The critical limiting temperature and selective brain cooling: neuroprotection during exercise?

Two winter-insulated Norwegian reindeer ( Rangifer tarandus tarandus) were exposed to air temperatures of 10, 20, 30, and 38°C while standing at rest in a climatic chamber. The direction of airflow through nose and mouth, and the total and the nasal minute volumes, respectively, were determined during both closed- and open-mouth panting. The animals alternated between closed- and open-mouth panting, but the proportion of open-mouth panting increased with increasing heat load. The shifts from closed- to open-mouth panting were abrupt and always associated with a rise in respiratory frequency and respiratory minute volume. During open-mouth panting, the direction of airflow was bidirectional in both nose and mouth, but only 2.4 ± (SD) 1.1% of the air was routed through the nose. Estimates suggest that the potential for selective brain cooling is markedly reduced during open-mouth panting in reindeer as a consequence of this airflow pattern.

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Selective brain cooling is affected by wearing headgear during exercise

Journal of Applied Physiology ◽

10.1152/jappl.1993.74.3.1229 ◽

1993 ◽

Vol 74 (3) ◽

pp. 1229-1233 ◽

Cited By ~ 14

Author(s):

W. Rasch ◽

M. Cabanac

Keyword(s):

Heat Loss ◽

Cycle Ergometer ◽

Tympanic Temperature ◽

Esophageal Temperature ◽

Brain Cooling ◽

Selective Brain Cooling ◽

Exercise Load ◽

Convective Heat Loss ◽

The Face ◽

Skin Temperatures

The purpose of this work is to relate the concept of selective brain cooling (SBC) during exercise to heat loss from the head while either bare or covered. During hyperthermia, SBC is considered to occur if tympanic temperature (Tty) is lower than esophageal temperature (Tes). In experiment I the head heat loss was measured with and without headgear. Each of four subjects took part in three sessions of exercise on a cycle ergometer. The face was cooled to simulate outdoor conditions. The first session (no headgear) served as control for the two following sessions in which a headband and a woolen cap were worn. Evaporative and radiative-convective heat loss were monitored from the head. Wearing a cap significantly reduced the heat loss from the head compared with the control condition. During the headband session the heat loss was not significantly lower than the control values. Tty, Tes, and head skin temperatures (T(sk)) were also recorded. Tty was significantly lower (-0.55 +/- 0.15 degrees C) than Tes at the end of exercise (150-W exercise load) when no headgear was worn. During headgear sessions, Tty was no longer significantly lower than Tes, either during the headband (-0.15 +/- 0.31 degrees C) or during the cap session (-0.30 +/- 0.13 degrees C). In experiment II the influence of wearing headgear on temperature regulation was studied. Hand skin blood flow, hand T(sk), and heat loss from the hand were recorded in addition to the variables monitored in experiment I. Wearing headgear elevated Tty and peripheral vasomotor responses, whereas Tes evolved in the opposite direction.(ABSTRACT TRUNCATED AT 250 WORDS)

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Rectal temperature measurement results in artifactual evidence of selective brain cooling

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.2001.281.1.r108 ◽

2001 ◽

Vol 281 (1) ◽

pp. R108-R114 ◽

Cited By ~ 18

Author(s):

Shane K. Maloney ◽

Andrea Fuller ◽

Graham Mitchell ◽

Duncan Mitchell

Keyword(s):

Rectal Temperature ◽

Brain Temperature ◽

Brain Cooling ◽

Selective Brain Cooling ◽

Blood Temperature ◽

Arterial Blood ◽

The Status ◽

Conscious Sheep ◽

Surrogate Measures ◽

Arterial Blood Temperature

Selective brain cooling (SBC) is defined as a brain temperature cooler than the temperature of arterial blood from the trunk. Surrogate measures of arterial blood temperature have been used in many published studies on SBC. The use of a surrogate for arterial blood temperature has the potential to confound proper identification of SBC. We have measured brain, carotid blood, and rectal temperatures in conscious sheep exposed to 40, 22, and 5°C. Rectal temperature was consistently higher than arterial blood temperature. Brain temperature was consistently cooler than rectal temperature during all exposures. Brain temperature only fell below carotid blood temperature during the final few hours of 40°C exposure and not at all during the 5°C exposure. Consequently, using rectal temperature as a surrogate for arterial blood temperature does not provide a reliable indication of the status of the SBC effector. We also show that rapid suppression of SBC can result if the animals are disturbed.

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