Effect of predosing versus slow administration of propofol on the dose required for anaesthetic induction and on physiologic variables in healthy dogs.

Objective: To investigate the effects of the timing of propofol administration on the dose required for induction of anaesthesia and commonly measured physiological effects.

Study Design: Randomized, investigator-blinded clinical study.

Animals: A group of 32 healthy dogs aged 6-144 months and weighing 3.5-47.2 kg.

Methods: Premedication was intramuscular acepromazine (0.025 mg kg) and methadone (0.25 mg kg). After 30 minutes, one of three treatments was administered to the dogs: propofol (0.5 mg kg; group PP), an equivalent volume of saline (group CP) or a propofol infusion (1.3 mg kg minute; group SI). Two minutes later, a propofol infusion (4 mg kg minute) was started in PP and CP, whereas the propofol infusion was continued in SI. At this stage an investigator, blinded to the group assignments, entered the room and decided when each animal was ready for intubation and stopped the propofol infusion. After intubation, management of anaesthesia was standardized. Pulse rate (PR), respiratory rate (f) and mean arterial pressure (MAP) were recorded before induction, 2 minutes later and 0, 2 and 5 minutes after intubation. Apnoea >30 seconds was recorded and managed. Sedation, quality of induction and endotracheal intubation were scored using simple descriptive scales. Data are presented as mean±standard deviation.

Results: Propofol dose requirement was lower in SI (3.5±1.2 mg kg) compared with PP and CP (5.0±0.9 and 4.8±0.6 mg kg; p=0.002 and 0.012), respectively. No statistically significant differences were found among groups for PR, f, MAP or incidence of apnoea. Sedation score and quality of induction were similar among groups.

Conclusions: Slow administration of propofol reduced the anaesthetic induction dose required compared with predosing and control groups. Effects on PR, f, MAP and apnoea were similar among groups.

Clinical Relevance: Slower injection of propofol reduces the dose required for induction of anaesthesia.