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The Use of Anatomical Position for Regional Block Description

To the Editor: We read with great interest the recent publication by Duma et al on nitrous oxide analgesic effect. In this post hoc analysis, they compared the early postsurgical opioid consumption and pain in patients receivingN2O (n = 353) with those who did not receive N2O (n = 89) during general anesthesia. They found no difference between the 2 groups either in pain intensity or equianalgesic morphine dosage throughout the postoperative postanesthesia care unit period. Although this finding might contradict certain laboratory works on Fischer rats, the lack of nitrous oxide– induced preemptive analgesic efficacy correlates well with our earlier studies in Sprague-Dawley rats receiving formalin injection. The formalin test consists of an early nociceptive phase (0–5 minutes) and the late spinal sensitization phase (10–60minutes). We administered pure oxygen (negative control), 75% nitrous oxide (0.5 MAC), 2% halothane (1 MAC), and 75% nitrous oxide with 2% halothane (1.5 MAC) during the early phase. Regardless of the gas in use, we found no difference in behavioral responses during the late phase (20–60 minutes) and the extent of c-fos expression in the spinal cord dorsal horn. In a subsequent study, rats were placed in an anesthetic chamber maintained with equianesthetic concentration (0.5 MAC) of N2O (75%) and halothane (0.5%) throughout the early and late phases. N2O and halothane increased the thermal nociceptive threshold, suppressed licking/biting behavior in both early and late phases of the formalin test but failed to suppress c-fos expression. Unlike fentanyl, 75% N2O was unable to suppress the spinal sensitization despite the apparent attenuation in behavioral hyperalgesic responses. Given these findings, we suggest that subanesthetic concentrations of inhalation anesthetic agents, both N2O and halothane, suppress the early and late phase behavioral response because of anesthetic but not analgesic effects.