Introduction

Propofol is widely used for induction of anaesthesia in dogs. A dose rate of 6.5 mg kg-1 is recommended for dogs without pre-anaesthetic medication. The aim of this study was to investigate whether the dose rate of propofol required for anaesthesia induction and tracheal intubation varies with the body weight of the dog.

Materials and Methods

Forty-one client-owned dogs scheduled for magnetic resonance imaging (ASA grades I and II) were included in the study. These dogs were the control group of another clinical study investigating the effect of intravenous lidocaine on responses to tracheal intubation in propofol-anaesthetised dogs. The dogs were weighed on admittance to the clinic. Following pre-oxygenation, anaesthesia was induced by intravenous injection of up to 6.5 mg kg-1 propofol administered to effect. Propofol injection was stopped when the dog reached an appropriate plane of anaesthesia, based on assessment of clinical signs. One minute after cessation of propofol injection and if the tongue could be protracted without resistance, the trachea was intubated. If necessary, additional increments of 1 mg kg-1 propofol were injected every 15 seconds to achieve this. The total dose of propofol was recorded.

Statistical Analysis

Linear regression was used to examine the relationship between body weight and dose rate of propofol. Results are reported as mean (standard deviation) with the minimum and maximum values.

Results

Forty-one dogs were enrolled in the study with a mean body weight of 25.4 kg (13.63) and a range of body weights from 6.2 to 68 kg. The mean dose rate of propofol administered was 6.2 (0.79) mg kg-1 with a range from 3.4 to 7.4 mg kg-1. Larger dogs required less propofol per kg of body weight than smaller dogs. For every 1 kg increase in body weight the propofol dose rate decreased by 0.046 mg kg-1 (95% CI: 0.035 to 0.058, P<0.0001, R2=0.63), with dogs weighing 20 kg requiring 6.5 mg kg-1 propofol for tracheal intubation. Examination of the residuals of the linear regression model suggested that the model fitted the data. The coefficient of determination (R2) indicated that 63% of the variation in the propofol dose rate was likely to be explained by its relationship with body weight.

Conclusions

In dogs the dose rate of propofol required for induction of anaesthesia and tracheal intubation varies inversely with bodyweight.