The micro-environment of the sleeping newborn piglet covered by bedclothes: gas exchange and temperature

Hyperthermia and Heat Stress as Risk Factors for Sudden Infant Death Syndrome: A Narrative Review

BACKGROUND AND OBJECTIVES

Heat stress and hyperthermia are common findings in sudden infant death syndrome (SIDS) victims. It has been suggested that thermal stress can increase the risk of SIDS directly via lethal hyperthermia or indirectly by altering autonomic functions. Major changes in sleep, thermoregulation, cardiovascular function, and the emergence of circadian functions occur at the age at which the risk of SIDS peaks-explaining the greater vulnerability at this stage of development. Here, we review the literature data on (i) heat stress and hyperthermia as direct risk factors for SIDS, and (ii) the indirect effects of thermal loads on vital physiological functions.

RESULTS

Various situations leading to thermal stress (i.e., outdoors temperatures, thermal insulation from clothing and bedding, the prone position, bed-sharing, and head covering) have been analyzed. Hyperthermia mainly results from excessive clothing and bedding insulation with regard to the ambient thermal conditions. The appropriate amount of clothing and bedding thermal insulation for homeothermia requires further research. The prone position and bed-sharing do not have major thermal impacts; the elevated risk of SIDS in these situations cannot be explained solely by thermal factors. Special attention should be given to brain overheating because of the head's major role in body heat losses, heat production, and autonomic functions. Thermal stress can alter cardiovascular and respiratory functions, which in turn can lead to life-threatening events (e.g., bradycardia, apnea with blood desaturation, and glottal closure). Unfortunately, thermal load impairs the responses to these challenges by reducing chemosensitivity, arousability, and autoresuscitation. As a result, thermal load (even when not lethal directly) can interact detrimentally with vital physiological functions.

CONCLUSIONS

With the exception of excessive thermal insulation (which can lead to lethal hyperthermia), the major risk factors for SIDS appears to be associated with impairments of vital physiological functions when the infant is exposed to thermal stress.

Animal models for assessment of infection and inflammation: contributions to elucidating the pathophysiology of sudden infant death syndrome

Sudden infant death syndrome (SIDS) is still not well understood. It is defined as the sudden and unexpected death of an infant without a definitive cause. There are numerous hypotheses about the etiology of SIDS but the exact cause or causes have never been pinpointed. Examination of theoretical pathologies might only be possible in animal models. Development of these models requires consideration of the environmental and/or developmental risk factors often associated with SIDS, as they need to explain how the risk factors could contribute to the cause of death. These models were initially developed in common laboratory animals to test various hypotheses to explain these infant deaths – guinea pig, piglet, mouse, neonatal rabbit, and neonatal rat. Currently, there are growing numbers of researchers using genetically altered animals to examine specific areas of interest. This review describes the different systems and models developed to examine the diverse hypotheses for the cause of SIDS and their potential for defining a causal mechanism or mechanisms.

Airway covering during bed-sharing

BACKGROUND

Parent-infant bed-sharing is a common practice in Western post-industrial nations with up to 50% of infants sleeping with their parents at some point during early infancy. However, researchers have claimed that infants may be at risk of suffocation or sudden infant death syndrome related to airway covering or compression in the bed-sharing environment. To further understand the role of airway covering and compression in creating risks for bed-sharing infants, we report here on a sleep-lab trial of two infant sleep conditions.

METHODS

In a sleep-lab environment 20 infants aged 2-3 months old slept in their parents' bed, and in a cot by the bed, on adjacent nights. Infants' oxygen saturation and heart rate were monitored physiologically while infant and parental behaviours were recorded via ceiling-mounted infra-red cameras. Infants served as their own controls. Continuous 8-h recordings were obtained for covering of infant external airways, levels of infant oxygen saturation, infant heart rate, evidence of parental compression/overlying of infant, circumstances leading up to potential infant airway obstruction, and parental awareness of and responses to infant airway covering.

RESULTS

The majority of infants (14/20) spent some part of the bed night with their airways (both mouth and nose) covered, compared with 2/20 on the cot night; however, no consistent effect on either oxygen saturation levels or heart rate was revealed, even during prolonged bouts of airway covering. All cases of airway covering were initiated by parents; 70% were terminated by parents, the remainder by infants. Seven bouts of potential compression were observed with parental limbs resting across infant bodies for lengthy periods, however, in only two cases was the full weight of a parental limb resting on an infant, both events lasting less than 15 s, both being terminated by infant movement.

CONCLUSION

Although numerous authors have suggested that bed-sharing infants face risks because of airway covering by bed-clothes or parental bodies, the present trial does not lend support to this hypothesis.

What is the mechanism of SIDS? Clues from epidemiology

The cause of sudden infant death syndrome (SIDS) is unknown. Many mechanisms have been postulated, although thermal stress, rebreathing of expired gases and infection/inflammation seem the most viable hypotheses for the causation of SIDS. Deaths from SIDS have reduced dramatically following the recommendation not to place infants to sleep prone. Epidemiological data have shown that prone sleeping position is more risky in winter, colder latitudes, higher altitudes, if the infant is unwell or has excessive bedding or clothing. This suggests prone sleeping position involves either directly or indirectly a thermal mechanism. SIDS caused by an infective/inflammatory mechanism might be associated with deaths occurring during the night. Rebreathing of expired gases, airway obstruction, long QT syndrome and other genetic conditions may explain a small number of sudden unexpected deaths in infancy.

Sleep practices and environment and the risk of sudden infant death syndrome in Turkey

PURPOSE

The aim of this study is to determine what mothers know about infant sleep practices and the environment.

DESIGN AND METHODS

Data were collected with a semistructured questionnaire. The research was conducted with 231 mothers who delivered an infant and who agreed to participate between September and December 2005.

RESULTS

It was determined that 70.6% (n = 163) of mothers would use a pillow with their sleeping infants, and 44.2% (n = 102) would cover their infants' faces. When infants were alone in a room, 96.5% of mothers would leave them in the supine position.

PRACTICE IMPLICATIONS

Public health interventions should alert women to the increased risk of sudden infant death syndrome associated with identifiable risk factors, and specific intervention campaigns for safe sleeping can be developed for Turkish families.

Effects of hyperthermia and muramyl dipeptide on IL-1beta, IL-6, and mortality in a neonatal rat model

The mechanism of sudden infant death syndrome (SIDS) may be linked to an interaction between the SIDS risk factors of hyperthermia and infection, and between their effect on cytokine production and arousal. This study investigated the effects of hyperthermia and a surrogate of infection (muramyl dipeptide or MDP) on cytokine production and mortality in a neonatal rat model. Four temperature groups were studied: 34 degrees C (baseline), 38 degrees C, 39 degrees C, and 40 degrees C. Body temperatures of neonatal rat pups in the hyperthermic groups were raised and maintained at the desired temperature (38 degrees C, 39 degrees C, or 40 degrees C) for 1 h and then returned to the baseline temperature (34 degrees C) for a further hour. The heat source was a covered, heatable aluminum metal plate in a Perspex heating chamber. Intraperitoneal (IP) injection of 0.1 mL normal saline was given 30 min before the start to control for MDP (protocol A). Four equivalent treatment groups were pretreated with MDP (25 nmol/animal) instead of normal saline (protocol B). IP ketamine (55 mg/kg) was used for anesthesia during the experiments and for euthanasia. Blood was collected by direct cardiac puncture immediately after the 2-h experiments and assayed for the cytokines IL-6 and IL-1beta by ELISA. Hyperthermia significantly increased the production of IL-6 (p = 0.049) but not IL-1beta and significantly increased mortality. Administration of MDP significantly increased the IL-1beta production (p = 0.006) but not IL-6. Cox regression analysis showed that MDP in combination with hyperthermia had a significant effect on mortality in the neonatal rat. The risk of experiencing mortality was two and half times higher in the MDP group than in the non-MDP group (p = 0.016) [hazard ratio (95% confidence interval) = 2.66 (1.20-5.92)]. We conclude that hyperthermia and a surrogate of infection (MDP) influence cytokine production and that the combination of heat stress and MDP increases mortality in the neonatal rat.

Influence of head position on thermal stress in newborns: simulation using a thermal mannequin

The influence of head position on thermal stress was assessed by using a heavily clothed thermal mannequin in three different body positions [supine, face straight up (FSU); supine, face to the side (FTS); prone, FTS] and with or without the head covered by a bonnet. The mannequin was exposed to air temperatures of 29, 32, 34, and 36 degrees C. When the head is uncovered, body or head position has no impact on heat loss. When the head is covered, dry heat loss from the mannequin as a whole (and that from the head in particular) is lower (-0.35 to -0.40 W) in the FTS position than in the FSU position as a result of decreased heat loss from the surface area of the face in contact with the mattress. In the FTS position and with the head covered, there was no difference in heat loss between the prone and supine positions. The results suggest that in heavily clothed newborns whose head is covered by a bonnet, thermal stress depends on the head position.

A review of the anatomy of the upper airway in early infancy and its possible relevance to SIDS

BACKGROUND

Since the danger of prone sleeping in the first 6 months of life has been publicised, there has been a dramatic and consistent reduction in the incidence of sudden infant death syndrome (SIDS). However, unexpected infant deaths and apparent life-threatening events (ALTEs) continue to occur that are clearly not associated with known epidemiological risk factors.

AIMS

To review the unique features of the anatomy and function of the upper airway of the young infant which contribute to increased vulnerability to hypoxia in this age group. We discuss the clinical identification of those infants at risk of obstruction or restriction of the upper airway and the management of the 'at risk' infant.

CONCLUSIONS

In the era after the "back to sleep" campaigns, it is likely that an increasing proportion of cases of ALTEs and SIDS will be related to obstruction or limitation of upper airway size leading to sleep hypoxia/asphyxia. This type of problem may be anticipated by evaluation and investigation of infants with signs or a clinical history consistent with possible upper respiratory tract compromise, including micrognathia.

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.

Thermal stress in sudden infant death: Is there an ambiguity with the rebreathing hypothesis?

OBJECTIVE

To assess the role of thermal stress in the cause of sudden infant death syndrome (SIDS), and to compare risk factors with those of rebreathing.

METHODOLOGY

Analysis of publications concerning the epidemiology and physiology of thermal stress in SIDS.

RESULTS

A strong association between thermal regulation and ventilatory control was found, specifically for prolonged apnea. Infections, excessive room heat and insulation, and prone sleeping produce significantly increased odds ratios for SIDS. Although some of the risk factors for rebreathing could be explained by the effects of thermal stress, several factors for thermal stress could not reasonably be explained by the rebreathing hypothesis.

CONCLUSIONS

Although the risk of thermal stress is widely accepted abroad, it has received relatively little attention in the United States. The incidence of SIDS in the United States can likely be further reduced by educating the public against the dangers of overheating, as an integral part of the back-to-sleep campaign.

CLINICAL REVIEW ARTICLE: Is changing the sleep environment enough? Current recommendations for SIDS

Sudden infant death syndrome (SIDS or cot death) was the major cause of post-neonatal infant death in many countries in the late 1970s and 1980s. There is now very strong evidence that public intervention campaigns targeting the prone sleeping position, which had been identified by epidemiological studies as a major risk factor, were followed by substantial falls in the rate of SIDS. In the present review we discuss the evidence on which current recommendations for the prevention of SIDS are based. The prone sleeping position is now clearly causally associated with SIDS. Further reductions in SIDS may be produced by recommending the back sleeping position as opposed to the side position. Maternal smoking in pregnancy and bed sharing by infants of mothers who smoke are also strongly associated with SIDS, but have been harder to influence. Paternal smoking has also been implicated, although the magnitude of the reported risk is small. Finally, breastfeeding, pacifier use and having the infant sharing the parents bedroom, but not the bed, may also reduce risk. Continued reductions in SIDS mortality will require innovative public health education to target these major risk factors, while building on the "back to sleep" approach.

Changing concepts of sudden infant death syndrome: implications for infant sleeping environment and sleep position. American Academy of Pediatrics. Task Force on Infant Sleep Position and Sudden Infant Death Syndrome

The American Academy of Pediatrics has recommended since 1992 that infants be placed to sleep on their backs to reduce the risk of sudden infant death syndrome (SIDS). Since that time, the frequency of prone sleeping has decreased from >70% to approximately 20% of US infants, and the SIDS rate has decreased by >40%. However, SIDS remains the highest cause of infant death beyond the neonatal period, and there are still several potentially modifiable risk factors. Although some of these factors have been known for many years (eg, maternal smoking), the importance of other hazards, such as soft bedding and covered airways, has been demonstrated only recently. The present statement is intended to review the evidence about prone sleeping and other risk factors and to make recommendations about strategies that may be effective for further reducing the risk of SIDS. This statement is intended to consolidate and supplant previous statements made by this Task Force.

Sleep influences on homeostatic functions: implications for sudden infant death syndrome

The mechanisms underlying the sudden infant death syndrome (SIDS) appear to have origins in the fetal environment resulting in neural damage which later compromises responses to breathing or blood pressure challenges during sleep. The deficits appear to involve alterations in neurotransmitter receptors within regions involved in chemoreception and cardiovascular control. SIDS risk is enhanced by pre- and postnatal nicotine exposure, and possibly by hypoxic experiences. The prone sleeping position plays a significant role in risk, as do head positions that minimize facial escape from enclosed spaces; elevated body temperature may also be a factor. Compensatory mechanisms, including diminished gasping ability, relative failure to arouse to a safer state, or a failure to recruit respiratory efforts to overcome a blood pressure loss have been the object of recent research efforts. The findings suggest that the fatal event involves a neurally-compromised infant, circumstances that challenge vital physiology, most likely during sleep, at a particular developmental period.

Case control study of thermal environment preceding haemorrhagic shock encephalopathy syndrome

The purpose of the study was to investigate whether the thermal environment in which babies slept before developing haemorrhagic shock encephalopathy syndrome (HSES) differed from that of other babies. Data were collected by standardised interview from parents of 31 babies who had had HSES before the age of 7 months and compared with equivalent data for 124 control babies, with matching for outside temperature on the relevant night and for age. Multivariate analysis showed a strong association between HSES and covering of the baby's head by bedding, the odds ratio being 30.7 (95% confidence interval, 2.5 to 384). There were weaker associations with other aspects of the thermal environment. This suggests a link between HSES and some cases of cot death, supports the suggestion that HSES may be caused by overheating, and reinforces advice that babies should be placed to sleep in such a way that they are less likely to become totally covered.

Are bedding and rebreathing suffocation a cause of SIDS?

Suffocation by bedclothes became a popular diagnosis in the 1940s but gradually became replaced with the diagnostic label of Sudden Infant Death Syndrome (SIDS). In 1991 a paper purported that, instead of SIDS, pillows filled with polystyrene beads had caused death by rebreathing suffocation; this conclusion was reached on the basis of experiments with anesthetized rabbits breathing through a doll's head that was placed face down on the pillow. Because of the anesthesia, rabbits could not change their face down position. The doll's nares could not collapse, which would have resulted in rapid death due to conventional suffocation. The rabbits required up to 3 hours or more to die of hypercarbia and hypoxia. Studies in normal infants revealed that they turned from the face-down position after only 2 minutes. (The only infant who retained CO2 soon died of a fatal neurologic disorder, with central hypoventilation). Using the rabbit/doll's head and mechanical models, a wide range of bedding was indicted, including cushions, sheepskins, pillows, comforters, foam mattresses, and even simple blankets and sheets as potentially causing fatal rebreathing. Except for the use of pillows in general, as well as mattresses filled with kapok and bark, there has been no epidemiologic support for these indictments. Although normal infants are unlikely to succumb to rebreathing suffocation, infants with blunted ventilatory responsiveness and delayed arousal due to prior hypoxia were hypothesized to be at increased risk. Support for this concept was found in the pathology of the brain stem in victims of SIDS that was attributed to prior hypoxic injury. In infants who survived prolonged apnea, less than 20% have demonstrated a diminished ventilatory responsiveness to hypercarbia, but, more significantly, none had an absent response. Arousal to hypercarbia, an abnormality which is crucial to the hypothesis of rebreathing suffocation, is regularly present in normal subjects, but the threshold is higher in near-SIDS infants; however, no instances of failure to arouse have been reported in near-SIDS. If the infant is placed on his or her back or side, the issue of bedding could become moot; unfortunately, a sizable percentage of infants are still being placed prone for sleep. Instead of confusing parents with an ever-expanding list of "dangerous bedding," the message "Back to Sleep" should be emphasized.

Pathophysiology of overheating in a piglet model: findings compared with sudden infant death syndrome

OBJECTIVE

To examine the nature of hyperthermia-induced pathophysiological changes in an animal model including effects on lung compliance.

METHODOLOGY

Piglets were randomly assigned to heated or non-heated groups. Heated animals were warmed to 4 degrees C above normal body temperature while sedated and breathing spontaneously. Cardiorespiratory variables were recorded serially and haematological assessments and blood cultures taken at 0 and 6 h. After 6 h the animals were killed and a limited postmortem was performed. Control animals had all procedures without heating.

RESULTS

Heated piglets developed tachycardia, hypotension and a metabolic acidosis in addition to tachypnoea, hypocapnic alkalosis and a neutrophil leucocytosis. Rectal temperature after death fell at the same rate in both groups. Lung histology revealed an excess of lung haemorrhage and alveolar oedema in the heated group. No significant group differences in dynamic lung compliance were demonstrated.

CONCLUSIONS

The pathological changes that occur during hyperthermia are non-specific but not incompatible with those found in sudden infant death syndrome. There was no confirmation of the thesis that hyperthermia causes death by altering lung compliance.

A potential danger of bedclothes covering the face

Investigations of infants dying unexpectedly have reported up to 28% being found completely under bedding. No detailed physiological studies looking at the possibilities of asphyxia in this situation are available. The aim was to determine the potential for asphyxia under different types and thicknesses of bedding. A mechanical model of a 3-month-old infant's respiratory system was used. Bedding was positioned over the head in a supine position, and inspired carbon dioxide recorded. With a fixed respiratory rate and tidal volume, carbon dioxide accumulation increased with increasing layers of blankets. Up to 8.3% inspired carbon dioxide was recorded with more than four layers of blankets. A cotton sheet between the face and blankets reduced the accumulation by half. An infant found dead under bedding may have been exposed to an asphyxial stress. Suffocation from rebreathing trapped, expired gases can be a cause of death in this situation.