An abstract was presented at the 2012 Postgraduate Assembly in Anesthesiology that measured the effects of giving lidocaine and propofol separately versus mixing the two in one syringe to lessen the pain most patients feel with injecting the drug. Presented by a group from the Robert Wood Johnson University Hospital, the study found that injecting lidocaine first before propofol was not as effective as mixing 60 mg of lidocaine into a syringe with 200 mg propofol. In fact, the mixture resulted in pain that was half of that experienced by the separate injections group.
The researchers theorized that the lidocaine may act as a stabilizer of the kinin cascade, resulting in less pain. Another possibility is that the lidocaine decreased the pH of the propofol, causing less burning.
I had previously written about this scourge of an otherwise wondrous drug. I still believe that it is the rate of injection of the propofol that determines the severity of the pain. When I push 30 mg of lidocaine followed by 30 mg of propofol then wait about 30 seconds before giving more, almost all patients feel no pain. After that I continue to give small boluses every few seconds until the patient falls asleep. Usually by then the patient has already become amnestic and giving a bigger bolus for a faster induction will not be remembered by the patient at that point. Again, if you're trying to achieve a rapid sequence induction, to hell with the patient feeling pain. You're trying to save his life, and that takes precedence over any coddling of the patient's feelings about the propofol burn.