RESUMEN
This study describes the pharmacokinetics of intravenously administered (i.v.) fentanyl citrate, and its primary metabolite norfentanyl in Holstein calves. Eight calves (58.6 ± 2.2 kg), aged 3-4 weeks, were administered fentanyl citrate at a single dose of 5.0 µg/kg i.v. Blood samples were collected from 0 to 24 hr. Plasma (nor)fentanyl concentrations were determined using liquid chromatography with mass spectrometry and a lower limit of quantification (LLOQ) of 0.03 ng/ml. To explore the effect of analytical performance on fentanyl parameter estimation, the noncompartmental pharmacokinetic analysis was then repeated with a hypothetical LLOQ value of 0.05 ng/ml. Terminal elimination half-life was estimated at 12.7 and 3.6 hr for fentanyl and norfentanyl, respectively. For fentanyl, systemic clearance was estimated at 2.0 L hr-1 kg-1 , volume of distribution at steady-state was 24.8 L/kg and extraction ratio was 0.42. At a hypothetical LLOQ of 0.05 ng/ml fentanyl half-life, volume of distribution at steady-state and clearance were, respectively, of 3.0 hr, 8.8 L/kg and 3.4 L kg-1 hr-1 . Fentanyl citrate administered i.v. at 5.0 µg/kg can reach levels associated with analgesia in other species. Pharmacokinetic parameters should be interpreted with respect to LLOQ, as lower limits can influence estimated parameters, such as elimination half-life or systemic clearance and have significant impact on dosage regimen selection in clinical practice.