Formulae Flashcards
How do you calculate maintenance dose?
maintenance dose = (clearance x desired concentration) divided by volume of distribution
How do you calculate loading dose?
LD = desired concentration x volume of distribution
what is the unit of clearance?
Volume per unit time
Total clearance is the sum of what?
hepatic + renal + other losses
Clearance in L/min equals what?
(Volume of distribution x elimination constant which is 0.693)/ half life
What does it mean when fraction excreted unchanged (FU) = 1?
Totally renally cleared
What does it mean when fraction excreted unchanged (FU) = 0?
No renal clearance
How to dose adjust in renal impairment?
How much of the renal function has changed, and how much of the excretion is due to renal function.
Examples of zero order kinetic drugs
PHENYTOIN (need careful small increments when changing dose) Alcohol Theophylline Aspirin Perhexilline in CYP2D6 poor metabolisers
If fraction excreted unchanged is one, how do you calculate the dose of your drug?
Dosing regimen patient/dosing regimen normal = CrCl patient/CrCl normal
CrCL normal is 1.5 ml/sec
If fraction excreted unchanged is less than one, how do you calculate the dose of your drug?
Dosing regimen patient/dosing regimen normal = (1-Fu) + fu (Cr Cl patient/CrCl normal)
Does Vd go up or down in the elderly?
Down
Reduced lean body mass, reduced water, increased fat
plasma concentration at steady state equals
(BIOAVAILABILTY X DOSE)/DOSING INTERVAL
elimination rate
elimination rate = clearance x plasma concentration
bioavailability equals
fraction absorbed x fraction escaping first pass metabolism
hepatic extraction ratio
hepatic clearance/hepatic blood flow
IV and oral dosing: estimating
IV: desired concentration = maintenance dose rate in mg/hr /clearance
Oral: same but x bioavailability
oral dose rate
is IV dose rate/bioavailability
effect of doublind the dose?
increase duration of action by one half life
What does level of enzyme activity have to do with hepatic clearance
if very high enzyme activity, then clearance is limited by blood flow
if low enzyme activity, then clearance is not limited by blood flow, but but
AUC equals
dose/clearance
or if oral AUC then (bioavail x dose)/clearance
