The analgesic effect of a vapocoolant stream spray in reducing heat nociception on the glabrous skin of rat pups.

BACKGROUND Blood sampling is a common screening and diagnostic test in newborn infants in the neonatal intensive care unit, and heel lancing accounts for two-thirds of these tests. Heel lancing causes acute pain and distress, and most infants rarely receive analgesics because of fear of respiratory depression from opioids and lack of effectiveness of topical local anesthetics on the glabrous skin. To circumvent this latter problem, we investigated the analgesic efficacy and safety of a topical vapocoolant spray. METHODS Forty Sprague-Dawley rat pups aged 7 days old were randomly assigned to receive either vapocoolant or saline spray on the plantar hindpaws for 5 to 6 seconds. Forty-five seconds later, the paws were subjected to a modified hotplate test to quantify the nociceptive flexor withdrawal (NFW) thresholds before and after treatment with the sprays. Seven days later, the animals were euthanized and the hindpaws were examined histologically. A nested analysis of variance approach was used to account for the triplicate measurements per animal. A 2-tailed P < 0.05 was considered significant. RESULTS At baseline, there were no differences in the NFW thresholds between the 2 groups (P = 0.22). After treatment, these thresholds were significantly lower in both vapocoolant (P < 0.001) and saline (P = 0.008) groups relative to baseline values. The vapocoolant group demonstrated a significantly longer NFW latency time compared to the saline group (P < 0.001). All specimens in both groups were examined and showed normal skin histology. CONCLUSIONS Vapocoolant spray treatment of the glabrous skin is effective and safe after a single treatment.