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I am seeking information concerning theophylline
clearance (population kinetics). I am building a kinetic module
for use at the hospital I work. Any information would be
greatly appreciated.
Thank You,
Doug Kidder R.Ph.
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Their are several factors that can influence the calculation of a
patients theophylline clearance in clinical practice. Drug interactions
can have abrupt (ie cimetidine after 24 hours) or delayed (e-mycin,
starting or stopping tobacco) onset. Factors such as liver disease or
heart failure depend on the severity and presence of symptoms. This can
some times change on a daily basis. If the patient received a flu
vaccination or has been recently under or over compliant (Some patients
will double up prior to arriving to the ER to avoid being tubed). Severe
COPD/Asthma requiring ventilation may depend on the presence of PEEP
>10mmhg (or presence of autoPEEP)(decreased cardiac output secondary to
depressed venous return) or changes in ABG's.
Infusion rates (was the rate equivalent to the total amount givin in the
past 24 hours) due to incompatibilities or administration error. Time of
level draw and the method used, especially if not at steady-state,
to calculate the clearance can also influence your predictions (ie Koup
vs Chu vs PK Bayesian software).
I hope this gives you a starting point and some things to consider. Feel
free to contact me if you have any questions.
William Dager, Pharm.D.,FCSHP
Coordinator, Pharmacokinetic Consult Service
UC Davis Medical Center
e-mail: wedager.-at-.ucdavis.edu
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Dear Doug,
From the sound of your question, the responses youv'e gotten our pretty
abstract. I, too, am a pharmacist in a hospital which routinely screens all
patients on theo for estimated levels and confirm them via actual levels.
we use very simple equations to determine estimates because like most
hospitals we have a lot of patients. This simple strait forward estimate may
be used if you like:
1. estimate Clearance in L/hr by:
using Ideal Body Wt x .05 x disease states
Here you'd factor in specific disease adjustments
examples: asthma = 1.0 smokers use 1.4 see applied therapeutics for a
complete list of factors associated w/variation in theophylline elimination.
2. Next calculate Vd using ABW * 0.5
3. Knowing the relationship b/w VD and cl we can
determine t 1/2 = vd/cl * 0.693 (usu 8hrs)
4. estimate Cpss using:
Cpss = (dose/24hr) divided by CL = mg/l
Hope this is what your after. It's fairly accurate and quick. Applied
therapeutics also discusses expected changes in levels when certain drugs
which interfere w/clearance are added.
Anna
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Copyright 1995-2005 David W. A. Bourne (david@boomer.org)