Gain and Maximum Response Intensity

Both the gain and maximum intensity of sweating remain normal during isoflurane^[8] and enflurane anesthesia.^[44] However, the gain of arteriovenous shunt vasoconstriction is reduced threefold during desflurane anesthesia ( Fig. 40-6 ), ^[26] even though the maximum vasoconstriction intensity remains normal.^[45]

Shivering is rare during surgical doses of general anesthesia, which is consistent with its threshold being roughly 1°C less than the vasoconstriction threshold.^{[21] [22] [23] [24] [25]} (Vasoconstriction usually prevents additional hypothermia,^[46] so even unwarmed patients rarely become cold enough to shiver.) Nonetheless, shivering can be induced by sufficient active cooling.

Gain and maximum shivering intensity remain normal during both meperidine and alfentanil administration.^[47] Gain also remains nearly intact during nitrous oxide administration, although maximum intensity is reduced.^[48] Isoflurane changes the macroscopic pattern of shivering to such an extent that it is no longer possible to easily determine gain. The drug does, however, reduce maximum shivering intensity. ^[27]

Taken together, sweating appears to be the thermoregulatory defense that is best preserved during anesthesia. Not only is the threshold only slightly increased, but the gain and maximum intensity are also well preserved. In contrast, the thresholds for vasoconstriction and shivering are markedly reduced, and furthermore, these responses are less effective than normal even after being activated.

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Figure 40-6 Finger blood flow without (open squares) and with (filled circles) desflurane administration. Values were computed relative to the thresholds (finger flow = 1.0 mL/min) in each subject. Flows of exactly 1.0 mL/min are not shown because flows in each individual were averaged over 0.1°C or 0.05°C increments; each data point thus includes both higher and lower flows. The horizontal standard deviation bars indicate variability in the thresholds among the volunteers; although errors bars are shown only at a flow near 1.0 mL/min, the same temperature variability applies to each data point. The slopes of the flowversus-core temperature relationships (1.0 to approximately 0.15 mL/min) were determined by linear regression. These slopes defined the gain of vasoconstriction with and without desflurane anesthesia. Gain was reduced by a factor of 3, from 2.4 to 0.8 mL/min/°C (P < .01). (Redrawn from Kurz A, Xiong J, Sessler DI, et al: Desflurane reduces the gain of thermoregulatory arteriovenous shunt vasoconstriction in humans. Anesthesiology 83:1212–1219, 1995.)