Does the use of sodium bicarbonate buffering of lignocaine reduce the pain associated with perineal infiltration prior to repair?

When someone gives birth and needs stitches for a perineal tear (a tear to the area between the vagina and the anus), a local anaesthetic injection is given first to numb the area. This injection can itself be painful because the local anaesthetic solution is slightly acidic. This study is testing whether adding a small amount of sodium bicarbonate (a common, safe compound) to the anaesthetic solution before it is injected can reduce the sting of that injection. Participants will be randomly assigned to receive either the buffered (with bicarbonate) or the standard (with saline) solution before their stitches, and will rate the pain of the injection on a scale. Neither the participant nor the midwife or doctor giving the injection will know which solution is being used. You may be eligible if you are a woman who has had an unassisted vaginal birth at term with a second-degree perineal tear, without using an epidural or spinal block. Women who had an assisted birth (forceps or vacuum), a more severe tear (third or fourth degree), an episiotomy, or a multiple pregnancy are not eligible.