Isothermal dialysis to control intradialytic hypotension and patient comfort: a pilot study

Isothermal hemodialysis to improve intradialytic tolerance in hypotension-prone patients has been effective in outpatient settings. The purpose of this pilot study was to examine thermal control in an acute care setting and describe comfort issues associated with thermal control Although complaints of cold or shivering occurred more frequently with the isothermal hemodialysis group, cold discomfort was managed by nursing interventions and was not a cause of significant discomfort. No statistically significant difference was observed in blood pressure or patient's comfort level between standard and isothermal dialysis. However, isothermal hemodialysis may be an appropriate hemodialysis option for control of intradialytic hypotension in the acute care setting. Further research is recommended.

Brown adipose tissue and the regulation of nonshivering thermogenesis

PURPOSE OF REVIEW

The recent rediscovery of functional cold activated brown adipose tissue (BAT) in adult humans fuelled an uprise in studies on this tissue. This review focuses on the contribution of human BAT to nonshivering thermogenesis and on factors other than cold that activate BAT.

RECENT FINDINGS

Earlier studies revealed BAT activity using a glucose tracer for positron emission tomography/computed tomography (PET/CT) scanning. Several recent studies, using a mix of tracers and PET/CT dynamic scanning showed that human brown fat is metabolically active and related to the perfusion of the tissue. The actual contribution of BAT to nonshivering thermogenesis still needs to be explored.The last few years, several new factors that activate human BAT have been described. These studies also highlight the plasticity of brown and white adipose tissue. Some of these factors may have pharmacological significance.

SUMMARY

New PET/CT studies provide information on oxidative human BAT metabolism in vivo. This new information in combination with the study on factors activating BAT are promising with respect to management of obesity and related disorders.

Shivering management during therapeutic temperature modulation: nurses' perspective

Therapeutic temperature modulation, which incorporates mild hypothermia and maintenance of normothermia, is being used to manage patients resuscitated after cardiac arrest. Methods of modulating temperature include intravenous infusion of cold fluids and surface or endovascular cooling. During this therapy, the shiver response is activated as a defense mechanism in response to an altered set-point temperature and causes metabolic and hemodynamic stress for patients. Recognition of shivering according to objective and subjective assessments is vital for early detection of the condition. Once shivering is detected, treatment is imperative to avoid deleterious effects. The Bedside Shivering Assessment Scale can be used to determine the efficacy of interventions intended to blunt thermoregulatory defenses and can provide continual evaluation of patients' responses to the interventions. Nurses' knowledge and understanding of the harmful effects of shivering are important to effect care and prevent injury associated with uncontrolled shivering.

Acute cold exposure and cognitive function: evidence for sustained impairment

Several industries experience periods of cold exposure and rewarming throughout the workday but mental performance under these conditions is unknown. A better understanding of cognition during the rewarming phase after cold exposure may help reduce accidents and improve performance. Ten young men (wearing ~0.1 clo) underwent three consecutive mornings trials where they were exposed to cold air (10°C) and then subsequently rewarmed (25°C air). A computerised test battery was administered during each stage of the protocol to determine working memory, choice reaction time, executive function and maze navigation. Rectal and skin temperature, oxygen consumption and thermal sensation were also measured throughout and showed a typical response. Relative to baseline performance, working memory, choice reaction time and executive function declined during exposure to 10°C, and these impairments persisted 60 min into the recovery period (i.e. once physiological parameters had returned to baseline). Further work is needed to develop countermeasures to this predicament.

PRACTITIONER SUMMARY

This study showed that working memory, choice reaction time and executive function declined during exposure to 10°C air, and these impairments persisted 60 min into the rewarming period (i.e. once measurable physiological parameters had returned to normal). Individuals may be at risk for injury after removal from a cold environment.

Pharmacological blockade of the cold receptor TRPM8 attenuates autonomic and behavioral cold defenses and decreases deep body temperature

We studied N-(2-aminoethyl)-N-(4-(benzyloxy)-3-methoxybenzyl)thiophene-2-carboxamide hydrochloride (M8-B), a selective and potent antagonist of the transient receptor potential melastatin-8 (TRPM8) channel. In vitro, M8-B blocked cold-induced and TRPM8-agonist-induced activation of rat, human, and murine TRPM8 channels, including those on primary sensory neurons. In vivo, M8-B decreased deep body temperature (T(b)) in Trpm8(+/+) mice and rats, but not in Trpm8(-/-) mice, thus suggesting an on-target action. Intravenous administration of M8-B was more effective in decreasing T(b) in rats than intrathecal or intracerebroventricular administration, indicating a peripheral action. M8-B attenuated cold-induced c-Fos expression in the lateral parabrachial nucleus, thus indicating a site of action within the cutaneous cooling neural pathway to thermoeffectors, presumably on sensory neurons. A low intravenous dose of M8-B did not affect T(b) at either a constantly high or a constantly low ambient temperature (T(a)), but the same dose readily decreased T(b) if rats were kept at a high T(a) during the M8-B infusion and transferred to a low T(a) immediately thereafter. These data suggest that both a successful delivery of M8-B to the skin (high cutaneous perfusion) and the activation of cutaneous TRPM8 channels (by cold) are required for the hypothermic action of M8-B. At tail-skin temperatures <23°C, the magnitude of the M8-B-induced decrease in T(b) was inversely related to skin temperature, thus suggesting that M8-B blocks thermal (cold) activation of TRPM8. M8-B affected all thermoeffectors studied (thermopreferendum, tail-skin vasoconstriction, and brown fat thermogenesis), thus suggesting that TRPM8 is a universal cold receptor in the thermoregulation system.

β₁-Adrenergic receptors increase UCP1 in human MADS brown adipocytes and rescue cold-acclimated β₃-adrenergic receptor-knockout mice via nonshivering thermogenesis

With the finding that brown adipose tissue is present and negatively correlated to obesity in adult man, finding the mechanism(s) of how to activate brown adipose tissue in humans could be important in combating obesity, type 2 diabetes, and their complications. In mice, the main regulator of nonshivering thermogenesis in brown adipose tissue is norepinephrine acting predominantly via β(3)-adrenergic receptors. However, vast majorities of β(3)-adrenergic agonists have so far not been able to stimulate human β(3)-adrenergic receptors or brown adipose tissue activity, and it was postulated that human brown adipose tissue could be regulated instead by β(1)-adrenergic receptors. Therefore, we have investigated the signaling pathways, specifically pathways to nonshivering thermogenesis, in mice lacking β(3)-adrenergic receptors. Wild-type and β(3)-knockout mice were either exposed to acute cold (up to 12 h) or acclimated for 7 wk to cold, and parameters related to metabolism and brown adipose tissue function were investigated. β(3)-knockout mice were able to survive both acute and prolonged cold exposure due to activation of β(1)-adrenergic receptors. Thus, in the absence of β(3)-adrenergic receptors, β(1)-adrenergic receptors are effectively able to signal via cAMP to elicit cAMP-mediated responses and to recruit and activate brown adipose tissue. In addition, we found that in human multipotent adipose-derived stem cells differentiated into functional brown adipocytes, activation of either β(1)-adrenergic receptors or β(3)-adrenergic receptors was able to increase UCP1 mRNA and protein levels. Thus, in humans, β(1)-adrenergic receptors could play an important role in regulating nonshivering thermogenesis.

Cold tolerance, cold-induced hyperphagia, and nonshivering thermogenesis are normal in α₁-AMPK-/- mice

Recent studies indicate that a substantial amount of metabolically active brown adipose tissue (BAT) exists in adult humans. Given the unique ability of BAT to convert calories to heat, there is intense interest in understanding the regulation of BAT metabolism in hopes that its manipulation might be an effective way of expending excess calories. Because of the established role of AMP-activated protein kinase (AMPK) as a "metabolic master switch" and its extremely high levels of activity in BAT, it was hypothesized that AMPK might play a central role in regulating BAT metabolism. To test this hypothesis, whole body α(1)-AMPK(-/-) (knockout) and wild-type mice were studied 1) under control (room temperature) conditions, 2) during chronic cold exposure (14 days at 4°C), and 3) during acute nonshivering thermogenesis (injection of a β(3)-adrenergic agonist). Under control conditions, loss of α(1)-AMPK resulted in downregulation of two important prothermogenic genes in BAT, thyrotropin-releasing hormone (-9.2-fold) and ciliary neurotrophic factor (-8.7-fold). Additionally, it caused significant upregulation of α(2)-AMPK activity in BAT, white adipose tissue, and liver, but not cardiac or skeletal muscle. During acute nonshivering thermogenesis and chronic cold exposure, body temperature was indistinguishable in the α(1)-AMPK(-/-) and wild-type mice. Similarly, the degree of cold-induced hyperphagia was identical in the two groups. We conclude that α(1)-AMPK does not play an obligatory role in these processes and that adaptations to chronic loss of α(1)-AMPK are able to compensate for its loss via several mechanisms.

Automated discrimination of brain pathological state attending to complex structural brain network properties: the shiverer mutant mouse case

Neuroimaging classification procedures between normal and pathological subjects are sparse and highly dependent of an expert's clinical criterion. Here, we aimed to investigate whether possible brain structural network differences in the shiverer mouse mutant, a relevant animal model of myelin related diseases, can reflect intrinsic individual brain properties that allow the automatic discrimination between the shiverer and normal subjects. Common structural networks properties between shiverer (C3Fe.SWV Mbp(shi)/Mbp(shi), n = 6) and background control (C3HeB.FeJ, n = 6) mice are estimated and compared by means of three diffusion weighted MRI (DW-MRI) fiber tractography algorithms and a graph framework. Firstly, we found that brain networks of control group are significantly more clustered, modularized, efficient and optimized than those of the shiverer group, which presented significantly increased characteristic path length. These results are in line with previous structural/functional complex brain networks analysis that have revealed topologic differences and brain network randomization associated to specific states of human brain pathology. In addition, by means of network measures spatial representations and discrimination analysis, we show that it is possible to classify with high accuracy to which group each subject belongs, providing also a probability value of being a normal or shiverer subject as an individual anatomical classifier. The obtained correct predictions (e.g., around 91.6-100%) and clear spatial subdivisions between control and shiverer mice, suggest that there might exist specific network subspaces corresponding to specific brain disorders, supporting also the point of view that complex brain network analyses constitutes promising tools in the future creation of interpretable imaging biomarkers.

Effect of ascorbic acid on fatigue of skeletal muscle fibres in long-term cold exposed Sprague Dawley rats

BACKGROUND

On exposure to prolonged cold temperature, the body responds for effective heat production both by shivering and non-shivering thermogenesis. Cold exposure increases the production of reactive oxygen species which influence the sarcoplasmic reticulum Ca++ release from the skeletal muscles and affect their contractile properties. The role of ascorbic acid supplementation on force of contraction during fatigue of cold exposed skeletal muscles was evaluated in this study.

METHOD

Ninety healthy, male Sprague Dawley rats were randomly divided into three groups of control (I), cold exposed (II), and cold exposed with ascorbic acid 500 mg/L supplementation mixed in drinking water (III). Group II and III were given cold exposure by keeping their cages in ice-filled tubs for 1 hr/day for one month. After one month, the extensor digitorum longus muscle was dissected out and force of contraction during fatigue in the skeletal muscle fibres was analysed on a computerised data acquisition system.

RESULTS

The cold exposed group showed a significant delay in the force of contraction during fatigue of skeletal muscle fibres compared to control group. Group III showed easy fatigability and a better force of contraction than the cold exposed group.

CONCLUSIONS

Ascorbic acid increases the force of contraction and decreases resistance to fatigue in the muscles exposed to chronic cold.

Shivering heat production and body fat protect the core from cooling during body immersion, but not during head submersion: a structural equation model

Previous studies showed that core cooling rates are similar when only the head or only the body is cooled. Structural equation modeling was used on data from two cold water studies involving body-only, or whole body (including head) cooling. Exposure of both the body and head increased core cooling, while only body cooling elicited shivering. Body fat attenuates shivering and core cooling. It is postulated that this protection occurs mainly during body cooling where fat acts as insulation against cold. This explains why head cooling increases surface heat loss with only 11% while increasing core cooling by 39%.

Metabolic requirements of shivering humans

This review examines the heat production component of thermoregulation in adult humans. It describes the energy requirements of shivering muscles as they attempt to provide sufficient heat to counterbalance increases in heat loss in cold environments. Emphasis is placed on types of metabolic substrates used under various shivering conditions as well as on the effects energy deficit and food consumption. During shivering, muscle recruitment intensity and pattern of fiber recruitment are highly variable between muscles and individuals. In addition, a number of studies have indicated that shivering can be sustained with different fuels for several hours under variable conditions of cold stress and CHO availability. However, little is still known on the effects of prolonged fasting and energy deficit in the cold on energy metabolism. Even though it is clear that food consumption increases the odds for survival, the metabolic fate of ingested substrates remains highly uncertain. Combining fundamental principles surrounding metabolic fuel selection with applied knowledge of human performance in the cold may allow important breakthroughs in this field of research.

Behavioral temperature regulation in humans during mild narcosis induced by inhalation of 30% nitrous oxide

In this study, we investigated the influence of mild narcosis on temperature perception, thermal comfort, and behavioral temperature regulation in humans. Twelve subjects (six males and six females) participated in two trials, during which they wore a water-perfused suit (WPS). The temperature of the WPS (TWPS) fluctuated sinusoidally from 27 degrees to 42 degrees C, at a heating and cooling rate of 1.2 degrees C x min(-1). In the first trial, the subjects had no control over TWPS: They determined their thermal comfort zone (TCZ) by providing a subjective response whenever they perceived the temperature changing from a comfortable to an uncomfortable level and vice versa; in addition, they provided subjective ratings of temperature perception and thermal comfort on a 7-point and 4-point scale, respectively, at each 3 degrees C change in TWPS. In the second trial, subjects could change the direction of TWPS whenever it became uncomfortable by depressing a button on a manual control. The protocols were conducted with subjects breathing either room air (AIR), or a normoxic breathing mixture containing 30% N2O. Subjects perceived increasing TWPS as equally warm and the decreasing TWPS as equally cold with AIR or N2O. However, equal changes in TWPS were perceived as significantly less discomforting (P<0.05) during N2O, and the magnitude of the TCZ significantly (P<0.01) increased. Thus, narcosis did not alter thermal sensation, but it significantly changed the perception of comfort. These changes were not reflected in the behavioral response. Subjects produced similar TWPS damped-oscillation patterns in the AIR and N2O trials. We conclude that the narcosis-induced alteration in the perception of thermal comfort does not change the preferred temperature, or the ability to behaviorally maintain thermal comfort.

The MT pool size ratio and the DTI radial diffusivity may reflect the myelination in shiverer and control mice

A quantitative magnetization transfer (qMT) technique was employed to quantify the ratio of the sizes of the bound and free water proton pools in ex vivo mouse brains. The goal was to determine the pool size ratio sensitivity to myelin. Fixed brains from both shiverer mice and control littermates were imaged. The pool size ratio in the corpus callosum of shiverer mice was substantially lower than that in the control mice, while there was no distinguishable difference in the pool size ratio in the gray matter. These results correlate with diffusion tensor imaging (DTI) derived radial diffusivity which previously was shown to reflect myelin integrity in this animal model. Histological study reveals the presence of myelin in control mice white matter and the absence of myelin in shiverer mice white matter, supporting the qMT and DTI results. Our findings support the view that qMT may be used for estimating myelin integrity.

Perioperative complications of hypothermia

Perioperative hypothermia is a common and serious complication of anesthesia and surgery and is associated with many adverse perioperative outcomes. It prolongs the duration of action of inhaled and intravenous anesthetics as well as the duration of action of neuromuscular drugs. Mild core hypothermia increases thermal discomfort, and is associated with delayed post anaesthetic recovery. Mild hypothermia significantly increases perioperative blood loss and augments allogeneic transfusion requirement. Only 1.9 degrees C core hypothermia triples the incidence of surgical wound infection following colon resection and increases the duration of hospitalization by 20%. Hypothermia adversely affects antibody- and cell-mediated immune defences, as well as the oxygen availability in the peripheral wound tissues. Furthermore mild hypothermia triples the incidence of postoperative adverse myocardial events. Thus, even mild hypothermia contributes significantly to patient care costs and needs to be avoided.

Ice-cold saline for the induction of mild hypothermia in patients with acute ischemic stroke: a pilot study

BACKGROUND AND PURPOSE

Neuroprotective effects of induced hypothermia depend on its time point of initiation after acute brain injury. Preliminary studies in cardiac arrest patients indicate that rapid infusion of ice cold saline (ICS) is safe and effective for induction of hypothermia. We investigated its use in patients with acute ischemic stroke (AIS).

METHODS

Patients (n=10) with AIS were included within 3 hours after symptom onset. After cranial CT, they were treated-if indicated-with rt-PA. ICS of 4 degrees C (25 mL/kg body weight) was administered via peripheral intravenous lines. Patients received buspirone/pethidine to prevent and treat shivering. After infusion of the target volume of ICS, no further efforts were made to maintain hypothermia by other methods.

RESULTS

Ten patients with a median National Institutes of Health Stroke Scale (NIHSS) score of 5.5 (range 4 to 12) on admission were included into the study. Nine patients were treated with thrombolysis within a time window of 104+/-25 minutes. A mean amount of 2163+/-256 mL ICS was infused 17+/-11 minutes after rt-PA infusion had started. Tympanic temperature dropped significantly by a maximum of 1.6+/-0.3 degrees C (P<0.005) at 52+/-16 minutes after ICS was started. The procedure was well tolerated. The NIHSS score improved significantly to a median of 1 (range 1 to 15) at discharge compared to admission (P<0.02).

CONCLUSIONS

This pilot study suggests that rapid ICS infusions in combination with pethidine and buspirone lower the body temperature significantly without major side effects.

Shaking rigors in parturients

OBJECTIVE

To describe the frequency, duration and timing of shaking rigors during parturition and their associations with several clinical variables.

STUDY DESIGN

A total of 467 term, singleton paturients at a single hospital over a 13-month recruitment period were observed by their labor nurses for shaking rigors. Thirteen clinical variables, including length of labor, were also recorded. Multiple regression procedures were used to evaluate associations with presence or absence, number and total duration of rigors. The relationship in time between shaking rigors and sentinel events in labor was also examined. With this sample size, we had > 80% power to detect differences as small as 0.26 effect size units for continuous measures, or ORs of 1.75 or greater for categorical measures, when comparing patients with and without rigors.

RESULTS

In total, 57% of parturients experienced at least 1 rigor. Epidural usage was related to risk, duration and number of rigors. Increased maximum temperature was associated with an increased chance of rigors. Epidural, birth and maternal fever were antecedent events.

CONCLUSION

Over half of parturients experienced shaking rigors. Epidural administration and fever were associated with an increased probability of rigors and an increased number of episodes.

Temperature monitoring and perioperative thermoregulation

Most clinically available thermometers accurately report the temperature of whatever tissue is being measured. The difficulty is that no reliably core-temperature-measuring sites are completely noninvasive and easy to use-especially in patients not undergoing general anesthesia. Nonetheless, temperature can be reliably measured in most patients. Body temperature should be measured in patients undergoing general anesthesia exceeding 30 min in duration and in patients undergoing major operations during neuraxial anesthesia. Core body temperature is normally tightly regulated. All general anesthetics produce a profound dose-dependent reduction in the core temperature, triggering cold defenses, including arteriovenous shunt vasoconstriction and shivering. Anesthetic-induced impairment of normal thermoregulatory control, with the resulting core-to-peripheral redistribution of body heat, is the primary cause of hypothermia in most patients. Neuraxial anesthesia also impairs thermoregulatory control, although to a lesser extent than does general anesthesia. Prolonged epidural analgesia is associated with hyperthermia whose cause remains unknown.

Fueling shivering thermogenesis during passive hypothermic recovery

In humans, the relative importance of oxidative fuels for sustaining shivering during passive hypothermic recovery or rewarming is still unclear. The main goals of this study were 1) to quantify the respective contributions of lipids and carbohydrates (CHO) during passive rewarming and 2) to determine the effects of precooling exercise on the pattern of fuel utilization. With indirect calorimetry methodologies, changes in fuel metabolism were quantified in nonacclimatized adult men shivering to rewarm from moderate hypothermia (core temperature ∼34.5°C) not following (Con) or following a precooling exercise at 75% V̇o2max for 15 min (Pre-CE). As hypothermic individuals shiver to normothermia, results showed that CHO dominate at all shivering intensities above 50% Shivpeak, while lipids were preferred at lower intensities. This change in the relative importance of CHO and lipids to total heat production was dictated entirely by modulating CHO oxidation rate, which decreased by as much as 10-fold from the beginning to the end of rewarming (from 1,611 ± 396 to 141 ± 361 mg/min for Con and 1,555 ± 230 to 207 ± 261 mg/min for Pre-CE). In contrast, lipid oxidation rate remained constant and low (relatively to maximal rates at exercise) throughout rewarming, averaging 183 ± 141 for Con and 207 ± 118 mg lipids/min for Pre-CE. In addition, this pattern of fuel selection remained the same between treatments. We concluded that fuel selection is regulated entirely by changes in CHO oxidation rate. Further research should focus on establishing the exact regulatory processes involved in achieving this large upregulation of CHO utilization rate following hypothermia.

Cold-induced alterations of phospholipid fatty acyl composition in brown adipose tissue mitochondria are independent of uncoupling protein-1

The recruitment process induced by acclimation of mammals to cold includes a marked alteration in the acyl composition of the phospholipids of mitochondria from brown adipose tissue: increases in 18:0, 18:2(n-6), and 20:4(n-6) and decreases in 16:0, 16:1, 18:1, and 22:6(n-3). A basic question is whether these alterations are caused by changes in the concentration of uncoupling protein-1 (UCP1) or the thermogenesis it mediates-implying that they are secondary effects-or whether they are an integrated, independent part of the recruitment process. This question was addressed here using wild-type and UCP1-ablated C57BL/6 mice acclimated to 24 degrees C or 4 degrees C. In wild-type mice, the phospholipid fatty acyl composition of mitochondria from brown adipose tissue showed the changes in response to cold that were expected from observations in other species and strains. The changes were specific, as different changes occurred in skeletal muscle mitochondria. In UCP1-ablated mice, cold acclimation induced acyl alterations in brown adipose tissue that were qualitatively identical and quantitatively similar to those in wild-type mice. Therefore, neither the increased content of UCP1 nor mitochondrial uncoupling altered the effect of cold on acyl composition. Cold acclimation in wild-type mice had little effect on phospholipid acyl composition in muscle mitochondria, but cold-acclimation in UCP1-ablated mice caused significant alterations, probably due to sustained shivering. Thus, the alterations in brown adipose tissue phospholipid acyl composition are revealed to be an independent part of the recruitment process, and their functional significance for thermogenesis should be elucidated.