Effects of altering the sequence of midazolam and propofol during co-induction of anaesthesia

Objective To assess the effects of varying the sequence of midazolam and propofol administration on the quality of induction, cardiorespiratory parameters and propofol requirements in dogs. Study design Randomized, controlled, clinical study. Animals Thirty-three client owned dogs (ASA I-III, 0.5-10years, 5-30kg). Methods Dogs were premedicated with acepromazine (0.02mg kg-1) and morphine (0.4mg kg-1) intramuscularly. After 30minutes, group midazolam-propofol (MP) received midazolam (0.25mg kg-1) intravenously (IV) before propofol (1mg kg-1) IV, group propofol-midazolam (PM) received propofol before midazolam IV at the same doses, and control group (CP) received saline IV, instead of midazolam, before propofol. Supplementary boluses of propofol (0.5mg kg-1) were administered to effect to all groups until orotracheal intubation was completed. Behaviour after midazolam administration, quality of sedation and induction, and ease of intubation were scored. Heart rate (HR), respiratory rate, and systolic arterial blood pressure were recorded before premedication, post-premedication, after midazolam or saline administration, and at 0, 2, 5, and 10minutes post-intubation. End-tidal CO2 and arterial oxygen haemoglobin saturation were recorded at 2, 5 and 10minutes post-intubation. Results Quality of sedation and induction, and ease of intubation were similar in all groups. Incidence of excitement was higher in the MP compared to CP (p=0.014) and PM (p=0.026) groups. Propofol requirements were decreased in MP and PM groups with respect to CP (p<0.001), and in PM compared to MP (p=0.022). The HR decreased after premedication in all groups, and increased after midazolam and subsequent times in MP (p=0.019) and PM (p=0.001) groups. Incidence of apnoea and paddling was higher in CP (p=0.005) and MP (p=0.031) groups than in PM. Conclusions and clinical relevance Administration of midazolam before propofol reduced propofol requirements although caused mild excitement in some dogs. Administration of propofol before midazolam resulted in less excitatory phenomena and greater reduction of propofol requirements.