Warming reduces pain, even when the anesthetic is buffered.
Injection of local anesthetics causes pain at the injection site before providing anesthesia. Postulated methods to mitigate this pain include slowing the rate of injection; avoiding epinephrine, when possible; buffering; and warming the local anesthetic to body temperature prior to injection. These authors assessed injection pain in a meta-analysis of 18 randomized studies involving 831 patients who received either warmed (body temperature) or unwarmed (room temperature) subcutaneous or intradermal local anesthetic injections. The anesthetic was unbuffered in ten studies.
Patients reported pain on either visual analog or numeric rating scales. Methods of warming included waterbaths, incubators, fluid warmers, baby food warmers, warming trays, and syringe warmers.
Patients reported less pain with warmed anesthetic than with room-temperature anesthetic (mean difference, 11 mm on a 100-mm scale). Even with buffered anesthetic, patients reported less pain with warming (mean difference, 7 mm).
Hogan ME et al. Systematic review and meta-analysis of the effect of warming local anesthetics on injection pain. Ann Emerg Med 2011 Jul; 58:86.