Small-dose ketamine reduces the pain of propofol injection

I.v. injection of propofol during anesthetic induction induces pain. Ketamine has been shown to reduce injection pain. In this study, we established the optimal dose of ketamine to prevent the pain of injection with propofol.

Two hundred forty patients presenting for elective surgery were randomly allocated into eight groups; five groups during the first part of the study and three groups during the second part. In Part 1, patients received saline (Group S), lidocaine (Group L), ketamine 10 microg/kg (Group K10), 50 microg/kg (Group K50), or 100 microg/kg (Group K100), respectively, immediately followed by propofol 2.5 mg/kg. In Part 2, the optimal dose of ketamine (100 microg/kg) was administered 3 min before propofol (Group Pre), mixed with propofol solution (Group KP), or after oral midazolam premedication (Group M). An anesthesiologist blinded to the study group monitored each patient's pain score at 5-s intervals.

In Part 1, the incidence and intensity of pain were the lowest in the K100 and L groups (P < 0.001). In Part 2, the patients in the K100 and M groups had significantly lower pain scores compared with the KP and Pre groups (P < 0.05). During induction, there were no significant intergroup differences in mean arterial blood pressure and heart rate in all groups.

Administration of ketamine 100 microg/kg immediately before propofol injection provided the optimal dose and timing to reduce propofol-induced pain on injection.