The role of antidiuretic hormone in cold-induced diuresis in the anaesthetized rat

The hypokalaemia that came from the cold

While electrolyte disorders are common in nephrologists' clinical practice, hypothermia is a condition that nephrologists rarely encounter. Hypothermia can induce several pathophysiological effects on the human body, including hypokalaemia, which is reversible with rewarming. Despite growing evidence from animal research and human studies, the underlying mechanisms of hypothermia-induced hypokalaemia remain unclear. Boubes and colleagues recently presented a case series of hypokalaemia during hypothermia and rewarming, proposing a novel hypothesis for the underlying mechanisms. In this editorial, we review the current knowledge about hypothermia and associated electrolyte changes with insights into the effects of hypothermia on renal physiology.

Influence of moderate-hypothermia on renal and cerebral haemodynamics and oxygenation during experimental cardiopulmonary bypass in sheep

AIM

Cardiac surgery requiring cardiopulmonary bypass (CPB) can result in renal and cerebral injury. Intra-operative tissue hypoxia could contribute to such organ injury. Hypothermia, however, may alleviate organ hypoxia. Therefore, we tested whether moderate-hypothermia (30^o C) improves cerebral and renal tissue perfusion and oxygenation during ovine CPB.

METHODS

Ten sheep were studied while conscious, under stable anaesthesia and during 3 hours of CPB. In a randomised within-animal cross-over design, 5 sheep commenced CPB at a target body temperature of 30 ^o C (moderate-hypothermia). After 90 minutes, body temperature was increased to 36 ^o C (standard-procedure). The remaining 5 sheep were randomised to the opposite order of target body temperature.

RESULTS

Compared with the standard-procedure, moderately-hypothermic CPB reduced renal oxygen delivery (-34.8 ± 19.6%, P = 0.003) and renal oxygen consumption (-42.7 ± 35.2%, P = 0.04). Nevertheless, moderately-hypothermic CPB did not significantly alter either renal cortical or medullary tissue PO₂ . Moderately-hypothermic CPB also did not significantly alter cerebral perfusion, cerebral tissue PO₂ , or cerebral oxygen saturation compared with the standard-procedure. Compared with anaesthetised state, standard-procedure reduced renal medullary PO₂ (-21.0 ± 13.8 mmHg, P = 0.014) and cerebral oxygen saturation (65.0 ± 7.0 to 55.4 ± 9.6%, P = 0.022) but did not significantly alter either renal cortical or cerebral PO₂ .

CONCLUSION

Ovine experimental CPB leads to renal medullary tissue hypoxia. Moderately-hypothermic CPB did not improve cerebral or renal tissue oxygenation. In the kidney, this is probably because renal tissue oxygen consumption is matched by reduced renal oxygen delivery.

Electrolyte profiles with induced hypothermia: A sub study of a clinical trial evaluating the duration of hypothermia after cardiac arrest

Background

Electrolyte disturbances can result from targeted temperature treatment (TTM) in out‐of‐hospital cardiac arrest (OHCA) patients. This study explores electrolyte changes in blood and urine in OHCA patients treated with TTM.

Methods

This is a sub‐study of the TTH48 trial, with the inclusion of 310 unconscious OHCA patients treated with TTM at 33°C for 24 or 48 h. Over a three‐day period, serum concentrations were obtained on sodium potassium, chloride, ionized calcium, magnesium and phosphate, as were results from a 24‐h diuresis and urine electrolyte concentration and excretion. Changes over time were analysed with a mixed‐model multivariate analysis of variance with repeated measurements.

Results

On admission, mean ± SD sodium concentration was 138 ± 3.5 mmol/l, which increased slightly but significantly (p < .05) during the first 24 h. Magnesium concentration stayed within the reference interval. Median ionized calcium concentration increased from 1.11 (IQR 1.1–1.2) mmol/l during the first 24 h (p < .05), whereas median phosphate concentration dropped to 1.02 (IQR 0.8–1.2) mmol/l (p < .05) and stayed low. During rewarming, potassium concentrations increased, and magnesium and ionizes calcium concentration decreased (p < .05). Median 24‐h diuresis results on days one and two were 2198 and 2048 ml respectively, and the electrolyte excretion mostly stayed low in the reference interval.

Conclusions

Electrolytes mostly remained within the reference interval. A temporal change occurred in potassium, magnesium and calcium concentrations with TTM’s different phases. No hypothermia effect on diuresis was detected, and urine excretion of electrolytes mostly stayed low.

Association between winter season and desmopressin treatment efficiency in children with monosymptomatic nocturnal enuresis: a pilot study

Objectives:

The purpose of our study was to assess the association between the winter season and desmopressin treatment failure in South Chinese children with monosymptomatic nocturnal enuresis (MNE).

Materials and Methods:

A retrospective study was conducted to analyze the clinical data of children with monosymptomatic nocturnal enuresis who have visited our urology clinic from January to December 2019. All patients received desmopressin treatment. Final treatment outcomes were categorized as successful (complete response) or failed (absent and partial response). The relationship between winter season and treatment response to desmopressin was evaluated. Additionally, associated risk factors were investigated with both univariate and multivariate regression analysis.

Results:

In total, 393 patients diagnosed with MNE were included in the present study. There were no statistically significant differences in pretreatment variables at first visit between patients who visited the clinic in winter and those who did so in other seasons. However, the treatment failure rate of MNE in the winter season was higher than that of other seasons (77.50% vs. 52.74%). Multivariate logistic regression analysis demonstrated that the severity of symptoms and an initial clinic visit in the winter season were significantly related to desmopressin treatment failure in MNE patients.

Conclusion:

Winter season and severity of symptoms are two risk factors associated with desmopressin treatment failure in MNE patients.

Impact of rewarming rate on the mortality of patients with accidental hypothermia: analysis of data from the J-Point registry

BACKGROUND

Accidental hypothermia (AH) is defined as an involuntary decrease in core body temperature to < 35 °C. The management of AH has been progressing over the last few decades, and numerous techniques for rewarming have been validated. However, little is known about the association between rewarming rate (RR) and mortality in patients with AH.

METHOD

This was a multicentre chart review study of patients with AH visiting the emergency department of 12 institutions in Japan from April 2011 to March 2016 (Japanese accidental hypothermia network registry, J-Point registry). We retrospectively registered patients using the International Classification of Diseases, Tenth Revision code T68: 'hypothermia'. We excluded patients whose body temperatures were unknown or ≥ 35 °C, who could not be rewarmed, whose rewarmed temperature or rewarming time was unknown, those aged < 18 years, or who or whose family members had refused to join the registry. RR was calculated based on the body temperature on arrival at the hospital, time of arrival at the hospital, the documented temperature during rewarming, and time of the temperature documentation. RR was classified into the following five groups: ≥2.0 °C/h, 1.5-< 2.0 °C/h, 1.0-< 1.5 °C/h, 0.5-< 1.0 °C/h, and < 0.5 °C/h. The primary outcome of this study was in-hospital mortality. The association between RR and in-hospital mortality was evaluated using multivariate logistic regression analysis.

RESULT

During the study, 572 patients were registered in the J-Point registry, and 481 patients were included in the analysis. The median body temperature on arrival to the hospital was 30.7 °C (interquartile range [IQR], 28.2 °C-32.4 °C), and the median RR was 0.85 °C/h (IQR, 0.53 °C/h-1.31 °C/h). The in-hospital mortality rates were 19.3% (11/57), 11.1% (4/36), 14.4% (15/104), 20.1% (35/175), and 34.9% (38/109) in the ≥2.0 °C/h, 1.5-< 2.0 °C/h, 1.0-< 1.5 °C/h, 0.5-< 1.0 °C/h, and < 0.5 °C/h groups, respectively. Multivariate regression analysis revealed that in-hospital mortality rate increased with each 0.5 °C/h decrease in RR (adjusted odds ratio, 1.49; 95% confidence interval, 1.15-1.94; P_trend < 0.01).

CONCLUSION

This study showed that slower RR is independently associated with in-hospital mortality.

Accidental hypothermia

Accidental hypothermia causes profound changes to the body's physiology. After an initial burst of agitation (e.g., 36-37°C), vital functions will slow down with further cooling, until they vanish (e.g. <20-25°C). Thus, a deeply hypothermic person may appear dead, but may still be able to be resuscitated if treated correctly. The hospital use of minimally invasive rewarming for nonarrested, otherwise healthy patients with primary hypothermia and stable vital signs has the potential to substantially decrease morbidity and mortality for these patients. Extracorporeal life support (ECLS) has revolutionized the management of hypothermic cardiac arrest, with survival rates approaching 100%. Hypothermic patients with risk factors for imminent cardiac arrest (i.e., temperature <28°C, ventricular arrhythmia, systolic blood pressure <90 mmHg), and those who have already arrested, should be transferred directly to an ECLS center. Cardiac arrest patients should receive continuous cardiopulmonary resuscitation (CPR) during transfer. If prolonged transport is required or terrain is difficult, mechanic CPR can be helpful. Intermittent CPR may be appropriate in hypothermic arrest when continuous CPR is impossible. Modern postresuscitation care should be implemented following hypothermic arrest. Structured protocols should be in place to optimize prehospital triage, transport, and treatment as well as in-hospital management, including detailed criteria and protocols for the use of ECLS and postresuscitation care.

Effect of nitric oxide on renal autoregulation during hypothermia in the rat

Hypothermia-induced reduction of metabolic rate is accompanied by depression of both glomerular perfusion and filtration. The present study investigated whether these changes are linked to changes in renal autoregulation and nitric oxide (NO) signalling. During hypothermia, renal blood flow (RBF) and glomerular filtration rate (GFR) were reduced and urine production was increased, and this was linked with reduced plasma cGMP levels and increased renal vascular resistance. Although stimulation of NO production decreased vascular resistance, blood pressure and urine flow, intravenous infusion of the NO precursor L-arginine or the NO donor sodium nitroprusside did not alter RBF or GFR. In contrast, inhibition of NO synthesis by N^w-nitro-L-arginine led to a further decline in both parameters. Functional renal autoregulation was apparent at both temperatures. Below the autoregulatory range, RBF in both cases increased in proportion to the perfusion ±pressure, although, the slope of the first ascending limb of the pressure-flow relationship was lower during hypothermia. The main difference was rather that the curves obtained during hypothermia levelled off already at a RBF of 3.9 ± 0.3 mL/min then remained stable throughout the autoregulatory pressure range, compared to 7.6 ± 0.3 mL/min during normothermia. This was found to be due to a threefold increase in, primarily, the afferent arteriolar resistance from 2.6 to 7.5 mmHg min mL⁻¹. Infusion of sodium nitroprusside did not significantly affect RBF during hypothermia, although a small increase at pressures below the autoregulatory range was observed. In conclusion, cold-induced rise in renal vascular resistance results from afferent arteriolar vasoconstriction by the autoregulatory mechanism, setting RBF and GFR in proportion to the metabolic rate, which cannot be explained by reduced NO production alone.

Saturation diving; physiology and pathophysiology

In saturation diving, divers stay under pressure until most of their tissues are saturated with breathing gas. Divers spend a long time in isolation exposed to increased partial pressure of oxygen, potentially toxic gases, bacteria, and bubble formation during decompression combined with shift work and long periods of relative inactivity. Hyperoxia may lead to the production of reactive oxygen species (ROS) that interact with cell structures, causing damage to proteins, lipids, and nucleic acid. Vascular gas-bubble formation and hyperoxia may lead to dysfunction of the endothelium. The antioxidant status of the diver is an important mechanism in the protection against injury and is influenced both by diet and genetic factors. The factors mentioned above may lead to production of heat shock proteins (HSP) that also may have a negative effect on endothelial function. On the other hand, there is a great deal of evidence that HSPs may also have a "conditioning" effect, thus protecting against injury. As people age, their ability to produce antioxidants decreases. We do not currently know the capacity for antioxidant defense, but it is reasonable to assume that it has a limit. Many studies have linked ROS to disease states such as cancer, insulin resistance, diabetes mellitus, cardiovascular diseases, and atherosclerosis as well as to old age. However, ROS are also involved in a number of protective mechanisms, for instance immune defense, antibacterial action, vascular tone, and signal transduction. Low-grade oxidative stress can increase antioxidant production. While under pressure, divers change depth frequently. After such changes and at the end of the dive, divers must follow procedures to decompress safely. Decompression sickness (DCS) used to be one of the major causes of injury in saturation diving. Improved decompression procedures have significantly reduced the number of reported incidents; however, data indicate considerable underreporting of injuries. Furthermore, divers who are required to return to the surface quickly are under higher risk of serious injury as no adequate decompression procedures for such situations are available. Decompression also leads to the production of endothelial microparticles that may reduce endothelial function. As good endothelial function is a documented indicator of health that can be influenced by regular exercise, regular physical exercise is recommended for saturation divers. Nowadays, saturation diving is a reasonably safe and well controlled method for working under water. Until now, no long-term impact on health due to diving has been documented. However, we still have limited knowledge about the pathophysiologic mechanisms involved. In particular we know little about the effect of long exposure to hyperoxia and microparticles on the endothelium.

Urine Output Changes During Postcardiac Arrest Therapeutic Hypothermia

While commonly described, no studies have characterized cold-induced diuresis or rewarm anti-diuresis occurring during the delivery of therapeutic hypothermia (TH). We sought to determine urine output changes during the provision of postcardiac arrest TH. We analyzed clinical data on patients receiving postcardiac arrest TH at an urban tertiary care center. TH measures included cooling by cold intravenous fluid, external ice packs, and a commercial external temperature management system. TH treatment was divided into phases: (1) induction, (2) maintenance, (3) rewarm, and (4) post-rewarm. The primary outcome measure was the mean urine output rate (mL/hour). We compared urine output rates between TH phases using a Generalized Estimating Equations model, defining urine output rate (mL/hour) as the dependent variable and TH phase (induction, maintenance, rewarm, and post-rewarm) as the primary exposure variable. We adjusted for age, sex, initial ECG rhythm, location of arrest, shock, acute kidney injury, rate of intravenous fluid input, and body mass index. Complete urine output data were available on 33 patients. Mean urine output rates during induction, maintenance, rewarm, and post-rewarm phases were 157 mL/hour (95% CI: 104-210), 103 mL/hour (95% CI: 82-125), 70 mL/hour (95% CI: 51-88), and 91 mL/hour (95% CI: 65-117), respectively. Compared with the post-rewarm phase, adjusted urine output was higher during the TH induction phase (output rate difference +51 mL/hour; 95% CI: 3-99). Adjusted urine output during the maintenance and rewarm phases did not differ from the post-rewarm phase. In this preliminary study, we observed modest increases in urine output during TH induction. We did not observe urine output changes during TH maintenance or rewarming.

Effects of transport duration on maintenance behavior, heart rate and gastrointestinal tract temperature of market-weight pigs in 2 seasons

Welfare and meat quality of market-weight pigs may be negatively affected by transport duration and environmental temperatures, which vary considerably between seasons. This study evaluated the effects of 3 transport durations (6, 12, and 18 h) on the physiology and behavior of pigs in summer and winter in western Canada. Market-weight pigs were transported using a pot-belly trailer at an average loading density of 0.375 m(2)/100 kg. Four replicates of each transport duration were conducted during each season. Heart rate and gastrointestinal tract temperature (GTT) were monitored from loading to unloading in 16 pigs from 4 selected trailer compartments (n = 96 groups, total of 384 animals, BW = 120.8 ± 0.4 kg), namely top front (C1), top back (C4), middle front (C5), and bottom rear (C10). Behavior was recorded for pigs (948 and 924 animals, in summer and winter, respectively) in C1, C4, and C5 during transportation (standing, sitting, lying), and during 90 min in lairage (sitting, lying, drinking, latency to rest) for pigs in all 4 compartments. Transport was split into 7 periods: loading, pre-travel (PT), initial travel (IT), pre-arrival 1 (PA1) and 2 (PA2), unloading, and lairage. During IT and PA2, pigs spent less time lying in winter than summer (P < 0.05 and P < 0.05, respectively). During PA1, PA2, and unloading, a greater (P < 0.001) heart rate was found in pigs transported in winter compared with summer. During PA2, pigs subjected to the 18-h transport treatment in winter had a greater (P < 0.05) GTT than the other groups. In lairage, pigs transported for 18 h in winter drank more (P < 0.001) and took longer to rest (P < 0.01) than pigs from other groups. During PA1, pigs transported for 18 h had the greatest GTT (P < 0.001). At unloading, pigs transported for 6 h had the lowest GTT (P < 0.001). In lairage, pigs transported for 18 h spent less time lying than those transported for 6 or 12 h (P < 0.001). These results suggest that in winter, pigs increased their metabolism and were reluctant to rest on cold floors. Pigs transported for 18 h in winter showed greater evidence of thirst. It may be concluded that under western Canadian climatic conditions, long transports (18 h) in cold weather appear to be more detrimental to pig welfare.

Ornithine decarboxylase activity in rat organs and tissues under artificial hypobiosis

The influence of hypothermia-hypoxia-hypercapnia on ornithine decarboxylase (ODC, EC 4.1.1.17) activities in rat organs and tissues and also on the thymocyte distribution throughout the cell cycle stages was studied. The state of artificial hypobiosis in rats on decrease in the body temperature to 14.4-18.0°C during 3.0-3.5 h was accompanied by drops in the ODC activities in the neocortex and liver by 50-60% and in rapidly proliferating tissues (thymus, spleen, and small intestine mucosa) by 80% of the control value. In kidneys the ODC activity raised to 200% of the control level. Twenty-four hours after termination of the cooling and replacing the rats under the standard conditions, the ODC activities in the neocortex, liver, kidneys, spleen, and intestinal mucosa returned to the control values, but remained decreased in the thymus. Forty-eight hours later the ODC activities in the thymus and spleen exceeded the normal level. The distribution of thymocytes throughout the cell cycle stages did not change in rats in the state of hypothermia (hypobiosis); 24 and 48 h after termination of the cooling the fraction of thymocytes in the S stage was decreased and the fraction of the cells in the G(0)+G(1) stage was increased. The normal distribution of thymocytes throughout the cell cycle stages recovered in 72 h. Thus, in the thymus the diminution of the ODC activity preceded the suppression of the cell proliferation rate. The tissue-specific changes in the ODC activity are suggested to reflect adaptive changes in the functional and proliferative activities of organs and tissues during the development of hypobiosis under conditions of hypothermia-hypoxia-hypercapnia.

[Therapeutic hypothermia]

The use of therapeutic hypothermia has been shown to improve survival and neurological outcome following cardiac arrest. Patients with traumatic brain injury or ischemic stroke also responded positively to therapeutic hypothermia, which may be induced by various procedures including surface cooling, endovascular cooling catheter and cold infusion. Possible side effects include infection and hemorrhage, as well as changes in water and electrolyte levels. It is the aim of this article to provide an overview of studies to date, as well as practical guidance for the application of therapeutic hypothermia.

Environmental Cold-Induced Injury

More than 650 deaths from hypothermia occur each year in the United States. Even minor deviation from normal temperature leads to important symptoms and disability. The most significant risk factors are advanced age, mental impairment, substance abuse, and injury. This article examines the incidence of hypothermia, its detrimental effect on trauma patients, and methods of rewarming the hypothermic patient. It also looks at the controversial protective role hypothermia might play in shock, organ transplantation, cardiac arrest, and brain injury. Finally, it examines cold injuries, including frostbite, chilblain, and trench foot, and makes recommendations for their treatment.

Genetic AVP deficiency abolishes cold-induced diuresis but does not attenuate cold-induced hypertension

Chronic cold exposure causes hypertension and diuresis. The aim of this study was to determine whether vasopressin (AVP) plays a role in cold-induced hypertension and diuresis. Two groups of Long-Evans (LE) and two groups of homozygous AVP-deficient Brattleboro (VD) rats were used. Blood pressure (BP) was not different among the four groups during a 2-wk control period at room temperature (25 degrees C, warm). After the control period, one LE group and one VD group were exposed to cold (5 degrees C); the remaining groups were kept at room temperature. BP and body weight were measured weekly during exposure to cold. Food intake, water intake, urine output, and urine osmolality were measured during weeks 1, 3, and 5 of cold exposure. At the end of week 5, all animals were killed and blood was collected for measurement of plasma AVP. Kidneys were removed for measurement of renal medulla V2 receptor mRNA and aquaporin-2 (AQP-2) protein expression. BP of LE and VD rats increased significantly by week 2 of cold exposure and reached a high level by week 5. BP elevations developed at approximately the same rate and to the same degree in LE and VD rats. AVP deficiency significantly increased urine output and solute-free water clearance and decreased urine osmolality. Chronic cold exposure increased urine output and solute-free water clearance and decreased urine osmolality in LE rats, indicating that cold exposure caused diuresis in LE rats. Cold exposure failed to affect these parameters in VD rats, suggesting that the AVP system is responsible for cold-induced diuresis. Cold exposure did not alter plasma AVP in LE rats. Renal medulla V2 receptor mRNA and AQP-2 protein expression levels were decreased significantly in the cold-exposed LE rats, suggesting that cold exposure inhibited renal V2 receptors and AVP-inducible AQP-2 water channels. We conclude that 1) AVP may not be involved in the pathogenesis of cold-induced hypertension, 2) the AVP system plays a critical role in cold-induced diuresis, and 3) cold-induced diuresis is due to suppression of renal V2 receptors and the associated AQP-2 water channels, rather than inhibition of AVP release.

[Therapeutic hypothermia after cardiac arrest]

The use of therapeutic hypothermia following different hypoxic-ischaemic insults has played an important role in various concepts of non-specific protection of cells for a long time. Although the use of deep therapeutic hypothermia after cardiac arrest in the last century did not lead to an improved outcome, recent data have demonstrated very positive effects of mild therapeutic hypothermia. The data from the European multicenter trial as well as those from Australia have clearly demonstrated a decrease in mortality and a better neurological outcome for patients being cooled to 32-34 degrees C for 12 or 24 h. In 2003, this led to the implementation of mild therapeutic hypothermia (32-34 degrees C) into the International Liaison Committee on Resuscitation (ILCOR) recommendations and guidelines for the treatment of unconscious patients after prehospital cardiac arrest. This article gives an overview on existing concepts and future perspectives of therapeutic mild hypothermia.

Mild hypothermia for acute liver failure: a review of mechanisms of action

Brain edema with intracranial hypertension is a major complication in patients with acute liver failure. Current therapies for this complication include a variety of pharmacologic and interventional measures, some of which are frequently associated with adverse effects or contraindications. Even though these measures usually allow the control of intracranial hypertension for a certain period of time, recurrence is common. New therapies are therefore needed. Increasing clinical and experimental evidence suggests that induction of mild hypothermia (32 degrees C-35 degrees C) may be a therapeutic alternative. Similar to traumatic brain injury or brain stroke, induction of mild hypothermia seems highly effective to reduce intracranial pressure in patients with acute liver failure. Several mechanisms by which mild hypothermia may prevent brain edema and intracranial hypertension in this condition have been disclosed and may include beneficial effects on ammonia metabolism, as well as on the disturbances of brain osmolarity, cerebrovascular hemodynamics, brain glucose metabolism, inflammation, and others. Improvement of systemic hemodynamics and amelioration of liver injury may be other benefits of the systemic induction of mild hypothermia, but the impact of potential adverse events, such as infection, should also be taken into account. At a time when mild hypothermia is increasingly used in several specialized centers, performance of a randomized controlled trial seems critical to confirm the benefits of mild hypothermia in acute liver failure and to provide adequate guidelines for its use.

The influence of acute hypothermia on renal function of anaesthetized euthermic and acclimatized rats

Acute hypothermia has a major impact on cardiovascular control and renal function, but the extent to which these can be correlated with and influenced by changes in the altered pattern of sympathetic outflow to the kidneys is unclear. Moreover, it is unknown whether these responses to acute hypothermia are altered by chronic cold exposure and this study aimed to examine these factors. Renal function and renal sympathetic nerve activity (RSNA) were measured in male Wistar rats, euthermic (control) or acclimatized (exposed to progressively lower environmental temperature and photoperiod over 8 weeks), anaesthetized with chloralose/urethane. Reduction of core temperature (Tc) to 25 degrees C caused approximately 40% reduction in heart rate (HR), approximately 10% fall in mean arterial blood pressure (MABP), and decreased glomerular filtration rate (GFR) by approximately 50% and approximately 5% in euthermic and acclimatized rats, respectively. At 25 degrees C, urine flow increased some two-fold and absolute and fractional sodium excretions by 4- to 6-fold in the euthermic rats and to a lesser extent in the cold acclimatized rats, while basal levels of fluid excretion were higher in the acclimatized rats. A loss of pulsatility in the RSNA signal with cooling was seen in both groups. One of the factors contributing to modest hypotension during acute hypothermia is a reduction in RSNA. There was a progressive fall in the proportion of RSNA power at HR frequency with cooling of 20% in euthermic and 80% in acclimatized rats. All variables were restored to basal levels on rewarming in both groups of rats. We conclude that natriuresis and diuresis in euthermic rats during hypothermia is a consequence of a reduction in nephron reabsorption, reduced urine osmolality and possibly altered patterning of RSNA. In acclimatized rats, the response was modified by altered renal haemodynamics and/or hormonal influences induced by chronic cold exposure to minimize the hypothermic stress on renal function.

Effects of desmopressin on prolonging survival in stable hypothermia in rats

The aim of the present study was to examine whether minimizing plasma volume loss due to cold-induced diuresis can increase the survival time of rats maintained in long-term stable hypothermia (~24 h at a body temperature of 19 degrees C). Infusion of desmopressin (0.5-2.0 microg), a potent antidiuretic agent, during the cooling period enhanced survival over saline controls in a dose-related manner. The enhanced survival was accompanied by a significant delay in the expected increase of hematocrit and decrease of plasma volume as compared with those seen in saline controls. In contrast, treating the rats with the same dose range of another vasopressin analog, [beta-mercapto-beta,beta-cyclopentamethyl enepropionyl]-vasopressin, which has no antidiuretic action, failed to enhance survival over saline control. Further, treating the rats with the optimal dose of desmopressin (1 microg) at the later stage of hypothermia failed to elicit any beneficial effect. Our results indicate that by using desmopressin early during the cooling phase of the hypothermia, plasma volume and rheological parameters important for sustaining microcirculation can be better maintained than those seen in saline controls. These improvements may have contributed to the observed longer survival time in hypothermia.

Renal responses to chronic cold exposure

The aim of this study was to assess our hypothesis that the release of antidiuretic hormone (ADH), the renal concentrating response to ADH, or both is decreased by prolonged cold exposure. Six groups (n = 6/group) of rats were used. Three groups were exposed to cold (5 degrees C), whilethe remaining three groups were kept at room temperature (25 degrees C). It was found that urine osmolality decreased significantly and serum osmolality increased significantly during cold exposure. The ratio of water/food intake was not affected by prolonged cold exposure. However, prolonged cold exposure increased the ratio of urine output/food intake in the cold-exposed rats, indicating that more urine flow is required by the cold-exposed rats to excrete the osmotic substance at a given food intake. The difference between water intake and urine output decreased significantly in the cold-exposed rats. Thus, prolonged cold exposure increases water loss from excretion. Renal concentrating responses to 24-h dehydration and Pitressin were decreased significantly in the cold-exposed rats. Plasma ADH levels remained unchanged, but renal ADH receptor (V2 receptor) mRNA was decreased significantly in the cold-exposed rats. The results strongly support the conclusion that cold exposure increases excretive water loss, and this may be due to suppression of renal V2 receptors rather than inhibition of ADH release.

Frostbite: pathogenesis and treatment

Frostbite, once almost exclusively a military problem, is becoming more prevalent among the general population and should now be considered to be within the scope of the civilian physician's practice. Studies into the epidemiology of civilian frostbite have identified several risk factors that may aid the clinician in the diagnosis and management of cold injuries. Research into the pathophysiology has revealed marked similarities in inflammatory processes to those seen in thermal burns and ischemia/reperfusion injury. Evidence of the role of thromboxanes and prostaglandins has resulted in more active approaches to the medical treatment of frostbite wounds. Although the surgical management of frostbite involves delayed debridement 1 to 3 months after demarcation, recent improvements in radiologic assessment of tissue viability have led to the possibility of earlier surgical intervention. In addition, several adjunctive therapies, including vasodilators, thrombolysis, hyperbaric oxygen, and sympathectomy, are discussed.