Core Temperature Fluctuations Research Articles

Thermal Sensation After the 10-km Open-Water Swimming in Cool Water Depends on the Skin's Thermal Sensitivity Rather Than Core Temperature.

To assess the core temperature fluctuations during 10-km open-water swimming (OWS) in cool water and the relationship between thermal sensation (TS) after 10-km OWS, core temperature, and local skin thermal sensitivity. Nine highly trained OWS swimmers (4 female; age 22 [3]y) completed a single 10-km trial in cool water (22.3°C) wearing swimsuitsfor OWS. During the trial, core temperature was continuously recorded via ingestible temperature sensors, and TS after trial was also measured. Then, local skin warm/cool sensitivity was measured in the forearm. All swimmers completed the 10-km OWS. Mean swimming speed for males and females were 1.39 (1.37-1.42m/s) and 1.33m/s (1.29-1.38m/s), respectively. Core temperature increased in 8 out of 9 swimmers during 10-km OWS (P = .047), with an average increase of 0.8°C. TS after 10-km OWS varied among swimmers. There were no correlations between post-OWS TS and post-OWS core temperature (P = .9333), whereas there was a negative correlation between post-OWS TS and local skin cool sensitivity (P = .0056). These results suggest that core temperature in elite swimmers might not decrease during 10-km OWS in the cool water temperature of official OWS. In addition, individual differences in TS after 10-km OWS may be related to skin cool sensitivity rather than core temperature.

Effects of core temperature, skin temperature, and inter-beat interval on resting metabolic rate measurements in thermoneutral conditions.

It is important to identify potential underlying factors that can affect the variability of resting metabolic rate (RMR) measurements. The RMRs of 20 college-aged men were tested twice in stable environmental conditions, with each measurement separated by 40 min. Skin temperature, core temperature, and inter-beat interval were monitored throughout the study as identified factors that could affect RMR measurements. Since environmental conditions in a clinic or laboratory can vary, skin temperature and core temperature can be affected which may affect RMR. Similarly, prior physical activity, stress, sleep, and caffeine intake can affect inter-beat interval and may be a co-variable affecting RMR. Higher RMR measurements were compared to lower RMR measurements. RMR for the higher trial was 2068 ± 66 kcal/day, the lower trial was 1975 ± 65 kcal/day (t = 4.23; p < 0.01). Core temperature for higher trial was 37.1 ± 0.1 °C, the lower trial was 36.8 ± 0.1 °C (s = 105.00; p < 0.01). Skin temperature measurements were significantly different for the anterior bicep site (t = -2.52; p = 0.02), but not for any other site. Inter-beat interval for the higher measurement was 1038 ± 33 ms, the lower measurement was 998 ± 32 ms (t = 3.82; p < 0.01). However, regression analysis found that none of the variables were significant predictors for the higher RMR, lower RMR, or change in RMR. While the factors affecting RMR measurement variability remain unclear, the results suggest that typical fluctuations in core temperature, skin temperature, and inter-beat interval do not effectively predict changes in RMR in a thermoneutral environment.

Model-independent astrophysical constraints on leptophilic Dark Matter in the framework of Tsallis statistics

We derive model-independent astrophysical constraints on leptophilic dark matter (DM), considering its thermal production in a supernova core and taking into account core temperature fluctuations within the framework of q-deformed Tsallis statistics. In an effective field theory approach, where the DM fermions interact with the Standard Model via dimension-six operators of either scalar-pseudoscalar, vector-axial vector, or tensor-axial tensor type, we obtain bounds on the effective cut-off scale Λ from supernova cooling and free-streaming of DM from supernova core, and from thermal relic density considerations, depending on the DM mass and the q-deformation parameter. Using Raffelt's criterion on the energy loss rate from SN1987A, we obtain a lower bound on Λ ≳ 3 (12) TeV corresponding to q = 1.0 (1.1) and an average supernova core temperature of 0TSN=3 MeV . From the optical depth criterion on the free-streaming of DM fermions from the outer 10% of the SN1987A core, the cooling bound is restricted to Λ ≳ 1 TeV . Both cooling and free-streaming bounds are insensitive to the DM mass mχ for mχ≲ TSN, whereas for mχ≫ TSN, the bounds weaken significantly due to the Boltzmann-suppression of the DM number density. We also calculate the thermal relic density of the DM particles in this setup and find that it imposes an upper bound on Λ4/mχ2, which together with the cooling/free-streaming bound significantly constrains light leptophilic DM.

Formation and evolution of quasi-interchange convection cell on the HL-2A tokamak

Formation and evolution of quasi-interchange convection cell have been observed for the first time by an electron cyclotron emission imaging (ECEI) system on the HL-2A tokamak. The instability with mode numbers of m/n = –1/–1 and a frequency of 2–4 kHz propagates in the electron diamagnetic drift direction. The mode downgrades the inner stored energy WE and enhances the divertor Hα signal, which indicates a degradation of plasma confinement. The ECEI reveals that the colder core temperature fluctuation distorts to a crescent shape, while a bubble originates from hot fluctuation gradually and draws into the concave side of the crescent. A finger-like structure can be observed during the convection process, and it presents a typical feature of the quasi-interchange model. The radial structure of the interchange mode obtained from electron cyclotron emission is quite narrow at the high field side but relatively wider at the low field side. Further analysis suggests that the discharge parameters are favorable for the excitation of interchange mode, and the stability criterion is violated in the core region. Moreover, the experimental frequency agrees with that given by the linear dispersion relation of interchange instability. The large grow rate suggests that the mode grows on a very short time scale.

Clock-dependent and system-driven oscillators interact in the suprachiasmatic nuclei to pace mammalian circadian rhythms.

Circadian clocks drive biological rhythms with a period of approximately 24 hours and keep in time with the outside world through daily resetting by environmental cues. While this external entrainment has been extensively investigated in the suprachiasmatic nuclei (SCN), the role of internal systemic rhythms, including daily fluctuations in core temperature or circulating hormones remains debated. Here, we show that lactating mice, which exhibit dampened systemic rhythms, possess normal molecular clockwork but impaired rhythms in both heat shock response gene expression and electrophysiological output in their SCN. This suggests that body rhythms regulate SCN activity downstream of the clock. Mathematical modeling predicts that systemic feedback upon the SCN functions as an internal oscillator that accounts for in vivo and ex vivo observations. Thus we are able to propose a new bottom-up hierarchical organization of circadian timekeeping in mammals, based on the interaction in the SCN between clock-dependent and system-driven oscillators.

Investigation of circulating temperature fluctuations of the primary coolant in order to develop an enhanced MTC estimator for VVER-440 reactors

Abstract Our aim was to develop a method based on noise diagnostics for the estimation of the moderator temperature coefficient of reactivity (MTC) for the Paks VVER-440 units in normal operation. The method requires determining core average neutron flux and temperature fluctuations. The circulation period of the primary coolant, transfer properties of the steam generators, as well as the source and the propagation of the temperature perturbations and the proportions of the perturbation components were investigated in order to estimate the feedback caused by the circulation of the primary coolant. Finally, after developing the new MTC estimator, determining its frequency range and optimal parameters, trends were produced based on an overall evaluation of measurements made with standard instrumentation during a one-year-long period at Paks NPP.

Acute Effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or Paraquat on Core Temperature in C57BL/6J Mice

Background: MPTP and paraquat are two compounds that have been used to model Parkinson’s disease in mice. Previous studies in two non-traditional strains of mice have shown that a single dose of MPTP can induce changes in body temperature, while the effects of paraquat have not been examined. Examination of body temperature is important since small fluctuations in an animal’s core temperature can significantly affect drug metabolism, and if significant enough can even culminate in an animal’s death.Objective: To determine how external heating can alter the survival of C57BL/6J mice following MPTP administration.Methods: In this study, we examine the effects of MPTP (4×20 mg/kg, 2 hours apart) and paraquat (2×10 mg/kg/week for 3 weeks) on core temperature of C57BL/6J mice. Correlations of purine and catecholamine levels were also done in mice treated with MPTP.Results: We find that MPTP induces a significant hypothermia in C57BL/6J mice that reduces their core temperature below the limit of fatal hypothermia. Unlike MPTP, paraquat did not induce a significant hypothermia. Placement of animals on heating pads significantly abrogates the loss of core temperature. In both heated and non-heated conditions, mice treated with MPTP showed a significant depletion of ATP within 2 hours of administration in both striatum and SN that started to recover 2 hours after MPTP administration was complete. Striatal DA and DOPAC are significantly reduced starting 4–6 hours after MPTP.Conclusions: The fatal hypothermic effects of MPTP can be abrogated through use of external heating.

Is propensity to obesity associated with the diurnal pattern of core body temperature?

Obesity affects more than half a billion people worldwide, but the underlying causes remain unresolved. It has been proposed that propensity to obesity may be associated with differences between individuals in metabolic efficiency and in the energy used for homeothermy. It has also been suggested that obese-prone individuals differ in their responsiveness to circadian rhythms. We investigated both these hypotheses by measuring the core body temperature at regular and frequent intervals over a diurnal cycle, using indigestible temperature loggers in two breeds of canines known to differ in propensity to obesity, but prior to divergence in fatness. Greyhounds (obesity-resistant) and Labradors (obesity-prone) were fed indigestible temperature loggers. Gastrointestinal temperature was recorded at 10-min intervals for the period of transit of the logger. Diet, body condition score, activity level and environment were similar for both groups. Energy digestibility was also measured. The mean core body temperature in obesity-resistant dogs (38.27 °C) was slightly higher (P<0.001) than in obesity-prone dogs (38.18 °C) and the former had a greater variation (P<0.001) in 24h circadian core temperature. There were no differences in diet digestibility. Canines differing in propensity to obesity, but prior to its onset, differed little in mean core temperature, supporting similar findings in already-obese and lean humans. Obese-prone dogs were less variable in daily core temperature fluctuations, suggestive of a degree of circadian decoupling.

Water Turnover and Core Temperature on Mount Rainier

Hydration is an important logistical consideration for persons performing in austere environments because water demands must be balanced with the burden of carrying water. Seven novice climbers participated in a study to determine the hydration kinetics and core temperatures associated with a successful summit of Mount Rainier. Ingestible radio-equipped thermometer capsules were swallowed to monitor core temperature, and an oral dose of deuterium (0.12 ± 0.02 g·kg⁻¹ body weight) was administered to determine hydration kinetics. Mean core temperature throughout the 5.5-hour climb to Camp Muir (3000 m) was 37.6 ± 0.3°C. Water turnover was 95.0 ± 17.5 mL·kg⁻¹·24 h⁻¹ over the duration of the 43-hour study. There was a trend for reduced body mass from before (75.9 ± 13.0 kg) to after (74.8 ± 12.5 kg) the climb (P = .06), and urine specific gravity increased from before (1.013 ± 0.002) to after (1.022 ± 0.006) the climb (P = .004). Hydration demands of climbing Mount Rainier are highly elevated despite modest fluctuations in core temperature. Participants experienced hypohydration but were able to maintain sufficient hydration to successfully summit Mount Rainier and return home safely.

Soft x-ray measurements of resistive wall mode behavior in NSTX**This paper was originally submitted as part of the special issue on MHD stability control in toroidal devices, see Plasma Phys. Control. Fusion 2010 52 100301–104006.

A multi-energy soft x-ray (ME-SXR) array is used for the characterization of resistive wall modes (RWMs) in the National Spherical Torus Experiment (NSTX). Modulations in the time history of the ME-SXR emissivity profiles indicate the existence of edge density and core temperature fluctuations in good agreement with the slow evolution of the n = 1 magnetic perturbation measured by the poloidal and radial RWM coils. The characteristic 20–25 Hz frequency in the SXR diagnostics is approximately that of the n = 1 stable RWM, which is also near the measured peak of the resonant field amplification (RFA) and inversely proportional to the wall time. Together with the magnetics, the ME-SXR measurements suggest that in NSTX the RWM is not restricted exclusively to the reactor wall and edge, and that acting with the stabilizing coils on its global structure may result in density and temperature fluctuations that can be taken into account when designing the feedback process.

Do alterations in temperature regulation contribute to protein‐energy malnutrition‐induced increase in reactive gliosis after global ischemia?

Protein‐energy malnutrition (PEM) impairs functional outcome after global ischemia, and this is associated with an increase in reactive gliosis. Since temperature is a key determinant of brain damage following an ischemic insult, the objective of this study was to determine if PEM modifies post‐ischemic temperature regulation. Bio‐electrical sensor transmitters were implanted into the peritoneal cavity of male, Sprague‐Dawley rats (31d) for continuous monitoring of core temperature. Rats were randomized for 7d to modified AIN‐93G control diet (CON; 18% protein) or PEM (2% protein). Animals were exposed to global ischemia (I) induced by 10 min bilateral carotid artery occlusion and hypotension (38 ± 0.2 mm Hg) or sham surgery (S) with tympanic temperature maintained at 37.4 ± 0.2°C. Physiological values were as follows: pH 7.29–7.41, pO2 119–144 mm Hg, pCO2 34–51 mm Hg, blood glucose 3.8–6.7 mmol/L. Preliminary results indicate mean (± SEM) core and daily temperature fluctuation from 5d of postsurgical monitoring as follows: CON‐S: 37.4 ± 0.04°C, 2.0 ± 0.1°C (n=3); PEM‐S: 37.4 ± 0.1°C, 2.6 ± 0.1°C (n=3); CON‐I: 37.7 ± 0.1°C, 1.9 ± 0.2 °C (n=3); PEM‐I: 37.5 ± 0.2°C, 2.3 ± 0.3°C (n=2). These data suggest that although PEM does not cause marked hypothermia or hyperthermia during the postsurgical period, it reduces thermoregulatory function in both sham and ischemic animals. Funded by the Heart and Stroke Foundation of SK.

MTC estimation based on neutron noise and propagating temperature perturbation in VVER-440 reactors

Abstract Moderator Temperature Coefficient (MTC) is an important parameter characterizing inherent safety in PWRs. Noise diagnostics provides a theoretically well established method to estimate its value without changing the reactor state with using fluctuation of the temperature and neutron flux measurements in frequency domain. However, several difficulties arise when determining the core average neutron and temperature fluctuation from the real measured signals, due to the specific instrumentation of each reactor type. Coolant temperature fluctuations originate from inhomogeneities traveling with the coolant flow and the treatment of this phenomenon requires an approach using the theory of propagating perturbations. Traditional MTC estimation methods do not consider these effects and they result in substantial under- and overestimations. This paper investigates MTC estimation in VVER-440 type reactors. For the evaluations the traditional H 1 estimator was used in a modified form: it was extended with factors taking into account the effects of propagating perturbations through geometry and coolant velocity. The 0.1–1.0 Hz frequency range of the traditional MTC evaluation was narrowed, as well, because of the relatively slow thermocouples and to decrease the effect of feedbacks. With these advancements results were quite close to the calculated values provided by reactor physics.

The Use Of Rectal Temperature Fluctuations In The Study Of Circadian Rhythm In Three Adult Vertebrate Species In Awka, Nigeria

Biological clocks allow organisms to assess and respond to the oscillating environmental rhythms that result from the earths movements via the generation of biological rhythms. Circadian clocks are also assumed to enhance survival and reproductive fitness in part by promoting optional timingof behaviour and physiology in relation to regular cycles in the environment. Temperature fluctuations were studied in three different vertebrate species – Sylvilagus floridanus, Rattus norvegicus and Columba livia in relation to the rhythmicity of rectal temperature. Clinical digital thermometer was used to record the core temperatue by inserting it 2 – 3 cm deep from the anal sphincter before taking readings 3 minutes after. The highest recorded mean rectal temperature for R. norvegicus and S. floridanus were 38.85 ± 0.40 0C and 39.83 ± 0.32 0C respectively with the lowest being 36.58 ± 0.74 0C and 36.63 ± 0.18 0C respectively. C. livia failed to exhibit core temperature fluctuation. There were not significant differences in the mean rectal temperature for both sexes for S. floridanus in relation to time of day. Differences in the circadian temperature fluctuations were traced to variation among the animals, of preferred temperature arising from differential behavioural and physiologcal regulation, in relation to environmental cues. It is also possible that daily changes in illuminaton were secondary to C. lvia. The results also suggest thatcircadian rhythmicity persists even in artificially imposed selective environment. Rectal temperatures are adequate for monitoring the biorhythms, with each species exhibiting endogenous peculiarities in the various circadian phases.Keywords: Circadian temperature, Rectal temperature, Vertebrate species, Physiological regulation

Heat Synch: Inter- and Independence of Body-Temperature Fluctuations and Brain-State Alternations in Urethane-Anesthetized Rats

During sleep, warm-blooded animals exhibit cyclic alternations between rapid-eye-movement (REM) and nonrapid-eye-movement (non-REM) states, characterized by distinct patterns of brain activity apparent in electroencephalographic (EEG) recordings coupled with corresponding changes in physiological measures, including body temperature. Recently we have shown that urethane-anesthetized rats display cyclic alternations between an activated state and a deactivated state that are highly similar in both EEG and physiological characteristics to REM and non-REM sleep states, respectively. Here, using intracranial local field potential recordings from urethane-anesthetized rats, we show that brain-state alternations were correlated to core temperature fluctuations induced using a feedback-controlled heating system. Activated (REM-like) states predominated during the rising phase of the temperature cycle, whereas deactivated (non-REM-like) states predominated during the falling phase. Brain-state alternations persisted following the elimination of core temperature fluctuations by the use of a constant heating protocol, but the timing and rhythmicity of state alternations were altered. In contrast, thermal fluctuations applied to the ventral surface (and especially the scrotum) of rats in the absence or independently of core temperature fluctuations appeared to induce brain-state alternations. Heating brought about activated patterns, whereas cooling produced deactivated patterns. This shows that although alternations of sleeplike brain states under urethane anesthesia can be independent of imposed temperature variations, they can also be entrained through the activation of peripheral thermoreceptors. Overall, these results imply that brain state and bodily metabolism are highly related during unconsciousness and that the brain mechanisms underlying sleep cycling and thermoregulation likely represent independent, yet coupled oscillators.

Technical Refinements and Drawbacks of a Surface Cooling Technique for the Treatment of Severe Acute Ischemic Stroke

To describe a technique for the induction of hypothermia and its complications for the treatment of acute ischemic stroke. Adults with acute (<8 hours), severe (National Institutes of Health Stroke Scale>14) ischemic stroke of the anterior circulation were enrolled. Patients were intubated, sedated, and paralyzed. Surface cooling to 32 degrees+/-1 degrees C was performed with a cooling blanket and an alcohol/ice bath. Hypothermia was maintained for 12-72 hours. Physiological parameters were measured continuously. A computed tomography scan of the brain was obtained at 24 hours. Rewarming was initiated 12 hours after middle cerebral artery recanalization at a rate of 0.25 degrees C/hour. All complications and adverse outcomes were documented from initiation of hypothermia until hospital discharge. Eighteen patients with a mean National Institutes of Health Stroke Scale=21.4+/-5.6 were treated. The goal temperature was reached within 3.2+/-1.5 hours. Cooling time was proportional to body weight (p=0.009) and decreased with immediate paralysis to prevent shivering (p=0.033). Maintenance and rewarming were characterized by fluctuations in core temperature. All patients developed a decrease in blood pressure, heart rate, and potassium values that were proportional to temperature (p<0.05). Complications were generally mild, but pneumonia and myocardial infarction or both occurred in five patients. There were trends for increased risk of complications with longer duration of hypothermia (p=0.08) and increasing age (p=0.0504). Rewarming was well-tolerated with rebound cerebral edema occurring in only one patient. Surface cooling for the treatment of acute ischemic stroke can be performed rapidly with early neuromuscular paralysis. Advanced age and prolonged hypothermia may be associated with an increased risk of complications.

Estrogen raises the sweating threshold in postmenopausal women with hot flashes

Objective: To determine if estrogen ameliorates hot flashes by raising the core body temperature sweating threshold, by reducing core body temperature fluctuations, and/or by reducing sympathetic activation (as measured by plasma 3-methoxy-4-hydroxyphenylglycol). Design: Laboratory physiological study. Setting: University medical center. Patient(s): Twenty-four healthy postmenopausal women reporting frequent hot flashes. Intervention(s): Participants were randomly assigned, in double-blind fashion, to receive 1 mg/d 17β-estradiol orally or placebo for 90 days. Main Outcome Measure(s): Core body temperature, core body temperature fluctuations, mean skin temperature, sternal sweat rate, laboratory hot flash counts (sternal skin conductance), plasma 3-methoxy-4-hydroxyphenylglycol. Result(s): The E 2 group had significant increases in plasma E 2 (8 ± 2 vs. 132 ± 22 pg/mL) and core body temperature sweating threshold (37.98 ± 0.09 vs. 38.14 ± 0.09°C) and decreases in plasma FSH (58.8 ± 8.9 vs. 40.1 ± 7.6 mIU/mL) and hot flashes (1.4 ± 0.5 vs. 0.6 ± 0.6). These changes did not occur in the placebo group. There were no significant changes in any other measure. Conclusion(s): E 2 ameliorates hot flashes by raising the core body temperature sweating threshold, but does not affect core temperature fluctuations or plasma 3-methoxy-4-hydroxyphenylglycol.

Sympathoadrenal hyperactivity and the etiology of neuroleptic malignant syndrome.

The author's goal was to develop a pathophysiological model for neuroleptic malignant syndrome with greater explanatory power than the alternative hypotheses of hypothalamic dopamine antagonism (elevated set point) and direct myotoxicity (malignant hyperthermia variant). Published clinical findings on neuroleptic malignant syndrome were integrated with data from human and animal studies of muscle physiology, thermoregulation, and autonomic nervous system function. The data show that the sympathetic nervous system's latent capacity for autonomous activity is expressed when tonic inhibitory inputs from higher central nervous system centers are disrupted. These tonic inhibitory inputs are relayed to preganglionic sympathetic neurons by way of dopaminergic hypothalamospinal tracts. The sympathetic nervous system mediates hypothalamic coordination of thermoregulatory activity and is a primary regulator of muscle tone and thermogenesis, augmenting both of these when stimulated. In addition, the sympathetic nervous system modulates all of the other end-organs that function abnormally in neuroleptic malignant syndrome. There is substantial evidence to support the hypothesis that dysregulated sympathetic nervous system hyperactivity is responsible for most, if not all, features of neuroleptic malignant syndrome. A predisposition to more extreme sympathetic nervous system activation and/or dysfunction in response to emotional or psychological stress may constitute a trait vulnerability for neuroleptic malignant syndrome, which, when coupled with state variables such as acute psychic distress or dopamine receptor antagonism, produces the clinical syndrome of neuroleptic malignant syndrome. This hypothesis provides a more comprehensive explanation for existing clinical data than do the current alternatives.

A Persistent Circhoral Ultradian Rhythm is Identified in Human Core Temperature

There have been inconclusive reports of intermittent rhythmic fluctuations in human core temperature, with the fluctuations having a period of about an hour. However, there has been no definitive demonstration of the phenomenon. This is likely due to the intermittency and seeming instability of the events. They have been assumed to be secondary rather than autonomous phenomena, putatively arising from the oscillation between rapid eye movement (REM) and non-REM (NREM) sleep. In this study, we report identification of a clear, persistent circhoral ultradian rhythm in core temperature with a period for this study sample of 64 ± 8 minutes. It appeared simultaneously with an intact circadian core temperature rhythm, persisted despite complex perturbations in core temperature brought about by the sequelae of 40h of sleep deprivation, and could not be attributed to sleep stage alternation or other endogenous or exogenous factors. Analysis of power spectra using the maximum entropy spectral analysis (MESA) method, which can uncover hidden rhythmicities, demonstrated that the apparent intermittency of the rhythm is due to periodic interference of this rhythm by other rhythmic events. The persistence of this oscillation suggests that, in this system as in the endocrine system, circhoral regulation is an integral component of thermoregulatory control. Identifying the source and functional role of this novel rhythm warrants further work.

High resolution electron cyclotron emission temperature profile and fluctuation diagnostic for Alcator C-Mod

A new second harmonic heterodyne electron cyclotron emission system with high spatial resolution (⩽15 mm) and a large number of channels (32) for dense profile coverage will be installed on Alcator C-Mod. The system will provide detailed radial electron temperature profiles and core temperature fluctuation measurements. The collection system is composed of elliptical and parabolic mirrors coupled to two overmoded waveguides to provide high poloidal spatial resolution necessary for T̃e measurements. Four radiometer sections cover the frequency range of 234–306 GHz, coupled to four optimized intermediate frequency modules with eight channels of detectors, amplifiers, dividers, and filters providing the profile information. A separate filter arrangement allows for temperature fluctuation measurements utilizing correlation techniques.

Continuous core temperature monitoring of search and rescue divers during extreme conditions

Objectives. To study the feasibility of continuously monitoring core temperatures of search and rescue divers, to compare core temperature gradients occurring during warm and cold diving events, and to identify conditions under which divers are subjected to extreme temperatures. Methods. Between June 1994 and March 1995, emergency medical technicians (EMTs) from two midwestern dive teams volunteered to ingest an encapsulated temperature sensor developed for the National Aeronautics and Space Administration. The capsule monitored continuous core temperatures and transmitted the data to a recorder worn under the divers' dry suits. Results. Twenty male EMTs, mean age 34 years and mean body weight 184 pounds, were monitored during training. Nine dives were ice dives, with ice thickness averaging 9 inches, and mean ambient temperatures of 33°F. Eleven dives were warm-water dives; average water temperature was 70°F and mean ambient temperature was 74°F. The average time spent in the dry suit was 95 minutes, and the mean total bottom time was 15 minutes. The ice divers showed a mean increase in core temperature of 1.0°C, while the warm-water divers showed a mean increase of 1.2°C. Divers experiencing the most extreme fluctuations were those in the role of safety diver, regardless of the season. Divers reported more subjective discomfort during ice dives than during warm-water dives. Conclusions. Continuous monitoring of body temperature during the duration of dive operations is possible, but labor-intensive. Safety measures to protect divers from extreme core temperature fluctuations should focus on the safety diver as well as the active diver.