rapid reversal with protamine may be needed, in severe renal impairment, and for cardiopulmonary bypass and dialysis circuits.