Minimum end-tidal sevoflurane concentration necessary to prevent movement during a constant rate infusion of morphine, or morphine plus dexmedetomidine in ponies

Objective To compare the effects of a constant rate infusion (CRI) of dexmedetomidine and morphine to those of morphine alone on the minimum end-tidal sevoflurane concentration necessary to prevent movement (MAC(NM)) in ponies. Study design Prospective, randomized, crossover, 'blinded', experimental study. Animals Five healthy adult gelding ponies were anaesthetized twice with a 3-week washout period. Methods After induction of anaesthesia with sevoflurane in oxygen (via nasotracheal tube), the ponies were positioned on a surgical table (T0), and anaesthesia was maintained with sevoflurane (Fe ' SEVO 2.5%) in 55% oxygen. Monitoring included pulse oximetry, electrocardiography and measurement of anaesthetic gases, arterial blood pressure and body temperature. The ponies were mechanically ventilated and randomly allocated to receive IV treatment M [morphine 0.15mgkg(-1) (T10-T15) followed by a CRI (0.1mgkg(-1)hour(-1))] or treatment DM [dexmedetomidine 3.5 mu gkg(-1) plus morphine 0.15mgkg(-1) (T10-T15) followed by a CRI of dexmedetomidine 1.75 mu gkg(-1)hour(-1) and morphine 0.1mgkg(-1)hour(-1)]. At T60, a stepwise MAC(NM) determination was initiated using constant current electrical stimuli at the skin of the lateral pastern region. Triplicate MAC(NM) estimations were obtained and then averaged in each pony. Wilcoxon signed-rank test was used to detect differences in MAC between treatments (alpha=0.05). Results Sevoflurane-morphine MAC(NM) values (median (range) and mean +/- SD) were 2.56 (2.01-4.07) and 2.79 +/- 0.73%. The addition of a continuous infusion of dexmedetomidine significantly reduced sevoflurane MAC(NM) values to 0.89 (0.62-1.05) and 0.89 +/- 0.22% (mean MAC(NM) reduction 67 +/- 11%). Conclusion and clinical relevance Co-administration of dexmedetomidine and morphine CRIs significantly reduced the MAC(NM) of sevoflurane compared with a CRI of morphine alone at the reported doses.