Thermoregulatory State Research Articles

Temperature management and its role in cardiac arrest patients—a review

Thermoregulation constitutes one of the most important homeostatic systems of the human body. The human thermoregulatory system is highly complex and intertwined with other regulatory homeostatic systems. Different evolutionary adaptations have evolved to ensure a well-regulated body temperature, encompassing simple behavioural (e.g., seeking shelter, going underground) as well as physiological changes (e.g., vasodilatation, sweating). However, when the heat, cold or other stimuli cause a disruption in the thermoregulatory state and our adaptations can no longer cope with the additional stress, the body enters a pathological state. In such instances other measures must be undertaken. In medicine there are several pathological states associated with disruptions in temperature homeostasis. Consequently, these patients have to be, in broad terms, thermoregulated. Speaking specifically, the most common application of thermoregulation is therapeutic temperature management. A prominent example is the utilisation of this technique in post-cardiac arrest patients, who remain comatose after resuscitation. This technique has been in use for almost 20 years since the first major reports on its benefits in improving out-of-hospital cardiac arrest and in-hospital cardiac arrest survival as well as improving neurological outcome. Recently, the findings from one of the biggest targeted temperature international and multicentre trials to date have been published (TTM2 trial; https://ttm2trial.org/). The study surprisingly showed no difference in mortality between patients after out of hospital cardiac arrest, who underwent normo- or hypothermia. Consequently, we might need to re-evaluate certain guidelines, recommendations, and perspectives. The aim of the current review is to present an overview of targeted temperature management in the field of intensive care medicine and cardiac arrest.

Metabolic rate, evaporative water loss and thermoregulatory state in four species of bats in the Negev desert

Life in deserts is challenging for bats because of their relatively high energy and water requirements; nevertheless bats thrive in desert environments. We postulated that bats from desert environments have lower metabolic rates (MR) and total evaporative water loss (TEWL) than their mesic counterparts. To test this idea, we measured MR and TEWL of four species of bats, which inhabit the Negev desert in Israel, one species mainly restricted to hyper-arid deserts (Otonycteris hemprichii), two species from semi-desert areas (Eptesicus bottae and Plecotus christii), and one widespread species (Pipistrellus kuhlii). We also measured separately, in the same individuals, the two components of TEWL, respiratory water loss (RWL) and cutaneous evaporative water loss (CEWL), using a mask. In all the species, MR and TEWL were significantly reduced during torpor, the latter being a consequence of reductions in both RWL and CEWL. Then, we evaluated whether MR and TEWL in bats differ according to their geographic distributions, and whether those rates change with Ta and the use of torpor. We did not find significant differences in MR among species, but we found that TEWL was lowest in the species restricted to desert habitats, intermediate in the semi-desert dwelling species, and highest in the widespread species, perhaps a consequence of adaptation to life in deserts. Our results were supported by a subsequent analysis of data collected from the literature on rates of TEWL for 35 bat species from desert and mesic habitats.

Validity and reliability of the Cold Discomfort Scale: a subjective judgement scale for the assessment of patient thermal state in a cold environment

Complementary measures for the assessment of patient thermoregulatory state, such as subjective judgement scales, might be of considerable importance in field rescue scenarios where objective measures such as body core temperature, skin temperature, and oxygen consumption are difficult to obtain. The objective of this study was to evaluate, in healthy subjects, the reliability of the Cold Discomfort Scale (CDS), a subjective judgement scale for the assessment of patient thermal state in cold environments, defined as test–retest stability, and criterion validity, defined as the ability to detect a difference in cumulative cold stress over time. Twenty-two healthy subjects performed two consecutive trials (test–retest). Dressed in light clothing, the subjects remained in a climatic chamber set to −20 °C for 60 min. CDS ratings were obtained every 5 min. Reliability was analysed by test–retest stability using weighted kappa coefficient that was 0.84 including all the 5-min interval measurements. When analysed separately at each 5-min interval the weighted kappa coefficients were was 0.48–0.86. Criterion validity was analysed by comparing median CDS ratings of a moving time interval. The comparison revealed that CDS ratings were significantly increased for every interval of 10, 15, and 30 min (p < 0.001) but not for every interval of 5 min. In conclusion, in a prehospital scenario, subjective judgement scales might be a valuable measure for the assessment of patient thermal state. The results of this study indicated that, in concious patients, the CDS may be both reliable and valid for such purpose.

The Relationship between Cutaneous Water Loss and Thermoregulatory State in Kuhl’s PipistrellePipistrellus kuhlii, a Vespertillionid Bat

Total evaporative water loss is the sum of respiratory water loss (RWL) and cutaneous water loss (CWL) and constitutes the main avenue of water loss in bats. Because bats fly and have large surface-to-volume ratios, they potentially have high rates of RWL and CWL. Most species of small insectivorous bats have the ability to reduce their body temperature (T(b)) at rest, which substantially reduces energy expenditure and water loss. We hypothesized that bats reduce evaporative water loss during bouts of deep hypothermia (torpor) by decreasing RWL and CWL. We measured T(b), RWL, CWL, and resting metabolic rate (RMR) in Kuhl's pipistrelle Pipistrellus kuhlii, a small insectivorous bat. In support of our hypothesis, we found that RWL decreased with decreasing RMR. We found that CWL was lower in torpid individuals than in normothermic bats; however, bats in deep torpor had similar or higher CWL than bats in shallow torpor, suggesting that they exert a less effective physiological control over CWL when in deep torpor. Because insectivorous bats spend most of their lives in torpor or hibernation, the regulation of CWL in different heterothermic states has relevant ecological and evolutionary consequences.

Thermoregulatory changes anticipate hibernation onset by 45 days: data from free-living arctic ground squirrels

Hibernation is a strategy of reducing energy expenditure, body temperature (T(b)) and activity used by endotherms to escape unpredictable or seasonally reduced food availability. Despite extensive research on thermoregulatory adjustments during hibernation, less is known about transitions in thermoregulatory state, particularly under natural conditions. Laboratory studies on hibernating ground squirrels have demonstrated that thermoregulatory adjustments may occur over short intervals when animals undergo several brief, preliminary torpor bouts prior to entering multiday torpor. These short torpor bouts have been suggested to reflect a resetting of hypothalamic regions that control T(b) or to precondition animals before they undergo deep, multiday torpor. Here, we examined continuous records of T(b) in 240 arctic ground squirrels (Urocitellus parryii) prior to hibernation in the wild and in captivity. In free-living squirrels, T(b) began to decline 45 days prior to hibernation, and average T(b) had decreased 4.28 °C at the onset of torpor. Further, we found that 75 % of free-living squirrels and 35 % of captive squirrels entered bouts of multiday torpor with a single T(b) decline and without previously showing short preliminary bouts. This study provides evidence that adjustments in the thermoregulatory component of hibernation begin far earlier than previously demonstrated. The gradual reduction in T(b) is likely a component of the suite of metabolic and behavioral adjustments, controlled by an endogenous, circannual rhythm, that vary seasonally in hibernating ground squirrels.

Arteriovenous anastomoses and the thermoregulatory shift between cutaneous vasoconstrictor and vasodilator reflexes

The reflex changes in skin blood flow which occur in response to various non-thermal stimuli (e.g., deep inspiratory gasps, arousing or painful stimuli, emotional stress) are profoundly influenced by the thermoregulatory state. The aim of the present study was to evaluate the involvement of arteriovenous anastomoses in the thermoregulatory modulation of skin vasomotor reflexes elicited by painful intraneural electrical stimulation and emotional stress (forced arithmetics), respectively. Vasomotor responses were recorded with laser Doppler flowmeters (LDF) placed on glabrous skin containing arteriovenous anastomoses (3rd finger and thenar eminence) and hairy skin which lack them (dorsal side of the first metacarpal bone). In some experiments, a laser Doppler flowmeter emitting laser light of two different wavelengths (infrared and green light) into the same skin site was used to record skin perfusion at different depths of glabrous skin on the thenar eminence. 40 subjects were investigated, both in the cold state (finger skin temperatures below 25°C) and after subsequent warming (finger skin temperatures) above 30°C). Thermoregulatory modulation of electrical stimulation- or stress-induced vasomotor reflexes occurred both in glabrous and hairy skin, but hairy skin differed from glabrous skin by showing no significant vasoconstrictions. Relative perfusion changes were most marked in laser Doppler flowmeter recordings using the deeper penetrating infrared light. The results suggests that arteriovenous anastomoses are major contributors to the vasoconstrictor component of vasomotor reflexes in glabrous skin of warm subjects. The reflex increase in perfusion, on the other hand, which occurs in both glabrous and hairy skin of cold subjects may be mediated by resistance vessels.

Determination of Thermoregulatory State in Full-Term Infants

A series of nine sets of axillary and tympanic (core) temperature readings were collected from each of 45 full-term infants during the first 4 hours after birth. The pattern of readings and the relationship between axillary and tympanic temperature readings were examined in each infant to determine whether the type of thermal state could be detected and categorized. The findings support the supposition that different thermal states are present and can be determined by the relationship between tympanic and axillary temperature readings. The ability to predict tympanic temperature decrease could not be predicted through examination of axillary-tympanic temperature patterns.

Temperature-dependent interaction between vasoconstrictor and vasodilator mechanisms in human skin.

The putative influence of the thermoregulatory state on skin blood-flow responses to various stimuli was studied in 17 healthy subjects exposed to different ambient temperatures. Skin blood flow was monitored by laser Doppler flowmeters and photoelectrical pulse plethysmographs. Stimuli included painful intraneural electrical stimulation (INS) in the median nerve at the wrist, mental stress, arousal stimuli and deep breaths. Intraneural electrical stimulation and mental stress were accompanied by virtually identical changes in skin blood flow, warm subjects responding with cutaneous vasoconstriction whereas cold subjects responded with vasodilatation. Similar but less pronounced responses were obtained with arousal stimuli and single deep breaths. The data indicate that the thermoregulatory state profoundly influences the extent and direction of various cutaneous vasomotor reflex responses. Furthermore, there were differences between responses in hands and feet, suggesting a spatial organization of vasomotor control.

Oxygen consumption, temperature, and water loss in bats from different environments.

Oxygen consumption, temperature, and water loss in bats from different environments.

CYTOLOGICAL RESPONSES OF BROWN FAT TISSUE IN COLD-EXPOSED RATS.

In young adult laboratory rats exposed to cold (6 degrees C) the brown adipose tissue undergoes time-dependent increases in cellularity, vascular supply, and total mass. These changes are largely complete after 16 days in the cold and concurrent generally with the development of a thermoregulatory state not greatly dependent upon shivering. Histologically the brown fat changes from a tissue having both unilocular and multilocular fat cell types to one having almost exclusively the latter. During the first 6 to 12 hours in cold, the multilocular cells lose their lipid vacuoles and decrease in size, but these features are restored to normal by 24 hours. Cell proliferation, as estimated by the DNA synthetic index method (using tritiated thymidine autoradiography), appears in the reticuloendothelial cells of the brown fat at 1 day of cold exposure, becomes maximal at 4 days, and returns to the control level by 16 days. In animals injected with tritiated thymidine on the 3rd day of cold exposure and then maintained for 1 or more additional days in the cold, autoradiographs indicate that new brown fat (multilocular) cells arise by cytogenesis from reticuloendothelial progenitor cells and not by proliferation of existing brown fat cells. Throughout this and subsequent periods, cells of the epididymal white adipose tissue slowly decrease in size. Because a thermogenic role in cold acclimation has been established for the brown fat, the reported changes are regarded as adaptive responses to a cold environment.