Example : A patient is receiving 120 mg gentamicin q 12 hr given as a constant rate iv infusion over 1 hr. Four plasma samples were obtained around the second dose:
| Time (hr) |
Conc (mg/L) |
| before drug administration |
1.46 |
| 1 hr after the end of administration |
7.9 |
| 3 hr after the end of administration |
5.8 |
| 5 hr after the end of administration |
4.3 |
Calculate gentamicin half life, and Vd in this patient.
t 1/2
slope = -k/2.303
The samples obtained after drug
administration can be used to determine
the and the drug conc after the
end of drug administration graphically.
The time scale starts at the end of drug
administration. So, the samples are
plotted at 1 hr, 3 hr, and 5 hr.
Graphically the half life = 4.5 hr
k = 0.693/4.5 hr = 0.154 hr-1
9.2 mg/L =
120 mg/hr
0.154 hr -1 Vd
= 1-e-0.154 hr -1 1hr + 1.46 mg/L e-0.154 hr -1 1hr
Vd = 14 L
