Objective To determine if the administration of atropine would reduce the measured minimum anaesthetic con-centration of isoflurane (MACisoflurane) in freshwater turtles -the yellow-bellied slider (Trachemys scripta scripta).

Study design Paired, blinded, randomized, prospective studies of 1) the effect of atropine in isoflurane anaes-thetized freshwater turtles (T. scripta scripta) and 2) the ef-fect of atropine in yellow-bellied sliders in which anaesthesia was induced with propofol and maintained with isoflurane.

Animals T. scripta scripta (n = 8), female, adult.

Methods Atropine (2 mg kg(-1)) or an isovolumetric control injection of saline was administered intraperitoneally 15 minutes prior to induction of anaesthesia with isoflurane. Individual MACisoflurane was then determined by end-tidal gas analysis in a bracketing design by an experimenter blinded to the administered drug, with a 2 week washout period. The experiment was repeated, with atropine (2 mg kg(-1)) or saline administered intravascularly in combination with propofol for anaesthetic induction. Linear mixed modelling was used to determine the effects of atropine and propofol on the individual MACisoflurane. Data are presented as mean +/- standard deviation.

Results Premedication with atropine significantly reduced MACisoflurane (p = 0.0039). In isoflurane-induced T. scripta scripta, MACisoflurane decreased from 4.2 +/- 0.4% to 3.3 +/- 0.8% when atropine had been administered. Propofol as an induction agent had a MAC-sparing effect (p < 0.001) such that MACisoflurane following propofol and a control injection of saline was 2.3 +/- 1.0%, which decreased further to 1.5 +/- 0.8% when propofol was combined with atropine.

Conclusions and clinical relevance Atropine, presumably by inhibiting parasympathetically mediated pulmonary ar-tery constriction, decreases right-to-left cardiac shunting and the MACisoflurane in yellow-bellied sliders, and thereby may facilitate control of inhalant anaesthesia. Propofol can be used for induction of anaesthesia and reduces the required concentration of inhaled anaesthesia assessed 1.5 hours following induction.