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

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.

How might non nutritional sucking protect from sudden infant death syndrome

Epidemiology has identified an association between the use of pacifiers and protection from sudden infant death syndrome (SIDS). The use of pacifiers for SIDS prevention fails to gain adoption partly because there is no widely accepted physiologic mechanism to explain the epidemiologic association. Additionally, the scientific literature available on pacifier use focuses largely on the probable adverse effects. We hypothesize that pacifier use and all other forms of non-nutritional sucking (specifically digit sucking, also known as thumb sucking) is a life saving defense mechanism meant to splint open and stabilize the collapsible portion of the upper airway in infants.The main objective of this review article is to propose a mechanism to explain how pacifiers might help prevent SIDS. If the medical community accepts this mechanism, it can help promote pacifier use by the public and potentially reduce the incidence of SIDS.

Conductive Thread-Based Textile Sensor for Continuous Perspiration Level Monitoring

Individual perspiration level indicates a person’s physical status as well as their comfort level. Therefore, continuous perspiration level measurement enables people to monitor these conditions for applications including fitness assessment, athlete physical status monitoring, and patient/elderly care. Prior work on perspiration (sweat) sensing required the user either to be static or to wear the adhesive sensor directly on the skin, which limits users’ mobility and comfort. In this paper, we present a novel conductive thread-based textile sensor that measures an individual’s on-cloth sweat quantity. The sensor consists of three conductive threads. Each conductive thread is surrounded by a braided cotton cover. An additional braided cotton cover is placed outside the three conductive threads, holding them in a position that is stable for measurement. the sensor can be embedded at various locations on a person’s clothing. When the person sweats, the cotton braids absorb the sweat and change the conductivity (resistance) between conductive threads. We used a voltage dividing circuit to measure this resistance as the sensor output (DC). We then conducted a sensor calibration to map this measured voltage to the quantity of electrolyte solution (with the same density as sweat) applied to the sensor. We used this sensor to measure individuals’ perspiration quantity and infer their perceived perspiration levels. The system is able to limit the average prediction error to 0.4 levels when compared to five pre-defined perceived perspiration levels.

Ambient Temperature and Sudden Infant Death Syndrome in the United States

BACKGROUND

Sudden infant death syndrome (SIDS) is a leading cause of infant mortality in the United States. While thermal stress is implicated in many risk factors for SIDS, the association between ambient temperature and SIDS remains unclear.

METHODS

We obtained daily individual-level infant mortality data and outdoor temperature data from 1972 to 2006 for 210 US cities. We applied a time-stratified case-crossover analysis to determine the effect of ambient temperature on the risk of SIDS by season. We stratified the analysis by race, infant age, and climate.

RESULTS

There were a total of 60,364 SIDS cases during our study period. A 5.6°C (10°F) higher daily temperature on the same day was associated with an increased SIDS risk of 8.6% (95% confidence interval [CI] = 3.6%, 13.8%) in the summer, compared with a 3.1% decrease (95% CI = -5.0%, -1.3%) in the winter. Summer risks were greater among black infants (18.5%; 95% CI = 9.3%, 28.5%) than white infants (3.6%; 95% CI = -2.3%, 9.9%), and among infants 3-11 months old (16.9%; 95% CI = 8.9%, 25.5%) than infants 0-2 months old (2.7%; 95% CI = -3.5%, 9.2%). The temperature-SIDS association was stronger in climate clusters in the Midwest and surrounding northern regions.

CONCLUSIONS

Temperature increases were associated with an elevated risk of SIDS in the summer, particularly among infants who were black, 3 months old and older, and living in the Midwest and surrounding northern regions.

Dynamics and complexity of body temperature in preterm infants nursed in incubators

BACKGROUND

Poor control of body temperature is associated with mortality and major morbidity in preterm infants. We aimed to quantify its dynamics and complexity to evaluate whether indices from fluctuation analyses of temperature time series obtained within the first five days of life are associated with gestational age (GA) and body size at birth, and presence and severity of typical comorbidities of preterm birth.

METHODS

We recorded 3h-time series of body temperature using a skin electrode in incubator-nursed preterm infants. We calculated mean and coefficient of variation of body temperature, scaling exponent alpha (Talpha) derived from detrended fluctuation analysis, and sample entropy (TSampEn) of temperature fluctuations. Data were analysed by multilevel multivariable linear regression.

RESULTS

Data of satisfactory technical quality were obtained from 285/357 measurements (80%) in 73/90 infants (81%) with a mean (range) GA of 30.1 (24.0-34.0) weeks. We found a positive association of Talpha with increasing levels of respiratory support after adjusting for GA and birth weight z-score (p<0.001; R2 = 0.38).

CONCLUSION

Dynamics and complexity of body temperature in incubator-nursed preterm infants show considerable associations with GA and respiratory morbidity. Talpha may be a useful marker of autonomic maturity and severity of disease in preterm infants.

Changes in Lung Surfactant Proteins in Rats With Lipopolysaccharide--Induced Fever

The study was designed to prove the hypothesis that lipopolysaccharide (LPS)-induced fever elicits the changes in surfactant specific proteins, potentially related to thermal tachypnea. In adult rats fever was induced by intraperitoneal administration of LPS at a dose 100 µg/kg of body weight; control group received saline. Respiratory parameters, arterial blood gases and pH and colonic body temperature (BT) were recorded. Five hours later, surfactant proteins (SP) A, B, C and D were evaluated in bronchoalveolar lavage fluid (BALF) and lung tissue (LT). LPS evoked monophasic thermic response (at 300 min 38.7±0.2 vs. 36.4±0.3 °C, P0.05) and an increase in minute ventilation due to changes in breathing rate and tidal volume. LPS-instilled animals had higher levels of SP-A and SP-D in LT (P0.05 and 0.01), and higher SP-D in BALF (P0.01) than controls. SP-B increased in LT and SP-C in BALF of animals with LPS (both P0.05 vs. controls). The changes in all surfactant specific proteins are present in LPS-induced fever. Alterations of proteins related to local immune mechanisms (SP-A, SP-D) are probably a part of general inflammatory response to pyrogen. Changes in proteins related to surface activity (SP-B and SP-C) might reflect the effort of the body to stabilize the lungs in thermal challenge.

Heat stress and sudden infant death syndrome--stress gene expression after exposure to moderate heat stress

The aim of the present study was to investigate stress gene expression in cultured primary fibroblasts established from Achilles tendons collected during autopsies from sudden infant death syndrome (SIDS) cases, and age-matched controls (infants dying in a traumatic event). Expression of 4 stress responsive genes, HSPA1B, HSPD1, HMOX1, and SOD2, was studied by quantitative reverse transcriptase PCR analysis of RNA purified from cells cultured under standard or various thermal stress conditions. The expression of all 4 genes was highly influenced by thermal stress in both SIDS and control cells. High interpersonal variance found in the SIDS group indicated that they represented a more heterogeneous group than controls. The SIDS group responded to thermal stress with a higher expression of the HSPA1B and HSPD1 genes compared to the control group, whereas no significant difference was observed in the expression of SOD2 and HMOX1 between the two groups. The differences were related to the heat shock treatment as none of the genes were expressed significantly different in SIDS at base levels at 37 °C. SOD2 and HMOX1 were up regulated in both groups, for SOD2 though the expression was lower in SIDS at all time points measured, and may be less related to heat stress. Being found dead in the prone position (a known risk factor for SIDS) was related to a lower HSPA1B up-regulation in SIDS compared to SIDS found on their side or back. The study demonstrates the potential usefulness of gene expression studies using cultured fibroblasts established from deceased individuals as a tool for molecular and pathological investigations in forensic and biomedical sciences.

The physiological determinants of sudden infant death syndrome

It is well-established that environmental and biological risk factors contribute to Sudden Infant Death Syndrome (SIDS). There is also growing consensus that SIDS requires the intersection of multiple risk factors that result in the failure of an infant to overcome cardio-respiratory challenges. Thus, the critical next steps in understanding SIDS are to unravel the physiological determinants that actually cause the sudden death, to synthesize how these determinants are affected by the known risk factors, and to develop novel ideas for SIDS prevention. In this review, we will examine current and emerging perspectives related to cardio-respiratory dysfunctions in SIDS. Specifically, we will review: (1) the role of the preBötzinger complex (preBötC) as a multi-functional network that is critically involved in the failure to adequately respond to hypoxic and hypercapnic challenges; (2) the potential involvement of the preBötC in the gender and age distributions that are characteristic for SIDS; (3) the link between SIDS and prematurity; and (4) the potential relationship between SIDS, auditory function, and central chemosensitivity. Each section underscores the importance of marrying the epidemiological and pathological data to experimental data in order to understand the physiological determinants of this syndrome. We hope that a better understanding will lead to novel ways to reduce the risk to succumb to SIDS.

A perspective on SIDS pathogenesis. the hypotheses: plausibility and evidence

Several theories of the underlying mechanisms of Sudden Infant Death Syndrome (SIDS) have been proposed. These theories have born relatively narrow beach-head research programs attracting generous research funding sustained for many years at expense to the public purse. This perspective endeavors to critically examine the evidence and bases of these theories and determine their plausibility; and questions whether or not a safe and reasoned hypothesis lies at their foundation. The Opinion sets specific criteria by asking the following questions: 1. Does the hypothesis take into account the key pathological findings in SIDS? 2. Is the hypothesis congruent with the key epidemiological risk factors? 3. Does it link 1 and 2? Falling short of any one of these answers, by inference, would imply insufficient grounds for a sustainable hypothesis. Some of the hypotheses overlap, for instance, notional respiratory failure may encompass apnea, prone sleep position, and asphyxia which may be seen to be linked to co-sleeping. For the purposes of this paper, each element will be assessed on the above criteria.

Potential Mechanisms of Failure in the Sudden Infant Death Syndrome

Current evidence suggests that multiple neural mechanisms contribute to the fatal lethal event in SIDS. The processes may develop from a range of otherwise seemingly-innocuous circumstances, such as unintended external airway obstruction or accidental extreme flexion of the head of an already-compromised structure of the infant upper airway. The fatal event may occur in a sleep state which can suppress muscle tone essential to restore airway patency or exert muscle action to overcome a profound loss of blood pressure. Neural processes that could overcome those transient events with reflexive compensation appear to be impaired in SIDS infants. The evidence ranges from subtle physiological signs that appear very early in life, to autopsy findings of altered neurotransmitter, including serotonergic, systems that have extensive roles in breathing, cardiovascular regulation, and thermal control. Determination of the fundamental basis of SIDS is critical to provide biologic plausibility to SIDS risk reduction messages and to develop specific prevention strategies.

Effects of body position on thermal, cardiorespiratory and metabolic activity in low birth weight infants

BACKGROUND

Low birth weight (LBW) infants sleeping prone are known to exhibit many physiological differences from those sleeping supine, including lower energy expenditure (heat production) and higher surface temperature. This apparent increase in heat storage suggests that heat loss may be inhibited in the prone position which, in turn, might influence cardiorespiratory activity.

AIMS

To determine the effects of body position (prone vs. supine) on absolute surface temperature profile (heat storage), central-peripheral (C-P) thermal gradients (vasomotor response), cardiorespiratory activity and metabolic gas exchange in growing LBW infants.

METHODS

Six-hour continuous recordings of absolute surface temperature profiles, cardiorespiratory activity and O2 and CO2 exchange, along with minute-to-minute assessment of behavioral sleep states were performed in 32 healthy growing LBW infants (birth weight 805-1590 g, gestational age 26-35 weeks and postconceptional age at study 33-38 weeks). Each infant was randomly assigned to the prone or supine position for the first 3 h of the study and then reversed for the second 3 h. Surface temperatures were recorded from 4 sites (forehead, flank, forearm and leg) and averaged each minute. Central (forehead and flank)-to-peripheral (forearm and leg) and forehead-to-environment (H-E) thermal gradients were calculated from the surface temperatures. Corresponding sleep states were aligned with minute averages obtained from the temperature and cardiorespiratory measurements. Data were then sorted for prone and supine positions during quiet (QS) and active sleep (AS) and compared using paired t-tests.

RESULTS

In the prone position during both AS and QS, infants had higher forehead, flank, forearm and leg surface temperatures, narrower C-P gradients, higher heart rates and respiratory frequency, and lower heart rate and respiratory variability. Despite similar environmental temperatures, the H-E gradient was higher in the prone position. In the prone position infants demonstrated lower O2 consumption and CO2 production and a higher respiratory quotient.

CONCLUSIONS

Despite thermoregulatory adjustments in cardiorespiratory function, infants sleeping prone have relatively higher body temperature. The cardiorespiratory responses to this modest increase in temperature indicate that thermal and metabolic control of cardiac and respiratory pumps seem to work in opposition. The consequences of any attendant changes in blood gas activity (e.g. hypocapnia and/or increased mixed venous oxygen concentration) due to this override of metabolic control remains speculative.

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.

A practical classification schema incorporating consideration of possible asphyxia in cases of sudden unexpected infant death

Although the rate of the sudden infant death syndrome (SIDS) has decreased over the last two decades, medical examiners and coroners are increasingly unwilling to use the SIDS diagnosis, particularly when there is an unsafe sleeping environment that might pose a risk for asphyxia. In order to reliably classify the infant deaths studied in a research setting in the mixed ancestory population in Cape Town, South Africa, we tested a classification system devised by us that incorporates the uncertainty of asphyxial risks at an infant death scene. We classified sudden infant deaths as: A) SIDS (where only a trivial potential for an overt asphyxial event existed); B) Unclassified-Possibly Asphyxial-Related (when any potential for an asphyxial death existed); C) Unclassified-Non-Asphyxial-Related (e.g., hyperthermia); D) Unclassified-No autopsy and/or death scene investigation; and E) Known Cause of Death. Ten infant deaths were classified according to the proposed schema as: SIDS, n = 2; Unclassified-Possibly Asphyxial-Related, n = 4; and Known Cause, n = 4. A conventional schema categorized the deaths as 6 cases, SIDS, and 4 cases, Known Cause, indicating that 4/6 (67%) of deaths previously classified as SIDS are considered related importantly to asphyxia and warrant their own subgroup. This new classification schema applies a simpler, more qualitative approach to asphyxial risk in infant deaths. It also allows us to test hypotheses about the role of asphyxia in sudden infant deaths, such as in brainstem defects in a range of asphyxial challenges.

The brainstem and serotonin in the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is the sudden death of an infant under one year of age that is typically associated with sleep and that remains unexplained after a complete autopsy and death scene investigation. A leading hypothesis about its pathogenesis is that many cases result from defects in brainstem-mediated protective responses to homeostatic stressors occurring during sleep in a critical developmental period. Here we review the evidence for the brainstem hypothesis in SIDS with a focus upon abnormalities related to the neurotransmitter serotonin in the medulla oblongata, as these are the most robust pathologic findings to date. In this context, we synthesize the human autopsy data with genetic, whole-animal, and cellular data concerning the function and development of the medullary serotonergic system. These emerging data suggest an important underlying mechanism in SIDS that may help lead to identification of infants at risk and specific interventions to prevent death.

Sudden infant death syndrome: risk factors for infants found face down differ from other SIDS cases

OBJECTIVE

To test the hypothesis that infants with sudden infant death syndrome (SIDS) found face down (FD) would have SIDS risk factors different from those found in other positions (non-face-down position, NFD).

STUDY DESIGN

We used the New Zealand Cot Death Study data, a 3-year, nationwide (1987 to 1990), case-control study. Odds ratios (univariate and multivariate) for FD (n = 154) and NFD SIDS (n = 239) were estimated separately, and statistical differences between the two groups were assessed.

RESULTS

Of 12 risk factors for SIDS, there were 8 with a statistically significant difference between FD and NFD infants. After adjustment for the potential confounders, younger infant age, Maori ethnicity, low birth weight, prone sleep position, use of a sheepskin, and pillow use were all associated with a greater risk of SIDS in the FD than the NFD group. Sleeping during the nighttime, maternal smoking, and bed-sharing were associated with a risk of SIDS only in the NFD group. Pacifier use was associated with a decreased risk for SIDS only in the NFD group, whereas being found with the head covered was associated with a decreased risk for SIDS for the FD group.

CONCLUSIONS

Infants with SIDS in the FD position appear to be a distinct subgroup of SIDS. These differences in risk factors provide clues to mechanisms of death in both SIDS subtypes.

Sudden cardiac arrest in an intubated premature infant with cerebellar and brainstem injury: is there a link?

The ventilated premature infant frequently exhibits unprovoked desaturation episodes accompanied by bradycardia. In most instances, these episodes are short-lived and recover spontaneously or with minimal interventions. However, in some infants these episodes may be more profound and require substantial interventions to restore cardiorespiratory status. Here we present the case of a ventilated premature infant who had experienced prolonged, multiple daily desaturation episodes accompanied by bradycardia that required significant interventions. Postoperatively, after placement of a tracheotomy and despite a patent airway, the infant developed acute bradycardia that progressed rapidly to sudden death. At autopsy, significant cerebellar and brainstem injury was noted. We hypothesize that the specific cerebellum and brainstem injury may have contributed to autonomic dysfunction and sudden death.

Case-control study of sudden infant death syndrome in Lithuania, 1997-2000

BACKGROUND

To identify risk factors for sudden infant death syndrome relevant in Lithuania.

METHODS

A nationwide case-control study surveying parents of 35 infants who died from sudden infant death syndrome during the period of 1997-2000 and parents of 145 control infants matched with SIDS infants for date of birth and for region of birth was carried out.

RESULTS

Deaths incidence was greater in the warm period (60%) vs. cold period (40%). Prone and side sleeping positions both carried no increased risk of sudden infant death syndrome compared with supine because of a rare prone sleeping (4.1% of controls vs. 0% of dead infants) and more prevalent side than supine sleeping (84.8% of controls vs. 94.3% of dead infants) in the controls as well as the cases. Bed sharing for the whole night as a risk factor for sudden infant death syndrome has not been confirmed, either, as bed sharing was common only for the controls (13.8% of controls vs. 0% of dead infants). Routine sleeping environment factors such as heavy wrapping (> or =4 togs) of an infant (odds ratio 8.49; 95% confidence interval 2.38 to 30.32), sleeping in a bassinet (4.22; 1.16 to 15.38) and maternal factors such as maternal education < or =12 years (4.48; 1.34 to 14.94), unplanned pregnancy (5.22; 1.49 to 18.18) and > or =2 previous live births (3.90; 1.00 to 15.10) were significantly associated with sudden infant death syndrome on multivariate analysis.

CONCLUSION

The results of this first population-based case-control study have shed some light on the epidemiology of the syndrome in Lithuania. Although the mortality of sudden infant death syndrome in Lithuania is not high, it might be lowered moreover by public informing about sudden infant death syndrome and related risk factors. Special attention must be paid to mothers with low education on potentially modifiable risk factors such as routine heavy wrapping of an infant during sleep, routine sleeping in a bassinet and unplanned pregnancy.

Influence of temperature on metabolism and breathing during mammalian ontogenesis

A literature survey of the ventilatory responses to changes in ambient temperature (T) in neonatal mammals reveals that, as in adults, the metabolic response to T is the major determining factor. In fact, the newborn's metabolic response to changes in T determines not only the pulmonary ventilation and the breathing pattern, but also the magnitude of the ventilatory responses to chemical stimuli and the intensity of the pulmonary reflexes at different T. The important difference from the adult is that in many neonatal mammals the control of body temperature (T(b)) is poorly developed. Hence, the metabolic response can be more similar to that of an ectothermic, rather than endothermic, animal, and T(b) can vary substantially with T. When hypoxia occurs in cold, T(b) can decrease greatly, because of the hypoxic drop in the thermoregulatory set-point, and this lowers pulmonary ventilation. Hence, in addition to the metabolic response, also the changes in T(b) are a factor modulating the ventilatory responses to T. Artificial warming of the newborn during hypoxia causes heat-dissipation responses that can be counterproductive. During ontogenesis, with prolonged cold conditions, the sustained alterations in metabolic rate and body growth do not modify the postnatal development of the respiratory control mechanisms. Presumably, this indicates that respiratory regulation develops independently from the individual's metabolic history.

Inhibitory effects of hyperthermia on mechanisms involved in autoresuscitation from hypoxic apnea in mice: a model for thermal stress causing SIDS

The physiological mechanisms that might be involved in an association between heat stress and sudden infant death syndrome (SIDS) are obscure. We tested the hypothesis that a combination of acute hypoxia and elevated body temperature (T(B)) might prevent autoresuscitation from hypoxic apnea (AR). We exposed 21-day-old mice (total = 216) to hyperthermia (40.5-43.5 degrees C), hypoxia, or a combination of the two. Neither hyperthermia alone (40.5-42.5 degrees C) nor hypoxia alone was found to be lethal, but the combination produced failure to AR during the first hypoxic exposure with increasing frequency as T(B) increased. The ability to withstand multiple hypoxic exposures was also reduced as T(B) increased. In contrast, heat stress causing moderate T(B) increase (40.5 degrees C) had no effect on survival. Increased T(B) (43.5 degrees C) reduced gasping duration and number of gasps. It increased heart rate during anoxia but did not alter gasping rate. Furthermore, the oxygen-independent increase in heart rate observed before gasping failure was usually delayed until after the last gasp in hyperthermic animals. Mild dehydration occurred during T(B) elevation, but this did not appear to be a primary factor in AR failure. We conclude that a thermal stress, which by itself is nonlethal, frequently prevents AR from hypoxic apnea. This may be due, at least in part, to decreased gasp number and duration as well as to hyperthermia-related asynchrony of reflexes regulating heart and gasping frequencies during attempted AR.

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.

Heat stress and sudden infant death syndrome incidence: a United States population epidemiologic study

OBJECTIVES

To determine the role of heat stress in sudden infant death syndrome (SIDS) by examining the SIDS rates during periods of extreme environmental temperatures in a period when most infants were placed prone for sleep.

DESIGN

A retrospective study of SIDS rates and mortality rates attributable to excessive environmental heat in relationship to climatologic temperature was performed. Data were collected for each of 454 counties in 4 states (Arkansas, Georgia, Kansas, and Missouri) from May 1 to September 30, 1980, and were then summed to yield the mortality rates for each 5 degrees F (2.8 degrees C) temperature range.

RESULTS

chi2 analyses revealed significant relationships for heat-related mortality rates and both the maximal daily temperature and mean daily temperature but demonstrated no such relationships for SIDS rates. No association between SIDS rates and heat-related mortality rates was found with Spearman's ranked correlation, for either the maximal daily temperature or the mean daily temperature.

CONCLUSIONS

On the basis of our findings of no significant association between SIDS and either measure of temperature during periods of high heat stress-related death rates, it seems unlikely that the heat stress associated with the combination of prone sleep positions and elevated environmental temperatures has a significant role in the development of SIDS.

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.

Unexpected death and critical illness in Prader-Willi syndrome: Report of ten individuals

Individuals with Prader-Willi syndrome (PWS) generally survive into adulthood. Common causes of death are obesity related cor pulmonale and respiratory failure. We report on a case series of eight children and two adults with unexpected death or critical illness. Our data show age-specific characteristics of PWS patients with fatal or life-threatening illnesses. Under the age of 2 years, childhood illnesses in general were associated with high fever and rapid demise or near-demise. Hypothalamic dysfunction likely plays a role in exaggerated fever response, but also perhaps in central regulation of adrenal function. Below average sized adrenal glands were found in three children, which raises the possibility of unrecognized adrenal insufficiency in a subset of individuals with PWS and emphasizes the vital role of autopsy. The tub drowning death of an adult patient could be related to central hypersomnia, which has been reported in PWS. We suggest that increased risk for critical illness be considered in the discussion of anticipatory guidance for the care of infants with PWS. Since a number of children died while hospitalized, particularly close observation of PWS children who are ill enough to warrant hospital admission is recommended.

The contribution of prone sleeping position to the racial disparity in sudden infant death syndrome: the Chicago Infant Mortality Study

BACKGROUND

Rates of sudden infant death syndrome (SIDS) are over twice as high among African Americans compared with Caucasians. Little is known, however, about the relationship between prone sleeping, other sleep environment factors, and the risk of SIDS in the United States and how differences in risk factors may account for disparities in mortality.

OBJECTIVE

To assess the contribution of prone sleeping position and other potential risk factors to SIDS risk in a primarily high-risk, urban African American population.

DESIGN, SETTING, AND POPULATION

Case-control study consisting of 260 infants ages birth to 1 year who died of SIDS between November 1993 and April 1996. The control group consists of an equal number of infants matched on race, age, and birth weight. Prospectively collected data from the death scene investigation and a follow-up home interview for case infants were compared with equivalent questions for living control participants to identify risk factors for SIDS.

MAIN OUTCOME MEASURES

Risk of SIDS related to prone sleeping position adjusting for potential confounding variables and other risk factors for SIDS, and comparisons by race-ethnicity.

RESULTS

Three quarters of the SIDS infants were African American. There was more than a twofold increased risk of SIDS associated with being placed prone for last sleep compared with the nonprone positions (odds ratio [OR]: 2.4; 95% confidence interval [CI]: 1.6-3.7). This OR increased after adjusting for potential confounding variables and other sleep environment factors (OR: 4.0; 95% CI: 1.8-8.8). Differences were found for African Americans compared with others (OR: 1.8; 95% CI: 1.2-2.6 and OR: 10.3, 95% CI: 10.3 [3.2-33.8, respectively]). The population attributable risk was 31%. Fewer case mothers (46%) than control mothers (64%) reported being advised about sleep position in the hospital after delivery. Of those advised, a similar proportion of case mothers as control mothers were incorrectly told or recalled being told to use the prone position, but prone was recommended in a higher proportion of black mothers (cases and controls combined) compared with nonblack mothers.

CONCLUSIONS

Prone sleeping was found to be a significant risk factor for SIDS in this primarily African American urban sample, and approximately one third of the SIDS deaths could be attributed to this factor. Greater and more effective educational outreach must be extended to African American families and the health personnel serving them to reduce prone prevalence during sleep, which appears, in part, to contribute to the higher rates of SIDS among African American infants.

Beyond the boundary between science and values: re-evaluating the moral dimension of the nurse's role in cot death prevention

This article combines a philosophical critique of the idea that public health nurses are primary technicians who neutrally hand over scientifically established facts on risks to the public and an empirical analysis of the actual work of public health nurses. It is argued that the relationship between facts and values in public health is complex and that, despite the introduction of several scientifically-based standards and guidelines, public health nurses are not technicians. They do moral work and experience ethical dilemmas. To get a grip on the specific character of this moral work, we distance ourselves from the idea that there are ethical dilemmas in public health nursing for which we can provide general ethical rules and principles. Instead we suggest a contextual ethical approach, in which several different kinds of consideration may be important. To illustrate this, we analysed 15 in-depth interviews with nurses involved in the prevention of cot deaths in the Netherlands. It is shown that these nurses do not neutrally pass on the epidemiological facts on the risks of prone sleeping, warm bed-clothes and passive smoking, but they are the moral architects of this preventive practice. It is also shown that this moral work and the ethical dilemmas they experience cannot be characterized in terms of general ethical rules and principles. It becomes clear that the moral work of nurses differs according to the three main risks at stake: the balance between virtue, risk taking and responsibility depends on the specific context.

Arousal from sleep in infants is impaired following an infection

Numerous studies have postulated a link between recent infection and Sudden Infant Death Syndrome (SIDS). In this study we contrasted arousal responses from sleep in infants on the day of discharge from hospital following an infection with those when fully recovered and also with well age-matched control infants. Thirteen term infants comprised the infection group and nine well infants acted as age-matched controls. All infants were studied using daytime polysomnography and multiple measurements of arousal threshold (cm H(2)O) in response to air-jet stimulation applied alternately to the nares were made in both active sleep (AS) and quiet sleep (QS). All infants were studied on two occasions: firstly, immediately before discharge from the Paediatric ward, and secondly, 10-15 days later when they were completely well in the case of the infection group.Arousal thresholds in QS in the infection group were significantly elevated on the day of discharge (262 +/- 48 cm H(2)O) compared with 10-15 days later (205 +/- 31 cm H(2)O, p<0.05). Thresholds in the control group were not different between studies. This study provides evidence that arousability from QS is impaired following an infection and we postulate that this may explain the increased risk for SIDS following infection observed in previous studies.

Changes in the epidemiological pattern of sudden infant death syndrome in southeast Norway, 1984-1998: implications for future prevention and research

AIM

To look for changes in risk factors for sudden infant death syndrome (SIDS) after decrease and stabilisation of the SIDS rate.

METHODS

Questionnaires were distributed to parents of 174 SIDS infants, dying between 1984 and 1998, and 375 age and sex matched controls in southeast Norway.

RESULTS

The proportion of infants sleeping prone has decreased, along with the decrease in SIDS rate for the region during the periods studied, but over half of the SIDS victims are still found in the prone position. As the number of SIDS cases has decreased, additional risk factors have become more significant. Thus, after 1993, a significantly increased risk of SIDS is seen when the mother smokes during pregnancy. After 1993, young maternal age carries an increased risk. Maternal smoking and young maternal age are associated with each other. For SIDS victims, an increase in the number of infants found dead while co-sleeping is seen, and the age peak between 2 and 4 months and the winter peak have become less pronounced.

CONCLUSION

Changes in risk factor profile following the decrease in SIDS rate in the early 1990s, as well as consistency of other factors, provides further clues to SIDS prevention and to the direction of further studies of death mechanisms.

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.

Infant stress and sleep deprivation as an aetiological basis for the sudden infant death syndrome

SIDS is almost invariably sleep-related. Viable syndrome aetiology must be compatible with its many epidemiologically diverse risk factors, each of which directly or indirectly associates with the creation of psychological and/or physiological infant stress, and the subsequent disruption of normal, contented sleep. During essential deep 'rebound' recovery sleep, arousal ability and upper airway muscle tone decrease further to that in normal sleep, with subsequent upper airway obstruction. When stress impact causes sufficient sleep disruption and physiological fatigue, a failure to arouse and so restore sufficient tone to overcome such obstruction results in sudden, unexpected death. SIDS has therefore many causes which share a final lethal mechanical pathway. Evidence is presented for obstructive apnoea during sleep as being the primary syndrome death mode, for sleep disruption, reduced arousal ability, and infant stress in SIDS, and for risk factor association with the creation of this stress. Specific infant vulnerability in the first 6 months of life to stress predominantly related to total dependency on a carer for gratification of need, and to obstructive sleep apnoea due to normal anatomical, physical, and respiratory immaturity, including rapid physiological fatigue, and peaks in sleep and thermal stress vulnerability, are discussed. Further reasons for the limited age period of SIDS, and for reduced neonatal risk, are given. Prone sleeping risk can relate to positional airway obstruction during normal sleep without prior infant stress. Much of SIDS aetiology appears to concern factors related to socio-economic deprivation and subsequent sub-optimal infant care.

Sudden infant death syndrome: a failure of compensatory cerebellar mechanisms?

The mechanisms underlying failure in sudden infant death syndrome may involve inadequate compensatory motor responses to a hypotensive challenge; the insult may result from a shock-like sequence, or from a ventilatory challenge that leads to a hypotensive event. Structures ordinarily not considered in mediating breathing or cardiovascular control, especially cerebellar-related structures, may play a critical role in compensatory responses, and underlie the position-dependent risk for SIDS. Dysfunction in affected brain areas appears to arise prenatally from a compromised fetal environment, with a nicotinic component contributing to the deficient mechanism. Physiologic characteristics of infants who later succumb to SIDS, and cardiovascular events associated with the fatal scenario suggest a failure of interaction between somatomotor and autonomic control mechanisms in infants at risk for the syndrome. A failure of compensatory motor actions to overcome a profound hypotension, perhaps mediated by cerebellar mechanisms that regulate blood pressure, may underlie-the fatal event.

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.

Living at high altitude and risk of sudden infant death syndrome

OBJECTIVE

To investigate the association between altitude of residence and risk of sudden infant death syndrome (SIDS).

METHODS

A retrospective, case control study in the Tyrol, Austria enrolled 99 infants with SIDS occurring between 1984 and 1994, and 136 randomly selected control cases. Data on pregnancy, delivery, child care practice, and socio-demographic characteristics including altitude of residence were collected with a standardised questionnaire.

RESULTS

The risk of SIDS increased gradually with increasing altitude of residence. This relation remained independently significant when the analysis was adjusted for gestational age, birth weight, prenatal care, mother's age at delivery, educational level of parents, and cigarette smoking during pregnancy. The prone sleeping position emerged as an obligatory cofactor in this association. In the whole of Austria, a similar trend of association emerged between the average altitudes in the 99 political counties and the rates of SIDS.

CONCLUSIONS

This study identified altitude of residence as a significant risk predictor of SIDS, primarily in combination with the prone sleeping position. Respiratory disturbances, reduced oxygen saturation, and lower temperatures at high altitude might explain this association.

SIDS--changes in the epidemiological pattern in eastern Norway 1984-1996

The objective was to analyse differences in the epidemiological pattern of sudden death in infancy during two time periods--the Sudden Infant Death Syndrome (SIDS) 'epidemic': 1984-1989, and the period of rapid decline in the SIDS rate 1990-1996. Sex distribution, age, sleeping position, signs of infection, day of the week and place of death were registered and compared for the two time periods studied in all SIDS cases autopsied at the Institute of Forensic Medicine, Oslo. There were significantly more deaths in the age group under four months in the period 1984-89 than in the second period. Prone sleeping position, signs of infection, death outdoors and during the winter were more frequent during the first period than in the second. These features also were more frequent in the age group under four months than in the older babies during the first period. The shift in the epidemiological pattern after 1990, when the risk factor campaign was launched, indicates that prone sleeping position, cold climate, sleeping outdoors and infections seem to be risk factors that are particularly harmful to the youngest infants.

The decline in the incidence of SIDS in Scandinavia and its relation to risk-intervention campaigns. Nordic Epidemiological SIDS Study

A prospective case-control study of sudden infant death syndrome (SIDS) in Norway, Denmark and Sweden between September 1, 1992 and August 31, 1995 comprised 244 cases and 869 matched controls. After the introduction of risk-intervention campaigns, the SIDS incidence decreased from 2.3/1000 live births in Norway, 1.6 in Denmark and 1.0 in Sweden to 0.6/1000 or fewer in all the Scandinavian countries in 1995. The decrease paralleled a decline in the prone sleeping position and there was an accompanying parallel fall in total postneonatal mortality in all three countries. Thus, the risk-reducing campaigns for SIDS have been successful not only in Norway and Denmark, starting from relatively high incidences, but also in Sweden, starting from a low incidence. During the study period, a gradual increase was observed for the effects of prone sleeping, smoking and bottle-feeding as risk factors for SIDS.

Temperature dependency of the vagal chronotropic response in the young puppy: an 'environmental-autonomic interaction'

We investigated the effects of mild hypothermia (34.3 +/- 0.2 degrees C [mean +/- SD]), hyperthermia (40.8 +/- 0.2 degrees C) and hypoxia (PaO2 = 43 +/- 4 mmHg) on the response to heart rate to continuous right vagus nerve stimulation (the 'vagal chronotropic response') in young puppies, aged 5-22 days. Puppies were anesthetized with alpha-chloralose, vagotomized and pre-treated with propranolol (1 mg/kg i.v.) and phentolamine (1 mg/kg, 1-2 mg/kg/h i.v.). Hypoxia (n = 9) did not significantly alter the resting sinus cycle length and did not alter the response of sinus cycle to a 30 s train of 8 Hz right vagal stimulation. Mild hypothermia (n = 8) increased the resting sinus cycle length by 16 +/- 4% and greatly augmented the vagal chronotropic response (from 76 +/- 27% change in the sinus cycle length (normothermia) to 155 +/- 38% (hypothermia)). Both the sinus cycle length and the vagal chronotropic response turned towards pre-hypothermia values with rewarming. Mild hypothermia also increased the negative chronotropic response to 20 micrograms/kg/min i.v. of methacholine (12 +/- 2% change in the sinus cycle length (normothermia) versus 24 +/- 14% (hypothermia)), suggesting that a postsynaptic mechanism is involved in the hypothermia-induced augmentation of the cardiac vagal chronotropic response. In contrast to hypothermia, mild hyperthermia (n = 8) decreased the resting sinus cycle length slightly (-5 +/- 5% change) and significantly attenuated the cardiac vagal chronotropic response (from 88 +/- 28% change in sinus cycle length (normothermia) to 50 +/- 26% (hyperthermia)). These changes were also reversible with the re-establishment of normothermia. This demonstrates that clinically relevant, environmentally-induced changes in body temperature can directly and reversibly modify parasympathetic efferent responses.

Dew-point hygrometry system for measurement of evaporative water loss in infants

Evaporation of water from the skin is an important mechanism in thermal homeostasis. Resistance hygrometry, in which the water vapor pressure gradient above the skin surface is calculated, has been the measurement method of choice in the majority of pediatric investigations. However, resistance hygrometry is influenced by changes in ambient conditions such as relative humidity, surface temperature, and convection currents. We have developed a ventilated capsule method that minimized these potential sources of measurement error and that allowed second-by-second, long-term, continuous measurements of evaporative water loss in sleeping infants. Air with a controlled reference humidity (dew-point temperature = 0 degree C) is delivered to a small, lightweight skin capsule and mixed with the vapor on the surface of the skin. The dew point of the resulting mixture is measured by using a chilled mirror dew-point hygrometer. The system indicates leaks, is mobile, and is accurate within 2%, as determined by gravimetric calibration. Examples from a recording of a 13-wk-old full-term infant obtained by using the system give evaporative water loss rates of approximately 0.02 mgH2O.cm-2.min-1 for normothermic baseline conditions and values up to 0.4 mgH2O.cm-2. min-1 when the subject was being warmed. The system is effective for clinical investigations that require dynamic measurements of water loss.

Can risk factors for over-heating explain epidemiological features of sudden infant death syndrome?

Sudden infant death syndrome is characterized by a number of epidemiological features that have defied simple explanation. Overheating is believed to play a role in its causation. Overheating can be caused by changes in heat production, changes in insulation of the infant, or changes in the external environmental temperature. Risk factors may influence any of these variables. A model is proposed which links risk factors for overheating to the epidemiological distribution of sudden infant death syndrome. This model might account for the frequently reported association between formula-feeding and sudden infant death syndrome.

The effect of temperature on the ventilatory response to carbon dioxide in the neonatal rat

The rectal temperature of newborn rats in the nest varies between 32 and 39 degrees C, depending on environmental temperature and the presence of the dam. Because rebreathing of CO2 occurs in the nest, experiments were carried out to examine the effects of body temperature on the ventilatory response (VR) to CO2 at different postnatal ages. The VR to 2 and 4% carbon dioxide was examined in rat pups by the body plethysmograph method whereas rectal temperature was manipulated to 32-33 degrees C, 36-37 degrees C, and 38-39 degrees C. At three different ages, 2-3 d, 5-6 d, and 8-9 d, rat pups had a VR to both concentrations of CO2 due, principally, to an increase in tidal volume (Vt). Ventilation (Vi) in air was highest at the lowest rectal temperature, and there was a significant, inverse correlation with temperature in the 2-3-d-old and 5-6-d-old rats. The VRs to 2 and 4% CO2 were significantly, inversely, related to body temperature in the youngest group. The direction of temperature modification also had a significant effect, because VRs to both 2 and 4% CO2 were significantly lower in rat pups which were being warmed compared with those being cooled.

Deaths of infants on polystyrene-filled beanbags

We have reviewed 35 deaths of infants who were found face down on polystyrene-filled beanbags or their equivalents. Most of the deaths were recorded as sudden infant death syndrome. One-half of the children died before the age of 2 months. It is suggested that the bags played a major factor in the babies' deaths and are dangerous for young children. Mechanisms involved in the deaths of infants on beanbags may also be applicable to deaths of infants on other types of bedding.

An assessment of behavioural characteristics in infants who died of sudden infant death syndrome using the Early Infancy Temperament Questionnaire

An assessment of the behavioural peculiarities of infants dying of sudden infant death syndrome (SIDS) was carried out using the Early Infancy Temperament Questionnaire (EITQ). Thirty-six infants (25 boys and 11 girls aged 1-4 months) in whom the diagnosis of SIDS was confirmed by full necropsy and who died in St Petersburg from 1990 to 1992 entered the study, and the families were approached contemporaneously. A control group consisted of an equal number of age, sex, geographic distribution and date of birth matched live infants from the same city. The infants who died of SIDS had significantly lower estimates of activity in comparison with the babies from the control group. Behavioural defects may be a contributory factor and a sign of increased risk in some babies presenting with SIDS.

Environment of infants during sleep and risk of the sudden infant death syndrome: results of 1993-5 case-control study for confidential inquiry into stillbirths and deaths in infancy. Confidential Enquiry into Stillbirths and Deaths Regional Coordinators and Researchers

OBJECTIVE

To investigate the role of sleeping arrangements as risk factors for the sudden infant death syndrome after a national risk reduction campaign.

DESIGN

Two year population based case-control study. Parental interviews were conducted for each infant who died and for four controls matched for age and date of interview.

SETTING

Three regions in England with a total population of 17 million people.

SUBJECTS

195 babies who died and 780 matched controls.

RESULTS

Prone and side sleeping positions both carried increased risks of death compared with supine when adjusted for maternal age, parity, gestation, birth weight, exposure to smoke, and other relevant factors in the sleeping environment (multivariate odds ratio = 9.00 (95% confidence interval 2.84 to 28.47) and 1.84 (1.02 to 3.31), respectively). The higher incidence of side rather than prone sleeping led to a higher population attributable risk (side 18.4%, prone 14.2%). More of the infants who died were found with bed covers over their heads (21.58; 6.21 to 74.99). The use of a dummy had an apparent protective effect (0.38; 0.21 to 0.70). Bed sharing for the whole night was a significant risk factor for infants whose mothers smoked (9.25; 2.31 to 34.02). No protective effect of breast feeding could be identified on multivariate analysis.

CONCLUSIONS

This study confirms the importance of certain risk factors for the sudden infant death syndrome and identifies others-for example, covers over the head, side sleeping position-which may be amenable to change by educating and informing parents and health care professionals.

Seasonality and the sudden infant death syndrome during 1987-9 and 1991-3 in Australia and Britain

OBJECTIVE

To determine whether seasonality of the sudden infant death syndrome persists now that rates have fallen, mostly after widespread adoption of the "face upwards" sleeping position.

DESIGN

Monthly data on the sudden infant death syndrome during 1987-9 were compared for seasonality with those of 1991-3; rates were studied as deaths per 1000 live births.

SETTING

Australia and Britain (England, Wales, and Scotland).

SUBJECTS

Infants under 1 year dying of the syndrome (2401 for Australia and 6630 for Britain).

MAIN OUTCOME MEASURE

Extent of seasonal variation (amplitude) was established by cosinor analysis; amplitudes for the earlier and later years were compared.

RESULTS

The rate fell in every month, and, though it did so relatively more in winter than summer, seasonality remained a distinctive feature. In the comparison of amplitudes the ratio between the earlier and later years was 1.4 in both Australia and Britain. Some differences between the hemispheres were noted.

CONCLUSIONS

Seasonality of the sudden infant death syndrome remains to be explained and continues to be an important aetiological lead. Studies from other countries are needed.

Breathing, sleep state, and rectal temperature oscillations

Overheating may cause terminal apnoea and cot death. Rectal temperature and breathing patterns were examined in normal infants at home during the first 6 months of life. Twenty one infants had continuous overnight rectal temperature and breathing recordings for 429 nights (mean 20.4 nights, range 7-30) spaced over the first six months of life. Periods when breathing was 'regular' were directly marked on single night records. Sleep state was determined from respiratory variables. 'Regular' breathing was a reliable marker of 'quiet' sleep (specificity 93%). The duration of 'quiet' sleep increased from 6 to 22 minutes from two weeks to three months of age and then remained static, as did the proportion of sleep spent in the quiet phase (9% to 34%). Rectal temperature fell during 66% of quiet sleep and usually rose during rapid eye movement (REM) sleep. The drop in rectal temperature was maximal at the start of quiet sleep, whereas the maximum rise during REM sleep was reached after 10 to 15 minutes. Oscillations in rectal temperature are associated with changes in sleep and breathing state. The maturation of rectal temperature patterns during the first six months of life are closely related to a maturation of sleep state and breathing patterns.

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.

Deficient heat shock protein expression: a potential mechanism for the sudden infant death syndrome

Induction of heat shock proteins follows a metabolic stress and protects from subsequent stresses. Stressors proposed for the sudden infant death syndrome include infection, environmentally-induced hyperthermia and hypoxia. Failure to express heat shock proteins to such stressful conditions may lead to reduced tolerance, and enhance inappropriate physiologic responses and vulnerability which ultimately may lead to infant death.

Vulnerability of respiratory control in healthy preterm infants placed supine

OBJECTIVE

We tested the hypothesis that healthy preterm infants have attenuated ventilatory responses to hypercapnia, associated with a decreased rib cage contribution to ventilation, in the supine versus prone position.

STUDY DESIGN

We elicited hypercapnic ventilatory responses from 19 healthy preterm infants (postconceptional age 35 +/- 1 weeks) who were being prepared for hospital discharge. The O2 saturation was continuously monitored. Before and during CO2 rebreathing, ventilation was measured with a nasal mask pneumotachygraph and was derived from chest wall motion as determined by respiratory inductance plethysmograph. This measuring method allowed us to compare both ventilation and the percentage rib cage contribution to ventilation between supine and prone positions. Statistical analysis employed analysis of variance with repeated measures.

RESULTS

The supine position was associated with a higher respiratory rate (p < 0.02) and lower O2 saturation (p < 0.007) than the prone position. The increase in ventilation in response to hypercapnia was lower in the supine than in the prone position. This was statistically significant for the respiratory inductance plethysmograph (p < 0.008) but not the pneumotachygraph (p = 0.077), and was associated with a smaller rib cage contribution to ventilation in the supine than in the prone position (p < 0.0001).

CONCLUSION

Respiratory control may be vulnerable when healthy preterm infants are placed supine. Widespread avoidance of the prone position may not be appropriate for such patients.