Example : Gentamicin is eliminated completely in urine and its elimination rate is dependent on the kidney function. Calculate the s.s. maximum and minimum plasma gentamicin conc after administration of 100 mg as a constant rate iv infusion of 1 hr duration q 12 hr in a patient with normal kidney function (k = 0.25 hr-1) and in a patient with reduced kidney function (k=0.06 hr-1). (Gentamicin Vd is 14 L).
Dose = 100 mg t' = 1 hr
Ko = Dose/t' = 100 mg/hr
k = 0.25 hr -1 (normal KF)
k = 0.06 hr -1(reduced KF)
Vd = 14 L
Kidney dysfunction slows the rate of
gentamicin elimination and results in
higher steady state concentration.
In the patient with normal kidney function
Cpmax ss =
100 mg/hr (1-e-0.25hr-1 1hr)
0.25 hr -1 14L(1-e -0.25hr -1 12 hr)
= 6.7 mg/L
Cpmin ss = 6.7 mg/L e -0.25 hr -1 (12hr - 1hr) = 0.43 mg/L
In the patient with reduced kidney function
Cpmax ss =
100 mg/hr (1-e-0.06hr-1 1hr)
0.06 hr -1 14L(1-e -0.06hr -1 12 hr)
= 13.5 mg/L
Cpmin ss = 13.5 mg/L e -0.06 hr -1 (12hr - 1hr) = 7.0 mg/L
