Minimizing injection pain in local anesthesia

A smaller diameter of needle and warming the injection solution can reduce pain

The choice of needle diameter can influence the pain felt by the patient. A well-designed randomized controlled trial (RCT) showed an inverse relation between needle diameter and pain.¹ A smaller needle also forces the injector to slow down, minimizing pain from volume expansion. A longer needle (e.g., 1 1/4″ v. 5/8″) is useful for injecting larger areas. In addition, warming the solution results in a less painful injection.²

Buffering lidocaine with sodium bicarbonate can minimize the typical burning sensation

With an acidic pH of 4.7, lidocaine can cause an unpleasant burning sensation. A Cochrane meta-analysis of several RCTs determined that the addition of sodium bicarbonate (10:1 lidocaine: sodium bicarbonate [8.4% NaHCO₃]) can considerably minimize pain.³ When buffering the anesthetic, it is worth noting that syringes can hold a higher volume than indicated (e.g., a 10-millilitre syringe can actually hold 11 millilitres).

The initial poke should be perpendicular to the skin, not angled

Sensory nerve endings in the skin branch out like a tree. By penetrating at an angle of 90°, the needle intersects fewer nerves (Appendix 1, available at www.cmaj.ca/lookup/suppl/doi:10.1503/cmaj.111780/-/DC1. One RCT involving more than 60 patients showed that there was significantly less pain with injections done at 90° than those done at 45°.⁴

A pause allows the anesthetic to alleviate the initial pain of the injection^*

The first 0.2–0.5 millilitres should be injected subdermally, rather than intradermally, followed by a pause. This pause allows the local anesthetic to alleviate the pain caused by the needle being in the skin. More anesthetic can then be injected slowly, before changing the needle’s angle and moving laterally. When properly done, a large wheal will be seen beneath the skin, and the patient will not have felt any pain since the first poke.⁵ It is important to ask your patients to tell you if they feel any further pain during the injection.

Keep some visible anesthetic ahead of the needle’s tip^*

Almost always, pain after the first poke is caused by the needle tip hitting nerves that have not yet been anesthetized. When moving laterally beneath the skin, it is important to keep five millimetres of palpable or visible local anesthetic ahead of the tip. This technique anesthetizes the subdermal nerve endings and should make a pain-free experience more likely for the patient.⁶

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