Chronic Dosing: IV infusion Flashcards
(13 cards)
Negatives of continuous IV
must be available as IV drug
limited to institutions normally
Positives of continuous IV
can keep C at steady state (no fluctuations)
easy to change dose
What happens when concentration in body reaches steady state
Drug in body rises in early stages (Ro> k*A) until reach plateau
— amount eliminated is little because not much in the body
drug in (Ro) = drug out (k *A) OR Css x Cl
- input of drug is zero order but elimination in 1st order
What is the only parameter than determines Css
Cl + Ro
—- Vd doesn’t impact it
How long does it take to reach Css
3-5 t1/2
What happens when duration of infusion is equal to t1/2
Css * 0.5
—- C will by 50% of Css
What if duration of infusion is 2 t1/2
1 t1/2 —- 50%
2 t1/2 - 75% of Css (aka increase by half
3 t1/2 - 87.5%
T or F: you reach Css at 3-5 t1/2
technically you never actually reach SS (just approach it)
time above 3.3 t1/2 —- basically SS (90%)
T or F: if you have a longer t1/2, it will take longer to get to Css
T
— time to get to Css depends on k/ t1/2
—- anything that changes t1/2 — changes time to Css
——- includes Cl and V (changes in these, impact)
Impact of increase in Cl
increase in Cl —- k increases
—- t1/2 decreases
reach C22 faster (plateau earlier ) + Css decreases
Increase in V
increase in V —- decrease in k , increase t1/2
—— takes longer to get to Css
— no change on Css itself
PK parameters needed
Cl
k: can get when look at concentrations in body once stop infusions (not realistic)
—- another way: if know Css + C prior to Css can use this OR Cl/V
What if drug in body before start IV
must take it into account
— as time increases, initial C contributions decrease bc eliminated + IV contributions increase
** doesn’t change Css itself
