Frédéric Telliez

Ventilatory response to a hyperoxic test is related to the frequency of short apneic episodes in late preterm neonates.

Chemoreception is frequently involved in the processes underlying apnea in premature infants. Apnea could result from a decrease in carotid body effectiveness. However, increased carotid body activity could also initiate apnea through hypocapnia following hyperventilation when the receptors are stimulated. The aim of this study was to analyze the relationship between carotid body effectiveness and short apneic episodes in older preterm neonates. Carotid body effectiveness was assessed at thermoneutrality in 36 premature neonates (2.07 ± 0.26 kg) by performing a 30-s hyperoxic test during sleep, the oxygen inhalation involving a ventilation decrease. Blood O2 saturation (Spo2) and ventilatory parameters were monitored before and during the hyperoxic test. Short episodes of apnea (frequency and mean duration) were recorded during the mornings 3-h interfeeding interval. Pretest Spo2 was not related to any of the measured respiratory parameters. A higher frequency of short apneic episodes was linked to a greater ventilation decrease in response to the hyperoxic test (ρ = −0.32; p = 0.01). Increased carotid body response is correlated with greater apneic episodes frequency, even in the absence of concomitant oxygen desaturation. Fetal or early postnatal hypoxemia could have increased peripheral chemoreceptor activity, which could initiate a “overshoot/undershoot” situation, which in turn could induce a critical Po2/Pco2 combination and apnea.

Effects of warm and cool thermal conditions on ventilatory responses to hyperoxic test in neonates

Body temperature interacts with respiratory control, but it is unclear what sites or mechanisms mediate those interactions. We hypothesized that warm and cool thermal conditions affect the decrease in ventilation (VE) seen during the hyperoxic test (HT), a breathing response believed to reflect the strength of the peripheral chemoreceptor drive. A breath-by-breath analysis during a 30 s HT was performed in eight premature neonates (postconceptional age: 36 +/- 1 weeks) under neutral, warm, and cool thermal conditions. Quiet sleep (QS) and active sleep (AS) were scored by neurophysiological criteria. The VE fall was higher in AS than in QS, and warm and cool conditions significantly enhanced the response only in AS (-24.2 +/- 6.0, -39.1 +/- 9.1, and -37.5 +/- 14.1% in neutral, warm, and cool conditions, respectively). Central control mechanisms of the respiratory chemoreflex may explain the increase in peripheral chemoreceptor drive during AS in response to thermal challenges, which may produce increased breathing instability leading to apnea in early life.

Interindividual differences in the thermoregulatory response to cool exposure in sleeping neonates.

Abstract The responses of the thermoregulatory effectors vary greatly among neonates. Therefore, we assume that a small decrease in air temperature from thermoneutrality induces various thermoregulatory responses within neonates that represent an energy cost due to the cold defence processes. To determine the importance of this variability in nursing, 26 neonates were explored at thermoneutrality and in a cool environment (−1.5 °C from thermoneutrality) similar to that which occurs currently in clinical procedure. Oxygen consumption (V˙O2), oesophageal and skin temperatures, as well as sleep parameters were recorded continuously in both conditions. Analysis of all of the data from all of the neonates revealed that the cool exposure induced thermal and sleep disturbances, but V˙O2 did not increase and was not negatively correlated to body temperature (as might be expected). Analyses of individual data showed large variability in body temperature regulation: the neonates could be assigned to one of three groups according to the direction of the individual slopes of V˙O2 versus oesophageal or skin temperature. The groups also differed according to the sleep changes recorded in the cool condition. The results show that the definition of thermoneutrality should be revised by incorporating non only changes in the body temperature, but also the sleep disturbances (increased wakefulness and active sleep, decreased quiet sleep), which are criteria that are more sensitive to mild cool exposure. Thermoneutrality should be defined for each individual, since the results stress that the variability does not help to predict a general pattern of thermoregulatory responses in cool-exposed neonates.

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.

In Utero Exposure to Smoking and Peripheral Chemoreceptor Function in Preterm Neonates

OBJECTIVE: We aimed to assess the involvement of peripheral chemoreceptor tonic activity in the ventilatory pattern during sleep in preterm neonates exposed in utero to maternal smoking. PATIENTS AND METHODS: Peripheral chemoreceptor activity was measured at thermoneutrality in neonates (postmenstrual age: 36.1 ± 1.2 weeks) born to nonsmoking (n = 21) or smoking (n = 16) mothers by performing a 30-second hyperoxic test during active and quiet sleep. Blood oxygen saturation, baseline ventilatory parameters, and central apnea were monitored. RESULTS: Prenatal smoking exposure did not modify baseline ventilation. It was interesting to note that prenatal smoking exposure decreased the peripheral chemoreceptor tonic activity during active sleep and increased the response time during quiet sleep. These changes could explain the increase in the time spent in apnea (both with and without blood oxygen desaturation) and in the mean duration of apneic episodes with desaturation found in neonates exposed to smoking in utero. The involvement of a change in the chemoreceptor function is supported by the fact that the peripheral chemoreceptor tonic activity was negatively correlated with the mean duration of apneic episodes with desaturation in the control group only. CONCLUSIONS: To our knowledge, this is the first study to reveal that prenatal smoking exposure does not directly modify baseline ventilatory parameters in the neonate but has a negative impact on peripheral chemoreceptor tonic activity. These alterations may increase the risk of sleep respiratory disorders, especially via apnea with desaturation.

Thermal acclimation of neonates to prolonged cool exposure as regards sleep stages

The thermal responses of neonates during a cool acclimation period were studied with regard to sleep stages. Sleep stages, body temperatures and metabolic rate (o2) were studied for seven neonates nursed in incubators and exposed to a cool temperature (thermoneutrality minus 2 °C) for 75 h. Each recording session lasted 3 h in the morning: firstly under thermoneutral baseline conditions, then during the first and last 3‐h periods of the cool acclimation and finally during the last 3 h of a 24‐h recovery period. Sleep structure was modified during the initial hours of cool exposure: the percentage of active sleep increased (AS: +13%, P = 0.028) at the expense of quiet sleep (QS: −11%, P = 0.043). This alteration in sleep structure persisted at the end of the acclimation period. Metabolic heat production only increased in the later period of cool acclimation. Throughout the cool exposure, o2 increased more (P = 0.040) in QS (+33%) than in AS (+20%) so that by the end of the cool period, o2 levels were similar in both sleep stages. During cool acclimation, the maintenance of homeothermy is related not only to a change in sleep organization but also to modifications in the thermoregulatory processes in both sleep stages. Considering the importance of AS/QS patterns in the neurobehavioral development of neonates, the present results could have clinical implications for the thermal management of neonates.

Peripheral chemoreceptor activity in sleeping neonates exposed to warm environments.

In neonates, it is often assumed that ventilatory control and heat stress interact. Thus the two factors have been implicated in various pathologies (apnoea, sudden infant death syndrome). However, little is known about the mechanisms of this interaction, and the influence of sleep is still debated. This study aimed at determining the influence of warm exposure on the decrease in ventilation during a hyperoxic test (HT), which is considered to be a measure of peripheral chemoreceptor activity. The test was performed in active (AS) and quiet sleep (QS) in 12 neonates exposed to thermoneutral or warm environments. The HT consisted of 30 s of inspired, 100% O(2). The ventilatory response was assessed in terms of a response time, defined as the time elapsing between HT onset and the first significant change in V(E). Our results show that, in both thermal conditions, the fall in V(E) was higher in AS than in QS. Warm exposure significantly enhanced the ventilatory response in AS (-27.5 +/- 8.7% vs. -38.3 +/- 8.8%, P < 0.01) but not in QS. A thermometabolic drive or inputs from thermoreceptors could be involved in the reinforcement of peripheral chemoreceptor activity in AS in warmer environments, which could contribute to an increasing risk of apnoea in neonates with altered chemoreceptor function. Since hypothalamic structures are involved in thermoregulatory, sleep processes and (probably) in respiratory control, it could well be the principal site where this interaction occurs.

Effects of medium‐ and long‐chain triglycerides on sleep and thermoregulatory processes in neonates

Sleep processes and body temperature regulation of neonates are never taken into account in the evaluation of nutrients, although these functions are implicated in the regulation of energy metabolism and are influenced by the nutritional state and its metabolic consequences. Medium‐chain triglycerides (MCT) are currently used in paediatric units during the first weeks of because they are considered to be a rapid source of energy, easy to assimilate for growing premature infants, whose digestive function is immature. However, no study has described the thermic effect of these nutrients on body temperature regulation and sleep. The present study aimed at analysing the influence of three feeding formulas with different content of MCT on sleep processes and on thermoregulation of neonates fed until desired intake was reached. Whatever the thermal conditions (thermal equilibrium or cool environment), the MCT‐fed groups had higher body temperatures and than groups fed without MCT, for whom total sleep time was reduced at thermal equilibrium. In this group, the large amount of quiet sleep seems to favour a strategy of conserving energy. Higher energy expenditure in MCT‐fed groups is not harmful to growth rate since nutritional efficiency is even better reflected by a larger body mass gain. The thermic effect of MCT contributes to lessening the vulnerability of neonates exposed to low incubator temperatures.

Heart Rate Variability in Sleeping Preterm Neonates Exposed to Cool and Warm Thermal Conditions

Sudden infant death syndrome (SIDS) remains the main cause of postneonatal infant death. Thermal stress is a major risk factor and makes infants more vulnerable to SIDS. Although it has been suggested that thermal stress could lead to SIDS by disrupting autonomic functions, clinical and physiopathological data on this hypothesis are scarce. We evaluated the influence of ambient temperature on autonomic nervous activity during sleep in thirty-four preterm neonates (mean ± SD gestational age: 31.4±1.5 weeks, postmenstrual age: 36.2±0.9 weeks). Heart rate variability was assessed as a function of the sleep stage at three different ambient temperatures (thermoneutrality and warm and cool thermal conditions). An elevated ambient temperature was associated with a higher basal heart rate and lower short- and long-term variability in all sleep stages, together with higher sympathetic activity and lower parasympathetic activity. Our study results showed that modification of the ambient temperature led to significant changes in autonomic nervous system control in sleeping preterm neonates. The latter changes are very similar to those observed in infants at risk of SIDS. Our findings may provide greater insight into the thermally-induced disease mechanisms related to SIDS and may help improve prevention strategies.

Head insulation and heat loss in naked and clothed newborns using a thermal mannequin

In newborns, large amounts of heat are lost from the head, due to its high skin surface area. Insulating the head (for example, with a hat or bonnet) can be a simple and effective method of reducing dry heat loss. In the present study, we evaluated the safety aspects of insulating the head of low-birth-weight naked or clothed newborns by using a heated mannequin that simulates a low-birth-weight newborn. Experimental conditions (comprising a nude and three clothed setups) were performed in a closed incubator at three different air temperatures (29 degrees C, 32 degrees C, and 34 degrees C) and with and without the head being covered with a bonnet in each case, i.e., 24 experimental conditions in all. The study shows that added clothing elements and insulating the head decreases the total heat loss of the mannequin as a whole. As regards the dry heat exchange from the head, wearing a bonnet decreases the local heat loss by an average of 18.9% in all clothed and thermal conditions. This phenomenon could be at the origin of brain overheating in heavily dressed newborns, when unrestricted heat loss is limited to the face only. Our results suggest that--apart from accidental hypothermia-in order to achieve thermal equilibrium of the body, it is preferable to leave the head unprotected and to increase the level of clothing insulation over the rest of the body.