Exam 1 Constant Rate Regimen Flashcards
What is a constant rate regimen?
- the drug is administered systemically at a constant/zero order rate
- constant plasma concentration can be best accomplished by infusing a drug at a constant rate
What are different examples of infusing a drug at a constant rate?
- IV infusion via infusion pump
- orally administered drugs: slow release tablets
- transdermal patches
Why is a constant rate regimen used over an IV bolus dose?
IV bolus dose has a rapid elimination which means that it falls under the therapeutic range quickly → want to keep the drug at a constant concentration to keep it in the therapeutic range via IV infusion
What is steady state of an IV infusion?
where the concentration does not change much → essentially where it plateaus
What happens when steady state is reached and the IV infusion is stopped?
the drug begins to be eliminated → elimination rate is like the elimination rate of an IV bolus dose
At steady state, what is dA/dt?
dA/dt = 0
What is the equation for the concentration at steady state?
Css = R0/CL (in which R0 is the infusion rate)
What is the time to reach steady state?
considered to be 90% of steady state
What is the relationship between % eliminated (IV bolus) and % steady state (IV infusion)?
they are equal! (for example, at 3 half lives, 90% of the IV bolus dose has been eliminated while 90% of steady state has been reached with the IV infusion)
What is the equation for time to reach steady state (Tss)?
Tss = 3.3 t1/2
What is the equation for the concentration reached before steady state?
C = Css(1-e^-k*t)
What is the equation of the concentration after stopping infusion?
C = Css*e^-kt (like the IV bolus dose equation)
Example question: What would be the plasma t-PA concentration at 10 min after stopping infusion? (Css is about 2, t1/2=5)
C = Csse^-kt = 2e^(-0.693/5*10) = 0.5 mg/L
What is the equation to find Cp at a certain time point after the end of the infusion?
Cp = Cp0*e^-kt (where Cp0 is the concentration when infusion stopped and t is the time since the end of the infusion)
If the dose (infusion rate) is doubled, what happens to the plasma concentration at steady state?
the plasma concentration at steady state would double as well (since Css = R0/CL)
What happens if steady state is already reached and you increased the dose to double?
the new steady state concentration would be doubled the original steady state concentration
How do you determine the concentration at steady state after infusing for a certain amount of time but it hasn’t reached the steady state concentration yet?
use the Css = R0/CL equation with the new infusion rate to get the steady state concentration
How do you calculate the IV bolus dose if you are infusing a drug but want to increase the Cp to a certain concentration immediately?
dose = Vd*Cp in which Vd should be given and Cp is the concentration you want subtracted by the concentration you already have at that time period from the infusion
After infusing a drug and given an IV bolus dose and then discontinued IV infusion, what is the expected Cp profile?
drops because the only thing happening is the drug elimination from the body
If you infused a drug for a certain amount of time, then give an IV bolus dose at the same time as continuing the infusion, how do you calculate Css?
Css = R0/CL and that is what the Cp profile would decline to and taper off (reaching steady state but not)
How do you find infusion rate to achieve a specific Css concentration?
Css = R0/CL so you would rearrange to get R0 = Css*CL to get the infusion rate (since kel and Vd is known and given)
What is the benefit of combining an IV infusion with an IV bolus dose?
combining the IV infusion with an IV bolus loading dose can maintain Cp near Css from the beginning of drug administration → so that it won’t be far off the therapeutic range and cause toxicity since IV bolus dose can be toxic at high concentrations when first starting out
What is the equation to find the concentration at a specific time point for the combined IV infusion and IV bolus dose?
Ccombined = Cinfusion + Cbolus = Css(1-e^-kt) + Css*e^-kt = Css
What equation would you use to find the IV bolus loading dose to maintain a certain concentration from the beginning?
dose = Vd*Cp (in which Cp is that certain concentration from the question)
How would you find the expected Css if the recommended regimen is changed?
calculate Css using Css = R0/CL → the Css should not change if the infusion rate remained the same
True/false: Time to reach steady state depends on the dose.
false → it depends on the half life (and kel)
True/false: Doubling the dose will double the steady state concentration.
true → Css = R0/CL so if R0 is doubled, Css would double as a result
True/false: Doubling the dose will shorten the time to reach steady state.
false → time to reach steady state only depends on the half life/kel which is not dependent on the dose
True/false: Doubling dosing interval will lead to greater fluctuation in plasma concentration.
true?
