Pharmacokinetics and applications of ampicillin sodium as an intravenous infusion in the horse.

A regime for administration of ampicillin sodium by continuous intravenous infusions to horses was designed. The aim was to achieve plasma ampicillin concentrations between 5 and 10 micrograms/ml over a 4-h period. A 2 mg/kg bodyweight loading dose of ampicillin sodium was administered intravenously at the beginning of the infusion in order to achieve steady-state plasma concentrations rapidly. The infusion system subsequently administered ampicillin at a rate of approximately 19.2 micrograms/min/kg bodyweight. The plasma concentrations obtained over the infusion period correlated very well with predicted calculations based on pharmacokinetic parameters. A mean +/- SEM steady-state plasma concentration (Cpss) of 5.94 +/- 0.33 was obtained and ampicillin was shown to have an apparent steady-state volume of distribution (Vdss) of 175.43 +/- 13.63 ml/kg. When the pump was disconnected the concentrations declined over the following 4 h in an exponential way with an elimination half-life (t1/2 beta) of 0.62 h. In addition, three different infusion dose rates (13.78, 19.34 and 24.48 micrograms/min/kg) were administered to a single animal showing that a good correlation (correlation coefficient > 0.99) existed between the dose administered the steady-state plasma concentrations and the corresponding areas under the plasma concentration versus time curve.