Question

A 35-year-old epileptic woman is brought to your clinic after suffering a seizure. You decide to use continuous intravenous infusion phenytoin for treatment. The dose is administered at a rate of 100 mg/hr. The elimination rate constant is 0.086625/hr. The clearance is 20 L/hr. What is the Css? (Enter answer as an integer with a space and appropriate unit) How long it will take to reach the steady state concentration? ___ hours (Enter answer as an integer, no units) What is the volume of distribution? ___ L (Enter answer as an integer, no units) What is the loading dose needed to immediately reach the steady state level? ___ mg (Enter answer as an integer, no units)

Answer 1

Given data

Dose of phenytoin given (Q)=100mg/hr

Elimination rate constant(Ke) = 0.086625/hr

Clearance of phenytoin(Cl) = 20L/hr

We have to calculate,

Steady state concentration (Css),

Time required to reach steady state concentration,

Volume of distribution (Vd),

Loading dose required to achieve steady state concentration immediately (X)

Now, formula to calculate Css is

Q=Css×Cl

Therefore, Css = Q÷Cl

Putting values into the equation

Css = 100mg/hr ÷ 20L/hr

= 5 mg/L

We have given Ke, from Ke we have to calculate half life of drug, t(1/2)

Ke=0.693 ÷t(1/2)

t(1/2)= 0.693÷Ke

Putting values in equation,

t (1/2) = 0.693÷ 0.087

=7.966 hr

Now, to calculate time required to reach steady state concentration

Time required to reach 99% steady state concentration is 6.65 half lives of drug

Therefore, time required to reach steady state concentration is

6.65× 7.966= 52.974 hrs

Volume of distribution (Vd) = Cl÷ Ke

Vd= 20L/hr÷ 0.0866/hr

=230.95L

Loading dose required to reach steady state concentration immediately (X)is calculated by formula ,

X = Css × Vd

= 5mg/L × 230.95 L

= 1154.75 mg

To conclude answers,

Steady state concentration = 5mg/L

Time required to reach steady state concentration = 53 hrs

Volume of distribution = 231 L

Loading dose required to reach steady state concentration immediately = 1155 mg