Human selective brain cooling during exercise in hot environment

Introduction

During normothermia, venous blood from the nasal mucosa and the face drains into the neck veins. When the body temperature is raised, however, the route of the venous return changes so that cooled blood from the nasal mucosa and the face enters into the cranium through the ophthalmic veins. This change in the route of venous return has been investigated in animals and humans passively heated and regarded as an important mechanism of selective brain cooling. However, it is not yet known whether this phenomenon occurs during exercise-induced hyperthermia in human subjects.

Methods

Five young men were subjected to either passive body warming or exercise on a cycle ergometer in a climatic chamber at 28*C and 40%RH. Prior to the experiment, maximal o2 consumption was determined for each subjects using a cycle ergometer in an upright position. Tympanic temperature (Tty) and skin temperature at 7 sites, and blood flow through the ophthalmic vein (Qov) were measured, and the mean skin and mean body temperatures were computed. Qov were measured using an ultrasonic-Doppler recorder with an 8 MHz sensor set in directional mode for venous flow. The probe was stereo-taxically held, almost perpendicular to the skin over the OV, with a stainless steel swivel holder attached to a metal head band (width:3cm) the inner surface of which was covered with foam rubber.

Results

During both tests, the direction of Qov was outward (from the intracranium to the face) before and the beginning of the heat tests, but the direction of Qov changed with the venous blood flowing from the face into the intracranium thereafter. Tty at the time of flow reversal was the same in both tests. The mean skin (and hence mean body) temperature at flow reversal was, however, significantly higher during passive body warming than during exercise.

Discussion/Conclusions

These results clearly show that switching of the direction of Qov also takes place in the hyperthermic human due to exercise. The mechanism for this switching appears to be triggered by a high temperature in the brain, but not by thermal input from the periphery of the body.