Pharmacokinetics software and free equations

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2 total messages Started by Koski, Greg,M.D. Mon, 09 Apr 2012 13:59

#2453

Author: Koski, Greg,M.D.
Date: Mon, 09 Apr 2012 13:59

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Hello, David.  I am a cardiac anesthesiologist who needs a simple way to calculate accurately the residual heparin in the circulation at any time after an initial loading dose and a fixed continuous infusion.  Heparin is essentially confined to the circulation and eliminated with an effective half like of about 1 hour.  We administer a loading dose of 3.5 mg/kg at time zero and start an infusion of 1 mg/kg/hg.  We generally stop the infusion at about 2.5 to 3 hours and need to know how much heparin is remaining assuming a simple one compartment first-order kinetic model so that we can calculate how much protamine is needed to reverse the remaining heparin.

May math is too many years old.  I know that the amount drops by about half every hour, but I'd like to do a bit better.  Can you give me a simple equation that can be easily programmed into a programmable calculator to estimate the residual heparin at nay time after the initial administration?

Please forgive my ignorance......

Greg



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Re: Pharmacokinetics software and free equations

#2455

Author: PiLS
Date: Mon, 16 Apr 2012 20:58

30 lines
3516 bytes

Le lun, 09 avr 2012 09:59:15, Koski, Greg,M.D. a plopp�:
> Hello, David.  I am a cardiac anesthesiologist who needs a simple 
> way to calculate accurately the residual heparin in the circulation 
> at any time after an initial loading dose and a fixed continuous 
> infusion.  Heparin is essentially confined to the circulation and 
> eliminated with an effective half like of about 1 hour.  We 
> administer a loading dose of 3.5 mg/kg at time zero and start an 
> infusion of 1 mg/kg/hg.  We generally stop the infusion at about 2.5 
> to 3 hours and need to know how much heparin is remaining assuming 
> a simple one compartment first-order kinetic model so that we can 
> calculate how much protamine is needed to reverse the remaining 
> heparin.

The quantity remaining will be something like:

Dt = (Dini + (Ir x Tinf)) / (2 x t)

With
Dt: the dose remaining at time t (in mg) 
Dini: the initial dose (in mg)
Ir: the infusion rate (in mg/h)
t: the time since time zero, in hours
Tinf: the time of infusion, in hours (=t before the end of the infusion;
    = 2.5 to 3 hours after the end of the infusion)

This formula should work at any time (including before the end of the 
infusion). That's assuming a half-life of 1 hr.

-- 
PiLS

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