Why do lidocaine injections hurt?

I believe that pain is a subjective experience of a noxious stimulus. If you penetrate the skin on the back of the hand one-third of the distance from the end of the the ring finger (a common place for venous cannulation) in one hundred people, some will experience this as a very painful event while others report little or no discomfort.

Recognizing that some people will experience more pain than others, how can we minimize the intensity of the stimulus?

Use a small needle to inject local anesthetic prior to incision or venous cannulation. A 1/2 inch 30 g needle works very well for numbing the skin prior to insertion of a venous cannula.

Lidocaine is a weak base. That means that a solution of lidocaine in water will be acidic. Commercial preparations of lidocaine are going to have twenty-five to one thousand times as much acid as human tissue. It is possible to reduce the amount of acid in the lidocaine by adding a solution of sodium bicarbonate to the vial of lidocaine prior to use. Adjusting the acidity of lidocaine has been shown to reduce the pain people experience with injection. Below find inks to two studies that addressed the issue of pain with lidocaine injection and how to minimize it.