Selective brain cooling in hyperthermia: the mechanisms and medical implications.

We hypothesize that selective brain cooling (SBC) can occur in hyperthermic humans despite the fact that humans have no carotid rete, a vascular structure that facilitates countercurrent heat exchange and that is located at the base of the skull in some mammals. We postulate that an increase in emissary and angular ocular venous flows contributes to SBC. The efficiency of SBC is increased by evaporation of sweat on the head and by ventilation through the nose. A body position that increases the intravenous pressure gradient across the skull increases emissary flows and hence enhances the efficiency of SBC. The validity of using tympanic temperature as an index of brain temperature is also postulated.

Rosacea: disturbed defense against brain overheating.

Tympanic (Tty), esophageal (Tes), forehead, and hand skin temperatures, as well as the forehead evaporation rate were recorded in six men (four suffering from rosacea and two healthy controls) before, during, and after 1 h of warm bath (38 degrees-39 degrees C). During the last 30 min of the bath, the subject's face was vigorously fanned (14 m/s). Blood flow was explored with ultrasonic Doppler in the emissary veins of the cranium during normothermia before entering the bath, and during hyperthermia just after leaving it. Under normothermic conditions, Tty was higher than Tes in all subjects. In three patients, no blood flow could be detected in the ophthalmic emissary veins whereas in the fourth patient as well as in both control subjects, blood flowed from the intracranium to the face. During hyperthermia, face fanning decreased Tty by 0.25 degrees +/- 0.05 degrees C (+/- SEM) below Tes in the control subjects whereas in all patients Tty remained warmer than Tes by 0.1 degrees C. Doppler recordings showed a rapid inward blood flow from the skin to the brain in the controls during hyperthermia. In patients, however, there was no change from normothermia in the blood flow patterns of vena angularis oculi. Their forehead temperature was permanently higher than in control subjects. Venous blood flow from the skin to the brain appears to be suppressed in rosacea, thus inhibiting selective brain cooling in hyperthermic conditions. The importance of this mechanism in the pathogenesis of rosacea and its significance as a means of investigation are discussed.

Beards, baldness, and sweat secretion.

The hypothesis according to which male common baldness has developed in the human species as a compensation for the growth of a beard in order to achieve heat loss has been tested. In 100 clean-shaven men direct measurement of the area of glabrous skin on the forehead and calvaria was found to be proportional to that of the hairy skin on the lips, cheeks, chin and neck. During light hyperthermia the evaporation rate on the bald scalp was 2 to 3 times higher than on the hairy scalp. Conversely the evaporation rate was practically equal on the foreheads and chins of women and unbearded young men, while in adult clean-shaven bearded men it was 40% less on the chin than the forehead. These results support the hypothesis that male baldness is a thermoregulatory compensation for the growth of a beard in adults.

Tympanic temperatures during hemiface cooling.

In adult men the left half of the head was covered with thick heat insulation, and the right hemiface was cooled by spraying a mist of water, and vigorous fanning. The subjects were immersed up to the waist in warm water (42 degrees) to achieve hyperthermia. In control sessions the subjects were rendered slightly hypothermic by preliminary exposure to cold. Under the hypothermic condition during right skin cooling, the right Tty remained low as compared with oesophageal temperature, while the left Tty was raised. Under the hyperthermic condition right hemiface cooling maintained not only the right Tty lower than oesophageal but also, to a lesser extent the left Tty, while the skin on the left side was close to core temperature. This latter result cannot be explained by conductive cooling from the skin to the tympanic membrane and implies a vascular cooling of the left Tty originating from the other side of the head. It is concluded that selective cooling of the brain takes place during hyperthermia. The main mechanism is forced vascular convection, but conductive cooling also occurs.

Enhanced brain protection during passive hyperthermia in humans.

Selective brain cooling during hyperthermia by emissary venous pathways from the skin of the head to the brain has been reported both in animals and humans. Heat protection of the brain extends tolerance to high deep body temperature in animals, and may be enhanced in humans if the head is cooled. In order to quantify to what extent brain protection could be obtained by face fanning, 9 non-anesthetized human volunteers were placed in ambient conditions as close as possible to those of passive therapeutic hyperthermia. Face-fanning maintained tympanic temperature 0.57 degrees C lower than esophageal temperature, and improved comfort. External head cooling techniques enhancing physiological brain cooling can therefore be useful for the protection of the human brain during heat stress or passive therapeutic hyperthermia.

Body weight set-point changes during the ovarian cycle: experimental study of rats using hoarding behavior.

Body weight, food intake and food hoarding behavior were measured in 12 female rats fed ad lib and correlated with the ovarian cycle of each. All three parameters fluctuated synchronously, being minimal at estrus and maximal at diestrus. The hypothesis was then tested that these fluctuations result from cyclical oscillation of the body weight set-point entrained by the ovarian cycle, as previously proposed by others. The linear correlation between hoarding behavior and body weight was used to extrapolate the computed threshold of body weight at which hoarding behavior occurred at diestrus and at estrus; this threshold has already been shown to be a good index of the body weight set-point. The results indicated that the regression of hoarding behavior on body weight was virtually identical at estrus and diestrus (same slope), but the critical level of body weight for the onset of hoarding behavior was 31.2 g lower at estrus than at diestrus. This provides direct evidence in the rat of fluctuation of the body weight set-point with the ovarian cycle.

Intracranial temperature during passive heat exposure in humans.

Intracranial and skin temperature, evaporative response and rating of thermal comfort/discomfort were monitored during passive therapeutic hyperthermia. Intracranial temperature at the end of normal therapeutic sessions was significantly lower when face-fanning had been performed. Although face-fanned subjects remained in the sweating box twice as long as patients, their intracranial temperature was not statistically different from that of patients. Thermal injury to the human brain during passive therapeutic hyperthermia may therefore be efficaciously retarded by face-fanning.

Blood flow in the emissary veins of the human head during hyperthermia.

The direction of the blood flowing in the emissary veins (vena emissaria mastoidea and v. e. partietalis) was recorded in human subjects during moderate hyperthermia and hypothermia. During hyperthermia the blood flowed rapidly from skin to brain. During hypothermia either no flow could be detected or the blood flowed slowly from brain to skin. On two fresh cadavers the calvaria was removed with the scalp adhering. Gentle massaging of the scalp produced abundant drops of blood on the inner surface of the bone each time the scalp was massaged, thus showing that cutaneous blood can flow inward through the bone. These results support the hypothesis of selective brain cooling in hyperthermic humans by offering a possible mechanism.

[Postoperative crural paralysis in gynecologic surgery]. / Paralysie crurale postopératoire en chirurgie gynécologique.

The authors describe one case of crural paralysis following a microsurgical salpingoplasty. The three causes of this usually reversible complication are discussed: the type of incision, the type of retractor, and positioning of the retractor by the surgeons over a long period of time.

[Is baldness a thermoregulatory adaptive process?]. / La calvitie est-elle un processus thermorégulateur adaptatif?

In 100 adult men the area of the face and neck where beard was growing was measured and compared to that of glabrous skin on the forehead and calvaria. In the population as a whole, forehead area was found to be proportional to bearded area. Forehead and calvaria sweat rate was measured on 10 baldheaded male subjects and compared with that of 10 hairy control subjects during mild hyperthermia. Bald skin was found to sweat more than twice as much as hairy skin. In the light of these results the hypothesis that baldness is a thermoregulatory adaptative process is proposed.