Reasons for performing study: Foal responses to a broader range of plasma fentanyl concentrations than currently reported are desirable to support (or not) clinical use. Objectives: To describe fentanyl plasma concentrations following an escalating i.v. fentanyl dosing schedule in foals aged 5-13 days and describe selected, associated dose- and time-related behavioural and physiological responses to plasma fentanyl concentration. Study design: Experimental. Methods: Fentanyl was administered i.v. in an escalating fashion (2, 4, 8, 16 and 32μg/kg bwt) at 10-min intervals. Blood samples were collected before and at selected times until 24h post administration. Blood samples were analysed for fentanyl and metabolite concentrations and correlated with behavioural and physiological observations and selected blood analytes. Results: Foals mostly appeared to be unaffected following 2μg/kg bwt (1.09 ± 0.41μg/l; average maximal plasma concentration) of fentanyl, but 6 of the 8 foals appeared to be sedated following 4μg/kg bwt (3.07 ± 1.11μg/l). Ataxia with increased locomotor activity, muscle rigidity and head pressing posture was observed in many foals at 8 (7.24 ± 3.22μg/l) and 16μg/kg bwt (17.4 ± 5.67μg/l). All foals were heavily sedated after 32μg/kg bwt (34.5 ± 10.3μg/l); 3 of the 8 foals became recumbent. The average (±s.d.) terminal half-life following administration of the final dose was 44.2 ± 9.85min. Conclusions: Behavioural and physiological responses to i.v. fentanyl in young foals are dose related. As with mature horses, the window of fentanyl plasma concentrations related to possible clinically desirable actions appears relatively narrow.
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