A mean body temperature of 37°C for incubated preterm infants is associated with lower energy costs in the first 11 days of life.

AIM: This randomised trial compared the energy costs of providing incubated preterm infants born before 32 weeks of gestation with homeothermia using either air temperature control (ATC) or skin servocontrol (SSC). METHODS: We studied 38 incubated preterm infants for the first 11 days of life, calculating the frequency of hypothermia (<36.0°C), hyperthermia (>37.5°C) and thermal challenge, together with energy costs, based on a change in incubator air temperature of 2°C above or below thermoneutrality. RESULTS: The daily mean incubator air temperature was higher in ATC than SSC (p < 0.05) for the first 6 days, and the mean body temperature was higher in ATC (37.0 ± 0.03°C) than SSC (36.8 ± 0.02; p < 0.01) over the whole study period. The frequency of moderate hyperthermia was higher in ATC (p < 0.001), whereas warm and cold thermal challenges were higher in SSC (p < 0.001). The two groups did not differ in terms of energy costs. The time to recover birthweight was shorter in ATC (p < 0.05). CONCLUSION: In incubators using ATC, a body temperature of 37°C was associated with lower energy costs and greater weight gain at 11 days of life for preterm infants. Future studies should test SSC shielded abdominal skin temperature set to 37°C.

Modulation of episodic renin release during sleep in humans.

We previously described a strong concordance between nocturnal oscillations in plasma renin activity and sleep cycles. To examine whether modifying renal renin content or release influences the response to central stimuli linked to sleep stage alternation, plasma renin activity was measured every 10 minutes from 11:00 PM to 8:00 AM in three groups of six subjects. The first group received one 40 mg dose of the diuretic furosemide; the second group underwent the night experiment after 3 days on a low sodium diet; the third group received one 100 mg dose of the beta-blocker atenolol. Each subject underwent a control night when a placebo was given. The nocturnal curves were analyzed with a pulse detection program. For the control nights, 74 of the 83 sleep cycles were associated with significant plasma renin activity oscillations; non-rapid eye movement sleep occurred in the ascending portions and rapid eye movement sleep in the declining portions of the oscillations. These oscillations persisted in the three groups of subjects during the experimental nights and the relation with the sleep stages was not disturbed. Acute stimulation by furosemide amplified the oscillations and led to a general upward trend of the nocturnal profiles. Similarly, a low sodium diet, which led to a slow increase in renal renin content, provoked large oscillations with high initial levels. However, in both cases the mean relative amplitude of the oscillations, expressed as a percentage of the nocturnal means, was similar to that of the control nights and approximated 60%.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of sleep and body temperature in response to exposure to cool and warm environments in neonates.

BACKGROUND AND OBJECTIVE: Thermoregulation is impaired during desynchronized sleep in animals and in adults. This can lead to a conflict between homeothermy and sleep in nonthermoneutral conditions. This study aimed to analyze thermoregulation during sleep, especially during desynchronized sleep (active sleep, AS) and to determine whether the conflict between thermoregulation and sleep might exist in the newborn sleeping in warm or cool conditions. METHODS: Esophageal and skin (cheek and abdomen) temperatures, local sweating rate (ventilated sweat collection capsule stuck on the abdomen), metabolism (indirect respiratory calorimetry), and sleep variables were recorded in 10 newborns exposed, in an incubator, to thermoneutral, warm, and cool environments. Body movements and apneas were also considered. Exposures were performed after a first habituation condition. RESULTS: Sleep structure was not modified by the first exposure nor by the warm environment. Exposure to cool temperatures increased AS duration (+13% of total sleep time) and the quantity of body movements during AS (+11.3% of AS duration), whereas these parameters were not modified during quiet sleep. The thermoregulatory response to warm and cool environments was not impaired during AS. During exposure to mild thermal load, analyses revealed large interindividual differences in the strategy for thermoregulation during AS. Depending on the newborn, the thermoregulatory response to cool temperatures could be described by an increase either in nonshivering thermogenesis or in frequency of body movement. In warm conditions, most newborns exhibited an increased sweating rate. The interindividual differences (lack of increase sweating in three newborns) seemed to be linked to changes in the sensitivity of the sweating response. CONCLUSION: Because thermoregulation is not impaired during AS, this sleep stage seems to be a well-protected one from a thermoregulatory point of view. This difference from adults and animals may be due to the important role of AS in newborn's nervous maturation.

Nocturnal oscillations in plasma renin activity and REM-NREM sleep cycles in humans: a common regulatory mechanism?

To establish the strength of the relationship between the nocturnal oscillations in plasma renin activity (PRA) and the sleep stage patterns, 42 PRA profiles from blood collected at 10-min intervals and the concomitant polygraphic sleep recordings were analyzed. In all cases, PRA curves exactly reflected the pattern of sleep stage distribution. When sleep cycles were complete, PRA levels oscillated at a regular 100-min period, with a strong spectral density. Declining PRA levels always coincided with REM sleep phases and increasing levels with NREM sleep phases. More precisely, peak levels corresponded to the transition from deep sleep stages toward lighter ones. The start of the rises in PRA generally marked the transition from REM sleep to stage 2. For incomplete sleep cycles, PRA curves reflected all disturbances and irregularities in the sleep structure. Spontaneous and provoked awakenings blunted the rise in PRA normally associated with NREM sleep, which indicates that disturbing sleep modifies the renin release from the kidneys. These results suggest that a common mechanism within the central nervous system controls both PRA oscillations and the REM-NREM sleep alternation.

Comparison of cardiovascular responses to noise during waking and sleeping in humans.

Eighty subjects, 40 men and 40 women, were allocated to one of two groups according to their self-estimated high or low sensitivity to noise. In the first part of the experiment, they were exposed to sequences of common noises during the morning or the afternoon. The heart-rate and finger-pulse responses were measured and recorded in relation to sensitivity, sex of subjects, and time of day. The different types of noise were compared for both responses. The heart-rate response showed differences between sensitivity groups but not between noises. In contrast, no significant differences were obtained between sensitivity groups when using the finger-pulse response, but clear differences were observed between noises. In a second part of the experiment, 10 men and 10 women subjects were selected from the previous two sensitivity groups. These 20 subjects were exposed during sleep to the same noises as during the daytime. Heart-rate and finger-pulse responses during sleep were significantly greater than during waking, and they did not differ significantly with respect to sensitivity to noise or gender. These two autonomic responses showed differences between noises that appeared to be related to their noise-equivalent-level value. Compared with the silent baseline night, the sleep pattern showed no significant modification in the night of noise disturbance, except for the frequency of transient activation phases, which was significantly increased in the latter.

Effect of continuous heat exposure on sleep stages in humans.

Six young men were exposed to a thermoneutral environment of air temperature (Ta) 20 degrees C for 5 days and nights followed by an acclimation period of 5 days and nights at Ta 35 degrees C and 2 recovery days and nights at Ta 20 degrees C. Electrophysiological measures of sleep, esophageal temperature, and mean skin temperature were continuously monitored. The total nocturnal body weight loss was measured by a sensitive platform scale. Compared with the 5 nights of the baseline period at 20 degrees C, sleep patterns showed disturbances at 35 degrees C. Total sleep time was significantly reduced, while the amount of wakefulness increased. The subjects exhibited fragmented sleep patterns. The mean duration of REM episodes was shorter at 35 degrees C than at 20 degrees C of Ta, while the REM cycle length shortened. In the acclimation period, there was no change in sleep pattern from night to night, despite adaptative adjustments of the thermoregulatory response. The protective mechanisms of deep body temperature occurring with heat adaptation did not interact with sleep processes. Upon return to baseline condition, a recovery effect was observed on a number of sleep parameters which were not significantly affected by the preceding exposure to prolonged heat. This would suggest that during exposure to dry heat, the demand for sleep could overcome that of other regulatory functions that are temperature-dependent. Therefore, a complete analysis of the effect of heat on sleep parameters can be assessed only if heat exposure is compared with both baseline and recovery periods.

Nocturnal cortisol release in relation to sleep structure.

The relationship between the temporal organization of cortisol secretion and sleep structure is controversial. To determine whether the cortisol profile is modified by 4 hours of sleep deprivation, which shifts slow-wave sleep (SWS) episodes, 12 normal men were studied during a reference night, a sleep deprivation night and a recovery night. Plasma cortisol was measured in 10-minute blood samples. Analysis of the nocturnal cortisol profiles and the concomitant patterns of sleep stage distribution indicates that the cortisol profile is not influenced by sleep deprivation. Neither the starting time of the cortisol increase nor the mean number and amplitude of pulses was significantly different between the three nights. SWS episodes were significantly associated with declining plasma cortisol levels (p less than 0.01). This was especially revealed after sleep deprivation, as SWS episodes were particularly present during the second half of the night, a period of enhanced cortisol secretion. In 73% of cases, rapid eye movement sleep phases started when cortisol was reflecting diminished adrenocortical activity. Cortisol increases were not concomitant with a specific sleep stage but generally accompanied prolonged waking periods. These findings tend to imply that cortisol-releasing mechanisms may be involved in the regulation of sleep.

Relative and combined effects of heat and noise exposure on sleep in humans.

In a counter-balanced design, the effects of daytime and/or nighttime exposure to heat and/or traffic noise on night sleep were studied in eight healthy young men. During the day, the subjects were exposed to baseline condition (ambient temperature = 20 degrees C; no noise) or to both heat (35 degrees C) and noise. The duration of the daytime exposure was 8 h ending 5 h before sleep onset. The following nights, the subjects slept either in undisturbed (20 degrees C; no noise) or in noise, heat, or noise plus heat-disturbed environments. During the day, the various types of traffic noise were distributed at a rate of 48/h with peak intensities ranging between 79 and 86 dB(A). The background noise level was at 45 dB(A). At night, the peak intensities were reduced by 15 dB(A), the rate was diminished to 9/h, and the background noise was at 30 dB(A). Electrophysiological measures of sleep and esophageal and mean skin temperatures were continuously recorded. The results showed that both objective and subjective measures of sleep were more disturbed by heat than by noise. The thermal load had a larger impact on sleep quality than on sleep architecture. In the nocturnal hot condition, total sleep time decreased while duration of wakefulness, number of sleep stage changes, stage 1 episodes, number of awakenings, and transitions toward waking increased. An increase in the frequency of transient activation phases was also found in slow-wave sleep and in stage 2. In the nocturnal noise condition, only total number of sleep stage changes, changes to waking, and number of stage 1 episodes increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic parameters of tyrosine hydroxylase in the rat's preoptic region during sleep deprivation and recovery induced by ambient temperature.

The role of catecholaminergic mechanisms in determining the changes in the rat's preoptic cyclic adenosine monophosphate (cAMP) concentration during sleep deprivation and recovery induced by ambient temperature was investigated in the present study. To this end, the activity of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, was measured in the preoptic region of rats maintained in: (a) control (22 degrees C for 52 h), (b) deprivation (-10 degrees C for 52 h), and (c) recovery (22 degrees C for 4 h after 48 h at -10 degrees C) conditions. The enzyme followed a Michaelis-Menten kinetic. The analysis of substrate-related kinetic parameters (Km and Vmax) did not show any clear-cut difference between experimental conditions, which, as already known, induce both sleep deprivation and recovery in relation to significant cAMP changes.

Effect of continuous heat exposure on sleep during partial sleep deprivation.

This study examined the effects of continuous heat exposure on sleep structure during a partial sleep-deprivation regime. The experimental protocol was divided into three periods. After a baseline period (5 days and nights at 20 degrees C), the sleep of the subjects was restricted to the second half of the night (3 a.m.-7 a.m.) for four consecutive nights. The restricted-sleep period was followed by two recovery days and nights. During the deprivation and recovery periods, the ambient temperature was 20 degrees C for six of the 12 subjects and 35 degrees C for the others. Sleep, esophageal and mean skin temperatures were continuously recorded. At 20 degrees C, the expected effect of sleep debt was apparent. There were significant reductions in time spent awake and in latencies for sleep and stage 4 sleep. The duration of stage 4 sleep significantly increased during the four successive restricted-sleep nights, whereas esophageal temperature significantly decreased over the successive days. When heat was added, esophageal temperature decrease was weakened, and the significant increase in stage 4 duration seen at 20 degrees C was not found. The findings suggest that the heat load imposed in our experimental condition has a suppressive effect on sleep stage 4 increase, which is induced by sleep restriction. The hypothesis that an increase in this sleep stage serves as a mechanism for energy conservation should be also considered.

Effects of triazolam on heart rate level and on phasic cardiac response to noise during sleep.

The influence of triazolam on cardiac and respiratory activity of healthy male subjects was examined during nights disturbed by airplane noises and during undisturbed nights. Twenty-four subjects, divided into three groups of eight, slept in the laboratory for 7 nights (N0-N6). Following a double blind design, group A (control group) received a placebo every night. Group B received 0.25 mg triazolam and group C received 0.5 mg on nights N3, N4 and N5. On the other nights, they received a placebo. For all three groups, the nights N0, N3 and N5 were disturbed by 32 semi-randomly distributed airplane noises. Air and wall temperatures (20 degrees C) and air humidity (10 degrees C, 52%) were kept constant. Sleep measures, heart rate and respiratory rate were continuously recorded. Results showed that the largest dose of hypnotic drug produced an increase in tonic heart rate in the first part of each night throughout the treatment period (N3, N4, N5). When compared to baseline disturbed night N0, the phasic cardiac response to the noises was significantly attenuated on only the 1st treatment night (N3). Triazolam had no significant effect on nocturnal respiratory rate. No after-effects of the drug were observed for cardiac and respiratory activity on the withdrawal night (N6). The results suggest that, with regard to the drug action, there was either an increase in arousal threshold or a dissociation between long-lasting and short-lasting modifications of heart rate. Contrary to the single night attenuation of phasic cardiac responses, there was no drug tolerance for the hypnotic-related increase in tonic heart rate.

Nocturnal pituitary hormone and renin profiles during chronic heat exposure.

To establish the principal endocrine systems involved in the initial adaptation to chronic passive heat exposure, responses of the hydromineral and pituitary systems were compared. Nocturnal plasma profiles of growth hormone, prolactin, thyrotropin, and plasma renin activity were determined during a period of prolonged heat exposure designed to simulate a hot climate. Two groups of six male subjects (G1 and G2) were kept for 11 days in a climatically controlled apartment. The G1 group was exposed to 20 degrees C throughout the study, and the G2 group was exposed to 20 degrees C for 5 days and to 35 degrees C for the following 6 days. Blood was collected continuously during the 1st and 10th nights from 2300 to 0700 h and was sampled in 10-min aliquots for hormone analysis. Heat exposure increased the nocturnal mean core temperature (P < 0.02) and stimulated plasma renin activity (P < 0.01) but had little effect on the pituitary hormones except for a small increase in prolactin (P < 0.05). For the G1 group there was no change in hormone profiles. These results demonstrate that the principal initial endocrine system involved in acclimatization to passive heat exposure is the renin-angiotensin system, reflecting counteractive measures against salt and water losses. In contrast, 5 days of continuous heat exposure does not provoke metabolic changes, indicated by the small effect on pituitary hormone profiles.

Amplification of nocturnal oscillations in PRA and aldosterone during continuous heat exposure.

To assess the effect of continuous heat exposure on the nocturnal patterns of renin, aldosterone, adrenocorticotropic hormone (ACTH), and cortisol, six young men were exposed to thermoneutral environment for 5 days, followed by a 5-day acclimation period in a hot dry environment (35 degrees C). Blood was collected at 10-min intervals during the second night at thermoneutrality (N0) and during the first (N1) and the last (N5) nights of heat exposure. Polygraphic recordings of sleep were scored according to established criteria. Continuous heat exposure led to progressive decreases in the 24-h urinary volume and in Na excretion, whereas urinary osmolality increased. After 5 days of uninterrupted heat, significant increases were found in plasma volume (P less than 0.05), osmolality (P less than 0.01), plasma Na (P less than 0.01), and protein levels (P less than 0.05). Sweat gland output increased during the first 3 days and then declined without any concomitant increases in body temperature. Compared with N0, there were no differences in plasma renin activity (PRA) and aldosterone (PA) profiles during N1 at 35 degrees C. However, during N5 the mean PRA and PA levels were significantly (P less than 0.05) enhanced, and their nocturnal oscillations were amplified (P less than 0.05). This amplification occurred mainly in the second part of the night when regular rapid-eye-movement and non-rapid-eye-movement sleep cycles were observed, leading to a general upward trend in the nocturnal profiles. The relationship between the nocturnal PRA oscillations and the sleep cycles was not modified. ACTH and cortisol patterns were not affected by continuous heat exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of dry heat exchanges in newborns: influence of body position and clothing in SIDS.

A dramatic decrease of sudden infant death syndrome (SIDS) has been noted following the issuance of recommendations to adopt the supine sleeping position for infants. It has been suggested that the increased risk could be related to heat stress associated with body position. In the present study, the dry heat losses of small-for-gestational-age newborns nude or clothed were assessed and compared to see whether there is a difference in the ability to lose heat between the prone and supine positions. An anthropomorphic thermal mannequin was exposed to six environmental temperatures, ranging between 25 and 37 degrees C, in a single-walled, air-heated incubator. The magnitudes of heat losses did not significantly differ between the two body positions for the nude (supine 103.46 +/- 29.67 vs. prone 85.78 +/- 34.91 W/m(2)) and clothed mannequin (supine 59.35 +/- 21.51 vs. prone 63.17 +/- 23.06 W/m(2)). With regard to dry heat exchanges recorded under steady-state conditions, the results show that there is no association between body position and body overheating.

Sleep modifications during cool acclimation in human neonates.

The present study aimed at testing in human neonates whether the thermal acclimation could reduce the sleep disturbances induced by brief cool exposure. Six neonates were exposed in incubator to a standardised cool thermal load of 75 h duration. The results show an increase of the metabolic heat production (VO2: +25% reaching 5.68 ml/min per kg) during cool acclimation which is not associated with a reduction of the sleep modifications observed on the first cool exposure: the increase of active sleep (+15%, +2 min) and the decrease of quiet sleep (-15%, -11 min) persist and wakefulness after sleep onset increases (+12%, +10 min). In conclusion, there is no sleep adaptation as cool acclimation progressed.

Neutral temperature range in incubators: performance of equipment in current use and new developments.

Low-birth-weight neonates should be nursed at thermoneutrality inside incubators. Thermoneutrality control is essential to enhance body growth and to reduce neonatal illnesses and mortality. Guidelines have been published to provide the thermoneutral range, but the recommendations did not always take into account all ambient and physiological parameters influencing thermoneutrality. In most marketed incubators, the heat supply is controlled through convective air flow (closed incubators) or through radiant power density (radiant warmer beds). The heating unit (on/off cycling or adjustable proportional control) is activated by an error signal calculated from the difference between a controlled temperature and a reference value preset by the clinician. The controlled variable can be either the incubator air or the skin temperature of the anterior abdominal region of the neonate. The neonate's size, thermal properties of the mattress and of incubator walls, air temperature and humidity, air velocity, incubator wall temperatures all influence the heat exchanges between the neonate and the surroundings, and, consequently, modify the obtention of thermoneutrality. Moreover, studies of the physiological mechanisms by which the neonate regulates body heat storage suggest that metabolic rate, behavior, vigilance level, nursing care, and heater control processes should also be taken into account. Little attention has been paid to these factors, and incubator performances are often disappointing. This article reviews the different factors that modify thermoneutral condition. An attempt is made to suggest new ways to design equipment incorporating these factors in algorithms controlling heater processes in order to reach the optimal thermal environment in which the neonate should be nursed.

Reliability of a new device to assess the oxygen consumption of human respiratory muscles.

PURPOSE: This study tests the reliability of a new device for assessing the oxygen consumption of the respiratory muscles (VO2 resp.). METHODS: Fourteen healthy male volunteers participated in the study. The device consists of an expandable external ventilatory dead space created with pieces of plastic tubing and a spirometer filled with 100% oxygen. It also incorporates a carbon dioxide absorber. Total VO2 (VO2 tot.) was recorded from the spirometric closed circuit and ventilation (V(E)), from the spirometer tracing. For each subject the test procedure was carried out in duplicate (T1 and T2) after an overnight fast. The dead space was increased at a constant rate of 260 mL every 90 s, and VO2 tot. and V(E) increased progressively. Because log VO2 tot. was linearly related to V(E), we calculated the slope value (log VO2-V(E)) and the Y-intercept (VE = 0) of the semilog regression representing, respectively, VO2 resp. and metabolic VO2 (VO2 met.). RESULTS: When compared with values in the literature, these values did not differ from those recorded in subjects of a similar age group. The VO2 resp. and VO2 met. calculated in T1 and T2 were not different (VO2 resp. = 0.0066 +/- 0.0005 for T1 vs 0.0067 +/- 0.0005 log mL x min(-1)/L x min(-1) for T2 and VO2 met. = 269.3 +/- 28.6 for T1 vs 281.9 +/- 24.1 mL x min(-1) for T2). The coefficients of variation were: 25% at T1 and 23% at T2 for VO2 resp. and 34% at T1 and 29% at T2 for VO2 met. Moreover, significant correlations (r = 0.96, P < 0.001 for VO2 resp., r = 0.95, P < 0.001 for VO2 met.), high coefficients of determination (r2 = 0.92 for VO2 resp., r2 = 0.90 for VO2 met.) and negligible SEE (0.0005 for VO2 resp., 0.2 mL x min(-1) for VO2 met.) were found between the two tests. When we plotted the mean values of VO2 resp. and VO2 met. measured at T1 and T2 against their respective differences, more than 95% of the slight differences ranged between the limits defined by mean value +/- 2 SD, reflecting the small discrepancy between duplicate measurements. CONCLUSION: The results confirm that the test performed with this device is useful and reliable for assessing the VO2 resp. in healthy subjects.

Measurement of functional residual capacity through the transient phase of He dilution in newborns.

Helium dilution maneuver is used to determine the functional residual capacity (FRC) 14 newborns ages 1-5 mo. The model equation describes the changing alveolar fractions of He and the ventilation promoted by a rebreathing procedure that does not exceed 40 s. The model does not involve the volume of the rebreathing bag usually needed when applying rebreathing technique and which is a source of error. The equation is discretized and solved for recorded data obtained with equipment adapted to newborns. Results show a strong relationship between FRC and the biometrical indexes, and confirm those found in the literature featuring that the measurement duration of FRC can be considerably shortened.

Determination of diffusing capacity by modeling the dynamics of C18O uptake in newborns.

Dynamic modeling of lung C18O diffusion is used to measure the C18O transfer factor (TLCO) of 14 newborns aged 1-4 mo. The model equation is based on the alveolar fractions of C18O and on changing alveolar ventilation induced by the rebreathing conditions. The model does not involve the volume of the rebreathing bag which is usually needed when applying rebreathing technique and which is a source of error. The equation is discretized and solved for recorded data obtained with equipment adapted to use in newborns. A least-square parameter calculation technique is applied to estimate TLCO. Results show a strong relationship between this index and the biometrical ones and confirm those found in the literature featuring that the measurement duration can be considerably shortened.

Body temperature regulation in the newborn infant: interaction with sleep and clinical implications.

Thermoregulation in newborn infant differs from that of adult. Comparisons between sleep stages show that, during rapid eye movements (REM) sleep, the impairment of thermoregulatory responses in adult is not observed in newborn. Both behavioral and autonomic temperature regulations are always operative in the range of air temperatures usually imposed. The interaction between sleep and thermoregulation seems to be less important in newborns than in adults, suggesting that sleep processes are well protected, reducing the probability of occurrence of central dysfunction. According to the model describing thermoregulation during sleep on the basis of changes in the hierarchical dominance of brain structures, either the influence of diencephalic structures is never depressed in REM sleep or the functional autonomy of the rhombencephalon is still relevant in the immature encephalon of the newborn. The thermoregulatory model also allows understanding of inter-individual differences in thermoregulation and levels of thermoneutrality. An attempt has also been made to learn the role of heat stroke in the production of sudden infant death syndrome when body heat loss is hampered.