Circadian changes in the sweating-to-vasoconstriction interthreshold range

Abstract

Thermoregulatory defenses are characterized by thresholds, the core temperatures triggering each response. Core body temperature is normally maintained within the interthreshold range, temperatures between the sweating and vasoconstriction thresholds that do not trigger autonomic defenses. This range usually spans only some 0.2 degrees C, but it remains unknown whether similar precision is maintained during the circadian core temperature cycle of about 0.8 degrees C. Accordingly, we evaluated the interthreshold range at four times of the day. We studied ten male volunteers, each at 3 a.m., 8 a.m., 3 p.m., and 8 p.m. At least 12 h elapsed between tests, and the order was randomly assigned. At each study time, volunteers were warmed peripherally until sweating was observed. Skin temperature was subsequently kept constant while core temperature was decreased by central-venous infusion of ice-cold fluid until peripheral vasoconstriction was detected. The volunteers were not permitted to sleep during threshold determinations, although sleep was not otherwise controlled. The core temperature triggering an evaporative water loss of 40 g.m-2.h-1 identified the sweating threshold. Similarly, the vasoconstriction threshold was defined by the core temperature triggering the initial decreases in plethysmographic finger tip blood flow. The interthreshold range at 3 a.m. was twice that observed at the other study times (P<0.05). Our data suggest that autonomic control of body temperature is reduced at 3 a.m., even when sleep is denied. This result contradicts the general perception that circadian variation alters the thermoregulatory target temperature, but not precision of body temperature control.

An evaluation of underbody forced‐air and resistive heating during hypothermic, on‐pump cardiac surgery*

“…Average core temperatures with passive insulation and either active warming system differed by £ 0.5°C at the end of the pre-CPB period and on arrival in the ICU. Such small temperature differences are not usually considered to be clinically important as they have yet to be linked to any important outcome [18]; furthermore, normal circadian temperature change exceeds 0.5°C and is presumably harmless [19,20]. Therefore, neither active underbody system made a clinically important contribution to thermal management in patients managed in this manner.…”

Thermoregulatory and Immune Responses During Cold Exposure: Effects of Repeated Cold Exposure and Acute Exercise

“…This observation is consistent with those reported from the only other investigation of circadian variations in thermoregulatory effector responses to cooling (103). Tayefeh et al (103) observed no difference in the cooling-induced vasoconstriction threshold between 0700 h and 1600 h, although a shift was apparent by 0300 h. However it is unclear to what extent that shift resulted from sleep deprivation as opposed to circadian rhythm effects (103). We…”

The accuracy of a disposable noninvasive core thermometer

“…[14][15][16] These limits reflect the normal magnitude of human circadian temperature variation. 17,18 We performed a Bland-Altman analysis to evaluate agreement between the temperature measurement methods, assuming that the true value changes from measurement to measurement. 13 In a Bland-Altman analysis, the bias and 95% limits of agreement are determined.…”