Total intravenous anesthesia with Ketofol in rabbits: a comparison of the effects of constant rate infusion of midazolam, fentanyl or dexmedetomidine

Background When inhalant anesthetic equipment is not available or during upper airway surgery, intravenous infusion of one or more drugs are commonly used to induce and/or maintain general anesthesia. Total intravenous anesthesia (TIVA) does not require endotracheal intubation, which may be more difficult to achieve in rabbits. A range of different injectable drug combinations have been used as continuous infusion rate in animals. Recently, a combination of ketamine and propofol (ketofol) has been used for TIVA in both human patients and animals. The purpose of this prospective, blinded, randomized, crossover study was to evaluate anesthetic and cardiopulmonary effects of ketofol total intravenous anesthesia (TIVA) in combination with constant rate infusion (CRI) of midazolam, fentanyl or dexmedetomidine in eight New Zealand White rabbits. Following IV induction with ketofol and endotracheal intubation, anesthesia was maintained with ketofol infusion in combination with CRIs of midazolam (loading dose [LD]: 0.3 mg/kg; CRI: 0.3 mg/kg/hr; KPM), fentanyl (LD: 6 mu g/kg; CRI: 6 mu g/kg/hr; KPF) or dexmedetomidine (LD: 3 mu g/kg; CRI: 3 mu g/kg/hr; KPD). Rabbits in the control treatment (KPS) were administered the same volume of saline for LD and CRI. Ketofol infusion rate (initially 0.6 mg kg(-1) minute(-1) [0.3 mg kg(-1) minute(-1) of each drug]) was adjusted to suppress the pedal withdrawal reflex. Ketofol dose and physiologic variables were recorded every 5 min. Results Ketofol induction doses were 14.9 +/- 1.8 (KPM), 15.0 +/- 1.9 (KPF), 15.5 +/- 2.4 (KPD) and 14.7 +/- 3.4 (KPS) mg kg(-1) and did not differ among treatments (p > 0.05). Ketofol infusion rate decreased significantly in rabbits in treatments KPM and KPD as compared with saline. Ketofol maintenance dose in rabbits in treatments KPM (1.0 +/- 0.1 mg/kg/min) and KPD (1.0 +/- 0.1 mg/kg/min) was significantly lower as compared to KPS (1.3 +/- 0.1 mg/kg/min) treatment (p < 0.05). Ketofol maintenance dose did not differ significantly between treatments KPF (1.1 +/- 0.3 mg/kg/min) and KPS (1.3 +/- 0.1 mg/kg/min). Cardiovascular variables remained at clinically acceptable values but ketofol infusion in combination with fentanyl CRI was associated with severe respiratory depression. Conclusions At the studied doses, CRIs of midazolam and dexmedetomidine, but not fentanyl, produced ketofol-sparing effect in rabbits. Mechanical ventilation should be considered during ketofol anesthesia, particularly when fentanyl CRI is used.