[A simple technique for estimating the level of analgesia in regional anesthesia]

In regional anesthesia the onset of analgesia is usually determined by stimulating the skin with sharp or cold objects: when sensations of sharp pain or cold are lost, all nociceptive afferents are regarded as blocked. Sharp pain and cold are mediated by thin, myelinated axons whereas the majority of nociceptor axons are unmyelinated. In peripheral nerve blocks unmyelinated fibers are blocked first, followed by those mediating sharp pain and cold. In spinal and epidural blocks the levels of anesthesia to sharp pain and cold correspond within 1-2 segments. Although pinprick seems to be a simple test for analgesia, it involves the risk of infection and is disliked by the patient. As the stimulus is spatially discontinuous, coarse testing may simulate analgesia. An ideal stimulus for testing analgesia should be noninvasive, give distinct sensations, not frighten the patient, and allow spatially continuous examination of larger skin areas. A stimulus that meets these conditions is cold applied to the skin by a metal roller (Fig. 1). If the roller is kept at room temperature (20 degrees-24 degrees C), it gives a strong cold sensation when it is slowly rolled (5-10 cm/s) over the warm skin (usually 30 degrees-35 degrees C on the trunk). With this noninvasive device, the levels of anesthesia to cold can be determined rapidly, with high precision, and without frightening the patient.