Pharmacology Flashcards
Volume of Distribution
Vd = Amount of Drug / Concentration of Drug
Clearance
Clearance = Rate of elimination / Concentration of Drug
Clearance is additive
Ke x Vd
Total body water
.6 L/Kg
Extracellular Water
.2 L/Kg
Blood
.08 L/Kg
Plasma
.04 L/Kg
Fat
.2-.35 L/Kg
Bone
.07 L/Kg
Three clinically active saturable drugs
Aspirin, Phenytoin, Ethanol
0th order rate of elimination
RoE = (Vmax X C) / (Km+C)
At concentrations much higher than Km the bottom part of the equation is negligible and the concentration continuously decreases near Vmax
Half Life
T1/2 = (.693 X Vd) / Clearance
Accumulation Factor
AF = 1 / Fraction lost in one dosing cycle
IV administration Characteristics
100 percent bioavailable
Most rapid onset
Intramuscular administration characteristics
> 75% bioavailability
Large volumes feasible
May be painful
Subcutaneous administration characteristics
> 75% bioavailability
Smaller volumes than IM
May be painful
Oral (PO) administration characteristics
> 5% bioavailability
Most convenient
First pass effect can be significant
Rectal (PR) administration characteristics
> 30% bioavailability
Less first pass effect than oral
Inhalation administration characteristics
> 5% bioavailability
Often very rapid onset
Transdermal administration characteristics
> 80% bioavailability
Very slow absorption
No first pass effect
Prolonged duration
Extraction Ratio (First pass effect)
ER = CL (liver) / Q (hepatic bloodflow, 90L/hr in 90kg)
Bioavailability of Drug
F = absorption X (1-ER)
Dosing rate
Dosing rate = CL X TC
When bioavailability is less than 1 you must multiply dosing rate by the percentage bioavailability for that route
Maintenance Dose
MD = Dosing Rate X Dosing Interval
Loading Dose
LD = Vd X TC
