Gernot Kuhnen

Activity, blood temperature and brain temperature of free-ranging springbok

Abstract We used miniature data loggers to record temperature and activity in free-ranging springbok (Antidorcas marsupialis) naturally exposed to severe nocturnal cold and moderate diurnal heat. The animals were active throughout the day and night, with short rests; the intensity of activity increased during daylight. Arterial blood temperature, averaged over many days, exhibited a circadian rhythm with amplitude <1 °C, but with a wide range which resulted from sporadic rapid deviations of body temperature. Peak blood temperature occurred after sunset. Environmental thermal loads had no detectable effect on blood temperature, even though globe temperature varied by >10 °C from day to day and >20 °C within a day. Brain temperature increased approximately linearly with blood temperature but with a slope <1, so that selective brain cooling tended to be activated at high body temperature, but without a precise threshold for the onset of brain cooling. Low activity attenuated selective brain cooling and high activity abolished it, even at high brain temperature. Our results support the concept that selective brain cooling serves to modulate thermoregulation rather than to protect the brain against heat injury.

Threshold and slope of selective brain cooling

Experiments (n=50) in three conscious goats were performed in a thermoneutral environment to determine the threshold (i.e. the point at which the brain temperature is equal to the carotid blood temperature) and slope (i.e. the difference between brain and carotid blood temperatures as a function of carotid blood temperature) of selective brain cooling (SBC) and analyse the thermal inputs affecting them. Prior to the experiments the animals received carotid loops and an arteriovenous shunt to manipulate head and trunk temperatures independently of each other. The mean SBC threshold was 38.75° C Tcarotis and independent of Ttrunk. When body core temperature was increased from a hypo- to a moderately hyperthermic level, the SBC threshold was passed before metabolic rate had reached its minimum and before cutaneous vasodilation occurred. The mean SBC slope was 0.78 and rose with increasing Ttrunk. The degree of SBC was principally independent of respiratory heat loss: high levels of heat loss were found without SBC, and large degrees of SBC were observed at low levels of heat loss. The effect of SBC in and around normothermia is to smooth the onset of shivering or panting and to establish a range of internal temperature within which metabolic rate and respiratory heat loss are simultaneously at low levels.

Coupling of Cerebral Blood Flow and Oxygen Metabolism in Infant Pigs During Selective Brain Hypothermia

Studies documenting the cerebral hemodynamic consequences of selective brain hypothermia (SBH) have yielded conflicting data. Therefore, the authors have studied the effect of SBH on the relation of cerebral blood flow (CBF) and CMRO2 in the forebrain of pigs. Selective brain hypothermia was induced in seven juvenile pigs by bicarotid perfusion of the head with extracorporally cooled blood. Cooling and stepwise rewarming of the brain to a Tbrain of 38°C, 25°C, 30°C, and 38°C at normothermic Ttrunk (38°C) decreased CBF from 71 ± 12 mL 100 g−1 min−1 at normothermia to 26 ± 3 mL 100 g−1 min−1 and 40 ± 12 mL 100 g−1 min−1 at a Tbrain of 25°C and 30°C, respectively. The decrease of CMRO2 during cooling of the brain to a Tbrain of 25°C resulted in a mean Q10 of 2.8. The ratio between CBF and CMRO2 was increased at a Tbrain of 25°C indicating a change in coupling of flow and metabolism. Despite this change, regional perfusion remained coupled to regional temperatures during deep cerebral hypothermia. The data demonstrate that SBH decreases CBF and oxygen metabolism to a degree comparable with the cerebrovascular and metabolic effects of systemic hypothermia. The authors conclude that, irrespective of a change in coupling of blood flow and metabolism during deep cerebral hypothermia, cerebral metabolism is a main determinant of CBF during SBH.

Effects of dehydration and rehydration on body temperatures in the black Bedouin goat

Abstract The temperatures of the arterial blood and the brain in black Bedouin goats were measured continuously by miniature data loggers. The animals were either euhydrated or dehydrated to 75–80% of the initial body mass by withholding water for 3–4 days during exposure to intense solar radiation. The daily blood temperature means and maxima of were significantly higher in dehydration than in euhydration, but 40°C was rarely exceeded even during the hot hours of the day. Selective brain cooling occurred in euhydration, but its extent was small when blood temperature was below 39.5°C. In dehydration, however, selective brain cooling was frequent and the standard response when blood temperature exceeded 39°C. We believe that selective brain cooling contributes to the inhibition of evaporative heat loss, which is the primary cause of the higher blood temperature in dehydration. Rapid rehydration with cold water induced long-lasting depression of blood temperature. No evidence was found for mechanisms attenuating the subsequent decrease of brain temperature which occurred a few minutes after the uptake of cold water.

Seasonal variations of body temperature in goats living in an outdoor environment

Abstract 1. 1. Blood and brain temperatures were measured continuously in three animals on 319, 248, and 127 days. 2. 2. Over a 30°C range of the 24-h mean of air temperature, the 24-h mean of body core temperature changed 0.5°C or less. 3. 3. The 24-h mean of body core temperature was, in the range between 38°C and 39°C, mainly determined by non-thermal factors. 4. 4. The 24-h amplitude of body core temperature was closely correlated with the 24-h amplitude of air temperature, and was small in winter and large in summer. 5. 5. The relationship between blood and brain temperatures was highly variable and did not reveal a consistent temperature effect within the range of the most frequently occurring body core temperatures.

The metabolic response to skin temperature

Experiments were done to assess that fraction of the metabolic response to external cold exposure, which is attributable to skin temperature. In 5 conscious and closely clipped goats the metabolic rate was determined at various stable levels of skin temperature in the range from 13 to 41°C, while core temperature was kept constant at 38.8°C. Skin temperature was manipulated by a rapidly circulating shower bath, while core temperature was controlled by means of heat exchangers acting on arterial blood temperature in a chronic arteriovenous shunt. The metabolic response to skin temperature fell into two clearly discernible sections: a first zone with skin temperatures above 25–30°C, within which the metabolic rate rose at a rate of −0.34±0.07 W/kg·°C with decreasing skin temperature, and a second zone with skin temperatures below 25–30°C, within which the metabolic rate either plateaued or even grew smaller with further decreasing skin temperature. It is concluded that the relationship between skin temperature and metabolic rate does not directly reproduce the temperature-response curve of cutaneous cold receptors but also reflects a complex interaction of several factors, including an unspecific temperature effect on muscle metabolism.

The effect of cage size and enrichment on core temperature and febrile response of the golden hamster

The aim of this study is to determine the effect of cage size and cage enrichment. Golden hamsters were individually housed in standard cages of four different sizes and in enriched cages of three different sizes since 3 weeks of age. Each of the seven housing groups consisted of 12 hamsters. After 14 weeks of housing in their respective environments the measurements started. The mean baseline rectal temperature was significantly higher in hamsters housed in small cages than in hamsters housed in large cages. After the injection of fever-inducing lipopolysaccharide rectal temperature increased by 1 to 2°C. The increase of rectal temperature and the fever index were the highest in animals housed in large cages and the smallest in animals housed in small cages. Through cage enrichment and increasing cage size the mean febrile response increased while the mean baseline rectal temperature decreased. Cage size and cage enrichment had no effect on the dispersion of the measured values. The differences in microclimate between large and small cages were too small to have an effect on thermoregulation. The results indicate that housing in small cages induce chronic stress which obviously affects thermoregulation. The findings demonstrate that the results of some physiological experiments are significantly influenced by the pre-experimental housing conditions.

Controlled brain hypothermia by extracorporeal carotid blood cooling at normothermic trunk temperatures in pigs.

Cerebral hypothermia improves outcomes after brain injury. A technique is presented for isolated brain cooling in pigs by cooling the natural blood supply of the brain. Under general anesthesia both common carotid arteries were exteriorized. One proximal carotid artery was connected to both distal carotid arteries and a heat exchanger in this line controlled brain temperature. The second proximal carotid artery was connected to an external jugular vein and a heat exchanger in this arteriovenous shunt was used to clamp trunk temperature. Thalamic brain temperatures of anesthetized juvenile pigs (N = 8) were clamped at 38, 25, and 30 degrees C while trunk core temperature was clamped at 38 degrees C. Approximately 7 min were needed to decrease brain temperature from 38 to 25 degrees C, reducing brain electric activity by 76% and increasing the temperature differences between different brain sites. Mean arterial blood pressure, heart rate, and cardiac output showed no significant change. Re-establishment of normothermic brain temperature led to a virtually complete recovery of brain electric activity. The technique is suitable for investigations of ischemic and traumatic injuries.

Effects of spinal cord temperature on the generation and transmission of temperature signals in the goat.

A series of 38 experiments were performed in five conscious goats at air temperatures of +20° C or +30° C to see whether a temperature dependence of spinal cord signal transmission affects the relationships between body temperature and metabolic rate (MR) or respiratory evaporative heat loss (REHL). Prior to the experiments the animals received peridural thermodes to clamp the spinal cord temperature by perfusion temperatures of 31 °C, 38° C or 43° C (45° C), carotid loops to clamp the brain temperature at 39° C or 39.5° C, and arteriovenous shunts to alter the trunk temperature and to determine thresholds and slopes of MR and REHL over trunk temperature. The trunk temperature thresholds, at which MR and REHL increased, were inversely related to the spinal cord temperature, thereby confirming previous observations on the generation of specific spinal temperature signals. The slopes at which MR rose below the threshold, increased with decreasing spinal cord temperature. The slopes of REHL over trunk temperature were independent of spinal cord temperature. Both observations are at variance with previously observed temperature effects on hypothalamic signal transmission and imply that temperature-dependent signal transmission at the spinal level cannot account for nonlinear interaction of various body temperatures in the control of MR and REHL.

Effects of selective brain cooling on mechanisms of respiratory heat loss

AbstractExperiments (n=36) in three conscious goats were performed at 35 °C air temperature and low (LH) or high (HH) humidity. Prior to the experiments the animals received carotid loops and an arteriovenous shunt, which made it possible to increase the temperature of the blood flowing to head and trunk (series A), or to increase the temperature of the trunk at constant carotid blood and hypothalamic temperature (Thyp), respectively (series B). Owing to the smaller cooling power of the inspired air in HH, the slope of respiratory evaporative heat loss versus aorta blood temperature (Taor) was reduced in series A and B. In series A the slopes of respiratory minute volume (