THE THRESHOLD FOR THERMOREGULATORY VASOCONSTRICTION DURING NITROUS-OXIDE ISOFLURANE ANESTHESIA IS LOWER IN ELDERLY THAN IN YOUNG-PATIENTS

Background: Thermoregulatory vasoconstriction minimizes further core h ypothermia during anesthesia. Elderly patients become more hypothermic during surgery than do younger patients, and take longer to rewarm po stoperatively. These data indicate that perianesthetic thermoregulator y responses may be especially impaired in the elderly. Accordingly, th e authors tested the hypothesis that the thermoregulatory threshold fo r vasoconstriction during nitrous oxide/isoflurane anesthesia is reduc ed more in elderly than in young patients. Methods. The authors studie d 12 young patients aged 30-50 yr and 12 elderly patients aged 60-80 y r. All were undergoing major orthopedic or open abdominal surgery. Ane sthesia was induced with thiopental and fentanyl, and maintained only with nitrous oxide (70%) and isoflurane (0.6-0.8%). Core temperature w as measured in the distal esophagus. Fingertip vasoconstriction was ev aluated using forearm minus fingertip, skin-temperature gradients. A g radient of 4-degrees-C identified significant vasoconstriction, and th e core temperature triggering vasoconstriction identified the thermore gulatory threshold. Results. The vasoconstriction threshold was signif icantly less in the elderly patients (33.9 +/- 0.6-degrees-C) than in the younger ones (35.1 +/- 0.3-degrees-C) (P < 0.01). The gender distr ibution, weight, and height of the elderly and young patients did not differ significantly. The end-tidal isoflurane concentration at the ti me of vasoconstriction did not differ significantly in the two groups. Conclusions: These data indicate that thermoregulatory responses in t he elderly are initiated at temperatures almost-equal-to 1.2-degrees-C less than that in younger patients. Thus, it is likely that elderly s urgical patients become more hypothermic than do younger patients, at least in part, because they fail to trigger protective thermoregulator y responses.