Print Medical Pharmacokinetics #2

 

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Types of Drugs Monitored

What kinds of drugs do we monitor?

 


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Steady State Concept

[Steady State Concept general, 09/03/96]

Steady State

Definition

Questions about why?

Questions about time required to plateau


Answers/Comments regarding steady state


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Major Pharmacokinetic Parameters


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Volume of Distribution
and Loading Dose


Calculating an Initial Dose

Amount of Drug = Desired Conc * Volume

{mg = (mg/L) * L}

Loading Dose

Information needed to calculate a DL


Finding Vd

Earlier formula rearranged!

Vd = Dose / Measured Conc

{L = mg / (mg/L) }

"Vd" is equivalent to "volume"
"dose" is equivalent to "amount of drug"
"measured" is equivalent to "desired"

{Note: in "real" pharmacokinetics, one would use
much more complex analyses to find what is known
as the volume of distribution at steady state (Vd(ss))}


Summarizing statements for Vd


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CLEARANCE

Clearance

major parameters

More on "Amounts", Extraction ratios, and Clearance

There may be confusion about how one relates extraction ratio and clearance with how much drug is actually eliminated during any specific time interval. It should be noted that if a drug is cleared by first order kinetics and if blood flow to the organ(s) of elimination is constant, then CL and ER are both constant regardless of how much drug there is in the plasma. However, THE ACTUAL AMOUNT OF DRUG ELIMINATED DURING EACH INTERVAL OF AN HOUR OR A MINUTE IS NOT CONSTANT. If the concentration of drug in the plasma is rising, the amount of drug eliminated (e.g., mg/min) is also rising. Vice versa, if the concentration of drug in the plasma is falling (when drug absorbed per min is less than the amount eliminated), then the amount of drug eliminated each successive minute will also decrease.

To find the actual amount of drug eliminated each minute, one must know the average concentration of drug in the arterial blood to the organ of elimination during that minute. One then multiplies that concentration times the CL to find the amount of drug eliminated during that minute.

Amount of drug eliminated in the minute = Average concentration in incoming plasma x CL

(mg per min (t) = average mg/L in (t) x L/min)

Return


Questions regarding clearance


Answers to Clearance Questions

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Elimination Rate Constant / Half-life

 


Ke (Elimination Rate) -- Half-Life

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Graphs -- Ke -- Half-life

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Answers to Graph Questions


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Bioavailability (F)

 

 


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