The thermal environment in which 3-4 month old infants sleep at home

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.

Thermoregulatory effects of swaddling in Mongolia: a randomised controlled study

OBJECTIVE

To investigate thermal balance of infants in a Mongolian winter, and compare the effects of traditional swaddling with an infant sleeping-bag in apartments or traditional tents (Gers).

DESIGN

A substudy within a randomised controlled trial.

SETTING

Community in Ulaanbaatar, Mongolia.

SUBJECTS

A stratified randomly selected sample of 40 swaddled and 40 non-swaddled infants recruited within 48 h of birth.

INTERVENTION

Sleeping-bags and baby outfits of total thermal resistance equivalent to that of swaddled babies.

OUTCOME MEASURE

Digital recordings of infants' core, peripheral, environmental and microenvironmental temperatures at 30-s intervals over 24 h at ages 1 month and 3 months.

RESULTS

In Gers, indoor temperatures varied greatly (<0->25°C), but remained between 20°C and 22°C, in apartments. Despite this, heavy wrapping, bed sharing and partial head covering, infant core and peripheral temperatures were similar and no infants showed evidence of significant heat or cold stress whether they were swaddled or in sleeping-bags. At 3 months, infants in sleeping-bags showed the 'mature' diurnal pattern of a fall in core temperature after sleep onset, accompanied by a rise in peripheral temperature, with a reverse pattern later in the night, just before awakening. This pattern was not related to room temperature, and was absent in the swaddled infants, suggesting that the mature diurnal pattern may develop later in them.

CONCLUSIONS

No evidence of cold stress was found. Swaddling had no identifiable thermal advantages over sleeping-bags during the coldest times, and in centrally heated apartments could contribute to the risk of overheating during the daytime.

TRIAL REGISTRATION NUMBER

ISRTN01992617.

Synthesis and validation of a weatherproof nursery design that eliminates tropical evening-Fever syndrome in neonates

Neonatal thermal stabilisation can become challenging when uncontrollable factors result in excessive body temperature. Hyperthermia can rapidly slow down baby's progress and response to treatment. High sunlight intensity in tropical countries such as Nigeria manifests in incessant high neonatal temperatures towards early evenings. The ugly consequences of this neonatal evening-fever syndrome (EFS) can only be eradicated by the development of a controlled weatherproof nursery environment. Two laboratories and a ‘control ward' were applied. Lab-2 was a renovation of an existing room in a manner that could correct an existing nursery. Lab-1 was an entirely new building idea. The laboratories were assessed based on comparative ability to maintain environmental coolness and neonatal thermal stability during hot days. Data collection continued for 12 full calendar months. On average, at evaluated out-wind peak temperature of 43°C (range: 41°C–46°C), the control-ward peak was at 39°C, Lab-2 peak at 36°C, and Lab-1 peak at 33°C. All incubators in the control overheated during the hot periods but there was no overheating in Lab-1. Forty-four (86%) of sampled babies were fever-quenched by water sponging 131 times in the control whilst only one baby received same treatment in Lab-1. Nursery designs patterned after Lab-1 can significantly reduce EFS-induced neonatal morbidity.

Evaluation of the thermal insulation of clothing of infants sleeping outdoors in Northern winter

It is a common practice in Northern countries that children aged about 2 weeks to 2 years take their daytime sleep outdoors in prams in winter. The aim was to evaluate the thermal insulation of clothing of infants sleeping outdoors in winter. Clothing data of infants aged 3.5 months was collected, and sleep duration, skin and microclimate temperatures, humidity inside middle wear, air temperature and velocity of the outdoor environment were recorded during sleep taken outdoors (n = 34) and indoors (n = 33) in families' homes. The insulation of clothing ensembles was measured by using a baby-size thermal manikin, and the values were used for defining clothing insulation of the observed infants. Required clothing insulation for each condition was estimated according to ISO 11079. Clothing insulation did not correlate with ambient air temperature. The observed and required insulation of the study group was equal at about -5 °C, but overdressing existed in warmer and deficiency in thermal insulation in colder temperatures (r (s) 0.739, p < 0.001). However, even at -5 °C a slow cooling (ca. 0.012 °C/min) of mean skin temperature (T (sk)) was observed. When the difference between observed and required insulation increased, the cooling rate of T (sk) increased linearly (r (s) 0.605, p < 0.001) and the infants slept for a shorter period (r (s) 0.524, p = 0.001). The results of this study show the difficulty of adjusting systematically the optimal thermal insulation for outdoor sleeping infants during northern winter. Therefore, the necessity for guidelines is obvious. The study provides information for adequate cold protection of infants sleeping in cold conditions.

Long-range correlations in rectal temperature fluctuations of healthy infants during maturation

Background

Control of breathing, heart rate, and body temperature are interdependent in infants, where instabilities in thermoregulation can contribute to apneas or even life-threatening events. Identifying abnormalities in thermoregulation is particularly important in the first 6 months of life, where autonomic regulation undergoes critical development. Fluctuations in body temperature have been shown to be sensitive to maturational stage as well as system failure in critically ill patients. We thus aimed to investigate the existence of fractal-like long-range correlations, indicative of temperature control, in night time rectal temperature (T_rec) patterns in maturing infants.

Methodology/Principal Findings

We measured T_rec fluctuations in infants every 4 weeks from 4 to 20 weeks of age and before and after immunization. Long-range correlations in the temperature series were quantified by the correlation exponent, α using detrended fluctuation analysis. The effects of maturation, room temperature, and immunization on the strength of correlation were investigated. We found that T_rec fluctuations exhibit fractal long-range correlations with a mean (SD) α of 1.51 (0.11), indicating that T_rec is regulated in a highly correlated and hence deterministic manner. A significant increase in α with age from 1.42 (0.07) at 4 weeks to 1.58 (0.04) at 20 weeks reflects a change in long-range correlation behavior with maturation towards a smoother and more deterministic temperature regulation, potentially due to the decrease in surface area to body weight ratio in the maturing infant. α was not associated with mean room temperature or influenced by immunization

Conclusions

This study shows that the quantification of long-range correlations using α derived from detrended fluctuation analysis is an observer-independent tool which can distinguish developmental stages of night time T_rec pattern in young infants, reflective of maturation of the autonomic system. Detrended fluctuation analysis may prove useful for characterizing thermoregulation in premature and other infants at risk for life-threatening events.

Bed-sharing and the infant's thermal environment in the home setting

AIMS

To study bed-sharing and cot-sleeping infants in the natural setting of their own home in order to identify differences in the thermal characteristics of the two sleep situations and their potential hazards.

METHODS

Forty routine bed-sharing infants and 40 routine cot-sleeping infants aged 5-27 weeks were individually matched between groups for age and season. Overnight video and physiological data of bed-share infants and cot-sleeping infants were recorded in the infants' own homes including rectal, shin, and ambient temperature.

RESULTS

The mean rectal temperature two hours after sleep onset for bed-share infants was 36.79 degrees C and for cot-sleeping infants, 36.75 degrees C (difference 0.05 degrees C, 95% CI -0.03 to 0.14). The rate of change thereafter was higher in the bed-share group than in the cot group (0.04 degrees C v 0.03 degrees C/h, difference 0.01, 0.00 to 0.02). Bed-share infants had a higher shin temperature at two hours (35.43 v 34.60 degrees C, difference 0.83, 0.18 to 1.49) and a higher rate of change (0.04 v -0.10 degrees C/h, difference 0.13, 0.08 to 0.19). Bed-sharing infants had more bedding. Face covering events were more common and bed-share infants woke and fed more frequently than cot infants (mean wake times/night: 4.6 v 2.5).

CONCLUSIONS

Bed-share infants experience warmer thermal conditions than those of cot-sleeping infants, but are able to maintain adequate thermoregulation to maintain a normal core temperature.

Physiologic studies on swaddling: an ancient child care practice, which may promote the supine position for infant sleep

OBJECTIVE

The calming effects of swaddling may help infants accept back sleeping and so reduce the risk of sudden infant death syndrome. We hypothesized that swaddling, with minimal leg restraint, would be accepted by postneonatal infants with minimal respiratory effects.

STUDY DESIGN

Postneonatal infants (n = 37)were studied for the introduction of swaddling. Four infants were studied by using traditional swaddling techniques. Swaddle tightness was increased in 13 infants, simulating traditional swaddles. Respiratory variables-respiratory rate, tidal volume, oxygen saturation, heart rate, sigh rate, and "grunting"-were measured.

RESULTS

Hips were flexed and abducted in the swaddle. The majority of infants accepted swaddling while supine, including 78% of infants who slept prone at home. Acceptance decreased with increasing age. With increased swaddle pressure, respiratory rate increased during quiet sleep (P <.05). In rapid eye movement sleep, a slight effect on heart rate was observed (P <.05). Other variables did not change.

CONCLUSIONS

Older infants including usual prone sleepers generally accept a form of swaddling that has minimal respiratory effects. The reintroduction of swaddling, without restricting hip movement or chest wall excursion, combined with supine sleeping, may promote further sudden infant death syndrome reduction.

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.

The relationship between inside and outside ambient temperatures in Christchurch, New Zealand

The objective of this study was to examine and identify relationships between hourly recorded meteorological temperature and ambient temperature, measured from within the home-sleeping environment of young infants' homes in Christchurch, New Zealand. From 1991 to 1994, home polysomnography recordings were conducted for up to 6 weeks on 32 infants aged between 2 and 24 weeks. One of the recorded signals was ambient room temperature. In total, 15735 hourly recordings of this temperature were available for analysis. The New Zealand Meteorological Service supplied hourly recordings of climatic temperature, collected over this time, from an exposed site that was considered to be representative of weather conditions for Christchurch. Temperature seasonality, hourly climatic temperature recordings and the interaction of these variables were found to be significantly related to the indoor ambient temperature recordings (all had P < 0.001). Fluctuations in hourly recorded indoor temperature appeared to lag outdoor temperature fluctuations by approximately 2 h; hence, a strong autocorrelation was identified in the regression residuals. The most parsimonious autoregression model accounted for 97% of the variability in the hourly indoor temperature measurements (r2 = 0.97). In Christchurch houses, which typically have poor thermal insulation properties, yet have no central heating capabilities, a very strong association between indoor and outdoor temperatures was clearly demonstrated.

Effect of prone sleeping on circulatory control in infants

BACKGROUND

The mechanism of death in sudden infant death syndrome (SIDS) remains unclear. Progressive bradycardia is the pre-eminent terminal event, suggesting that circulatory failure might be a crucial factor. Vasomotor tone regulates the circulatory system by controlling blood volume distribution while maintaining venous return and blood pressure.

AIM

To examine whether prone sleeping, the most consistently identified risk factor for SIDS, has a measurable influence on vasomotor/circulatory control.

METHODS

44 full term infants (mean age, 7.9 weeks) were studied during an overnight sleep. Recordings were made while the infants were horizontal and asleep in the supine and prone positions, and repeated after a head up tilt to 60 degrees, maintained for 30 minutes, while in both sleep positions. Blood pressure, heart rate, anterior shin, and anterior abdominal wall skin temperatures were measured.

RESULTS

Systolic blood pressure was lower, but peripheral skin temperature and heart rate were higher during sleep, while horizontal, in the prone rather than the supine position. After tilting, there was a greater reduction in blood pressure and a greater increase in peripheral skin temperature and heart rate when in the prone position. Anterior abdominal wall skin temperature did not vary in either sleeping positions while horizontal or tilted.

CONCLUSION

Prone sleeping has a measurable effect on circulatory control, with a reduction in vasomotor tone resulting in peripheral vasodilatation, a higher peripheral skin temperature, a lower blood pressure, and a higher resting heart rate. Because vasomotor tone is crucially important in circulatory control this could be a factor in increasing the risk of SIDS.

Is baby too warm? The use of infant clothing, bedding and home heating in Victoria, Australia

Overheating of infants has been associated with a raised risk for sudden infant death syndrome (SIDS). Data on the use of heating, bedding and clothing and other measures affecting the thermal environment of 4 week old infants were collected at a home interview for infants of women born in Australia (Anglo-Celtic background), in Southern Europe, in Asia and of women who had a planned out-of-hospital birth. These groups have different risks of SIDS not explained by the classic social and perinatal risk factors nor associated with the currently promoted 'new' risk factors. Thermal insulation of the infant's bedding and clothing and excess thermal insulation (for any observed room temperature) were calculated. Bed sharing differed significantly between the groups as did the use of a sheepskin, tucking in firmly, the closing of doors and windows and the use of heating in the infant's room. After stratifying by bed sharing practice and season of interview, it was found that bed sharing infants had more thermal insulation than those sleeping alone irrespective of season of interview. Infants sleeping alone in the Asian-born and Southern European-born groups were kept warmer than infants in the other two groups. Cultural factors appear to affect the thermal environment in which infants are raised. Some bed sharing infants in all four groups were inappropriately warm, particularly in colder weather, but this was more likely in the Asian-born (low risk) group than in the home birth (high risk) group. These results do not explain the differences in SIDS incidence between the groups.

Behaviour and physiological responses during prone and supine sleep in early infancy

AIMS

To study the effect of prone and supine sleep on infant behaviour, peripheral skin temperature, and cardiorespiratory parameters to aid understanding of why prone sleeping is associated with an increased risk of sudden infant death syndrome.

METHODS

Of 33 enrolled infants, 32 were studied at 2.5 and 28 at 5 months of age. A computer aided multichannel system was used for polysomnographic recordings. Behaviour was charted separately.

RESULTS

Prone REM (active) sleep was associated with lower frequencies of short arousals, body movements and sighs, and a shorter duration of apnoeas than supine REM sleep at both ages. At 2.5 months there were less frequent episodes of periodic breathing during prone sleep in non-REM (quiet) and REM sleep. Heart rate and peripheral skin temperature were higher in the prone position during both sleep states at both ages.

CONCLUSIONS

The observation of decreased variation in behaviour and respiratory pattern, increased heart rate, and increased peripheral skin temperature during prone compared with supine sleep may indicate that young infants are less able to maintain adequate respiratory and metabolic homoeostasis during prone sleep.

Hyperthermia in sudden infant death

To determine whether preterminal hyperthermia is significantly associated with sudden infant death (SID), 140 structured interviews with parents of SID victims were compared with questionnaires filled in by a control group of parents living in the same area. All SID autopsies were performed between 1986 and 1992 at the Institute of Legal Medicine of Hannover Medical School according to the same protocol. Signs of profuse sweating (i.e. moist head, damp clothing or bedding) were present at the scene of death in 35.7% of cases. SID victims with signs of profuse sweating were more frequently found under their bedding (p < 0.001), were older (178 vs. 130 days) and the time period between when they were last seen alive and when they were found dead was longer (6.5 vs. 4.5 hours p < 0.01) compared to cases without sweating. Sweat on the head [odds ratio (OR) = 1.9; 95% confidence interval (CI) = 1.0, 3.6], and sweaty clothing and bedding (OR = 17.9; 95% CI = 8.7; 37.1) showed a significant association with the risk for SID. The pathophysiological basis for hyperthermia is SID remains to be determined. Hyperthermia could result from infection, overinsulation from excessive clothing with high environmental temperatures, covering of the infant's head or immature central thermoregulatory centres. The influence on the fatal outcome and the role in the pathogenesis of these deaths requires further research.

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.

Sudden infant death syndrome — a ‘diagnosis’ in search of a disease

Although Sudden Infant Death Syndrome (SIDS) is the major cause of death in infants aged between 1 week and 1 year in Western countries, it is one of the most enigmatic conditions encountered in paediatric forensic practice. SIDS has been recognized since Biblical times, and yet the definition continues to be debated and the aetiology remains obscure. In addition, there are no accepted pathognomonic features at post-mortem and the diagnosis is still one of exclusion. Emery once asked whether the term 'SIDS' is in reality more of a 'diagnostic dustbin' into which are placed a variety of unrelated entities.(1) To a certain extent this is true, as it is now recognized that a range of disorders can result in the sudden and unexpected death of an infant in a cot. It is also likely that the aetiology of SIDS is heterogeneous and that the term SIDS is not so much a diagnosis but a term covering a variety of mechanisms which result in a common lethal outcome. There are a number of controversies in the SIDS field which complicate the use of the term and which confound the assessment of causes and mechanisms of sudden infant death.

Ethnic differences in infant care practices and in the incidence of sudden infant death syndrome in Birmingham

Certain infant care practices have consistently been shown to play an important role in the aetiology of the sudden infant death syndrome (SIDS). In the UK, the incidence of SIDS amongst Asians is less than half that of whites. We conducted a questionnaire-based survey of 374 multiparous mothers from a multiracial community in Birmingham. We found that the majority of Asian infants slept in the parental bedroom at night, 94% compared to 61% of whites, whilst 33% of white infants slept alone compared to 4% of Asians. Three times as many white mothers as Asians placed infants prone (31% vs. 11%). We, therefore, observed marked differences in the infant care practices of Asians compared to whites in the UK, which may partly account for the low incidence of SIDS amongst infants of Asian origin.

Acute changes in oxygen consumption and body temperature after burn injury

This study describes the pattern of oxygen consumption (VO2), rectal temperature (Tr), and acral skin temperature (Tac) in sleeping and resting (awake) burned children nursed in a thermoneutral environment. Measurements of respiratory gas exchange (VO2 and carbon dioxide production (VCO2)) were made using an open circuit, flow through system of indirect calorimetry. Tr and Tac were monitored continuously. Sixteen patients were studied during the first 18 hours after being burned. Three phases of change in VO2, Tr, and Tac are described. The first was a stable period and there was little change from admission values. The second (7-10 hours after burn) was a phase of rapid heat storage. It started with a fall in Tac. Peak values of Tr (38.8-41.1, median 40.0 degrees C) and VO2 (8.5-11.8 ml/min/kg) occurred either in phase 2 or in the later phase 3. At its peak VO2 was 12-61% above values in phase 1. In phase 3, Tac returned towards admission values but Tr and VO2 were variable. These changes suggest that both an increase in metabolic heat production as well as heat conservation at the extremities may be involved in the generation of early fever after a burn.

Thermal balance and metabolic rate during upper respiratory tract infection in infants

Sequential recordings were made in the first five months after birth of metabolic rate, environmental temperature, and body temperature during sleep at home in 17 infants, each with an older sibling. Further recordings were made whenever an older sibling developed an upper respiratory tract infection (URTI), again four to six days later, and again two weeks later, aiming to achieve recordings before, during, and after an URTI in the infant. The temperature of the room and wrapping of the infant were determined according to their usual practice by the parents. Parents added appropriate wrapping to achieve thermal neutrality based on our calculated values and the measured oxygen consumption. In five of the six infants who developed an URTI in the first three months after birth there was no change or a decrease in metabolic rate at the time of the infection; for infants older than 3 months the metabolic rate increased in three of the five episodes recorded. Peripheral skin temperature decreased at the time of URTI at all ages, though in the older infants it usually increased in parallel with rectal temperature during the latter part of the night, when pyrexia was most common. Infants thus respond to URTI by heat conservation. In the younger infants the lower metabolic rate and the further decrease in this rate with URTI means that fever is rare, and their temperature may decrease on infection. In the older infants the increase in metabolic rate (from an already higher baseline) may result in fever. These differences may contribute to the increased vulnerability of the older infants to heat stress, particularly at the time of acute viral infections.

Ethnic differences in infant-rearing practices and their possible relationship to the incidence of sudden infant death syndrome (SIDS)

The aetiology of sudden infant death syndrome (SIDS) is still uncertain, although associations with overheating and the prone sleeping position have been reported. In the UK, the incidence of SIDS is considerably lower in infants of Asian origin, but as yet no explanation for this has been suggested. We have studied a group of 202 white and 172 Asian multiparous mothers attending an antenatal clinic to compare the sleeping position and home environment of infants in each ethnic group. We found that significantly more white infants (31%) than Asians (11%) were placed in the prone position at night and that 94% of Asian infants slept in their parents' bedroom, compared with 61% of whites. These observations demonstrate marked differences in the infant rearing practices favoured by Asians and whites and lend support to the concept that the prone position and separate bedrooms may be contributors to the development of sudden infant death.

Interactions between thermoregulation and the control of respiration in infants: possible relationship to sudden infant death

In summary, there is evidence that developmental changes in respiratory control and in thermoregulation have effects upon each other. Theoretically, such effects could give rise to failure of the respiratory system and there is some circumstantial evidence to support the concept that on occasions such interactions may be of importance in sudden unexpected death in infancy. Future research in this area should focus on the nature of the interactions between thermoregulation and respiration, and on the effects of infection and infection-related mediators on those interactions.

Thermal environment and sudden infant death syndrome: case-control study

OBJECTIVE

To compare the thermal environment of infants who died of the sudden infant death syndrome with that of age matched control infants.

DESIGN

Case-control study. Infants who died were matched with two controls, one for age and one for age and birth weight. Thermal measurements were conducted at the death scene for cases and at the scene of last sleep for control infants, who were visited unexpectedly within four weeks of the index infant's death on a day of similar climatic conditions. A follow up questionnaire was administered to parents of cases and controls.

SETTING

The geographical area served by the professional Tasmanian state ambulance service, which includes 94% of the Tasmanian population.

SUBJECTS

41 infants died of the sudden infant death syndrome at home; thermal observations at death scene were available for 28 (68%), parental questionnaire data were available for 40 (96%). 38 controls matched for age and 41 matched for age and birth weight.

RESULTS

Cases had more excess thermal insulation for their given room temperature (2.3 togs) than matched controls (0.6 togs) (p = 0.009). For every excess thermal insulation unit (tog) the relative risk of the sudden infant death syndrome was 1.26 (95% confidence interval 1.05 to 1.52). The average thermal bedding value calculated from parental recall was similar to that observed by attendant ambulance officers (mean difference = 0.4 tog, p = 0.39). Cases were more likely to have been found prone (odds ratio 4.58; 1.48 to 14.11). Prone sleeping position was not a confounder or effect modifier of the relation between excess thermal insulation and the syndrome.

CONCLUSIONS

Overheating and the prone sleeping position are independently associated with an increased risk of the sudden infant death syndrome. Further work on infant thermal balance and sudden infant death is required and guidelines for appropriate infant thermal care need to be developed.

Characteristics of the infant thermal environment in the control population of a case-control study of SIDS

This report examines the thermal environment during last sleep of a control population to investigate how the thermal environment of the infant's bedroom varies by season, external temperature and by certain maternal and infant characteristics. Two age-matched control infants were chosen for each case, one of which was also matched on birthweight. The home visits were not pre-arranged and were matched on climatic conditions, time of year and time period of day for the index case. The initial response rate for controls (n = 108) was 86%. Although there was a large amount of variation in the infant thermal environment, thermal insulation correlated with room temperature (r = -0.44, P = 0.0001) and external temperature (r = -0.30, P = 0.002). The thermal environment of the infant, as defined by excess thermal insulation for room temperature, did not vary by indoor or outdoor temperature, but higher average values were observed in teenage mothers (mean difference = 2.7 tog [95% Cl = 0.3, 5.2]), infants who slept in an adult bed (mean difference = 2.6 tog [-0.1, 5.4]) and infants with an illness (mean difference = 0.8 tog [-0.3, 1.9]). There was a tendency for the thermal environment of infants to be higher and more variable during winter, supporting previous hypotheses that paradoxical overheating may occur in some infants during winter. Further work is required to provide a set of recommendations on the optimal thermal conditions for post-neonatal infants.

The influence of ambient temperature on some commonly measured physiological variables in infants

Fifty-four infants underwent recording of heart rate, respiratory rate, long term and short term heart rate variability and end tidal carbon dioxide measurements at three different ambient temperatures (20 degrees C, 25-26 degrees C, 30 degrees C) under standardized conditions. The infants' skin and rectal temperature remained normal throughout the study. The ambient temperature of the room where the study was conducted significantly influenced the infants' heart rate, respiratory rate and long and short term heart rate variability but not end tidal carbon dioxide values. The thermal environment in which an infant resides has a significant influence on some physiological variables and should be standardized in future physiological studies.

Sleeping position and rectal temperature

The effects of sleeping position upon body temperature were assessed by continuous monitoring of rectal temperature in 137 babies sleeping at home under conditions chosen by their parents. There were three groups of subjects: (1) normal babies aged 12-22 weeks whose temperature rhythms were developed, (2) normal babies aged 6-12 weeks who were developing their night time temperature rhythms, and (3) babies the night after diphtheria, pertussis, and tetanus immunisation, whose temperature rhythms were disturbed. Sleeping in the prone position was not associated with higher rectal temperatures at any time of night in young babies, nor did it exaggerate the disturbance of rectal temperature rhythm after immunisation. In older normal babies the prone position did not disturb rectal temperature in the first part of the night, though prone sleepers warmed a little faster prior to walking, especially in warm conditions. Prone sleepers were, however, born earlier in gestation and tended to be of lower birth weight. Normal babies can therefore thermoregulate effectively whatever their sleeping posture, even in warm conditions, though the prone position may make it slightly more difficult to lose heat. It is difficult to see how the prone position, even interacting with warm conditions, could induce lethal hyperthermia in otherwise normal babies. Perhaps the prone position is associated with other risk factors for sudden infant death syndrome.

Home environmental temperatures in an Irish infant population

In this study the ambient temperature in which thirty-three Irish infants, within six weeks of birth, were nursed was documented. The study was conducted from October to March and a total of 1,250 individual room temperatures were recorded with a mean of 19.3 degrees C and a range of 8 degrees C, to 28 degrees C. The highest ambient temperature occurred from 1800 to 2100 hours and the lowest from midnight to 0300 hours. There was no central heating in 60% of the homes in this study. Comparing the various methods of home heating we found that houses with a coal fire and back boiler had the greatest range of temperatures recorded (from 10 degrees C to 23.5 degrees C) and also the lowest mean temperature recorded (17.14 degrees C). There was little relationship found between the ambient temperature and the amount of clothing and bedding used and some infants were heavily clothed despite a high ambient temperature. The purpose of this paper is to provide background data on which to base future studies examining the effects of varying ambient temperatures and amounts of clothing on an infant's physiological homeostasis.

How mothers keep their babies warm

Details of room temperature, clothing, and bedding used by night and by day and in winter and in summer were recorded for 649 babies aged 8 to 26 weeks. Room temperature at night was significantly related to outside temperature and duration of heating. Total insulation was significantly related to outside temperature and to minimum room temperature, but there was wide variation in insulation at the same room temperature. High levels of insulation for a given room temperature were found particularly at night and in winter, and were associated with the use of thick or doubled duvets and with swaddling. At least half the babies threw off some or all of their bedding at night, and at least a quarter sweated. Younger mothers and mothers in the lower social groups put more bedclothes over their babies, and the latter also kept their rooms warmer. Many mothers kept their babies warmer during infections.

Development of night time temperature rhythms over the first six months of life

Continuous recordings of night time rectal temperature were made at regular intervals over the first six months of life in 49 babies. In the first two weeks of life rectal temperature changed little overnight, but by 6 weeks of age rectal temperature at bedtime was significantly higher than later in the night. By around 12 weeks of age sleeping deep body temperature fell below 36.5 degrees C, and by 16 weeks of age all babies exhibited a consistent rhythm of rectal temperature. This fell by about 0.8 degrees C within two hours of bedtime, and then remained low until an hour or two before waking. As babies got older the mean interval between bedtime and first disturbance of parents got longer. Sleeping rectal temperature fell below 36.5 degrees C at about the time babies slept for seven hours. From 6 weeks of age, as individual baby's rectal temperatures fell more with sleep, sleep got longer.

Sudden infant death syndrome related to climate

In Australia the single most important factor influencing the incidence of SIDS is the climate. The incidence in mid summer in South Australia is 0.7 per 1,000 live births, in mid winter in Tasmania it is 6.3 per 1,000 live births. It is predicted that if infants under 6 months of age in cold weather were tied into swaddling and placed supine to sleep as in Asia and Czechoslovakia, and older infants who may object to restriction of movement, had the cot made up with the infants' feet touching the lower end, warm clothing and no more than a single thin porous cover, the incidence of SIDS as in Asia and Czechoslovakia could be reduced to less than 1.0 per 1,000.

Rectal temperature of normal babies the night after first diphtheria, pertussis, and tetanus immunisation

Continuous rectal temperature recordings were made from 32 babies the night after their first diphtheria, pertussis, and tetanus immunisation and compared with recordings made before immunisation. Tog values of clothes and wrapping and room temperatures were also recorded. We found that immunisation the day before disturbs the normal night time rhythm of deep body temperature. The rectal temperature of immunised babies was significantly higher than non-immunised babies from two hours into the night. We also found that there were considerable individual variations in the extent of disturbance of temperature rhythm. They were not correlated with thermal environment. There is no reason to suppose that these mild physiological responses to immunisation are in any way harmful.

Factors influencing the body temperature of 3-4 month old infants at home during the day

Continuous recordings of rectal temperature were made from 40 normal infants, aged 3-4 months, at home during two days of normal activities. We found that the rectal temperature of a normal, healthy baby may vary from 36.0 degrees C at night to 37.8 degrees C during active periods of the day. During daytime sleep rectal temperature fell, but to a lesser extent, and for less time than during night time sleeps. Feeds raised the temperature unless the baby slept, when they reduced the rate of fall of temperature. Bottle feeds affected temperature more quickly than breast feeds. The changes in temperature during sleep and after feeds were independent of the room temperature or thermal insulation of clothing and wrapping.

Use of thermographic imaging to study babies sleeping at home

Two 3 month old infants sleeping under different thermal conditions were found to maintain similar deep body temperatures. Thermographic imaging suggested that though the uncovered head is the main source of heat transfer, other parts of the body such as the hands may be used when necessary.

Interaction between bedding and sleeping position in the sudden infant death syndrome: a population based case-control study

OBJECTIVE

To determine the relation between sleeping position and quantity of bedding and the risk of sudden unexpected infant death.

DESIGN

A study of all infants dying suddenly and unexpectedly and of two controls matched for age and date with each index case. The parents of control infants were interviewed within 72 hours of the index infant's death. Information was collected on bedding, sleeping position, heating, and recent signs of illness for index and control infants.

SETTING

A defined geographical area comprising most of the county of Avon and part of Somerset.

SUBJECTS

72 Infants who had died suddenly and unexpectedly (of whom 67 had died from the sudden infant death syndrome) and 144 control infants.

RESULTS

Compared with the control infants the infants who had died from the sudden infant death syndrome were more likely to have been sleeping prone (relative risk 8.8; 95% confidence interval 7.0 to 11.0; p less than 0.001), to have been more heavily wrapped (relative risk 1.14 per tog above 8 tog; 1.03 to 1.28; p less than 0.05), and to have had the heating on all night (relative risk 2.7; 1.4 to 5.2; p less than 0.01). These differences were less pronounced in the younger infants (less than 70 days) than the older ones. The risk of sudden unexpected death among infants older than 70 days, nursed prone, and with clothing and bedding of total thermal resistance greater than 10 tog was increased by factors of 15.1 (2.6 to 89.6) and 25.2 (3.7 to 169.0) respectively compared with the risk in infants of the same age nursed supine or on their side and under less than 6 tog of bedding.

CONCLUSIONS

Overheating and the prone position are independently associated with an increased risk of sudden unexpected infant death, particularly in infants aged more than 70 days. Educating parents about appropriate thermal care and sleeping position of infants may help to reduce the incidence of the sudden infant death syndrome.

Review of the epidemiology of sudden infant death syndrome and its relationship to temperature regulation

Infants dying suddenly and unexpectedly now account for 20% of all infant deaths in England and Wales, and the incidence shows no sign of falling. Recent work relating sudden infant death to a raised environmental temperature and a raised body temperature, implicates fever as a possible contributory cause of death; some infants may be unable to control their febrile response to infection, or to thermoregulate effectively, when well wrapped and heated. Death might then result from apnoea, occurring in a critical sleep state. These ideas have increased the interest in describing the normal practices of parents in caring for the environment of their infants in health and disease, and the effect of their behaviour on the child's temperature. Studies of these areas depend on collecting and interpreting data from young children during their day to day lives, and present a challenge of great relevance to primary care research.

Sleeping body temperatures in 3-4 month old infants

Rectal, skin, and ambient temperatures were continuously recorded overnight from 3-4 month old normal infants in their home cots under conditions of room temperature and wrapping chosen freely by parents. It was found that rectal temperature was above 37 degrees C when infants were put down, but fell rapidly to 36.4 degrees C within one and a half hours, then stabilised for a few hours before rising steadily. This pattern was tied more closely to the time of putting down than time of day. The extent and rate of temperature fall did not correlate with any feature of the thermal environment. We also found that skin temperature changed much less than rectal temperature over the night, and for the first two hours in the cot there was no relation between skin and rectal temperature. There is therefore a well organised, endogenous rhythm of temperature in 4 month old infants.