Week 3C IV infusion

Question 1

IV infusion

Answer 1

a single, bolus dose –> unsuitable to maintain constant plasma concentrations and pharmacodynamic effect
• to maintain a constant concentration, drugs may often be given at a constant rate/ zero order –> IV drip/pump
• some drugs are unsafe to give by rapid injection and require slow infusion

Question 2

if elimination follows first-order PK, as the drug is infused at a constant rate,

Answer 2

the amount of drug in the body increases until the rate of elimination equals the rate of administration
rate of drug elimination = k x X

Question 3

as the drug is infused, the value for time (t) increases,

steady state during continuous infusion?

Answer 3

Css = k0/k x V = k0/CL
the steady state conc during a continuous infusion is directly proportional to the infusion rate and inversely proportional to the clearance
-if you double the clearance it will half the steady state conc

Question 4

Factors Affecting Steady State Plasma Concentration

infusion rate

Answer 4

– steady-state plasma concentration is directly proportional to the infusion rate
• time to reach steady state is not affected by infusion rate (Dose).

Question 5

Factors Affecting Steady State Plasma Concentration

volume of distribution

Answer 5

• V does not affect the steady-state plasma concentration, but does alter time to reach steady-state through alteration of k (and t1⁄2). The bigger the volume of distribution, the greater the half life

Question 6

Factors Affecting Steady State Plasma Concentration

clearance

Answer 6

• steady-state plasma concentration is inversely proportional to clearance
• time and conc are effected
• lowest clearance –> highest drug conc

Question 7

•thus time to steady state is

controlled by drug elimination

Answer 7

not infusion process

however, the value of Css IS controlled by k0

Question 8

• Theophylline is used in the treatment of asthma
and for sleep apnea. What is the steady state
concentration during a constant rate infusion of
35 mg/hr? How long will it take to achieve 90%
of steady state? (Vd = 40L, k = 0.10 h-1)

Answer 8

Css = k0/k x V
35/0.10 x 40 = 8.8 mg/L

C/css= 0.9 = (1-e^-0.1t)
t= ln (1-0.9)/-0.1 = 23

Question 9

if you wait 4 half lives

Answer 9

you get to 94% of the steady state concentration

Question 10

A steady-state plasma theophylline
concentration of 15 mg/L is desired in a patient.
If the theophylline clearance is 4 L/h in this
patient, what is the required infusion rate?
• Answer:

Answer 10

Css = k0/CL
k0 = Css x CL
= 15 x 4 = 60 mg/h

Question 11

• We know from a previous case that an infusion rate of 60 mg/h of theophylline resulted in a Css of 15 mg/L. If a Css of 10 mg/L is needed, what is the required infusion rate?

Answer 11

Css,1 x k0,2/k0,1

60 x 10/15 = 40 mg/h

Question 12

If you want to change the rate of infusion of a drug because it is toxic or not working,

Answer 12

it depends on half life t1/2 of the drug