The physiology of hypothermia in the black-capped Chickadee,Parus atricapillus

Summary The shivering, body temperature, and metabolic response to stable and decreasing ambient temperature were measured in winter acclimatized Black-capped Chickadees,Parus atricapillus. Shivering activity, measured by duration and amplitude of bursts, increased curvilinearly from thermoneutral temperatures of 27°C down to 0°C. This parabolic shivering response may be a major component of the curvilinear response of metabolism to decreasing ambient temperature.Birds exposed to 0°C exhibited metabolism 32–45% lower than predicted for a 12-g homeotherm and body temperatures 10°C below the pre-experimental nocturnal body temperature. This hypothermia was not the result of a breakdown in thermoregulation, but was a controlled effort serving to reduce overnight energy expenditure. It is suggested that (1) hypothermia was achieved by decreased shivering by pectoral muscles during exposure to decreasing ambient temperatures, (2) the rate of body temperature decline was moderated by intermittent and reduced bursts during the cooling period, and (3) body temperature was maintained at a particular level during exposure to a stable low ambient temperature by intense bursts lasting one to three minutes.The physiology of hypothermia in chickadees is similar to torpor; however, chickadees did not arouse to a normal diurnal body temperature in the laboratory, and their hypothermia was not induced by inanition or prolonged exposure to cold, as reported for other species capable of torpor.     

Endothelial ultrastructure of myocardium microvessels affected by various methods of artificial hypothermia

A comparative analysis of the endothelial ultrastructure of myocardium microvessels affected by various methods of artificial hypothermia was carried out. Tissue samples were harvested in children with a congenital ventricular septum defect after cooling the whole body under the conditions of hypothermic artificial circulation and perfusionless (immersion) hypothermia. It was found out that the shifts in population composition of endothelial cells, as well as the changes in the ultrastructure of organelles participating in endocellular syntheses and transendothelial transfer of macromolecules depended upon the rate body cooling. Under perfusionless hypothermia and of moderately low cooling rate, morphological signs of inhibition of endothelial cells metabolism were observed alongside with quantitative reduction of their micropinocytic transport indicators. Under hypothermic artificial circulation these reactions tended to lag behind due to the high cooling rate that initiates a heterogenic response of various endothelial processes to the changes of body temperature.     

Shallow hypothermia depends on the level of fatty acid unsaturation in adipose and liver tissues in a tropical heterothermic primate

Optimal levels of unsaturated fatty acids have positive impacts on the use of prolonged bouts of hypothermia in mammalian hibernators, which generally have to face low winter ambient temperatures. Unsaturated fatty acids can maintain the fluidity of fat and membrane phospholipids at low body temperatures. However, less attention has been paid to their role in the regulation of shallow hypothermia, and in tropical species, which may be challenged more by seasonal energetic and/or water shortages than by low temperatures. The present study assessed the relationship between the fatty acids content of white adipose and liver tissues and the expression of shallow hypothermia in a tropical heterothermic primate, the gray mouse lemur (Microcebus murinus). The adipose tissue is the main tissue for fat storage and the liver is involved in lipid metabolism, so both tissues were expected to influence hypothermia dependence on fatty acids. As mouse lemurs largely avoid deep hypothermia (i.e. torpor) use under standard captive conditions, the expression of hypothermia was triggered by food-restricting experimental animals. Hypothermia depth increased with time, with a stronger increase for individuals that exhibited higher contents of unsaturated fatty acids suggesting that they were more flexible in their use of hypothermia. However these same animals delayed the use of long hypothermia bouts relative to individuals with a higher level of saturated fatty acids. This study evidences for the first time that body fatty acids unsaturation levels influence the regulation of body temperature not only in cold-exposed hibernators but also in tropical, facultative heterotherms.     

Urban hypothermia and hyperglycemia in the elderly

From December 1993 to March 1999 we treated 18 elderly patients aged 66-87 years, suffering from urban hypothermia: 11 women and 7 men. Ten patients suffered from moderate hypothermia (rectal temperature 32-35 degrees C), and eight from severe hypothermia (rectal temperature < 32 degrees C). Regarding consciousness, in the group suffering from moderate hypothermia, 3 were somnolent and 6 in various degrees of comatose states. In the group suffering from severe hypothermia, 3 patients were somnolent or soporous and 5 in comatose states of various degrees. Values of arterial blood pressure in the group with moderate hypothermia was normal in one, in 3 arterial hypotension was observed and 6 were in a state of shock. In the group with severe hypothermia, 3 presented arterial hypotension and 5 were in a state of shock. In the group with moderate hypothermia the blood glucose level was elevated in six: 9.3-10.2-10.7-17.9-21.3-99.0, and in one patient the blood glucose level was low: 2.3 mmol/L, in correlation with hypoglycemic coma. In the group with severe hypothermia in all eight patients the values were elevated: 6.7-7.4-7.6-8.7-9.1-11.2-12.4-17.9 mmol/L.     

Protein and lipid utilization during fasting with shallow and deep hypothermia in the European hedgehog (Erinaceus europaeus)

We investigated whether the relative contributions of body protein and lipid reserves differ according to the level of energy expenditure in fasting animals. Protein and lipid utilization was therefore quantified and compared in hedgehogs which fasted with shallow and deep hypothermia, i.e. by exposure at 5 or 20 °C ambient temperature. Body composition was determined for every 150-g decrease in mass throughout the experiment, allowing the calculation of regression lines between body mass (independent variable, x) and body composition (dependent variable, y: water, protein, neutral lipids, phospholipids and cholesterol). There were highly significant (P<0.001) linear decreases in all body components with decreasing body mass in both groups of hedgehogs. Neutral lipids were the main component of the total body mass loss (54%) in fasted animals with shallow and deep hypothermia, percentages of water (26–30%) and protein (10–11%) being lower, and those of phospholipid and cholesterol negligible (<0.5%). In spite of different levels in energy expenditure (2.54 and 1.07 W·kg^-1 in shallow-and deep-hypothermal fasting hedgehogs, respectively), the energy sources were identical in both groups, neutral lipid being the main fuel (91–92%) and body protein accounting for the remainder (8–9%). Prolonged fasting with shallow and deep hypothermia were marked by low alaninemia and glycemia, while plasma free fatty acids and -hydroxybutyrate were elevated. These data therefore indicate that the relative contribution of lipid and protein is similar during prolonged fasting with shallow and deep hypothermia, i.e. there is no specific effect of deep hypothermia on body fuel utilization. The tolerance of a much longer fast in deep-hypothermal hedgehogs can simply be attributed to the lower rates of lipid and protein utilization as a result of the lower level of energy expenditure.Abbreviations bm body mass     

Influence of restricted food intake on brown adipose tissue function in genetically obese mice (genotype, ob/ob)

Measurements were made of cytochrome c oxidase activity and the GDP-binding capacity of mitochondria in brown adipose tissue of genetically obese mice and wild-type siblings, to estimate the thermogenic capacity of the tissue. The binding capacity was decreased in ad libitum fed obese animals compared with wild-type animals. Limited feeding of obese animals to restrict their body weight caused a large increase in the binding capacity of the tissue, which was greater than that in wild-type animals fed either ad limitum or on a limited diet. The decreased binding capacity of brown adipose tissue mitochondria in obese mice appears to be a consequence of ad libitum feeding and therefore not a cause of the obesity. Limit feeding of obese animals also corrected their characteristic hypothermia at low ambient temperature. The large increase in the thermogenic capacity of brown adipose tissue in obese animals, induced by limited feeding, may account for the vital improvement of their thermoregulation. However, close similarities were found between obesity hypothermia and hypothermia induced in wild-type animals by restraint. It is suggested that changes in posture caused by obesity, resulting in increased loss of body heat, may be important in the development of obesity hypothermia. Obese animals fed less than wild-type grained more weight than wild-type animals, indicating that the high thermogenic capacity of their brown adipose tissue did not function to regulate their calorie intake.     

围术期体温监测及保温的现状分析

体温是人体重要的生命体征,其过低或过高都会对患者的预后造成不利影响,在临床中最常见也最容易被忽视的是围术期低体温。围术期长时间的体温过低会引起麻醉苏醒延迟、心律失常、影响切口愈合等多种并发症,增加死亡率,延迟出院时间。体温监测及相应的保温措施对于防治低体温至关重要,围术期及时有效地体温监测能反映病人术中病情的变化,但目前临床中对于体温的监控却普遍重视不足。本文对近年来临床常用的体温监测方法及保温手段做一综述。     

Effects of regional body temperature variation during asphyxial cardiac arrest on mortality and brain damage in a rat model

To date, hypothermia has focused on improving rates of resuscitation to increase survival in patients sustaining cardiac arrest (CA). Towards this end, the role of body temperature in neuronal damage or death during CA needs to be determined. However, few studies have investigated the effect of regional temperature variation on survival rate and neurological outcomes. In this study, adult male rats (12 week-old) were used under the following four conditions: (i) whole-body normothermia (37 ± 0.5 °C) plus (+) no asphyxial CA, (ii) whole-body normothermia + CA, (iii) whole-body hypothermia (33 ± 0.5 °C)+CA, (iv) body hypothermia/brain normothermia + CA, and (v) brain hypothermia/body normothermia + CA. The survival rate after resuscitation was significantly elevated in groups exposed to whole-body hypothermia plus CA and body hypothermia/brain normothermia plus CA, but not in groups exposed to whole-body normothermia combined with CA and brain hypothermia/body normothermia plus CA. However, the group exposed to hypothermia/brain normothermia combined with CA exhibited higher neuroprotective effects against asphyxial CA injury, i.e. improved neurological deficit and neuronal death in the hippocampus compared with those involving whole-body normothermia combined with CA. In addition, neurological deficit and neuronal death in the group of rat exposed to brain hypothermia/body normothermia and CA were similar to those in the rats subjected to whole-body normothermia and CA. In brief, only brain hypothermia during CA was not associated with effective survival rate, neurological function or neuronal protection compared with those under body (but not brain) hypothermia during CA. Our present study suggests that regional temperature in patients during CA significantly affects the outcomes associated with survival rate and neurological recovery.     

The corrected Q-T interval in the elderly with urban hypothermia

During winter time in the period from 1993 to 1998, 18 elderly patients: 11 female and 7 male aged 65-88 years, were treated because of hypothermia. Rectal temperature on admission was 20-34.5 degrees C. Ten patients suffered from moderate hypothermia (35-32 degrees C), and eight suffered of severe hypothermia (< 32 degrees C). Arterial hypotension was recorded in 7, and shock in 11 patients. In all of them, and in 18 controls, an electrocardiogram was analyzed with the special reference to the corrected Q-T interval. Decompensated metabolic acidosis was observed in 7/8 patients with severe hypothermia and in 4/10 with moderate hypothermia. Among patients with moderate hypothermia, sinus tachycardia was present in 2, sinus bradycardia in 2, idioventricular rhythm in 2 and atrial fibrillation in 4/10 patients. In patients with severe hypothermia, sinus tachycardia was present in 2, sinus bradycardia in 3, idioventricular rhythm in one, and atrial fibrillation in 2/8 patients. In moderate hypothermia Osborn's or Tomaszewski's J wave was present in 7/10, and it only appeared in 3/10 patients; in severe hypothermia it was present in 6/8 and only appeared in 2/8 patients. The corrected Q-T interval in the group with hypothermia ranged 0.450-0.688 s, in the control group 0.343-0.444 s. The X minimum (s) in the group with hypothermia was 0.508 +/- 0.079, in the control group it was 0.371-0-139 s, and the difference was statistically significant (p < 0.001). The X maximum (s) in the group with hypothermia was 0.576 +/- 0.067 s, in the control group 0.390 +/- 0.019 s, and the difference was also statistically significant (p < 0.0001). In both groups the most significant prolongation of the corrected Q-T interval in the majority of patients was found in anteroseptal leads. The dispersion of the corrected Q-T interval in the group with hypothermia was 87.19 +/- 28.44 ms, in the control group it was 32.06 +/- 8.94 ms, and the difference was statistically significant (p < 0.001).     

Evidence of facultative daytime hypothermia in a small passerine wintering at northern latitudes

The use of hypothermia as a means to save energy is well documented in birds. This energy‐saving strategy is widely considered to occur exclusively at night in diurnally active species. However, recent studies suggest that facultative hypothermia may also occur during the day. Here, we document the use of daytime hypothermia in foraging Black‐capped Chickadees Poecile atricapillus wintering in eastern Canada. We measured the body temperature (T_b) of 126 individuals (plus 48 repeated measures) during a single winter and related values to ambient temperature (T_a) at the time of capture. We also tested whether daytime hypothermia was correlated with the size of body reserves (residuals of mass on structural size and fat score) and levels of metabolic performance (basal metabolic rate and maximum thermogenic capacity). We found that T_b of individual birds was lower when captured at low T_a, reaching values as low as 35.5 °C in actively foraging individuals. T_b was unrelated to metabolic performance or measures of body reserves. Therefore, daytime hypothermia does not result from individuals being unable to maintain T_b during cold spells or to a lack of body reserves. Our data also demonstrated a high level of individual variation in the depth of hypothermia, the causes of which remain to be explored.     

Treatment of accidental hypothermia: a prospective clinical study.

A 15-year prospective study was carried out of 44 patients with accidental hypothermia (mean age 60 years) admitted to an intensive therapy unit. The lowest core temperature recorded in each patient ranged from 20.0 to 34.3 degrees C. The precipitating factors were poisoning (by drugs, alcohol, or coal gas) in 25 cases and various illnesses in 19. Rewarming was achieved in 42 patients by applying a radiant heat cradle over the torso, and in two patients by mediastinal irrigation with warmed fluids. Twelve patients died, but only two during the period of rewarming. Thus rewarming may be consistently and safely achieved irrespective of the cause of hypothermia, and normal body temperature may be regained as rapidly as is compatible with adequate tissue perfusion and oxygenation. Surface rewarming of the torso is perhaps the simplest technique available, but internal rewarming procedures may be desirable or essential in the presence of, for example, profound hypothermia, severe hypotension, or ventricular fibrillation. Mortality was attributable to underlying factors or disease and not to hypothermia.     

Determinants of heat production in newborn lambs

Measurement of summit metabolism (the maximum rate of heat production) in lambs aged 1 or 4h revealed considerable between animal variation. Summit metabolism per unit body weight decreased as body weight increased whereas summit metabolism per unit body surface area was independent of body weight. Severe pre-partum hypoxia was apparently associated with a low summit metabolism at 1 or 4h of age which made such lambs very susceptible to hypothermia. This deficiency in heat production capacity did not appear to be a permanent featuresince most lambs so affected recovered full thermoregulatory ability by 12h of age. Feeding of colostrum conferred an immediate 18% increase in summit metabolism. The significance of these findings to the prevention of hypothermia in the newborn lamb is discussed.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Accidental hypothermia and impaired temperature homoeostasis in the elderly.

A longitudinal study of the age-related decline in thermoregulatory capacity was made in 47 elderly people to try to identify those at risk from spontaneous hypothermia. During the winters of 1971-2 and 1975-6 environmental and body temperature profiles were obtained in the home, and thermoregulatory function was investigated by cooling and warming tests. Environmental temperature and socioeconomic conditions had not changed but the body core-shell temperature gradients were smaller in 1976, indicating progressive thermoregulatory impairment. People at risk of developing hypothermia also seem to have low resting peripheral blood flows, a nonconstrictor pattern of vasomotor response to cold, and a higher incidence of orthostatic hypotension.     

Hypothermia induction and survival in hamsters: the role of temperature acclimation and an anesthetic (Holothane).

In summary, it has been shown that a reduction of biological insult during hypothermic induction with light anesthesia (halothane) will greatly enhance the survival ability of hamsters in hypothermia. Heat acclimation and cold acclimation prior to hypothermia drastically alter the thermogenic capacity of these animals at low body temperatures while apparently not altering the hypothermic survival capabilities. Normalization and shortening of induction times was also achieved with the halothane-helox method which will permit future comparative studies where it is essential to eliminate effects caused by variations in the induction time.     

Implication of prostaglandins and histamine H1 and H2 receptors in radiation-induced temperature responses of rats

Exposure of rats to 1-15 Gy gamma radiation (60Co) induced hyperthermia, whereas 20-200 Gy induced hypothermia. Exposure either to the head or to the whole body to 10 Gy induced hyperthermia, while body-only exposure produced hypothermia. This observation indicates that radiation-induced fever is a result of a direct effect on the brain. The hyperthermia due to 10 Gy was significantly attenuated by the pre- or post-treatment with a cyclooxygenase inhibitor, indomethacin. Hyperthermia was also altered by the central administration of a mu-receptor antagonist naloxone but only at low doses of radiation. These findings suggest that radiation-induced hyperthermia may be mediated through the synthesis and release of prostaglandins in the brain and to a lesser extent to the release of endogenous opioid peptides. The release of histamine acting on H1 and H2 receptors may be involved in radiation-induced hypothermia, since both the H1 receptor antagonist, mepyramine, and H2 receptor antagonist, cimetidine, antagonized the hypothermia. The results of these studies suggest that the release of neurohumoral substances induced by exposure to ionizing radiation is dose dependent and has different consequences on physiological processes such as the regulation of body temperature. Furthermore, the antagonism of radiation-induced hyperthermia by indomethacin may have potential therapeutic implications in the treatment of fever resulting from accidental irradiations.     

NPY Y1 receptor antagonist prevents NPY-induced torpor-like hypothermia in cold-acclimated Siberian hamsters

Siberian hamsters (Phodopus sungorus) undergo bouts of daily torpor during which body temperature decreases by as much as 20 degrees C and provides a significant savings in energy expenditure. Natural torpor in this species is normally triggered by winterlike photoperiods and low ambient temperatures. Intracerebroventricular injection of neuropeptide Y (NPY) reliably induces torporlike hypothermia that resembles natural torpor. NPY-induced torporlike hypothermia is also produced by intracerebroventricular injections of an NPY Y1 receptor agonist but not by injections of an NPY Y5 receptor agonist. In this research, groups of cold-acclimated Siberian hamsters were either coinjected with a Y1 receptor antagonist (1229U91) and NPY or were coinjected with a Y5 receptor antagonist (CGP71683) and NPY in counterbalanced designs. Paired vehicle + NPY induced torporlike hypothermia in 92% of the hamsters, whereas coinjection of Y1 antagonist + NPY induced torporlike hypothermia in 4% of the hamsters. In contrast, paired injections of vehicle + NPY and Y5 antagonist + NPY induced torporlike hypothermia in 100% and 91% of the hamsters, respectively. Although Y5 antagonist treatment alone had no effect on body temperature, Y1 antagonist injections produced hyperthermia compared with controls. Both Y1 antagonist and Y5 antagonist injections significantly reduced food ingestion 24 h after treatment. We conclude that activation of NPY 1 receptors is both sufficient and necessary for NPY-induced torporlike hypothermia.     

Moderate hypothermia induces a preferential increase in pancreatic islet blood flow in anesthetized rats

The aim of the study was to characterize the effects of induced moderate hypothermia on splanchnic blood flow, with particular reference to that of the pancreas and the islets of Langerhans. We also investigated how interference with the autonomic nervous system at different levels influenced the blood perfusion during hypothermia. For this purpose, hypothermia (body temperature of 28 degrees C) was induced by external cooling, whereas normothermic (37.5 degrees C) anesthetized Sprague-Dawley rats were used as controls. Some rats were pretreated with either propranolol, yohimbine, atropine, hexamethonium, or a bilateral abdominal vagotomy. Our findings suggest that moderate hypothermia elicits complex, organ-specific circulatory changes, with increased perfusion noted in the pylorus, as well as the whole pancreas and the pancreatic islets. The pancreatic islets maintain their high blood perfusion through mechanisms involving both sympathetic and parasympathetic mediators, whereas the increased pyloric blood flow is mediated through parasympathetic mechanisms. Renal blood flow was decreased, and this can be prevented by ganglionic blockade and is also influenced by beta-adrenoceptors.     

The Management of Accidental Hypothermia

Accidental general hypothermia is defined as an unintentional lowering of the body temperature in a previously conscious patient due to exposure. Even mild degrees of hypothermia may be followed by death if treatment is not instituted promptly. Hypothermic patients who are still conscious may rewarm spontaneously. They should not be left unattended and, if the facilities are available, rapid rewarming appears to be the treatment of choice. Unconscious patients who are presumed to have a lower temperature of prolonged duration may not benefit from rapid rewarming. All hypothermia victims showing signs of life are potential survivors, but even with good facilities the mortality rate may be high.     

Effect of hypothermia on reuptake of neuromediators by synaptosomes

Cranio-cerebral hypothermia (temperature of the body 32-30 degrees C, of the brain 29-27 degrees C) was studied for its effect on the reuptake of neuromediators (3H-noradrenaline and [14C]GABA) by the cortex and hypothalamus synaptosomes of the rat brain. It was found that the reuptake of [3H]noradrenaline by the cortex synaptosomes under narcosis and cranio-cerebral hypothermia was inhibited much stronger than that by the hypothalamus synaptosomes. At the same time GABA-ergic synapses of the cortex and hypothalamus were not sensitive to narcosis. Cranio-cerebral hypothermia essentially inhibited the reuptake of [14C] GABA by synaptosomes and hypothalamus.