Heparin formulas

 

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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.

 

Initial dosing

 

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

 

aPTT ratio

 

aPTT ratio

Loading dose or

Hold time

Maintenance dose

Less than 1.2 times control

50 u/kg

Increase by 4 u/kg/hr

1.2 to 1.3 times control

25 u/kg

Increase by 3 u/kg/hr

1.3 to 1.4 times control

None

Increase by 2 u/kg/hr

1.5 to 2.5 times control

None

No change

2.6 to 2.9 times control

None

Decrease by 2 u/kg/hr

3 to 4 times control

Hold for 1 hour

Decrease by 3 u/kg/hr

Greater than 4 times control

Hold for 2 hours

Decrease by 4 u/kg/hr

 

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)

 

aPTT corresponding to Heparin concentration of

Loading dose

or Hold time

Maintenance dose

Less than 0.1 u/ml

50 u/kg

Increase by 3 u/kg/hr

0.1 to 0.15 u/ml

25 u/kg

Increase by 2 u/kg/hr

0.16 to 0.19 u/ml

None

Increase by 2 u/kg/hr

0.2 to 0.4 u/ml

None

No change

0.41 to 0.5 u/ml

None

Decrease by 2 u/kg/hr

0.51 to 0.6 u/ml

Hold for 1 hour

Decrease by 2 u/kg/hr

Greater than 0.6 u/ml

Hold for 2 hours

Decrease by 3 u/kg/hr

 

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).

 


See also:

Introduction

Monitoring parameters

Precautions

Bibliography

 

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