Assessment of needle insertion pain with flexor reflex responses in anesthetized rats

Abstract

Needle insertion pain is a serious problem for many patients, especially young patients with insulin–dependent diabetes mellitus. Although a reliable assessment method is essential for analyzing and reducing the pain caused by needle insertion, there are very few studies on the measurement of needle pain, because the mechanism of pain is complicated and its assessment is affected by the assessment methodologies. In this study, we assessed flexor reflex responses evoked by insertion of needles into the hind paws of anesthetized rats, and investigated the factors that affect needle pain. We studied the process of needle insertion into the skin using a high–speed microscope and load cell along with a constant insertion speed–controlled device. Needles of two sizes, 27 gauge (G) and 31G (outer diameter: 0.40 and 0.25 mm, respectively), were inserted into the hind paws of anesthetized rats at two velocities. The resultant action poten- tials evoked by the flexor reflex response in the semitendinosus muscles were recorded by electromyography (EMG). The EMG magnitude evoked by 27G needle insertion was greater than that with the 31G needle, irrespective of insertion speed. Penetration force, which increased as the needle penetrated the plantar skin, might be indirectly related to EMG magnitude. Our method demonstrates that thicker–diameter needles evoke more intense flexor reflex responses, as assessed by EMG in anesthetized rats. This is the first study to assess needle insertion pain in animals, and this method could advance studies on reducing needle insertion pain in humans.