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The heparin model is not hard-coded into the program. The parameters are found in the drug model database and are fully user-editable. You can tailor each drug model to fit your patient population, or you can create your own models. See the Edit drug models section of the help file for further information.
1. Calculate dosing weight
DW = LBW + ((TBW - LBW)/3
where TBW = total body weight
LBW = lean body weight
2. Determine loading dose
LD = 80 units x DW x F
where F = 0.67 if age >= 70
3. Determine initial infusion rate (ko)
ko = 18 units/hr x DW
4. Calculate heparin concentration
a. Calculate blood volume (Volume of distribution)
• M= 0.3669 x Ht^3 + 0.03219 x TBW +0.604
• F= 0.3561 x Ht^3 + 0.03308 x TBW + 0.1833
where Ht = height (meters)
TBW= total body weight (kg)
b. Calculate elimination rate
• kel = 0.693 x F x F'
where F = 1.2 if smoker
F' = 0.8 if age >= 70
c. Calculate heparin concentration
Cp = ko / (Vd x kel)
where ko is heparin infusion rate
Adjust maintenance dose
The control value used to determine the ratio is poorly defined, it may be the mean value of the normal range for the aPTT, or the patient's baseline aPTT (the aPTT measured before any heparin). Use of the patient's baseline as the control has gained popularity based on the fact that many patients undergoing thrombotic episodes have shortened aPTTs.
aPTT range (Heparin response curve)
The heparin response curve is determined by the local lab. An approximation of the heparin concentration can be made by testing the aPTT reagent in a plasma system that has been calibrated by addition of a range of clinically relevant concentrations of heparin. This heparin response curve can then be entered into the Kinetics program and used as the basis for dosage adjustments (highly recommended).