IV Infusion

Question 1

What is IV infusion used for?

Answer 1

To directly administer drug into systemic circulation

Question 2

What are advantages to IV infusion?

Answer 2

• Precise control of blood drug concentration
• Excellent for drugs w/ narrow therapeutic windows
• Duration can be terminated easily

Question 3

What is the value for dCp / dt for IV infusions? (change in concentration / change in time)
Why?

Answer 3

• 0

- Rate of drug input = rate of drug output

Question 4

What does R mean?

Answer 4

Infusion rate

Question 5

What does Db mean?

Answer 5

Amount of drug in body

Question 6

What can be determined based on the sign in front of e^-kt?

Answer 6

• Negative e^-kt means it is a rising function

- e^-kt means it is a declining function

Question 7

How many half lives is needed to reach 90% Css? When is this used?

Answer 7

• 3.3 half lives

- Used in the clinic

Question 8

How many half lives is needed to reach 95% and 99% Css? When are these used?

Answer 8

• 95% = 4.3 half lives
• 99% = 6.6 half lives
• Used in the lab

Question 9

How do you know that it takes 6.65 half lives to reach 99% steady state? (how to derive it)

Answer 9

• Css = (R/VdK) (1 - e^-kt), but e^-kt = 0 and 1-e^-kt = 1, so Css = R/VdK
• At 99% Css, Css = 0.99R/Vd*K
• Cp = R/VdK (1-e^-kt), so 0.99R/VdK = R/VdK (1-e^-kt)
• e^-kt = 0.01, so -kt = ln(0.01)
• t 99% = ln0.01 / -k = -4.61 = (0.692/t1/2)
• t 0.01 = 6.65 * t1/2

Question 10

What does increasing the rate of infusion affect and what does it not affect?

Answer 10

• Affects final concentration

- Doesn’t shorten time to reach steady state, b/c that depends on half life

Question 11

When calculating Css, which Cp value will you use?

Answer 11

• The Cp from the time that is 1-2 half lives away from Css (Css can be assumed to be 3 half lives)
• Want Cp as far away from Css as possible to better estimate k

Question 12

What happens when a px receives an IV infusion plus a loading dose?

Answer 12

• As loading dose is declining, infusion is increasing at the same rate b/c it is the same drug w/ the same half life
• Steady state will be immediately reached

Question 13

What was done if the concentration is above steady state for the first 1-2 half lives? What must be done?

Answer 13

• Too high of a loading dose was given

- Must wait 3 half lives for concentration to fall to 90% of Css before turning on infusion

Question 14

What was done if the concentration is below steady state for the first 1-2 half lives? What must be done?

Answer 14

• Too low of a loading dose was given
• Will take 3 half lives to reach steady state
• If this is an emergency situation, calculate another loading dose and continue infusion

Question 15

What is the difference between aminophylline and theophylline?

Answer 15

Aminophylline is the ethylenediamine salt of theophylline and contains 80% theophylline base

Question 16

What should be assumed for IV infusion?

Answer 16

One-compartment model

Question 17

What is the equation for IV infusion of one-compartment model drugs?

Answer 17

dDb / dt = R - k*Db

Question 18

What is the infusion equation?

Answer 18

Css = R/Vd*K (1-e^-kt)

Question 19

When are loading doses commonly used?

Answer 19

Emergency situation to reach steady state faster

Question 20

What happens to Css if volume of distribution decreases?

Answer 20

• Decreased Vd means increased K

- So time to reach steady state decreases