Example : A patient is receiving 120 mg gentamicin q 12 hr given as a constant rate iv infusion over 1 hr. At steady state the plasma conc before drug administration was 1.73 mg/L and the plasma conc 1 hr after the end of drug administration was 8.1 mg/L. Calculate gentamicin t1/2 and Vd in this patient.
t 1/2
slope = -k/2.303
The conc measured before drug
administration should be equal to the
conc 11 hr after administration of the
following dose (true only at ).
The time scale starts at the end of drug
administration. So, the samples are
plotted at 1 hr and 11 hr.
Graphically the half life = 4.5 hr
k = 0.693/4.5 hr = 0.154 hr-1
Graphically Cpmax ss = 9.4 mg/L
9.4 mg/L =
120 mg/hr( 1-e-0.154 hr -1 1hr)
0.154 hr -1 Vd (1-e-0.154 hr -1 12hr)
Vd = 14 L
(time difference between samples = 10 hr)
1.74 mg/L = 8.1 mg/L e-k 10 hr
k = 0.154 hr-1 Half life = 4.5 hr
8.1 mg/L = Cpmax ss e-0.154 hr-1 1 hr
Cpmax = 9.4 mg/L
9.4 mg/L =
120 mg/hr( 1-e-0.154 hr -1 1hr)
0.154 hr -1 Vd (1-e-0.154 hr -1 12hr)
Vd = 14 L
